JVM系列(三)：JVM建立過程解析

等你歸去來發表於2021-02-16

　　上兩篇中梳理了整個java啟動過程中，jvm大致是如何執行的。即釐清了我們認為的jvm的啟動過程。但那裡面僅為一些大致的東西，比如引數解析，驗證，dll載入等等。把最核心的loadJavaVM()交給了一個dll或者so庫。也就是真正的jvm我們並沒有接觸到，我們僅看了一個包裝者或者是上層應用的實現。即我們僅是在jdk的角度看了下虛擬機器，這需要更深入一點。

1. 回顧jvm載入框架

　　雖然jvm的載入核心並不在jdk中，但它確實沒有自己的簡易入口。也就是說jvm想要啟動，還得依靠jdk. 所以，讓我們回顧下jdk是如何帶動jvm的？

1.1. java啟動框架

　　自然是在 JLI_Launch 的入口檢視了。

// share/bin/java.c
/*
 * Entry point.
 */
int
JLI_Launch(int argc, char ** argv,              /* main argc, argc */
        int jargc, const char** jargv,          /* java args */
        int appclassc, const char** appclassv,  /* app classpath */
        const char* fullversion,                /* full version defined */
        const char* dotversion,                 /* dot version defined */
        const char* pname,                      /* program name */
        const char* lname,                      /* launcher name */
        jboolean javaargs,                      /* JAVA_ARGS */
        jboolean cpwildcard,                    /* classpath wildcard*/
        jboolean javaw,                         /* windows-only javaw */
        jint ergo                               /* ergonomics class policy */
)
{
    int mode = LM_UNKNOWN;
    char *what = NULL;
    char *cpath = 0;
    char *main_class = NULL;
    int ret;
    InvocationFunctions ifn;
    jlong start, end;
    char jvmpath[MAXPATHLEN];
    char jrepath[MAXPATHLEN];
    char jvmcfg[MAXPATHLEN];
    _fVersion = fullversion;
    _dVersion = dotversion;
    _launcher_name = lname;
    _program_name = pname;
    _is_java_args = javaargs;
    _wc_enabled = cpwildcard;
    _ergo_policy = ergo;
    // 初始化啟動器
    InitLauncher(javaw);
    // 列印狀態
    DumpState();
    // 跟蹤呼叫啟動
    if (JLI_IsTraceLauncher()) {
        int i;
        printf("Command line args:\n");
        for (i = 0; i < argc ; i++) {
            printf("argv[%d] = %s\n", i, argv[i]);
        }
        AddOption("-Dsun.java.launcher.diag=true", NULL);
    }
    /*
     * Make sure the specified version of the JRE is running.
     *
     * There are three things to note about the SelectVersion() routine:
     *  1) If the version running isn't correct, this routine doesn't
     *     return (either the correct version has been exec'd or an error
     *     was issued).
     *  2) Argc and Argv in this scope are *not* altered by this routine.
     *     It is the responsibility of subsequent code to ignore the
     *     arguments handled by this routine.
     *  3) As a side-effect, the variable "main_class" is guaranteed to
     *     be set (if it should ever be set).  This isn't exactly the
     *     poster child for structured programming, but it is a small
     *     price to pay for not processing a jar file operand twice.
     *     (Note: This side effect has been disabled.  See comment on
     *     bugid 5030265 below.)
     */
    // 解析命令列引數，選擇一jre版本
    SelectVersion(argc, argv, &main_class);
    CreateExecutionEnvironment(&argc, &argv,
                               jrepath, sizeof(jrepath),
                               jvmpath, sizeof(jvmpath),
                               jvmcfg,  sizeof(jvmcfg));
    if (!IsJavaArgs()) {
        // 設定一些特殊的環境變數
        SetJvmEnvironment(argc,argv);
    }
    ifn.CreateJavaVM = 0;
    ifn.GetDefaultJavaVMInitArgs = 0;
    if (JLI_IsTraceLauncher()) {
        start = CounterGet();
    }
    // 載入VM, 重中之重
    if (!LoadJavaVM(jvmpath, &ifn)) {
        return(6);
    }
    if (JLI_IsTraceLauncher()) {
        end   = CounterGet();
    }
    JLI_TraceLauncher("%ld micro seconds to LoadJavaVM\n",
             (long)(jint)Counter2Micros(end-start));
    ++argv;
    --argc;
    // 解析更多引數資訊
    if (IsJavaArgs()) {
        /* Preprocess wrapper arguments */
        TranslateApplicationArgs(jargc, jargv, &argc, &argv);
        if (!AddApplicationOptions(appclassc, appclassv)) {
            return(1);
        }
    } else {
        /* Set default CLASSPATH */
        cpath = getenv("CLASSPATH");
        if (cpath == NULL) {
            cpath = ".";
        }
        SetClassPath(cpath);
    }
    /* Parse command line options; if the return value of
     * ParseArguments is false, the program should exit.
     */
    // 解析引數
    if (!ParseArguments(&argc, &argv, &mode, &what, &ret, jrepath))
    {
        return(ret);
    }
    /* Override class path if -jar flag was specified */
    if (mode == LM_JAR) {
        SetClassPath(what);     /* Override class path */
    }
    /* set the -Dsun.java.command pseudo property */
    SetJavaCommandLineProp(what, argc, argv);
    /* Set the -Dsun.java.launcher pseudo property */
    SetJavaLauncherProp();
    /* set the -Dsun.java.launcher.* platform properties */
    SetJavaLauncherPlatformProps();
    // 進行jvm初始化操作，一般是新開一個執行緒，然後呼叫 JavaMain() 實現java程式碼的權力交接
    return JVMInit(&ifn, threadStackSize, argc, argv, mode, what, ret);
}

　　以上就是java啟動jvm的核心框架。和真正的jvm相關的兩個：1. SelectVersion() 會查詢系統中存在的jvm即jre版本，是否可以被當前使用，以及main_class的驗證；2. 在初始化時會呼叫jvm的 CreateJavaVM()方法，進行jvm真正的建立交接，這是通過函式指標實現的；

　　具體兩個相關操作需要分解下，因為這些過程還是略微複雜的。

1.2. jre的查詢定位與驗證

　　要執行jvm，首先就是要確定系統中是否安裝了相應的jre環境，並確定版本是否正確。

// java.c
/*
 * The SelectVersion() routine ensures that an appropriate version of
 * the JRE is running.  The specification for the appropriate version
 * is obtained from either the manifest of a jar file (preferred) or
 * from command line options.
 * The routine also parses splash screen command line options and
 * passes on their values in private environment variables.
 */
static void
SelectVersion(int argc, char **argv, char **main_class)
{
    char    *arg;
    char    **new_argv;
    char    **new_argp;
    char    *operand;
    char    *version = NULL;
    char    *jre = NULL;
    int     jarflag = 0;
    int     headlessflag = 0;
    int     restrict_search = -1;               /* -1 implies not known */
    manifest_info info;
    char    env_entry[MAXNAMELEN + 24] = ENV_ENTRY "=";
    char    *splash_file_name = NULL;
    char    *splash_jar_name = NULL;
    char    *env_in;
    int     res;
    /*
     * If the version has already been selected, set *main_class
     * with the value passed through the environment (if any) and
     * simply return.
     */
    // _JAVA_VERSION_SET=
    if ((env_in = getenv(ENV_ENTRY)) != NULL) {
        if (*env_in != '\0')
            *main_class = JLI_StringDup(env_in);
        return;
    }
    /*
     * Scan through the arguments for options relevant to multiple JRE
     * support.  For reference, the command line syntax is defined as:
     *
     * SYNOPSIS
     *      java [options] class [argument...]
     *
     *      java [options] -jar file.jar [argument...]
     *
     * As the scan is performed, make a copy of the argument list with
     * the version specification options (new to 1.5) removed, so that
     * a version less than 1.5 can be exec'd.
     *
     * Note that due to the syntax of the native Windows interface
     * CreateProcess(), processing similar to the following exists in
     * the Windows platform specific routine ExecJRE (in java_md.c).
     * Changes here should be reproduced there.
     */
    new_argv = JLI_MemAlloc((argc + 1) * sizeof(char*));
    new_argv[0] = argv[0];
    new_argp = &new_argv[1];
    argc--;
    argv++;
    while ((arg = *argv) != 0 && *arg == '-') {
        if (JLI_StrCCmp(arg, "-version:") == 0) {
            version = arg + 9;
        } else if (JLI_StrCmp(arg, "-jre-restrict-search") == 0) {
            restrict_search = 1;
        } else if (JLI_StrCmp(arg, "-no-jre-restrict-search") == 0) {
            restrict_search = 0;
        } else {
            if (JLI_StrCmp(arg, "-jar") == 0)
                jarflag = 1;
            /* deal with "unfortunate" classpath syntax */
            if ((JLI_StrCmp(arg, "-classpath") == 0 || JLI_StrCmp(arg, "-cp") == 0) &&
              (argc >= 2)) {
                *new_argp++ = arg;
                argc--;
                argv++;
                arg = *argv;
            }
            /*
             * Checking for headless toolkit option in the some way as AWT does:
             * "true" means true and any other value means false
             */
            if (JLI_StrCmp(arg, "-Djava.awt.headless=true") == 0) {
                headlessflag = 1;
            } else if (JLI_StrCCmp(arg, "-Djava.awt.headless=") == 0) {
                headlessflag = 0;
            } else if (JLI_StrCCmp(arg, "-splash:") == 0) {
                splash_file_name = arg+8;
            }
            *new_argp++ = arg;
        }
        argc--;
        argv++;
    }
    if (argc <= 0) {    /* No operand? Possibly legit with -[full]version */
        operand = NULL;
    } else {
        argc--;
        *new_argp++ = operand = *argv++;
    }
    while (argc-- > 0)  /* Copy over [argument...] */
        *new_argp++ = *argv++;
    *new_argp = NULL;
    /*
     * If there is a jar file, read the manifest. If the jarfile can't be
     * read, the manifest can't be read from the jar file, or the manifest
     * is corrupt, issue the appropriate error messages and exit.
     *
     * Even if there isn't a jar file, construct a manifest_info structure
     * containing the command line information.  It's a convenient way to carry
     * this data around.
     */
    if (jarflag && operand) {
        if ((res = JLI_ParseManifest(operand, &info)) != 0) {
            if (res == -1)
                JLI_ReportErrorMessage(JAR_ERROR2, operand);
            else
                JLI_ReportErrorMessage(JAR_ERROR3, operand);
            exit(1);
        }
        /*
         * Command line splash screen option should have precedence
         * over the manifest, so the manifest data is used only if
         * splash_file_name has not been initialized above during command
         * line parsing
         */
        if (!headlessflag && !splash_file_name && info.splashscreen_image_file_name) {
            splash_file_name = info.splashscreen_image_file_name;
            splash_jar_name = operand;
        }
    } else {
        info.manifest_version = NULL;
        info.main_class = NULL;
        info.jre_version = NULL;
        info.jre_restrict_search = 0;
    }
    /*
     * Passing on splash screen info in environment variables
     */
    if (splash_file_name && !headlessflag) {
        char* splash_file_entry = JLI_MemAlloc(JLI_StrLen(SPLASH_FILE_ENV_ENTRY "=")+JLI_StrLen(splash_file_name)+1);
        JLI_StrCpy(splash_file_entry, SPLASH_FILE_ENV_ENTRY "=");
        JLI_StrCat(splash_file_entry, splash_file_name);
        putenv(splash_file_entry);
    }
    if (splash_jar_name && !headlessflag) {
        char* splash_jar_entry = JLI_MemAlloc(JLI_StrLen(SPLASH_JAR_ENV_ENTRY "=")+JLI_StrLen(splash_jar_name)+1);
        JLI_StrCpy(splash_jar_entry, SPLASH_JAR_ENV_ENTRY "=");
        JLI_StrCat(splash_jar_entry, splash_jar_name);
        putenv(splash_jar_entry);
    }
    /*
     * The JRE-Version and JRE-Restrict-Search values (if any) from the
     * manifest are overwritten by any specified on the command line.
     */
    if (version != NULL)
        info.jre_version = version;
    if (restrict_search != -1)
        info.jre_restrict_search = restrict_search;
    /*
     * "Valid" returns (other than unrecoverable errors) follow.  Set
     * main_class as a side-effect of this routine.
     */
    if (info.main_class != NULL)
        *main_class = JLI_StringDup(info.main_class);
    /*
     * If no version selection information is found either on the command
     * line or in the manifest, simply return.
     */
    if (info.jre_version == NULL) {
        JLI_FreeManifest();
        JLI_MemFree(new_argv);
        return;
    }
    /*
     * Check for correct syntax of the version specification (JSR 56).
     */
    if (!JLI_ValidVersionString(info.jre_version)) {
        JLI_ReportErrorMessage(SPC_ERROR1, info.jre_version);
        exit(1);
    }
    /*
     * Find the appropriate JVM on the system. Just to be as forgiving as
     * possible, if the standard algorithms don't locate an appropriate
     * jre, check to see if the one running will satisfy the requirements.
     * This can happen on systems which haven't been set-up for multiple
     * JRE support.
     */
    jre = LocateJRE(&info);
    JLI_TraceLauncher("JRE-Version = %s, JRE-Restrict-Search = %s Selected = %s\n",
        (info.jre_version?info.jre_version:"null"),
        (info.jre_restrict_search?"true":"false"), (jre?jre:"null"));
    if (jre == NULL) {
        if (JLI_AcceptableRelease(GetFullVersion(), info.jre_version)) {
            JLI_FreeManifest();
            JLI_MemFree(new_argv);
            return;
        } else {
            JLI_ReportErrorMessage(CFG_ERROR4, info.jre_version);
            exit(1);
        }
    }
    /*
     * If I'm not the chosen one, exec the chosen one.  Returning from
     * ExecJRE indicates that I am indeed the chosen one.
     *
     * The private environment variable _JAVA_VERSION_SET is used to
     * prevent the chosen one from re-reading the manifest file and
     * using the values found within to override the (potential) command
     * line flags stripped from argv (because the target may not
     * understand them).  Passing the MainClass value is an optimization
     * to avoid locating, expanding and parsing the manifest extra
     * times.
     */
    if (info.main_class != NULL) {
        if (JLI_StrLen(info.main_class) <= MAXNAMELEN) {
            (void)JLI_StrCat(env_entry, info.main_class);
        } else {
            JLI_ReportErrorMessage(CLS_ERROR5, MAXNAMELEN);
            exit(1);
        }
    }
    (void)putenv(env_entry);
    ExecJRE(jre, new_argv);
    JLI_FreeManifest();
    JLI_MemFree(new_argv);
    return;
}

// jre的定位過程
// solaris/bin/java_md_common.c
/*
 *      This is the global entry point. It examines the host for the optimal
 *      JRE to be used by scanning a set of directories.  The set of directories
 *      is platform dependent and can be overridden by the environment
 *      variable JAVA_VERSION_PATH.
 *
 *      This routine itself simply determines the set of appropriate
 *      directories before passing control onto ProcessDir().
 */
char*
LocateJRE(manifest_info* info)
{
    char        *path;
    char        *home;
    char        *target = NULL;
    char        *dp;
    char        *cp;

    /*
     * Start by getting JAVA_VERSION_PATH
     */
    if (info->jre_restrict_search) {
        path = JLI_StringDup(system_dir);
    } else if ((path = getenv("JAVA_VERSION_PATH")) != NULL) {
        path = JLI_StringDup(path);
    } else {
        if ((home = getenv("HOME")) != NULL) {
            path = (char *)JLI_MemAlloc(JLI_StrLen(home) + \
                        JLI_StrLen(system_dir) + JLI_StrLen(user_dir) + 2);
            sprintf(path, "%s%s:%s", home, user_dir, system_dir);
        } else {
            path = JLI_StringDup(system_dir);
        }
    }

    /*
     * Step through each directory on the path. Terminate the scan with
     * the first directory with an acceptable JRE.
     */
    cp = dp = path;
    while (dp != NULL) {
        cp = JLI_StrChr(dp, (int)':');
        if (cp != NULL)
            *cp = '\0';
        if ((target = ProcessDir(info, dp)) != NULL)
            break;
        dp = cp;
        if (dp != NULL)
            dp++;
    }
    JLI_MemFree(path);
    return (target);
}

// 嘗試執行jre, 以驗證其是否有效
// solaris/bin/java_md_common.c
/*
 * Given a path to a jre to execute, this routine checks if this process
 * is indeed that jre.  If not, it exec's that jre.
 *
 * We want to actually check the paths rather than just the version string
 * built into the executable, so that given version specification (and
 * JAVA_VERSION_PATH) will yield the exact same Java environment, regardless
 * of the version of the arbitrary launcher we start with.
 */
void
ExecJRE(char *jre, char **argv)
{
    char    wanted[PATH_MAX];
    const char* progname = GetProgramName();
    const char* execname = NULL;

    /*
     * Resolve the real path to the directory containing the selected JRE.
     */
    if (realpath(jre, wanted) == NULL) {
        JLI_ReportErrorMessage(JRE_ERROR9, jre);
        exit(1);
    }

    /*
     * Resolve the real path to the currently running launcher.
     */
    SetExecname(argv);
    execname = GetExecName();
    if (execname == NULL) {
        JLI_ReportErrorMessage(JRE_ERROR10);
        exit(1);
    }

    /*
     * If the path to the selected JRE directory is a match to the initial
     * portion of the path to the currently executing JRE, we have a winner!
     * If so, just return.
     */
    if (JLI_StrNCmp(wanted, execname, JLI_StrLen(wanted)) == 0)
        return;                 /* I am the droid you were looking for */


    /*
     * This should never happen (because of the selection code in SelectJRE),
     * but check for "impossibly" long path names just because buffer overruns
     * can be so deadly.
     */
    if (JLI_StrLen(wanted) + JLI_StrLen(progname) + 6 > PATH_MAX) {
        JLI_ReportErrorMessage(JRE_ERROR11);
        exit(1);
    }

    /*
     * Construct the path and exec it.
     */
    (void)JLI_StrCat(JLI_StrCat(wanted, "/bin/"), progname);
    argv[0] = JLI_StringDup(progname);
    if (JLI_IsTraceLauncher()) {
        int i;
        printf("ReExec Command: %s (%s)\n", wanted, argv[0]);
        printf("ReExec Args:");
        for (i = 1; argv[i] != NULL; i++)
            printf(" %s", argv[i]);
        printf("\n");
    }
    JLI_TraceLauncher("TRACER_MARKER:About to EXEC\n");
    (void)fflush(stdout);
    (void)fflush(stderr);
    execv(wanted, argv);
    JLI_ReportErrorMessageSys(JRE_ERROR12, wanted);
    exit(1);
}

　　接下來有個環境準備的過程，有點複雜。主要就是根據不同的平臺要求，設定一些環境變數，想看更多的同學自行展開。

void
CreateExecutionEnvironment(int *pargc, char ***pargv,
                           char jrepath[], jint so_jrepath,
                           char jvmpath[], jint so_jvmpath,
                           char jvmcfg[],  jint so_jvmcfg) {
  /*
   * First, determine if we are running the desired data model.  If we
   * are running the desired data model, all the error messages
   * associated with calling GetJREPath, ReadKnownVMs, etc. should be
   * output.  However, if we are not running the desired data model,
   * some of the errors should be suppressed since it is more
   * informative to issue an error message based on whether or not the
   * os/processor combination has dual mode capabilities.
   */
    jboolean jvmpathExists;

    /* Compute/set the name of the executable */
    SetExecname(*pargv);

    /* Check data model flags, and exec process, if needed */
    {
      char *arch        = (char *)GetArch(); /* like sparc or sparcv9 */
      char * jvmtype    = NULL;
      int  argc         = *pargc;
      char **argv       = *pargv;
      int running       = CURRENT_DATA_MODEL;

      int wanted        = running;      /* What data mode is being
                                           asked for? Current model is
                                           fine unless another model
                                           is asked for */
#ifdef SETENV_REQUIRED
      jboolean mustsetenv = JNI_FALSE;
      char *runpath     = NULL; /* existing effective LD_LIBRARY_PATH setting */
      char* new_runpath = NULL; /* desired new LD_LIBRARY_PATH string */
      char* newpath     = NULL; /* path on new LD_LIBRARY_PATH */
      char* lastslash   = NULL;
      char** newenvp    = NULL; /* current environment */
#ifdef __solaris__
      char*  dmpath     = NULL;  /* data model specific LD_LIBRARY_PATH,
                                    Solaris only */
#endif /* __solaris__ */
#endif  /* SETENV_REQUIRED */

      char** newargv    = NULL;
      int    newargc    = 0;

      /*
       * Starting in 1.5, all unix platforms accept the -d32 and -d64
       * options.  On platforms where only one data-model is supported
       * (e.g. ia-64 Linux), using the flag for the other data model is
       * an error and will terminate the program.
       */

      { /* open new scope to declare local variables */
        int i;

        newargv = (char **)JLI_MemAlloc((argc+1) * sizeof(char*));
        newargv[newargc++] = argv[0];

        /* scan for data model arguments and remove from argument list;
           last occurrence determines desired data model */
        for (i=1; i < argc; i++) {

          if (JLI_StrCmp(argv[i], "-J-d64") == 0 || JLI_StrCmp(argv[i], "-d64") == 0) {
            wanted = 64;
            continue;
          }
          if (JLI_StrCmp(argv[i], "-J-d32") == 0 || JLI_StrCmp(argv[i], "-d32") == 0) {
            wanted = 32;
            continue;
          }
          newargv[newargc++] = argv[i];

          if (IsJavaArgs()) {
            if (argv[i][0] != '-') continue;
          } else {
            if (JLI_StrCmp(argv[i], "-classpath") == 0 || JLI_StrCmp(argv[i], "-cp") == 0) {
              i++;
              if (i >= argc) break;
              newargv[newargc++] = argv[i];
              continue;
            }
            if (argv[i][0] != '-') { i++; break; }
          }
        }

        /* copy rest of args [i .. argc) */
        while (i < argc) {
          newargv[newargc++] = argv[i++];
        }
        newargv[newargc] = NULL;

        /*
         * newargv has all proper arguments here
         */

        argc = newargc;
        argv = newargv;
      }

      /* If the data model is not changing, it is an error if the
         jvmpath does not exist */
      if (wanted == running) {
        /* Find out where the JRE is that we will be using. */
        if (!GetJREPath(jrepath, so_jrepath, arch, JNI_FALSE) ) {
          JLI_ReportErrorMessage(JRE_ERROR1);
          exit(2);
        }
        JLI_Snprintf(jvmcfg, so_jvmcfg, "%s%slib%s%s%sjvm.cfg",
                     jrepath, FILESEP, FILESEP,  arch, FILESEP);
        /* Find the specified JVM type */
        if (ReadKnownVMs(jvmcfg, JNI_FALSE) < 1) {
          JLI_ReportErrorMessage(CFG_ERROR7);
          exit(1);
        }

        jvmpath[0] = '\0';
        jvmtype = CheckJvmType(pargc, pargv, JNI_FALSE);
        if (JLI_StrCmp(jvmtype, "ERROR") == 0) {
            JLI_ReportErrorMessage(CFG_ERROR9);
            exit(4);
        }

        if (!GetJVMPath(jrepath, jvmtype, jvmpath, so_jvmpath, arch, 0 )) {
          JLI_ReportErrorMessage(CFG_ERROR8, jvmtype, jvmpath);
          exit(4);
        }
        /*
         * we seem to have everything we need, so without further ado
         * we return back, otherwise proceed to set the environment.
         */
#ifdef SETENV_REQUIRED
        mustsetenv = RequiresSetenv(wanted, jvmpath);
        JLI_TraceLauncher("mustsetenv: %s\n", mustsetenv ? "TRUE" : "FALSE");

        if (mustsetenv == JNI_FALSE) {
            JLI_MemFree(newargv);
            return;
        }
#else
        JLI_MemFree(newargv);
        return;
#endif /* SETENV_REQUIRED */
      } else {  /* do the same speculatively or exit */
#ifdef DUAL_MODE
        if (running != wanted) {
          /* Find out where the JRE is that we will be using. */
          if (!GetJREPath(jrepath, so_jrepath, GetArchPath(wanted), JNI_TRUE)) {
            /* give up and let other code report error message */
            JLI_ReportErrorMessage(JRE_ERROR2, wanted);
            exit(1);
          }
          JLI_Snprintf(jvmcfg, so_jvmcfg, "%s%slib%s%s%sjvm.cfg",
                       jrepath, FILESEP, FILESEP, GetArchPath(wanted), FILESEP);
          /*
           * Read in jvm.cfg for target data model and process vm
           * selection options.
           */
          if (ReadKnownVMs(jvmcfg, JNI_TRUE) < 1) {
            /* give up and let other code report error message */
            JLI_ReportErrorMessage(JRE_ERROR2, wanted);
            exit(1);
          }
          jvmpath[0] = '\0';
          jvmtype = CheckJvmType(pargc, pargv, JNI_TRUE);
          if (JLI_StrCmp(jvmtype, "ERROR") == 0) {
            JLI_ReportErrorMessage(CFG_ERROR9);
            exit(4);
          }

          /* exec child can do error checking on the existence of the path */
          jvmpathExists = GetJVMPath(jrepath, jvmtype, jvmpath, so_jvmpath, GetArchPath(wanted), 0);
#ifdef SETENV_REQUIRED
          mustsetenv = RequiresSetenv(wanted, jvmpath);
#endif /* SETENV_REQUIRED */
        }
#else /* ! DUALMODE */
        JLI_ReportErrorMessage(JRE_ERROR2, wanted);
        exit(1);
#endif /* DUAL_MODE */
        }
#ifdef SETENV_REQUIRED
        if (mustsetenv) {
            /*
             * We will set the LD_LIBRARY_PATH as follows:
             *
             *     o          $JVMPATH (directory portion only)
             *     o          $JRE/lib/$LIBARCHNAME
             *     o          $JRE/../lib/$LIBARCHNAME
             *
             * followed by the user's previous effective LD_LIBRARY_PATH, if
             * any.
             */

#ifdef __solaris__
            /*
             * Starting in Solaris 7, ld.so.1 supports three LD_LIBRARY_PATH
             * variables:
             *
             * 1. LD_LIBRARY_PATH -- used for 32 and 64 bit searches if
             * data-model specific variables are not set.
             *
             * 2. LD_LIBRARY_PATH_64 -- overrides and replaces LD_LIBRARY_PATH
             * for 64-bit binaries.
             *
             * 3. LD_LIBRARY_PATH_32 -- overrides and replaces LD_LIBRARY_PATH
             * for 32-bit binaries.
             *
             * The vm uses LD_LIBRARY_PATH to set the java.library.path system
             * property.  To shield the vm from the complication of multiple
             * LD_LIBRARY_PATH variables, if the appropriate data model
             * specific variable is set, we will act as if LD_LIBRARY_PATH had
             * the value of the data model specific variant and the data model
             * specific variant will be unset.  Note that the variable for the
             * *wanted* data model must be used (if it is set), not simply the
             * current running data model.
             */

            switch (wanted) {
                case 0:
                    if (running == 32) {
                        dmpath = getenv("LD_LIBRARY_PATH_32");
                        wanted = 32;
                    } else {
                        dmpath = getenv("LD_LIBRARY_PATH_64");
                        wanted = 64;
                    }
                    break;

                case 32:
                    dmpath = getenv("LD_LIBRARY_PATH_32");
                    break;

                case 64:
                    dmpath = getenv("LD_LIBRARY_PATH_64");
                    break;

                default:
                    JLI_ReportErrorMessage(JRE_ERROR3, __LINE__);
                    exit(1); /* unknown value in wanted */
                    break;
            }

            /*
             * If dmpath is NULL, the relevant data model specific variable is
             * not set and normal LD_LIBRARY_PATH should be used.
             */
            if (dmpath == NULL) {
                runpath = getenv("LD_LIBRARY_PATH");
            } else {
                runpath = dmpath;
            }
#else /* ! __solaris__ */
            /*
             * If not on Solaris, assume only a single LD_LIBRARY_PATH
             * variable.
             */
            runpath = getenv("LD_LIBRARY_PATH");
#endif /* __solaris__ */

            /* runpath contains current effective LD_LIBRARY_PATH setting */

            jvmpath = JLI_StringDup(jvmpath);
            new_runpath = JLI_MemAlloc(((runpath != NULL) ? JLI_StrLen(runpath) : 0) +
                    2 * JLI_StrLen(jrepath) + 2 * JLI_StrLen(arch) +
                    JLI_StrLen(jvmpath) + 52);
            newpath = new_runpath + JLI_StrLen("LD_LIBRARY_PATH=");


            /*
             * Create desired LD_LIBRARY_PATH value for target data model.
             */
            {
                /* remove the name of the .so from the JVM path */
                lastslash = JLI_StrRChr(jvmpath, '/');
                if (lastslash)
                    *lastslash = '\0';

                sprintf(new_runpath, "LD_LIBRARY_PATH="
                        "%s:"
                        "%s/lib/%s:"
                        "%s/../lib/%s",
                        jvmpath,
#ifdef DUAL_MODE
                        jrepath, GetArchPath(wanted),
                        jrepath, GetArchPath(wanted)
#else /* !DUAL_MODE */
                        jrepath, arch,
                        jrepath, arch
#endif /* DUAL_MODE */
                        );


                /*
                 * Check to make sure that the prefix of the current path is the
                 * desired environment variable setting, though the RequiresSetenv
                 * checks if the desired runpath exists, this logic does a more
                 * comprehensive check.
                 */
                if (runpath != NULL &&
                        JLI_StrNCmp(newpath, runpath, JLI_StrLen(newpath)) == 0 &&
                        (runpath[JLI_StrLen(newpath)] == 0 || runpath[JLI_StrLen(newpath)] == ':') &&
                        (running == wanted) /* data model does not have to be changed */
#ifdef __solaris__
                        && (dmpath == NULL) /* data model specific variables not set  */
#endif /* __solaris__ */
                        ) {
                    JLI_MemFree(newargv);
                    JLI_MemFree(new_runpath);
                    return;
                }
            }

            /*
             * Place the desired environment setting onto the prefix of
             * LD_LIBRARY_PATH.  Note that this prevents any possible infinite
             * loop of execv() because we test for the prefix, above.
             */
            if (runpath != 0) {
                JLI_StrCat(new_runpath, ":");
                JLI_StrCat(new_runpath, runpath);
            }

            if (putenv(new_runpath) != 0) {
                exit(1); /* problem allocating memory; LD_LIBRARY_PATH not set
                    properly */
            }

            /*
             * Unix systems document that they look at LD_LIBRARY_PATH only
             * once at startup, so we have to re-exec the current executable
             * to get the changed environment variable to have an effect.
             */

#ifdef __solaris__
            /*
             * If dmpath is not NULL, remove the data model specific string
             * in the environment for the exec'ed child.
             */
            if (dmpath != NULL)
                (void)UnsetEnv((wanted == 32) ? "LD_LIBRARY_PATH_32" : "LD_LIBRARY_PATH_64");
#endif /* __solaris */

            newenvp = environ;
        }
#endif /* SETENV_REQUIRED */
        {
            char *newexec = execname;
#ifdef DUAL_MODE
            /*
             * If the data model is being changed, the path to the
             * executable must be updated accordingly; the executable name
             * and directory the executable resides in are separate.  In the
             * case of 32 => 64, the new bits are assumed to reside in, e.g.
             * "olddir/LIBARCH64NAME/execname"; in the case of 64 => 32,
             * the bits are assumed to be in "olddir/../execname".  For example,
             *
             * olddir/sparcv9/execname
             * olddir/amd64/execname
             *
             * for Solaris SPARC and Linux amd64, respectively.
             */

            if (running != wanted) {
                char *oldexec = JLI_StrCpy(JLI_MemAlloc(JLI_StrLen(execname) + 1), execname);
                char *olddir = oldexec;
                char *oldbase = JLI_StrRChr(oldexec, '/');


                newexec = JLI_MemAlloc(JLI_StrLen(execname) + 20);
                *oldbase++ = 0;
                sprintf(newexec, "%s/%s/%s", olddir,
                        ((wanted == 64) ? LIBARCH64NAME : ".."), oldbase);
                argv[0] = newexec;
            }
#endif /* DUAL_MODE */
            JLI_TraceLauncher("TRACER_MARKER:About to EXEC\n");
            (void) fflush(stdout);
            (void) fflush(stderr);
#ifdef SETENV_REQUIRED
            if (mustsetenv) {
                execve(newexec, argv, newenvp);
            } else {
                execv(newexec, argv);
            }
#else /* !SETENV_REQUIRED */
            execv(newexec, argv);
#endif /* SETENV_REQUIRED */
            JLI_ReportErrorMessageSys(JRE_ERROR4, newexec);

#ifdef DUAL_MODE
            if (running != wanted) {
                JLI_ReportErrorMessage(JRE_ERROR5, wanted, running);
#ifdef __solaris__
#ifdef __sparc
                JLI_ReportErrorMessage(JRE_ERROR6);
#else  /* ! __sparc__ */
                JLI_ReportErrorMessage(JRE_ERROR7);
#endif  /* __sparc */
#endif /* __solaris__ */
            }
#endif /* DUAL_MODE */

        }
        exit(1);
    }
}

View Code

1.3. 裝載jvm

　　經過前面的查詢與驗證，已經確認系統上有相應的jre環境了。但還沒有進行真正的呼叫，這是重中之重。不過其實現卻也是簡單的，因為，它只是載入一個外部動態庫而已。其主要目的在於獲取與動態庫的聯絡，直接些就是獲取幾個jvm的函式指標入口，以便後續可以呼叫。這和我們常說的面向介面程式設計，也一脈相承。

// 我們們就只看linux版本的實現好了，原理都一樣，各平臺實現不同而已（API規範不同）
// solaris/bin/java_md_solinux.c
jboolean
LoadJavaVM(const char *jvmpath, InvocationFunctions *ifn)
{
    void *libjvm;

    JLI_TraceLauncher("JVM path is %s\n", jvmpath);
    // jvmpath 是在前面解析出的地址, 直接載入開啟即可獲得
    libjvm = dlopen(jvmpath, RTLD_NOW + RTLD_GLOBAL);
    if (libjvm == NULL) {
#if defined(__solaris__) && defined(__sparc) && !defined(_LP64) /* i.e. 32-bit sparc */
      FILE * fp;
      Elf32_Ehdr elf_head;
      int count;
      int location;

      fp = fopen(jvmpath, "r");
      if (fp == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
      }

      /* read in elf header */
      count = fread((void*)(&elf_head), sizeof(Elf32_Ehdr), 1, fp);
      fclose(fp);
      if (count < 1) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
      }

      /*
       * Check for running a server vm (compiled with -xarch=v8plus)
       * on a stock v8 processor.  In this case, the machine type in
       * the elf header would not be included the architecture list
       * provided by the isalist command, which is turn is gotten from
       * sysinfo.  This case cannot occur on 64-bit hardware and thus
       * does not have to be checked for in binaries with an LP64 data
       * model.
       */
      if (elf_head.e_machine == EM_SPARC32PLUS) {
        char buf[257];  /* recommended buffer size from sysinfo man
                           page */
        long length;
        char* location;

        length = sysinfo(SI_ISALIST, buf, 257);
        if (length > 0) {
            location = JLI_StrStr(buf, "sparcv8plus ");
          if (location == NULL) {
            JLI_ReportErrorMessage(JVM_ERROR3);
            return JNI_FALSE;
          }
        }
      }
#endif
        JLI_ReportErrorMessage(DLL_ERROR1, __LINE__);
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }
    // 載入jvm的目的，主要就是為了獲取 JNI_CreateJavaVM, 
    //         JNI_GetDefaultJavaVMInitArgs, JNI_GetCreatedJavaVMs 這些個指標
    ifn->CreateJavaVM = (CreateJavaVM_t)
        dlsym(libjvm, "JNI_CreateJavaVM");
    if (ifn->CreateJavaVM == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }

    ifn->GetDefaultJavaVMInitArgs = (GetDefaultJavaVMInitArgs_t)
        dlsym(libjvm, "JNI_GetDefaultJavaVMInitArgs");
    if (ifn->GetDefaultJavaVMInitArgs == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }

    ifn->GetCreatedJavaVMs = (GetCreatedJavaVMs_t)
        dlsym(libjvm, "JNI_GetCreatedJavaVMs");
    if (ifn->GetCreatedJavaVMs == NULL) {
        JLI_ReportErrorMessage(DLL_ERROR2, jvmpath, dlerror());
        return JNI_FALSE;
    }

    return JNI_TRUE;
}

　　可見裝載jvm的過程顯得很清晰明瞭，因為並沒有做真正的呼叫，所以也只是算是處理初始化階段。有簡單的系統api提供，一切都很輕量級。

　　而jvm的真正建立，是在進行JvmInit()時，準備載入 main_class 時，才進行的的。

1.4. 回顧JavaMain執行框架

　　JavaMain是真正接入java程式碼的地方，它一般是會開啟一個新執行緒去執行。前置呼叫可自行展開。

// 只看在linux中的實現
// solaris/bin/java_solinux.c
int
JVMInit(InvocationFunctions* ifn, jlong threadStackSize,
        int argc, char **argv,
        int mode, char *what, int ret)
{
    ShowSplashScreen();
    return ContinueInNewThread(ifn, threadStackSize, argc, argv, mode, what, ret);
}
// java.c
int
ContinueInNewThread(InvocationFunctions* ifn, jlong threadStackSize,
                    int argc, char **argv,
                    int mode, char *what, int ret)
{

    /*
     * If user doesn't specify stack size, check if VM has a preference.
     * Note that HotSpot no longer supports JNI_VERSION_1_1 but it will
     * return its default stack size through the init args structure.
     */
    if (threadStackSize == 0) {
      struct JDK1_1InitArgs args1_1;
      memset((void*)&args1_1, 0, sizeof(args1_1));
      args1_1.version = JNI_VERSION_1_1;
      ifn->GetDefaultJavaVMInitArgs(&args1_1);  /* ignore return value */
      if (args1_1.javaStackSize > 0) {
         threadStackSize = args1_1.javaStackSize;
      }
    }

    { /* Create a new thread to create JVM and invoke main method */
      JavaMainArgs args;
      int rslt;

      args.argc = argc;
      args.argv = argv;
      args.mode = mode;
      args.what = what;
      args.ifn = *ifn;
      // 傳入 JavaMain, 在新執行緒中呼叫
      rslt = ContinueInNewThread0(JavaMain, threadStackSize, (void*)&args);
      /* If the caller has deemed there is an error we
       * simply return that, otherwise we return the value of
       * the callee
       */
      return (ret != 0) ? ret : rslt;
    }
}

// solaris/bin/java_md_solinux.c
/*
 * Block current thread and continue execution in a new thread
 */
int
ContinueInNewThread0(int (JNICALL *continuation)(void *), jlong stack_size, void * args) {
    int rslt;
#ifdef __linux__
    pthread_t tid;
    pthread_attr_t attr;
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

    if (stack_size > 0) {
      pthread_attr_setstacksize(&attr, stack_size);
    }
    // 常見的 pthread_xx 方式 建立執行緒
    if (pthread_create(&tid, &attr, (void *(*)(void*))continuation, (void*)args) == 0) {
      void * tmp;
      pthread_join(tid, &tmp);
      rslt = (int)tmp;
    } else {
     /*
      * Continue execution in current thread if for some reason (e.g. out of
      * memory/LWP)  a new thread can't be created. This will likely fail
      * later in continuation as JNI_CreateJavaVM needs to create quite a
      * few new threads, anyway, just give it a try..
      */
      rslt = continuation(args);
    }

    pthread_attr_destroy(&attr);
#else /* ! __linux__ */
    thread_t tid;
    long flags = 0;
    if (thr_create(NULL, stack_size, (void *(*)(void *))continuation, args, flags, &tid) == 0) {
      void * tmp;
      thr_join(tid, NULL, &tmp);
      rslt = (int)tmp;
    } else {
      /* See above. Continue in current thread if thr_create() failed */
      rslt = continuation(args);
    }
#endif /* __linux__ */
    return rslt;
}

View Code

　　JavaMain() 是執行java程式碼或者建立jvm的核心入口。

// share/bin/java.c
// 載入 main 函式類
// 通過引入 JavaMain(), 接入java方法
// #define JNICALL __stdcall
int JNICALL
JavaMain(void * _args)
{
    JavaMainArgs *args = (JavaMainArgs *)_args;
    int argc = args->argc;
    char **argv = args->argv;
    int mode = args->mode;
    char *what = args->what;
    // 一些jvm的呼叫例項，在之前的步驟中，通過載入相應動態連結方法，儲存起來的
    /** 
     * ifn->CreateJavaVM =
     *   (void *)GetProcAddress(handle, "JNI_CreateJavaVM");
     * ifn->GetDefaultJavaVMInitArgs =
     *   (void *)GetProcAddress(handle, "JNI_GetDefaultJavaVMInitArgs");
     */
    InvocationFunctions ifn = args->ifn;
    JavaVM *vm = 0;
    JNIEnv *env = 0;
    jclass mainClass = NULL;
    jclass appClass = NULL; // actual application class being launched
    jmethodID mainID;
    jobjectArray mainArgs;
    int ret = 0;
    jlong start, end;
    // collector
    RegisterThread();
    /* Initialize the virtual machine */
    start = CounterGet();
    // 重點1：初始化jvm，失敗則退出
    // 此處會將重要變數 *env 程式初始化，從而使後續可用
    if (!InitializeJVM(&vm, &env, &ifn)) {
        JLI_ReportErrorMessage(JVM_ERROR1);
        exit(1);
    }
    // jvm檢查完畢，如果只是一些展示類請求，則展示資訊後，退出jvm
    if (showSettings != NULL) {
        ShowSettings(env, showSettings);
        /**
         * 巨集是神奇的操作，此處 *env 直接引用
#define CHECK_EXCEPTION_LEAVE(CEL_return_value) \
    do { \
        if ((*env)->ExceptionOccurred(env)) { \
            JLI_ReportExceptionDescription(env); \
            ret = (CEL_return_value); \
            LEAVE(); \
        } \
    } while (JNI_FALSE)
         */
        CHECK_EXCEPTION_LEAVE(1);
    }
    // 呼叫 LEAVE() 方法的目的在於主動銷燬jvm執行緒
    // 且退出當前方法呼叫，即 LEAVE() 後方法不再被執行
/*
 * Always detach the main thread so that it appears to have ended when
 * the application's main method exits.  This will invoke the
 * uncaught exception handler machinery if main threw an
 * exception.  An uncaught exception handler cannot change the
 * launcher's return code except by calling System.exit.
 *
 * Wait for all non-daemon threads to end, then destroy the VM.
 * This will actually create a trivial new Java waiter thread
 * named "DestroyJavaVM", but this will be seen as a different
 * thread from the one that executed main, even though they are
 * the same C thread.  This allows mainThread.join() and
 * mainThread.isAlive() to work as expected.
 */
    /**
     *
     * 
#define LEAVE() \
    do { \
        if ((*vm)->DetachCurrentThread(vm) != JNI_OK) { \
            JLI_ReportErrorMessage(JVM_ERROR2); \
            ret = 1; \
        } \
        if (JNI_TRUE) { \
            (*vm)->DestroyJavaVM(vm); \
            return ret; \
        } \
    } while (JNI_FALSE)
     */
    if (printVersion || showVersion) {
        PrintJavaVersion(env, showVersion);
        CHECK_EXCEPTION_LEAVE(0);
        if (printVersion) {
            LEAVE();
        }
    }
    /* If the user specified neither a class name nor a JAR file */
    if (printXUsage || printUsage || what == 0 || mode == LM_UNKNOWN) {
        PrintUsage(env, printXUsage);
        CHECK_EXCEPTION_LEAVE(1);
        LEAVE();
    }
    // 釋放記憶體
    FreeKnownVMs();  /* after last possible PrintUsage() */
    if (JLI_IsTraceLauncher()) {
        end = CounterGet();
        JLI_TraceLauncher("%ld micro seconds to InitializeJVM\n",
               (long)(jint)Counter2Micros(end-start));
    }
    /* At this stage, argc/argv have the application's arguments */
    if (JLI_IsTraceLauncher()){
        int i;
        printf("%s is '%s'\n", launchModeNames[mode], what);
        printf("App's argc is %d\n", argc);
        for (i=0; i < argc; i++) {
            printf("    argv[%2d] = '%s'\n", i, argv[i]);
        }
    }
    ret = 1;
    /*
     * Get the application's main class.
     *
     * See bugid 5030265.  The Main-Class name has already been parsed
     * from the manifest, but not parsed properly for UTF-8 support.
     * Hence the code here ignores the value previously extracted and
     * uses the pre-existing code to reextract the value.  This is
     * possibly an end of release cycle expedient.  However, it has
     * also been discovered that passing some character sets through
     * the environment has "strange" behavior on some variants of
     * Windows.  Hence, maybe the manifest parsing code local to the
     * launcher should never be enhanced.
     *
     * Hence, future work should either:
     *     1)   Correct the local parsing code and verify that the
     *          Main-Class attribute gets properly passed through
     *          all environments,
     *     2)   Remove the vestages of maintaining main_class through
     *          the environment (and remove these comments).
     *
     * This method also correctly handles launching existing JavaFX
     * applications that may or may not have a Main-Class manifest entry.
     */
    // 重點2：載入 main 指定的class類
    mainClass = LoadMainClass(env, mode, what);
    CHECK_EXCEPTION_NULL_LEAVE(mainClass);
    /*
     * In some cases when launching an application that needs a helper, e.g., a
     * JavaFX application with no main method, the mainClass will not be the
     * applications own main class but rather a helper class. To keep things
     * consistent in the UI we need to track and report the application main class.
     */
    appClass = GetApplicationClass(env);
    NULL_CHECK_RETURN_VALUE(appClass, -1);
    /*
     * PostJVMInit uses the class name as the application name for GUI purposes,
     * for example, on OSX this sets the application name in the menu bar for
     * both SWT and JavaFX. So we'll pass the actual application class here
     * instead of mainClass as that may be a launcher or helper class instead
     * of the application class.
     */
    // 載入main() 方法前執行初始化
    PostJVMInit(env, appClass, vm);
    CHECK_EXCEPTION_LEAVE(1);
    /*
     * The LoadMainClass not only loads the main class, it will also ensure
     * that the main method's signature is correct, therefore further checking
     * is not required. The main method is invoked here so that extraneous java
     * stacks are not in the application stack trace.
     */
    // 重點3：執行 main(args[]) java方法
    // 獲取main()方法id, main(String[] args)
    mainID = (*env)->GetStaticMethodID(env, mainClass, "main",
                                       "([Ljava/lang/String;)V");
    CHECK_EXCEPTION_NULL_LEAVE(mainID);
    /* Build platform specific argument array */
    // 構建args[] 引數
    mainArgs = CreateApplicationArgs(env, argv, argc);
    CHECK_EXCEPTION_NULL_LEAVE(mainArgs);
    /* Invoke main method. */
    // 呼叫java實現的main()方法
    // XX:: 重要實現
    (*env)->CallStaticVoidMethod(env, mainClass, mainID, mainArgs);
    /*
     * The launcher's exit code (in the absence of calls to
     * System.exit) will be non-zero if main threw an exception.
     */
    ret = (*env)->ExceptionOccurred(env) == NULL ? 0 : 1;
    LEAVE();
}
/*
 * Loads a class and verifies that the main class is present and it is ok to
 * call it for more details refer to the java implementation.
 */
static jclass
LoadMainClass(JNIEnv *env, int mode, char *name)
{
    jmethodID mid;
    jstring str;
    jobject result;
    jlong start, end;
    // sun/launcher/LauncherHelper
    jclass cls = GetLauncherHelperClass(env);
    NULL_CHECK0(cls);
    if (JLI_IsTraceLauncher()) {
        start = CounterGet();
    }
    // checkAndLoadMain(String) 方法作為中間main()呼叫
    NULL_CHECK0(mid = (*env)->GetStaticMethodID(env, cls,
                "checkAndLoadMain",
                "(ZILjava/lang/String;)Ljava/lang/Class;"));
    str = NewPlatformString(env, name);
    CHECK_JNI_RETURN_0(
        result = (*env)->CallStaticObjectMethod(
            env, cls, mid, USE_STDERR, mode, str));
    if (JLI_IsTraceLauncher()) {
        end   = CounterGet();
        printf("%ld micro seconds to load main class\n",
               (long)(jint)Counter2Micros(end-start));
        printf("----%s----\n", JLDEBUG_ENV_ENTRY);
    }
    return (jclass)result;
}    
jclass
GetLauncherHelperClass(JNIEnv *env)
{
    if (helperClass == NULL) {
        NULL_CHECK0(helperClass = FindBootStrapClass(env,
                "sun/launcher/LauncherHelper"));
    }
    return helperClass;
}

　　JavaMain 框架大概就是初始化建立jvm, 查詢mainClass類, 找到函式指定, 構建args引數, 執行main(), 以及其他的一些相容性處理。當JavaMain 執行完成時，則意味著整個jvm就完成了。所以，這也成為了我們要研究的重中之重。

2. 真正的jvm建立

　　jdk是我們看到的jvm前端，而背後的jre或者jvm才是大佬。它是在 JavaMain() 中觸發呼叫的。也就是上節中看到的框架結構。初始化JVM的過程，實際就是呼叫jvm的函式指標 JNI_CreateJavaVM 地過程。

// 初始化jvm, 主要是呼叫 CreateJavaVM() 方法，進行建立jvm操作
/*
 * Initializes the Java Virtual Machine. Also frees options array when
 * finished.
 */
static jboolean
InitializeJVM(JavaVM **pvm, JNIEnv **penv, InvocationFunctions *ifn)
{
    JavaVMInitArgs args;
    jint r;
    memset(&args, 0, sizeof(args));
    args.version  = JNI_VERSION_1_2;
    args.nOptions = numOptions;
    args.options  = options;
    args.ignoreUnrecognized = JNI_FALSE;
    if (JLI_IsTraceLauncher()) {
        int i = 0;
        printf("JavaVM args:\n    ");
        printf("version 0x%08lx, ", (long)args.version);
        printf("ignoreUnrecognized is %s, ",
               args.ignoreUnrecognized ? "JNI_TRUE" : "JNI_FALSE");
        printf("nOptions is %ld\n", (long)args.nOptions);
        for (i = 0; i < numOptions; i++)
            printf("    option[%2d] = '%s'\n",
                   i, args.options[i].optionString);
    }
    // 轉交給jvm執行
    r = ifn->CreateJavaVM(pvm, (void **)penv, &args);
    JLI_MemFree(options);
    return r == JNI_OK;
}

　　單是這 CreateJavaVM(), 就將jvm接入進來了。它的作用，就像是很多語言的 main() 入口一樣，看似簡單，卻包羅永珍。

2.1. jni.h概述

　　jni.h 中定義了許多的jdk可以呼叫的方法。比如上面提到 JNI_CreateJavaVM() 就是建立jvm的核心入口。在openjdk中，是在hotspot中實現的。

　　其中定義了各種java的型別，各種需要的介面。當我們想要自定義寫一些native介面時，則jni.h是我們必須要引入的。其開頭部分如下：

// share/vm/prims/jni.h
/*
 * We used part of Netscape's Java Runtime Interface (JRI) as the starting
 * point of our design and implementation.
 */

/******************************************************************************
 * Java Runtime Interface
 * Copyright (c) 1996 Netscape Communications Corporation. All rights reserved.
 *****************************************************************************/

#ifndef _JAVASOFT_JNI_H_
#define _JAVASOFT_JNI_H_

#include <stdio.h>
#include <stdarg.h>

/* jni_md.h contains the machine-dependent typedefs for jbyte, jint
   and jlong */

#include "jni_md.h"

#ifdef __cplusplus
extern "C" {
#endif

/*
 * JNI Types
 */

#ifndef JNI_TYPES_ALREADY_DEFINED_IN_JNI_MD_H

typedef unsigned char   jboolean;
typedef unsigned short  jchar;
typedef short           jshort;
typedef float           jfloat;
typedef double          jdouble;

typedef jint            jsize;

#ifdef __cplusplus
// c++版本的物件定義
class _jobject {};
class _jclass : public _jobject {};
class _jthrowable : public _jobject {};
class _jstring : public _jobject {};
class _jarray : public _jobject {};
class _jbooleanArray : public _jarray {};
class _jbyteArray : public _jarray {};
class _jcharArray : public _jarray {};
class _jshortArray : public _jarray {};
class _jintArray : public _jarray {};
class _jlongArray : public _jarray {};
class _jfloatArray : public _jarray {};
class _jdoubleArray : public _jarray {};
class _jobjectArray : public _jarray {};

typedef _jobject *jobject;
typedef _jclass *jclass;
typedef _jthrowable *jthrowable;
typedef _jstring *jstring;
typedef _jarray *jarray;
typedef _jbooleanArray *jbooleanArray;
typedef _jbyteArray *jbyteArray;
typedef _jcharArray *jcharArray;
typedef _jshortArray *jshortArray;
typedef _jintArray *jintArray;
typedef _jlongArray *jlongArray;
typedef _jfloatArray *jfloatArray;
typedef _jdoubleArray *jdoubleArray;
typedef _jobjectArray *jobjectArray;

#else
// c版本的物件定義
struct _jobject;

typedef struct _jobject *jobject;
typedef jobject jclass;
typedef jobject jthrowable;
typedef jobject jstring;
typedef jobject jarray;
typedef jarray jbooleanArray;
typedef jarray jbyteArray;
typedef jarray jcharArray;
typedef jarray jshortArray;
typedef jarray jintArray;
typedef jarray jlongArray;
typedef jarray jfloatArray;
typedef jarray jdoubleArray;
typedef jarray jobjectArray;

#endif

typedef jobject jweak;
// java各型別的定義簡寫
typedef union jvalue {
    jboolean z;
    jbyte    b;
    jchar    c;
    jshort   s;
    jint     i;
    jlong    j;
    jfloat   f;
    jdouble  d;
    jobject  l;
} jvalue;

struct _jfieldID;
typedef struct _jfieldID *jfieldID;

struct _jmethodID;
typedef struct _jmethodID *jmethodID;

/* Return values from jobjectRefType */
typedef enum _jobjectType {
     JNIInvalidRefType    = 0,
     JNILocalRefType      = 1,
     JNIGlobalRefType     = 2,
     JNIWeakGlobalRefType = 3
} jobjectRefType;


#endif /* JNI_TYPES_ALREADY_DEFINED_IN_JNI_MD_H */

/*
 * jboolean constants
 */

#define JNI_FALSE 0
#define JNI_TRUE 1

/*
 * possible return values for JNI functions.
 */

#define JNI_OK           0                 /* success */
#define JNI_ERR          (-1)              /* unknown error */
#define JNI_EDETACHED    (-2)              /* thread detached from the VM */
#define JNI_EVERSION     (-3)              /* JNI version error */
#define JNI_ENOMEM       (-4)              /* not enough memory */
#define JNI_EEXIST       (-5)              /* VM already created */
#define JNI_EINVAL       (-6)              /* invalid arguments */

/*
 * used in ReleaseScalarArrayElements
 */

#define JNI_COMMIT 1
#define JNI_ABORT 2

/*
 * used in RegisterNatives to describe native method name, signature,
 * and function pointer.
 */

typedef struct {
    char *name;
    char *signature;
    void *fnPtr;
} JNINativeMethod;
...

　　大概就是相容各平臺，可能使用C實現，也可能使用C++實現。完整版本請參考官網: http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/6ea5a8067d1f/src/share/vm/prims/jni.h

　　其中，有兩個比較重的結構體的定義：JNINativeInterface_ 是jni呼叫的大部分介面定義，基本上可以通過它呼叫任意方法。JNIEnv_ 是每個方法呼叫時的上下文管理器，它負責呼叫 JNINativeInterface_ 的方法，相當於是C++版本的JNINativeInterface_。

3. jvm的建立過程

　　在jni.h中，還有很多C++或者C的判斷，但對於CreateJavaVM這件事，就變成了一個純粹C++的實現了。

// hotspot/src/share/vm/prims/jni.cpp
_JNI_IMPORT_OR_EXPORT_ jint JNICALL JNI_CreateJavaVM(JavaVM **vm, void **penv, void *args) {
#ifndef USDT2
  HS_DTRACE_PROBE3(hotspot_jni, CreateJavaVM__entry, vm, penv, args);
#else /* USDT2 */
  HOTSPOT_JNI_CREATEJAVAVM_ENTRY(
                                 (void **) vm, penv, args);
#endif /* USDT2 */

  jint result = JNI_ERR;
  DT_RETURN_MARK(CreateJavaVM, jint, (const jint&)result);

  // We're about to use Atomic::xchg for synchronization.  Some Zero
  // platforms use the GCC builtin __sync_lock_test_and_set for this,
  // but __sync_lock_test_and_set is not guaranteed to do what we want
  // on all architectures.  So we check it works before relying on it.
#if defined(ZERO) && defined(ASSERT)
  {
    // java 魔術頭
    jint a = 0xcafebabe;
    jint b = Atomic::xchg(0xdeadbeef, &a);
    void *c = &a;
    void *d = Atomic::xchg_ptr(&b, &c);
    assert(a == (jint) 0xdeadbeef && b == (jint) 0xcafebabe, "Atomic::xchg() works");
    assert(c == &b && d == &a, "Atomic::xchg_ptr() works");
  }
#endif // ZERO && ASSERT

  // At the moment it's only possible to have one Java VM,
  // since some of the runtime state is in global variables.

  // We cannot use our mutex locks here, since they only work on
  // Threads. We do an atomic compare and exchange to ensure only
  // one thread can call this method at a time

  // We use Atomic::xchg rather than Atomic::add/dec since on some platforms
  // the add/dec implementations are dependent on whether we are running
  // on a multiprocessor, and at this stage of initialization the os::is_MP
  // function used to determine this will always return false. Atomic::xchg
  // does not have this problem.
  if (Atomic::xchg(1, &vm_created) == 1) {
    return JNI_EEXIST;   // already created, or create attempt in progress
  }
  if (Atomic::xchg(0, &safe_to_recreate_vm) == 0) {
    return JNI_ERR;  // someone tried and failed and retry not allowed.
  }

  assert(vm_created == 1, "vm_created is true during the creation");

  /**
   * Certain errors during initialization are recoverable and do not
   * prevent this method from being called again at a later time
   * (perhaps with different arguments).  However, at a certain
   * point during initialization if an error occurs we cannot allow
   * this function to be called again (or it will crash).  In those
   * situations, the 'canTryAgain' flag is set to false, which atomically
   * sets safe_to_recreate_vm to 1, such that any new call to
   * JNI_CreateJavaVM will immediately fail using the above logic.
   */
  bool can_try_again = true;
  // 核心: 建立vm
  result = Threads::create_vm((JavaVMInitArgs*) args, &can_try_again);
  if (result == JNI_OK) {
    JavaThread *thread = JavaThread::current();
    /* thread is thread_in_vm here */
    *vm = (JavaVM *)(&main_vm);
    // 將jvm資訊儲存到 penv 中，以備外部使用
    *(JNIEnv**)penv = thread->jni_environment();

    // Tracks the time application was running before GC
    RuntimeService::record_application_start();

    // Notify JVMTI
    if (JvmtiExport::should_post_thread_life()) {
       JvmtiExport::post_thread_start(thread);
    }

    EventThreadStart event;
    if (event.should_commit()) {
      event.set_javalangthread(java_lang_Thread::thread_id(thread->threadObj()));
      event.commit();
    }

#ifndef PRODUCT
  #ifndef TARGET_OS_FAMILY_windows
    #define CALL_TEST_FUNC_WITH_WRAPPER_IF_NEEDED(f) f()
  #endif

    // Check if we should compile all classes on bootclasspath
    if (CompileTheWorld) ClassLoader::compile_the_world();
    if (ReplayCompiles) ciReplay::replay(thread);

    // Some platforms (like Win*) need a wrapper around these test
    // functions in order to properly handle error conditions.
    CALL_TEST_FUNC_WITH_WRAPPER_IF_NEEDED(test_error_handler);
    CALL_TEST_FUNC_WITH_WRAPPER_IF_NEEDED(execute_internal_vm_tests);
#endif

    // Since this is not a JVM_ENTRY we have to set the thread state manually before leaving.
    ThreadStateTransition::transition_and_fence(thread, _thread_in_vm, _thread_in_native);
  } else {
    // 建立VM失敗, 還原標識位資訊
    if (can_try_again) {
      // reset safe_to_recreate_vm to 1 so that retrial would be possible
      safe_to_recreate_vm = 1;
    }

    // Creation failed. We must reset vm_created
    *vm = 0;
    *(JNIEnv**)penv = 0;
    // reset vm_created last to avoid race condition. Use OrderAccess to
    // control both compiler and architectural-based reordering.
    OrderAccess::release_store(&vm_created, 0);
  }

  return result;
}

　　看C++程式碼果然有點費勁，不過有著註釋的加持，還算可以理解。大致就是測試環境，然後上CAS鎖,保證vm載入時的執行緒安全性，進行vm建立，然後將vm的環境資訊賦值給外部 penv, 測試下vm有效性, 返回建立狀態。

　　核心仍然是被包裹著的：Threads::create_vm()

// share/vm/runtime/thread.cpp
jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {

  extern void JDK_Version_init();

  // Check version
  if (!is_supported_jni_version(args->version)) return JNI_EVERSION;

  // Initialize the output stream module
  ostream_init();

  // Process java launcher properties.
  Arguments::process_sun_java_launcher_properties(args);

  // Initialize the os module before using TLS
  os::init();

  // Initialize system properties.
  Arguments::init_system_properties();

  // So that JDK version can be used as a discrimintor when parsing arguments
  JDK_Version_init();

  // Update/Initialize System properties after JDK version number is known
  Arguments::init_version_specific_system_properties();

  // Parse arguments
  jint parse_result = Arguments::parse(args);
  if (parse_result != JNI_OK) return parse_result;

  os::init_before_ergo();

  jint ergo_result = Arguments::apply_ergo();
  if (ergo_result != JNI_OK) return ergo_result;

  if (PauseAtStartup) {
    os::pause();
  }

#ifndef USDT2
  HS_DTRACE_PROBE(hotspot, vm__init__begin);
#else /* USDT2 */
  HOTSPOT_VM_INIT_BEGIN();
#endif /* USDT2 */

  // Record VM creation timing statistics
  TraceVmCreationTime create_vm_timer;
  create_vm_timer.start();

  // Timing (must come after argument parsing)
  TraceTime timer("Create VM", TraceStartupTime);

  // Initialize the os module after parsing the args
  jint os_init_2_result = os::init_2();
  if (os_init_2_result != JNI_OK) return os_init_2_result;

  jint adjust_after_os_result = Arguments::adjust_after_os();
  if (adjust_after_os_result != JNI_OK) return adjust_after_os_result;

  // intialize TLS
  ThreadLocalStorage::init();

  // Bootstrap native memory tracking, so it can start recording memory
  // activities before worker thread is started. This is the first phase
  // of bootstrapping, VM is currently running in single-thread mode.
  MemTracker::bootstrap_single_thread();

  // Initialize output stream logging
  ostream_init_log();

  // Convert -Xrun to -agentlib: if there is no JVM_OnLoad
  // Must be before create_vm_init_agents()
  if (Arguments::init_libraries_at_startup()) {
    convert_vm_init_libraries_to_agents();
  }

  // Launch -agentlib/-agentpath and converted -Xrun agents
  if (Arguments::init_agents_at_startup()) {
    create_vm_init_agents();
  }

  // Initialize Threads state
  _thread_list = NULL;
  _number_of_threads = 0;
  _number_of_non_daemon_threads = 0;

  // Initialize global data structures and create system classes in heap
  vm_init_globals();

  // Attach the main thread to this os thread
  JavaThread* main_thread = new JavaThread();
  main_thread->set_thread_state(_thread_in_vm);
  // must do this before set_active_handles and initialize_thread_local_storage
  // Note: on solaris initialize_thread_local_storage() will (indirectly)
  // change the stack size recorded here to one based on the java thread
  // stacksize. This adjusted size is what is used to figure the placement
  // of the guard pages.
  main_thread->record_stack_base_and_size();
  main_thread->initialize_thread_local_storage();

  main_thread->set_active_handles(JNIHandleBlock::allocate_block());

  if (!main_thread->set_as_starting_thread()) {
    vm_shutdown_during_initialization(
      "Failed necessary internal allocation. Out of swap space");
    delete main_thread;
    *canTryAgain = false; // don't let caller call JNI_CreateJavaVM again
    return JNI_ENOMEM;
  }

  // Enable guard page *after* os::create_main_thread(), otherwise it would
  // crash Linux VM, see notes in os_linux.cpp.
  main_thread->create_stack_guard_pages();

  // Initialize Java-Level synchronization subsystem
  ObjectMonitor::Initialize() ;

  // Second phase of bootstrapping, VM is about entering multi-thread mode
  MemTracker::bootstrap_multi_thread();

  // Initialize global modules
  jint status = init_globals();
  if (status != JNI_OK) {
    delete main_thread;
    *canTryAgain = false; // don't let caller call JNI_CreateJavaVM again
    return status;
  }

  // Should be done after the heap is fully created
  main_thread->cache_global_variables();

  HandleMark hm;

  { MutexLocker mu(Threads_lock);
    Threads::add(main_thread);
  }

  // Any JVMTI raw monitors entered in onload will transition into
  // real raw monitor. VM is setup enough here for raw monitor enter.
  JvmtiExport::transition_pending_onload_raw_monitors();

  // Fully start NMT
  MemTracker::start();

  // Create the VMThread
  { TraceTime timer("Start VMThread", TraceStartupTime);
    VMThread::create();
    Thread* vmthread = VMThread::vm_thread();

    if (!os::create_thread(vmthread, os::vm_thread))
      vm_exit_during_initialization("Cannot create VM thread. Out of system resources.");

    // Wait for the VM thread to become ready, and VMThread::run to initialize
    // Monitors can have spurious returns, must always check another state flag
    {
      MutexLocker ml(Notify_lock);
      os::start_thread(vmthread);
      while (vmthread->active_handles() == NULL) {
        Notify_lock->wait();
      }
    }
  }

  assert (Universe::is_fully_initialized(), "not initialized");
  if (VerifyDuringStartup) {
    // Make sure we're starting with a clean slate.
    VM_Verify verify_op;
    VMThread::execute(&verify_op);
  }

  EXCEPTION_MARK;

  // At this point, the Universe is initialized, but we have not executed
  // any byte code.  Now is a good time (the only time) to dump out the
  // internal state of the JVM for sharing.
  if (DumpSharedSpaces) {
    MetaspaceShared::preload_and_dump(CHECK_0);
    ShouldNotReachHere();
  }

  // Always call even when there are not JVMTI environments yet, since environments
  // may be attached late and JVMTI must track phases of VM execution
  JvmtiExport::enter_start_phase();

  // Notify JVMTI agents that VM has started (JNI is up) - nop if no agents.
  JvmtiExport::post_vm_start();

  {
    TraceTime timer("Initialize java.lang classes", TraceStartupTime);

    if (EagerXrunInit && Arguments::init_libraries_at_startup()) {
      create_vm_init_libraries();
    }

    initialize_class(vmSymbols::java_lang_String(), CHECK_0);

    // Initialize java_lang.System (needed before creating the thread)
    initialize_class(vmSymbols::java_lang_System(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ThreadGroup(), CHECK_0);
    Handle thread_group = create_initial_thread_group(CHECK_0);
    Universe::set_main_thread_group(thread_group());
    initialize_class(vmSymbols::java_lang_Thread(), CHECK_0);
    oop thread_object = create_initial_thread(thread_group, main_thread, CHECK_0);
    main_thread->set_threadObj(thread_object);
    // Set thread status to running since main thread has
    // been started and running.
    java_lang_Thread::set_thread_status(thread_object,
                                        java_lang_Thread::RUNNABLE);

    // The VM creates & returns objects of this class. Make sure it's initialized.
    initialize_class(vmSymbols::java_lang_Class(), CHECK_0);

    // The VM preresolves methods to these classes. Make sure that they get initialized
    initialize_class(vmSymbols::java_lang_reflect_Method(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ref_Finalizer(),  CHECK_0);
    call_initializeSystemClass(CHECK_0);

    // get the Java runtime name after java.lang.System is initialized
    JDK_Version::set_runtime_name(get_java_runtime_name(THREAD));
    JDK_Version::set_runtime_version(get_java_runtime_version(THREAD));

    // an instance of OutOfMemory exception has been allocated earlier
    initialize_class(vmSymbols::java_lang_OutOfMemoryError(), CHECK_0);
    initialize_class(vmSymbols::java_lang_NullPointerException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ClassCastException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ArrayStoreException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_ArithmeticException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_StackOverflowError(), CHECK_0);
    initialize_class(vmSymbols::java_lang_IllegalMonitorStateException(), CHECK_0);
    initialize_class(vmSymbols::java_lang_IllegalArgumentException(), CHECK_0);
  }

  // See        : bugid 4211085.
  // Background : the static initializer of java.lang.Compiler tries to read
  //              property"java.compiler" and read & write property "java.vm.info".
  //              When a security manager is installed through the command line
  //              option "-Djava.security.manager", the above properties are not
  //              readable and the static initializer for java.lang.Compiler fails
  //              resulting in a NoClassDefFoundError.  This can happen in any
  //              user code which calls methods in java.lang.Compiler.
  // Hack :       the hack is to pre-load and initialize this class, so that only
  //              system domains are on the stack when the properties are read.
  //              Currently even the AWT code has calls to methods in java.lang.Compiler.
  //              On the classic VM, java.lang.Compiler is loaded very early to load the JIT.
  // Future Fix : the best fix is to grant everyone permissions to read "java.compiler" and
  //              read and write"java.vm.info" in the default policy file. See bugid 4211383
  //              Once that is done, we should remove this hack.
  initialize_class(vmSymbols::java_lang_Compiler(), CHECK_0);

  // More hackery - the static initializer of java.lang.Compiler adds the string "nojit" to
  // the java.vm.info property if no jit gets loaded through java.lang.Compiler (the hotspot
  // compiler does not get loaded through java.lang.Compiler).  "java -version" with the
  // hotspot vm says "nojit" all the time which is confusing.  So, we reset it here.
  // This should also be taken out as soon as 4211383 gets fixed.
  reset_vm_info_property(CHECK_0);

  quicken_jni_functions();

  // Must be run after init_ft which initializes ft_enabled
  if (TRACE_INITIALIZE() != JNI_OK) {
    vm_exit_during_initialization("Failed to initialize tracing backend");
  }

  // Set flag that basic initialization has completed. Used by exceptions and various
  // debug stuff, that does not work until all basic classes have been initialized.
  set_init_completed();

#ifndef USDT2
  HS_DTRACE_PROBE(hotspot, vm__init__end);
#else /* USDT2 */
  HOTSPOT_VM_INIT_END();
#endif /* USDT2 */

  // record VM initialization completion time
#if INCLUDE_MANAGEMENT
  Management::record_vm_init_completed();
#endif // INCLUDE_MANAGEMENT

  // Compute system loader. Note that this has to occur after set_init_completed, since
  // valid exceptions may be thrown in the process.
  // Note that we do not use CHECK_0 here since we are inside an EXCEPTION_MARK and
  // set_init_completed has just been called, causing exceptions not to be shortcut
  // anymore. We call vm_exit_during_initialization directly instead.
  SystemDictionary::compute_java_system_loader(THREAD);
  if (HAS_PENDING_EXCEPTION) {
    vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));
  }

#if INCLUDE_ALL_GCS
  // Support for ConcurrentMarkSweep. This should be cleaned up
  // and better encapsulated. The ugly nested if test would go away
  // once things are properly refactored. XXX YSR
  if (UseConcMarkSweepGC || UseG1GC) {
    if (UseConcMarkSweepGC) {
      ConcurrentMarkSweepThread::makeSurrogateLockerThread(THREAD);
    } else {
      ConcurrentMarkThread::makeSurrogateLockerThread(THREAD);
    }
    if (HAS_PENDING_EXCEPTION) {
      vm_exit_during_initialization(Handle(THREAD, PENDING_EXCEPTION));
    }
  }
#endif // INCLUDE_ALL_GCS

  // Always call even when there are not JVMTI environments yet, since environments
  // may be attached late and JVMTI must track phases of VM execution
  JvmtiExport::enter_live_phase();

  // Signal Dispatcher needs to be started before VMInit event is posted
  os::signal_init();

  // Start Attach Listener if +StartAttachListener or it can't be started lazily
  if (!DisableAttachMechanism) {
    AttachListener::vm_start();
    if (StartAttachListener || AttachListener::init_at_startup()) {
      AttachListener::init();
    }
  }

  // Launch -Xrun agents
  // Must be done in the JVMTI live phase so that for backward compatibility the JDWP
  // back-end can launch with -Xdebug -Xrunjdwp.
  if (!EagerXrunInit && Arguments::init_libraries_at_startup()) {
    create_vm_init_libraries();
  }

  // Notify JVMTI agents that VM initialization is complete - nop if no agents.
  JvmtiExport::post_vm_initialized();

  if (TRACE_START() != JNI_OK) {
    vm_exit_during_initialization("Failed to start tracing backend.");
  }

  if (CleanChunkPoolAsync) {
    Chunk::start_chunk_pool_cleaner_task();
  }

  // initialize compiler(s)
#if defined(COMPILER1) || defined(COMPILER2) || defined(SHARK)
  CompileBroker::compilation_init();
#endif

  if (EnableInvokeDynamic) {
    // Pre-initialize some JSR292 core classes to avoid deadlock during class loading.
    // It is done after compilers are initialized, because otherwise compilations of
    // signature polymorphic MH intrinsics can be missed
    // (see SystemDictionary::find_method_handle_intrinsic).
    initialize_class(vmSymbols::java_lang_invoke_MethodHandle(), CHECK_0);
    initialize_class(vmSymbols::java_lang_invoke_MemberName(), CHECK_0);
    initialize_class(vmSymbols::java_lang_invoke_MethodHandleNatives(), CHECK_0);
  }

#if INCLUDE_MANAGEMENT
  Management::initialize(THREAD);
#endif // INCLUDE_MANAGEMENT

  if (HAS_PENDING_EXCEPTION) {
    // management agent fails to start possibly due to
    // configuration problem and is responsible for printing
    // stack trace if appropriate. Simply exit VM.
    vm_exit(1);
  }

  if (Arguments::has_profile())       FlatProfiler::engage(main_thread, true);
  if (MemProfiling)                   MemProfiler::engage();
  StatSampler::engage();
  if (CheckJNICalls)                  JniPeriodicChecker::engage();

  BiasedLocking::init();

  if (JDK_Version::current().post_vm_init_hook_enabled()) {
    call_postVMInitHook(THREAD);
    // The Java side of PostVMInitHook.run must deal with all
    // exceptions and provide means of diagnosis.
    if (HAS_PENDING_EXCEPTION) {
      CLEAR_PENDING_EXCEPTION;
    }
  }

  {
      MutexLockerEx ml(PeriodicTask_lock, Mutex::_no_safepoint_check_flag);
      // Make sure the watcher thread can be started by WatcherThread::start()
      // or by dynamic enrollment.
      WatcherThread::make_startable();
      // Start up the WatcherThread if there are any periodic tasks
      // NOTE:  All PeriodicTasks should be registered by now. If they
      //   aren't, late joiners might appear to start slowly (we might
      //   take a while to process their first tick).
      if (PeriodicTask::num_tasks() > 0) {
          WatcherThread::start();
      }
  }

  // Give os specific code one last chance to start
  os::init_3();

  create_vm_timer.end();
#ifdef ASSERT
  _vm_complete = true;
#endif
  return JNI_OK;
}

　　以上，就是vm建立的框架程式碼，也已經這麼複雜了。大體有這麼幾個步驟：

1. 檢查jdk版本號, 不支援則退出;
2. 輸出流初始化;
3. sun.java.launcher屬性配置檢查接入;
4. 初始化一些系統模組，如隨機數...;
5. 初始化系統屬性如java.ext.dirs...;
6. 引數解析;
7. 系統頁初始化;
8. 再初始化更多平臺相關的系統模組, 如執行緒,頁設定,mmap,PV機制,maxfd,優先順序等;
9. ThreadLocalStorage初始化;
10. 記憶體跟蹤器初始化;
11. agentlib 初始化;
12. 全域性變數初始化;
13. 建立JavaThread, 初始化資訊;
14. 將java執行緒對映到系統執行緒JavaThread -> OSThread;
15. ObjectMonitor物件監視器建立初始化;
16. 進入多執行緒模式;
17. 初始化java的全域性模組，如bytecode,classloader...;
18. 新增main_thread到執行緒表中;
19. 建立 VMThread 執行緒, 啟動vmThread執行緒並等待其事務處理完成;
20. JvmtiExport開始執行, 保證agent開始切入;
21. 初始化java系統類庫,如system,string...;
22. 初始化編譯器compiler;
23. jni函式資訊設定;
24. jdwp除錯模組執行;
25. AttachListener啟動執行;
26. BiasedLocking 初始化;
27. WatcherThread 啟動;
28. PeriodicTask 任務執行;
29. 執行完成, 返回建立成功;

　　細節就不說了（哈哈，因為說也說不清楚）。我們只需理解流程即可，真正想理解，那麼就需要指定一個特定的小點，來進行探討了。以後再說咯！

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