經過前面幾章的簡單介紹，我們已經大致瞭解了jvm的啟動框架和執行流程了。不過，這些都是些無關痛癢的問題，幾行文字描述一下即可。
所以，今天我們從另一個角度來講解jvm的一些東西，以便可以更多一點認知。即如題：jvm是如何找到對應的java方法，然後執行的呢？（但是執行太複雜，太重要，我們就不說了。我們單看如何找到對應的java方法吧）
1. 回顧核心變數JNIEnv的初始化
如上一篇系列文章中講到的，jdk執行的核心方法，實際上也是呼叫jvm或者hotspot的介面方法實現的，這其中有個重要變數，供jdk使用。即：JNIEnv* env 。可見其重要性。我們再來回顧下它的初始化過程。
//實際上，我們可以通過前面對 JNIEnv **penv 的賦值中查到端倪： // hotspot/src/share/vm/prims/jni.cpp ... // 將jvm資訊儲存到 penv 中，以備外部使用 *(JNIEnv**)penv = thread->jni_environment(); ... // 而檢視 jni_environment() 方法可知，其由一個類變數 _jni_environment 處理 // share/vm/runtime/thread.hpp // Returns the jni environment for this thread JNIEnv* jni_environment() { return &_jni_environment; } // 所以，我們只需找出 _jni_environment 是如何賦值初始化，即可知道如何獲取這個關鍵變數的邏輯了。結果是，在建立JavaThread, 在進行初始化時，便會設定該值。 // share/vm/runtime/thread.cpp JavaThread::JavaThread(ThreadFunction entry_point, size_t stack_sz) : Thread() #if INCLUDE_ALL_GCS , _satb_mark_queue(&_satb_mark_queue_set), _dirty_card_queue(&_dirty_card_queue_set) #endif // INCLUDE_ALL_GCS { if (TraceThreadEvents) { tty->print_cr("creating thread %p", this); } // 初始化執行緒變數資訊, 如 JNIEnv initialize(); _jni_attach_state = _not_attaching_via_jni; set_entry_point(entry_point); // Create the native thread itself. // %note runtime_23 os::ThreadType thr_type = os::java_thread; thr_type = entry_point == &compiler_thread_entry ? os::compiler_thread : os::java_thread; os::create_thread(this, thr_type, stack_sz); _safepoint_visible = false; // The _osthread may be NULL here because we ran out of memory (too many threads active). // We need to throw and OutOfMemoryError - however we cannot do this here because the caller // may hold a lock and all locks must be unlocked before throwing the exception (throwing // the exception consists of creating the exception object & initializing it, initialization // will leave the VM via a JavaCall and then all locks must be unlocked). // // The thread is still suspended when we reach here. Thread must be explicit started // by creator! Furthermore, the thread must also explicitly be added to the Threads list // by calling Threads:add. The reason why this is not done here, is because the thread // object must be fully initialized (take a look at JVM_Start) } // A JavaThread is a normal Java thread void JavaThread::initialize() { // Initialize fields // Set the claimed par_id to -1 (ie not claiming any par_ids) set_claimed_par_id(-1); set_saved_exception_pc(NULL); set_threadObj(NULL); _anchor.clear(); set_entry_point(NULL); // 取數jni_functions, 初始化到 _jni_environment set_jni_functions(jni_functions()); set_callee_target(NULL); set_vm_result(NULL); set_vm_result_2(NULL); set_vframe_array_head(NULL); set_vframe_array_last(NULL); set_deferred_locals(NULL); set_deopt_mark(NULL); set_deopt_nmethod(NULL); clear_must_deopt_id(); set_monitor_chunks(NULL); set_next(NULL); set_thread_state(_thread_new); #if INCLUDE_NMT set_recorder(NULL); #endif _terminated = _not_terminated; _privileged_stack_top = NULL; _array_for_gc = NULL; _suspend_equivalent = false; _in_deopt_handler = 0; _doing_unsafe_access = false; _stack_guard_state = stack_guard_unused; (void)const_cast<oop&>(_exception_oop = NULL); _exception_pc = 0; _exception_handler_pc = 0; _is_method_handle_return = 0; _jvmti_thread_state= NULL; _should_post_on_exceptions_flag = JNI_FALSE; _jvmti_get_loaded_classes_closure = NULL; _interp_only_mode = 0; _special_runtime_exit_condition = _no_async_condition; _pending_async_exception = NULL; _thread_stat = NULL; _thread_stat = new ThreadStatistics(); _blocked_on_compilation = false; _jni_active_critical = 0; _do_not_unlock_if_synchronized = false; _cached_monitor_info = NULL; _parker = Parker::Allocate(this) ; #ifndef PRODUCT _jmp_ring_index = 0; for (int ji = 0 ; ji < jump_ring_buffer_size ; ji++ ) { record_jump(NULL, NULL, NULL, 0); } #endif /* PRODUCT */ set_thread_profiler(NULL); if (FlatProfiler::is_active()) { // This is where we would decide to either give each thread it's own profiler // or use one global one from FlatProfiler, // or up to some count of the number of profiled threads, etc. ThreadProfiler* pp = new ThreadProfiler(); pp->engage(); set_thread_profiler(pp); } // Setup safepoint state info for this thread ThreadSafepointState::create(this); debug_only(_java_call_counter = 0); // JVMTI PopFrame support _popframe_condition = popframe_inactive; _popframe_preserved_args = NULL; _popframe_preserved_args_size = 0; pd_initialize(); } // Returns the function structure struct JNINativeInterface_* jni_functions() { #if INCLUDE_JNI_CHECK if (CheckJNICalls) return jni_functions_check(); #endif // INCLUDE_JNI_CHECK return &jni_NativeInterface; } // thread.hpp //JNI functiontable getter/setter for JVMTI jni function table interception API. void set_jni_functions(struct JNINativeInterface_* functionTable) { _jni_environment.functions = functionTable; }
所以，核心的初始化變成了 jni_NativeInterface 的具體值問題了。剛好我們可以通過這個方法去這個 JNIEnv 都定義了啥。這對於我們以後的分析工作有非常大的幫助。
// jni.cpp // Structure containing all jni functions struct JNINativeInterface_ jni_NativeInterface = { NULL, NULL, NULL, NULL, jni_GetVersion, jni_DefineClass, jni_FindClass, jni_FromReflectedMethod, jni_FromReflectedField, jni_ToReflectedMethod, jni_GetSuperclass, jni_IsAssignableFrom, jni_ToReflectedField, jni_Throw, jni_ThrowNew, jni_ExceptionOccurred, jni_ExceptionDescribe, jni_ExceptionClear, jni_FatalError, jni_PushLocalFrame, jni_PopLocalFrame, jni_NewGlobalRef, jni_DeleteGlobalRef, jni_DeleteLocalRef, jni_IsSameObject, jni_NewLocalRef, jni_EnsureLocalCapacity, jni_AllocObject, jni_NewObject, jni_NewObjectV, jni_NewObjectA, jni_GetObjectClass, jni_IsInstanceOf, jni_GetMethodID, jni_CallObjectMethod, jni_CallObjectMethodV, jni_CallObjectMethodA, jni_CallBooleanMethod, jni_CallBooleanMethodV, jni_CallBooleanMethodA, jni_CallByteMethod, jni_CallByteMethodV, jni_CallByteMethodA, jni_CallCharMethod, jni_CallCharMethodV, jni_CallCharMethodA, jni_CallShortMethod, jni_CallShortMethodV, jni_CallShortMethodA, jni_CallIntMethod, jni_CallIntMethodV, jni_CallIntMethodA, jni_CallLongMethod, jni_CallLongMethodV, jni_CallLongMethodA, jni_CallFloatMethod, jni_CallFloatMethodV, jni_CallFloatMethodA, jni_CallDoubleMethod, jni_CallDoubleMethodV, jni_CallDoubleMethodA, jni_CallVoidMethod, jni_CallVoidMethodV, jni_CallVoidMethodA, jni_CallNonvirtualObjectMethod, jni_CallNonvirtualObjectMethodV, jni_CallNonvirtualObjectMethodA, jni_CallNonvirtualBooleanMethod, jni_CallNonvirtualBooleanMethodV, jni_CallNonvirtualBooleanMethodA, jni_CallNonvirtualByteMethod, jni_CallNonvirtualByteMethodV, jni_CallNonvirtualByteMethodA, jni_CallNonvirtualCharMethod, jni_CallNonvirtualCharMethodV, jni_CallNonvirtualCharMethodA, jni_CallNonvirtualShortMethod, jni_CallNonvirtualShortMethodV, jni_CallNonvirtualShortMethodA, jni_CallNonvirtualIntMethod, jni_CallNonvirtualIntMethodV, jni_CallNonvirtualIntMethodA, jni_CallNonvirtualLongMethod, jni_CallNonvirtualLongMethodV, jni_CallNonvirtualLongMethodA, jni_CallNonvirtualFloatMethod, jni_CallNonvirtualFloatMethodV, jni_CallNonvirtualFloatMethodA, jni_CallNonvirtualDoubleMethod, jni_CallNonvirtualDoubleMethodV, jni_CallNonvirtualDoubleMethodA, jni_CallNonvirtualVoidMethod, jni_CallNonvirtualVoidMethodV, jni_CallNonvirtualVoidMethodA, jni_GetFieldID, jni_GetObjectField, jni_GetBooleanField, jni_GetByteField, jni_GetCharField, jni_GetShortField, jni_GetIntField, jni_GetLongField, jni_GetFloatField, jni_GetDoubleField, jni_SetObjectField, jni_SetBooleanField, jni_SetByteField, jni_SetCharField, jni_SetShortField, jni_SetIntField, jni_SetLongField, jni_SetFloatField, jni_SetDoubleField, jni_GetStaticMethodID, jni_CallStaticObjectMethod, jni_CallStaticObjectMethodV, jni_CallStaticObjectMethodA, jni_CallStaticBooleanMethod, jni_CallStaticBooleanMethodV, jni_CallStaticBooleanMethodA, jni_CallStaticByteMethod, jni_CallStaticByteMethodV, jni_CallStaticByteMethodA, jni_CallStaticCharMethod, jni_CallStaticCharMethodV, jni_CallStaticCharMethodA, jni_CallStaticShortMethod, jni_CallStaticShortMethodV, jni_CallStaticShortMethodA, jni_CallStaticIntMethod, jni_CallStaticIntMethodV, jni_CallStaticIntMethodA, jni_CallStaticLongMethod, jni_CallStaticLongMethodV, jni_CallStaticLongMethodA, jni_CallStaticFloatMethod, jni_CallStaticFloatMethodV, jni_CallStaticFloatMethodA, jni_CallStaticDoubleMethod, jni_CallStaticDoubleMethodV, jni_CallStaticDoubleMethodA, jni_CallStaticVoidMethod, jni_CallStaticVoidMethodV, jni_CallStaticVoidMethodA, jni_GetStaticFieldID, jni_GetStaticObjectField, jni_GetStaticBooleanField, jni_GetStaticByteField, jni_GetStaticCharField, jni_GetStaticShortField, jni_GetStaticIntField, jni_GetStaticLongField, jni_GetStaticFloatField, jni_GetStaticDoubleField, jni_SetStaticObjectField, jni_SetStaticBooleanField, jni_SetStaticByteField, jni_SetStaticCharField, jni_SetStaticShortField, jni_SetStaticIntField, jni_SetStaticLongField, jni_SetStaticFloatField, jni_SetStaticDoubleField, jni_NewString, jni_GetStringLength, jni_GetStringChars, jni_ReleaseStringChars, jni_NewStringUTF, jni_GetStringUTFLength, jni_GetStringUTFChars, jni_ReleaseStringUTFChars, jni_GetArrayLength, jni_NewObjectArray, jni_GetObjectArrayElement, jni_SetObjectArrayElement, jni_NewBooleanArray, jni_NewByteArray, jni_NewCharArray, jni_NewShortArray, jni_NewIntArray, jni_NewLongArray, jni_NewFloatArray, jni_NewDoubleArray, jni_GetBooleanArrayElements, jni_GetByteArrayElements, jni_GetCharArrayElements, jni_GetShortArrayElements, jni_GetIntArrayElements, jni_GetLongArrayElements, jni_GetFloatArrayElements, jni_GetDoubleArrayElements, jni_ReleaseBooleanArrayElements, jni_ReleaseByteArrayElements, jni_ReleaseCharArrayElements, jni_ReleaseShortArrayElements, jni_ReleaseIntArrayElements, jni_ReleaseLongArrayElements, jni_ReleaseFloatArrayElements, jni_ReleaseDoubleArrayElements, jni_GetBooleanArrayRegion, jni_GetByteArrayRegion, jni_GetCharArrayRegion, jni_GetShortArrayRegion, jni_GetIntArrayRegion, jni_GetLongArrayRegion, jni_GetFloatArrayRegion, jni_GetDoubleArrayRegion, jni_SetBooleanArrayRegion, jni_SetByteArrayRegion, jni_SetCharArrayRegion, jni_SetShortArrayRegion, jni_SetIntArrayRegion, jni_SetLongArrayRegion, jni_SetFloatArrayRegion, jni_SetDoubleArrayRegion, jni_RegisterNatives, jni_UnregisterNatives, jni_MonitorEnter, jni_MonitorExit, jni_GetJavaVM, jni_GetStringRegion, jni_GetStringUTFRegion, jni_GetPrimitiveArrayCritical, jni_ReleasePrimitiveArrayCritical, jni_GetStringCritical, jni_ReleaseStringCritical, jni_NewWeakGlobalRef, jni_DeleteWeakGlobalRef, jni_ExceptionCheck, jni_NewDirectByteBuffer, jni_GetDirectBufferAddress, jni_GetDirectBufferCapacity, // New 1_6 features jni_GetObjectRefType };
以上就是 JNIEnv* env 變數的設值過程了，它藉助於java執行緒的建立時機進行初始化。而後續的使用中，幾乎都會仰仗它來執行，可見其重要性。
但總結一下，這裡面提供的介面，實際上都是一些非常基礎的操作，比如變數新建，初始化，異常處理，鎖處理，native註冊等。型別實際並不多。這也提示了我們一點，越是基礎的東西，實際上越不會那麼複雜。它更多的是做好抽象工作，打好基礎，比什麼都好。
2. main方法的查詢實現
要談其他方法，著實也太泛了。因為，你可以定義這個方法，他可以定義一個別的方法。這裡面的特性就太難找了。但，對於每個java應用的啟動，都會去載入main（）方法執行，所以，以這個main()方法的查詢為出發點，定然能找到些端倪來。
我們先來看看main()的呼叫地方如何：
// 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(); }
JVM的初始化，我們在上篇系列文章中已窺得簡要。這篇，我們就以 *env 作為入口進行。因為jvm初始化完成後，就會給 *env 的賦值。
2.1. GetStaticMethodID 的實現
而，載入main()方法，最核心的就是上面最後幾行：
// 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;
很明顯，我們的目的就是看jvm如何找到main()方法，這也是執行main()邏輯的第一步工作。下面來細聊下，它使用的是 (*env)->GetStaticMethodID(), 而這個方法，在上一節中，我們可以看到其實現為：jni_GetStaticMethodID 。 所以，知道這個 jni_GetStaticMethodID 的實現就知道了如何查詢java靜態方法了。
// share/vm/prims/jni.cpp JNI_ENTRY(jmethodID, jni_GetStaticMethodID(JNIEnv *env, jclass clazz, const char *name, const char *sig)) JNIWrapper("GetStaticMethodID"); #ifndef USDT2 DTRACE_PROBE4(hotspot_jni, GetStaticMethodID__entry, env, clazz, name, sig); #else /* USDT2 */ HOTSPOT_JNI_GETSTATICMETHODID_ENTRY( env, (char *) clazz, (char *) name, (char *)sig); #endif /* USDT2 */ jmethodID ret = get_method_id(env, clazz, name, sig, true, thread); #ifndef USDT2 DTRACE_PROBE1(hotspot_jni, GetStaticMethodID__return, ret); #else /* USDT2 */ HOTSPOT_JNI_GETSTATICMETHODID_RETURN( (uintptr_t) ret); #endif /* USDT2 */ return ret; JNI_END
我們通過這個實現，能看到什麼呢？好像什麼也看不懂。實際上是因為，其中有太多的巨集定義了，要想讀懂這程式碼，必須將巨集定義展開。而這些巨集，基本都是是在 interfaceSupport.hpp 中定義的。
下面我們來看下 JNI_ENTRY|JNI_END 的定義拆解：
// share/vm/runtime/interfaceSupport.hpp // JNI_ENTRY 的定義，又依賴於 JNI_ENTRY_NO_PRESERVE 的定義 #define JNI_ENTRY(result_type, header) \ JNI_ENTRY_NO_PRESERVE(result_type, header) \ WeakPreserveExceptionMark __wem(thread); // JNI_ENTRY_NO_PRESERVE 的定義，又依賴於 VM_ENTRY_BASE 的定義 #define JNI_ENTRY_NO_PRESERVE(result_type, header) \ extern "C" { \ result_type JNICALL header { \ JavaThread* thread=JavaThread::thread_from_jni_environment(env); \ assert( !VerifyJNIEnvThread || (thread == Thread::current()), "JNIEnv is only valid in same thread"); \ ThreadInVMfromNative __tiv(thread); \ debug_only(VMNativeEntryWrapper __vew;) \ VM_ENTRY_BASE(result_type, header, thread) // VM_ENTRY_BASE 的定義 #define VM_ENTRY_BASE(result_type, header, thread) \ TRACE_CALL(result_type, header) \ HandleMarkCleaner __hm(thread); \ Thread* THREAD = thread; \ os::verify_stack_alignment(); \ /* begin of body */ // Close the routine and the extern "C" #define JNI_END } }
此時，如上的函式實現可以轉換為：
extern "C" { jmethodID JNICALL header { JavaThread* thread=JavaThread::thread_from_jni_environment(env); assert( !VerifyJNIEnvThread || (thread == Thread::current()), "JNIEnv is only valid in same thread"); ThreadInVMfromNative __tiv(thread); debug_only(VMNativeEntryWrapper __vew;) TRACE_CALL(jmethodID, jni_GetStaticMethodID(JNIEnv *env, jclass clazz, const char *name, const char *sig)) HandleMarkCleaner __hm(thread); Thread* THREAD = thread; os::verify_stack_alignment(); WeakPreserveExceptionMark __wem(thread); // 預設為空 JNIWrapper("GetStaticMethodID"); #ifndef USDT2 DTRACE_PROBE4(hotspot_jni, GetStaticMethodID__entry, env, clazz, name, sig); #else /* USDT2 */ // 預設為空, 在 hotspot/src/share/vm/utilities/dtrace_usdt2_disabled.hpp 中定義 HOTSPOT_JNI_GETSTATICMETHODID_ENTRY( env, (char *) clazz, (char *) name, (char *)sig); #endif /* USDT2 */ // 核心查詢方法 jmethodID ret = get_method_id(env, clazz, name, sig, true, thread); #ifndef USDT2 DTRACE_PROBE1(hotspot_jni, GetStaticMethodID__return, ret); #else /* USDT2 */ // 預設為空 HOTSPOT_JNI_GETSTATICMETHODID_RETURN( (uintptr_t) ret); #endif /* USDT2 */ return ret; } }
經過這一層層的巨集展開，工作就變得清晰起來，重點在於 get_method_id() 了。
// jni.cpp 根據方法簽名，找到方法id static jmethodID get_method_id(JNIEnv *env, jclass clazz, const char *name_str, const char *sig, bool is_static, TRAPS) { // %%%% This code should probably just call into a method in the LinkResolver // // The class should have been loaded (we have an instance of the class // passed in) so the method and signature should already be in the symbol // table. If they're not there, the method doesn't exist. const char *name_to_probe = (name_str == NULL) ? vmSymbols::object_initializer_name()->as_C_string() : name_str; TempNewSymbol name = SymbolTable::probe(name_to_probe, (int)strlen(name_to_probe)); // sig如: "([Ljava/lang/String;)V" TempNewSymbol signature = SymbolTable::probe(sig, (int)strlen(sig)); if (name == NULL || signature == NULL) { THROW_MSG_0(vmSymbols::java_lang_NoSuchMethodError(), name_str); } // Throw a NoSuchMethodError exception if we have an instance of a // primitive java.lang.Class if (java_lang_Class::is_primitive(JNIHandles::resolve_non_null(clazz))) { THROW_MSG_0(vmSymbols::java_lang_NoSuchMethodError(), name_str); } // 初始化類例項 KlassHandle klass(THREAD, java_lang_Class::as_Klass(JNIHandles::resolve_non_null(clazz))); // Make sure class is linked and initialized before handing id's out to // Method*s. klass()->initialize(CHECK_NULL); Method* m; // "main" // "<init>" "<clinit>" if (name == vmSymbols::object_initializer_name() || name == vmSymbols::class_initializer_name()) { // Never search superclasses for constructors if (klass->oop_is_instance()) { m = InstanceKlass::cast(klass())->find_method(name, signature); } else { m = NULL; } } else { // 只是在本類中進行方法id查詢 m = klass->lookup_method(name, signature); if (m == NULL && klass->oop_is_instance()) { m = InstanceKlass::cast(klass())->lookup_method_in_ordered_interfaces(name, signature); } } if (m == NULL || (m->is_static() != is_static)) { THROW_MSG_0(vmSymbols::java_lang_NoSuchMethodError(), name_str); } // 返回id return m->jmethod_id(); } // share/vm/oops/klass.cpp public: Method* lookup_method(Symbol* name, Symbol* signature) const { return uncached_lookup_method(name, signature); } Method* Klass::uncached_lookup_method(Symbol* name, Symbol* signature) const { #ifdef ASSERT tty->print_cr("Error: uncached_lookup_method called on a klass oop." " Likely error: reflection method does not correctly" " wrap return value in a mirror object."); #endif ShouldNotReachHere(); return NULL; } // oops/method.hpp // Get this method's jmethodID -- allocate if it doesn't exist jmethodID jmethod_id() { methodHandle this_h(this); return InstanceKlass::get_jmethod_id(method_holder(), this_h); } // oops/instanceKlass.cpp // Lookup or create a jmethodID. // This code is called by the VMThread and JavaThreads so the // locking has to be done very carefully to avoid deadlocks // and/or other cache consistency problems. // jmethodID InstanceKlass::get_jmethod_id(instanceKlassHandle ik_h, methodHandle method_h) { size_t idnum = (size_t)method_h->method_idnum(); jmethodID* jmeths = ik_h->methods_jmethod_ids_acquire(); size_t length = 0; jmethodID id = NULL; // We use a double-check locking idiom here because this cache is // performance sensitive. In the normal system, this cache only // transitions from NULL to non-NULL which is safe because we use // release_set_methods_jmethod_ids() to advertise the new cache. // A partially constructed cache should never be seen by a racing // thread. We also use release_store_ptr() to save a new jmethodID // in the cache so a partially constructed jmethodID should never be // seen either. Cache reads of existing jmethodIDs proceed without a // lock, but cache writes of a new jmethodID requires uniqueness and // creation of the cache itself requires no leaks so a lock is // generally acquired in those two cases. // // If the RedefineClasses() API has been used, then this cache can // grow and we'll have transitions from non-NULL to bigger non-NULL. // Cache creation requires no leaks and we require safety between all // cache accesses and freeing of the old cache so a lock is generally // acquired when the RedefineClasses() API has been used. if (jmeths != NULL) { // the cache already exists if (!ik_h->idnum_can_increment()) { // the cache can't grow so we can just get the current values get_jmethod_id_length_value(jmeths, idnum, &length, &id); } else { // cache can grow so we have to be more careful if (Threads::number_of_threads() == 0 || SafepointSynchronize::is_at_safepoint()) { // we're single threaded or at a safepoint - no locking needed get_jmethod_id_length_value(jmeths, idnum, &length, &id); } else { MutexLocker ml(JmethodIdCreation_lock); get_jmethod_id_length_value(jmeths, idnum, &length, &id); } } } // implied else: // we need to allocate a cache so default length and id values are good if (jmeths == NULL || // no cache yet length <= idnum || // cache is too short id == NULL) { // cache doesn't contain entry // This function can be called by the VMThread so we have to do all // things that might block on a safepoint before grabbing the lock. // Otherwise, we can deadlock with the VMThread or have a cache // consistency issue. These vars keep track of what we might have // to free after the lock is dropped. jmethodID to_dealloc_id = NULL; jmethodID* to_dealloc_jmeths = NULL; // may not allocate new_jmeths or use it if we allocate it jmethodID* new_jmeths = NULL; if (length <= idnum) { // allocate a new cache that might be used size_t size = MAX2(idnum+1, (size_t)ik_h->idnum_allocated_count()); new_jmeths = NEW_C_HEAP_ARRAY(jmethodID, size+1, mtClass); memset(new_jmeths, 0, (size+1)*sizeof(jmethodID)); // cache size is stored in element[0], other elements offset by one new_jmeths[0] = (jmethodID)size; } // allocate a new jmethodID that might be used jmethodID new_id = NULL; if (method_h->is_old() && !method_h->is_obsolete()) { // The method passed in is old (but not obsolete), we need to use the current version Method* current_method = ik_h->method_with_idnum((int)idnum); assert(current_method != NULL, "old and but not obsolete, so should exist"); new_id = Method::make_jmethod_id(ik_h->class_loader_data(), current_method); } else { // It is the current version of the method or an obsolete method, // use the version passed in new_id = Method::make_jmethod_id(ik_h->class_loader_data(), method_h()); } if (Threads::number_of_threads() == 0 || SafepointSynchronize::is_at_safepoint()) { // we're single threaded or at a safepoint - no locking needed id = get_jmethod_id_fetch_or_update(ik_h, idnum, new_id, new_jmeths, &to_dealloc_id, &to_dealloc_jmeths); } else { MutexLocker ml(JmethodIdCreation_lock); id = get_jmethod_id_fetch_or_update(ik_h, idnum, new_id, new_jmeths, &to_dealloc_id, &to_dealloc_jmeths); } // The lock has been dropped so we can free resources. // Free up either the old cache or the new cache if we allocated one. if (to_dealloc_jmeths != NULL) { FreeHeap(to_dealloc_jmeths); } // free up the new ID since it wasn't needed if (to_dealloc_id != NULL) { Method::destroy_jmethod_id(ik_h->class_loader_data(), to_dealloc_id); } } return id; }
查詢 methodID的實現就挖到這裡吧，拆不下去了，尷尬。
但有一點很明瞭，就是查詢methodID是在mainClass例項中進行的。那麼，mainClass又是如何查詢到的，我們需要看下。這個要從 LoadMainClass()說起。
2.2. LoadMainClass 查詢啟動類
上一節我們找到了方法id, 但卻未找到類。所以，得重頭開始再來。
/* * 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) { // 查詢 helplerClass, 並快取 NULL_CHECK0(helperClass = FindBootStrapClass(env, "sun/launcher/LauncherHelper")); } return helperClass; } // solaris/bin/java_md_common.c // 查詢啟動類 jclass FindBootStrapClass(JNIEnv *env, const char* classname) { // 先找到jvm的 JVM_FindClassFromBootLoader 函式地址，然後呼叫即可 if (findBootClass == NULL) { findBootClass = (FindClassFromBootLoader_t *)dlsym(RTLD_DEFAULT, "JVM_FindClassFromBootLoader"); if (findBootClass == NULL) { JLI_ReportErrorMessage(DLL_ERROR4, "JVM_FindClassFromBootLoader"); return NULL; } } return findBootClass(env, classname); }
具體怎麼呼叫，我們略去不說。但如何查詢啟動類，可以一起來看看。即 JVM_FindClassFromBootLoader。
// jvm.cpp // Returns a class loaded by the bootstrap class loader; or null // if not found. ClassNotFoundException is not thrown. // // Rationale behind JVM_FindClassFromBootLoader // a> JVM_FindClassFromClassLoader was never exported in the export tables. // b> because of (a) java.dll has a direct dependecy on the unexported // private symbol "_JVM_FindClassFromClassLoader@20". // c> the launcher cannot use the private symbol as it dynamically opens // the entry point, so if something changes, the launcher will fail // unexpectedly at runtime, it is safest for the launcher to dlopen a // stable exported interface. // d> re-exporting JVM_FindClassFromClassLoader as public, will cause its // signature to change from _JVM_FindClassFromClassLoader@20 to // JVM_FindClassFromClassLoader and will not be backward compatible // with older JDKs. // Thus a public/stable exported entry point is the right solution, // public here means public in linker semantics, and is exported only // to the JDK, and is not intended to be a public API. JVM_ENTRY(jclass, JVM_FindClassFromBootLoader(JNIEnv* env, const char* name)) JVMWrapper2("JVM_FindClassFromBootLoader %s", name); // Java libraries should ensure that name is never null... // 類名稱最長不超過65535 if (name == NULL || (int)strlen(name) > Symbol::max_length()) { // It's impossible to create this class; the name cannot fit // into the constant pool. return NULL; } // 常量池檢查 // 建立啟動類例項 TempNewSymbol h_name = SymbolTable::new_symbol(name, CHECK_NULL); Klass* k = SystemDictionary::resolve_or_null(h_name, CHECK_NULL); if (k == NULL) { return NULL; } if (TraceClassResolution) { trace_class_resolution(k); } // 建立jclass版本例項返回 return (jclass) JNIHandles::make_local(env, k->java_mirror()); JVM_END
整個方法定義，除去各複雜的巨集定義，基本還是邏輯比較清晰的。 分三步走：1. 從常量池拿類名資訊；2. 查詢類資訊例項化Klass；3. 轉換為jclass返回。
2.3. 新增或查詢常量池字元
在查詢啟動類時，看到有常量池的處理，這也是每個類的初始化時必須的過程，所以來看看常量池的使用吧。
// share/vm/classfile/symbolTable.hpp // Symbol creation static Symbol* new_symbol(const char* utf8_buffer, int length, TRAPS) { assert(utf8_buffer != NULL, "just checking"); return lookup(utf8_buffer, length, THREAD); } // symbolTable.cpp // We take care not to be blocking while holding the // SymbolTable_lock. Otherwise, the system might deadlock, since the // symboltable is used during compilation (VM_thread) The lock free // synchronization is simplified by the fact that we do not delete // entries in the symbol table during normal execution (only during // safepoints). Symbol* SymbolTable::lookup(const char* name, int len, TRAPS) { unsigned int hashValue = hash_symbol(name, len); int index = the_table()->hash_to_index(hashValue); Symbol* s = the_table()->lookup(index, name, len, hashValue); // Found if (s != NULL) return s; // 上鎖新增常量池 // Grab SymbolTable_lock first. MutexLocker ml(SymbolTable_lock, THREAD); // Otherwise, add to symbol to table return the_table()->basic_add(index, (u1*)name, len, hashValue, true, CHECK_NULL); } // This version of basic_add adds symbols in batch from the constant pool // parsing. bool SymbolTable::basic_add(ClassLoaderData* loader_data, constantPoolHandle cp, int names_count, const char** names, int* lengths, int* cp_indices, unsigned int* hashValues, TRAPS) { // Check symbol names are not too long. If any are too long, don't add any. for (int i = 0; i< names_count; i++) { if (lengths[i] > Symbol::max_length()) { THROW_MSG_0(vmSymbols::java_lang_InternalError(), "name is too long to represent"); } } // Cannot hit a safepoint in this function because the "this" pointer can move. No_Safepoint_Verifier nsv; for (int i=0; i<names_count; i++) { // Check if the symbol table has been rehashed, if so, need to recalculate // the hash value. unsigned int hashValue; if (use_alternate_hashcode()) { hashValue = hash_symbol(names[i], lengths[i]); } else { hashValue = hashValues[i]; } // Since look-up was done lock-free, we need to check if another // thread beat us in the race to insert the symbol. int index = hash_to_index(hashValue); Symbol* test = lookup(index, names[i], lengths[i], hashValue); if (test != NULL) { // A race occurred and another thread introduced the symbol, this one // will be dropped and collected. Use test instead. cp->symbol_at_put(cp_indices[i], test); assert(test->refcount() != 0, "lookup should have incremented the count"); } else { // Create a new symbol. The null class loader is never unloaded so these // are allocated specially in a permanent arena. bool c_heap = !loader_data->is_the_null_class_loader_data(); Symbol* sym = allocate_symbol((const u1*)names[i], lengths[i], c_heap, CHECK_(false)); assert(sym->equals(names[i], lengths[i]), "symbol must be properly initialized"); // why wouldn't it be??? HashtableEntry<Symbol*, mtSymbol>* entry = new_entry(hashValue, sym); add_entry(index, entry); cp->symbol_at_put(cp_indices[i], sym); } } return true; }
通過hash的方式，將字串新增到常量池中。下一次進行字串獲取時，也就直接從常量池中獲取即可。hash作為查詢最快的方式，非常有效。因為類資訊本身就會反覆使用，所以使用常量池或者快取的方式儲存，再好不過。
2.4. 類的查詢與初始化
經過常量池處理後，進行例項查詢和建立。有點複雜，有可能還涉及到java程式碼的互動。我們只看大概。
// share/vm/classfile/systemDictionary.cpp Klass* SystemDictionary::resolve_or_null(Symbol* class_name, TRAPS) { return resolve_or_null(class_name, Handle(), Handle(), THREAD); } // Forwards to resolve_instance_class_or_null Klass* SystemDictionary::resolve_or_null(Symbol* class_name, Handle class_loader, Handle protection_domain, TRAPS) { assert(!THREAD->is_Compiler_thread(), err_msg("can not load classes with compiler thread: class=%s, classloader=%s", class_name->as_C_string(), class_loader.is_null() ? "null" : class_loader->klass()->name()->as_C_string())); if (FieldType::is_array(class_name)) { return resolve_array_class_or_null(class_name, class_loader, protection_domain, CHECK_NULL); } else if (FieldType::is_obj(class_name)) { ResourceMark rm(THREAD); // Ignore wrapping L and ;. // 類的命名，一定是 Ljava/lang/String; TempNewSymbol name = SymbolTable::new_symbol(class_name->as_C_string() + 1, class_name->utf8_length() - 2, CHECK_NULL); return resolve_instance_class_or_null(name, class_loader, protection_domain, CHECK_NULL); } else { return resolve_instance_class_or_null(class_name, class_loader, protection_domain, CHECK_NULL); } } Klass* SystemDictionary::resolve_instance_class_or_null(Symbol* name, Handle class_loader, Handle protection_domain, TRAPS) { assert(name != NULL && !FieldType::is_array(name) && !FieldType::is_obj(name), "invalid class name"); Ticks class_load_start_time = Ticks::now(); // UseNewReflection // Fix for 4474172; see evaluation for more details class_loader = Handle(THREAD, java_lang_ClassLoader::non_reflection_class_loader(class_loader())); ClassLoaderData *loader_data = register_loader(class_loader, CHECK_NULL); // Do lookup to see if class already exist and the protection domain // has the right access // This call uses find which checks protection domain already matches // All subsequent calls use find_class, and set has_loaded_class so that // before we return a result we call out to java to check for valid protection domain // to allow returning the Klass* and add it to the pd_set if it is valid unsigned int d_hash = dictionary()->compute_hash(name, loader_data); int d_index = dictionary()->hash_to_index(d_hash); Klass* probe = dictionary()->find(d_index, d_hash, name, loader_data, protection_domain, THREAD); if (probe != NULL) return probe; // Non-bootstrap class loaders will call out to class loader and // define via jvm/jni_DefineClass which will acquire the // class loader object lock to protect against multiple threads // defining the class in parallel by accident. // This lock must be acquired here so the waiter will find // any successful result in the SystemDictionary and not attempt // the define // ParallelCapable Classloaders and the bootstrap classloader, // or all classloaders with UnsyncloadClass do not acquire lock here bool DoObjectLock = true; if (is_parallelCapable(class_loader)) { DoObjectLock = false; } unsigned int p_hash = placeholders()->compute_hash(name, loader_data); int p_index = placeholders()->hash_to_index(p_hash); // Class is not in SystemDictionary so we have to do loading. // Make sure we are synchronized on the class loader before we proceed Handle lockObject = compute_loader_lock_object(class_loader, THREAD); check_loader_lock_contention(lockObject, THREAD); ObjectLocker ol(lockObject, THREAD, DoObjectLock); // Check again (after locking) if class already exist in SystemDictionary bool class_has_been_loaded = false; bool super_load_in_progress = false; bool havesupername = false; instanceKlassHandle k; PlaceholderEntry* placeholder; Symbol* superclassname = NULL; { MutexLocker mu(SystemDictionary_lock, THREAD); Klass* check = find_class(d_index, d_hash, name, loader_data); if (check != NULL) { // Klass is already loaded, so just return it class_has_been_loaded = true; k = instanceKlassHandle(THREAD, check); } else { placeholder = placeholders()->get_entry(p_index, p_hash, name, loader_data); if (placeholder && placeholder->super_load_in_progress()) { super_load_in_progress = true; if (placeholder->havesupername() == true) { superclassname = placeholder->supername(); havesupername = true; } } } } // If the class is in the placeholder table, class loading is in progress if (super_load_in_progress && havesupername==true) { k = SystemDictionary::handle_parallel_super_load(name, superclassname, class_loader, protection_domain, lockObject, THREAD); if (HAS_PENDING_EXCEPTION) { return NULL; } if (!k.is_null()) { class_has_been_loaded = true; } } bool throw_circularity_error = false; if (!class_has_been_loaded) { bool load_instance_added = false; // add placeholder entry to record loading instance class // Five cases: // All cases need to prevent modifying bootclasssearchpath // in parallel with a classload of same classname // Redefineclasses uses existence of the placeholder for the duration // of the class load to prevent concurrent redefinition of not completely // defined classes. // case 1. traditional classloaders that rely on the classloader object lock // - no other need for LOAD_INSTANCE // case 2. traditional classloaders that break the classloader object lock // as a deadlock workaround. Detection of this case requires that // this check is done while holding the classloader object lock, // and that lock is still held when calling classloader's loadClass. // For these classloaders, we ensure that the first requestor // completes the load and other requestors wait for completion. // case 3. UnsyncloadClass - don't use objectLocker // With this flag, we allow parallel classloading of a // class/classloader pair // case4. Bootstrap classloader - don't own objectLocker // This classloader supports parallelism at the classloader level, // but only allows a single load of a class/classloader pair. // No performance benefit and no deadlock issues. // case 5. parallelCapable user level classloaders - without objectLocker // Allow parallel classloading of a class/classloader pair { MutexLocker mu(SystemDictionary_lock, THREAD); if (class_loader.is_null() || !is_parallelCapable(class_loader)) { PlaceholderEntry* oldprobe = placeholders()->get_entry(p_index, p_hash, name, loader_data); if (oldprobe) { // only need check_seen_thread once, not on each loop // 6341374 java/lang/Instrument with -Xcomp if (oldprobe->check_seen_thread(THREAD, PlaceholderTable::LOAD_INSTANCE)) { throw_circularity_error = true; } else { // case 1: traditional: should never see load_in_progress. while (!class_has_been_loaded && oldprobe && oldprobe->instance_load_in_progress()) { // case 4: bootstrap classloader: prevent futile classloading, // wait on first requestor if (class_loader.is_null()) { SystemDictionary_lock->wait(); } else { // case 2: traditional with broken classloader lock. wait on first // requestor. double_lock_wait(lockObject, THREAD); } // Check if classloading completed while we were waiting Klass* check = find_class(d_index, d_hash, name, loader_data); if (check != NULL) { // Klass is already loaded, so just return it k = instanceKlassHandle(THREAD, check); class_has_been_loaded = true; } // check if other thread failed to load and cleaned up oldprobe = placeholders()->get_entry(p_index, p_hash, name, loader_data); } } } } // All cases: add LOAD_INSTANCE holding SystemDictionary_lock // case 3: UnsyncloadClass || case 5: parallelCapable: allow competing threads to try // LOAD_INSTANCE in parallel if (!throw_circularity_error && !class_has_been_loaded) { PlaceholderEntry* newprobe = placeholders()->find_and_add(p_index, p_hash, name, loader_data, PlaceholderTable::LOAD_INSTANCE, NULL, THREAD); load_instance_added = true; // For class loaders that do not acquire the classloader object lock, // if they did not catch another thread holding LOAD_INSTANCE, // need a check analogous to the acquire ObjectLocker/find_class // i.e. now that we hold the LOAD_INSTANCE token on loading this class/CL // one final check if the load has already completed // class loaders holding the ObjectLock shouldn't find the class here Klass* check = find_class(d_index, d_hash, name, loader_data); if (check != NULL) { // Klass is already loaded, so return it after checking/adding protection domain k = instanceKlassHandle(THREAD, check); class_has_been_loaded = true; } } } // must throw error outside of owning lock if (throw_circularity_error) { assert(!HAS_PENDING_EXCEPTION && load_instance_added == false,"circularity error cleanup"); ResourceMark rm(THREAD); THROW_MSG_NULL(vmSymbols::java_lang_ClassCircularityError(), name->as_C_string()); } if (!class_has_been_loaded) { // Do actual loading k = load_instance_class(name, class_loader, THREAD); // For UnsyncloadClass only // If they got a linkageError, check if a parallel class load succeeded. // If it did, then for bytecode resolution the specification requires // that we return the same result we did for the other thread, i.e. the // successfully loaded InstanceKlass // Should not get here for classloaders that support parallelism // with the new cleaner mechanism, even with AllowParallelDefineClass // Bootstrap goes through here to allow for an extra guarantee check if (UnsyncloadClass || (class_loader.is_null())) { if (k.is_null() && HAS_PENDING_EXCEPTION && PENDING_EXCEPTION->is_a(SystemDictionary::LinkageError_klass())) { MutexLocker mu(SystemDictionary_lock, THREAD); Klass* check = find_class(d_index, d_hash, name, loader_data); if (check != NULL) { // Klass is already loaded, so just use it k = instanceKlassHandle(THREAD, check); CLEAR_PENDING_EXCEPTION; guarantee((!class_loader.is_null()), "dup definition for bootstrap loader?"); } } } // If everything was OK (no exceptions, no null return value), and // class_loader is NOT the defining loader, do a little more bookkeeping. if (!HAS_PENDING_EXCEPTION && !k.is_null() && k->class_loader() != class_loader()) { check_constraints(d_index, d_hash, k, class_loader, false, THREAD); // Need to check for a PENDING_EXCEPTION again; check_constraints // can throw and doesn't use the CHECK macro. if (!HAS_PENDING_EXCEPTION) { { // Grabbing the Compile_lock prevents systemDictionary updates // during compilations. MutexLocker mu(Compile_lock, THREAD); update_dictionary(d_index, d_hash, p_index, p_hash, k, class_loader, THREAD); } if (JvmtiExport::should_post_class_load()) { Thread *thread = THREAD; assert(thread->is_Java_thread(), "thread->is_Java_thread()"); JvmtiExport::post_class_load((JavaThread *) thread, k()); } } } } // load_instance_class loop if (HAS_PENDING_EXCEPTION) { // An exception, such as OOM could have happened at various places inside // load_instance_class. We might have partially initialized a shared class // and need to clean it up. if (class_loader.is_null()) { // In some cases k may be null. Let's find the shared class again. instanceKlassHandle ik(THREAD, find_shared_class(name)); if (ik.not_null()) { if (ik->class_loader_data() == NULL) { // We didn't go as far as Klass::restore_unshareable_info(), // so nothing to clean up. } else { Klass *kk; { MutexLocker mu(SystemDictionary_lock, THREAD); kk = find_class(d_index, d_hash, name, ik->class_loader_data()); } if (kk != NULL) { // No clean up is needed if the shared class has been entered // into system dictionary, as load_shared_class() won't be called // again. } else { // This must be done outside of the SystemDictionary_lock to // avoid deadlock. // // Note that Klass::restore_unshareable_info (called via // load_instance_class above) is also called outside // of SystemDictionary_lock. Other threads are blocked from // loading this class because they are waiting on the // SystemDictionary_lock until this thread removes // the placeholder below. // // This need to be re-thought when parallel-capable non-boot // classloaders are supported by CDS (today they're not). clean_up_shared_class(ik, class_loader, THREAD); } } } } } if (load_instance_added == true) { // clean up placeholder entries for LOAD_INSTANCE success or error // This brackets the SystemDictionary updates for both defining // and initiating loaders MutexLocker mu(SystemDictionary_lock, THREAD); placeholders()->find_and_remove(p_index, p_hash, name, loader_data, PlaceholderTable::LOAD_INSTANCE, THREAD); SystemDictionary_lock->notify_all(); } } if (HAS_PENDING_EXCEPTION || k.is_null()) { return NULL; } post_class_load_event(class_load_start_time, k, class_loader); #ifdef ASSERT { ClassLoaderData* loader_data = k->class_loader_data(); MutexLocker mu(SystemDictionary_lock, THREAD); Klass* kk = find_class(name, loader_data); assert(kk == k(), "should be present in dictionary"); } #endif // return if the protection domain in NULL if (protection_domain() == NULL) return k(); // Check the protection domain has the right access { MutexLocker mu(SystemDictionary_lock, THREAD); // Note that we have an entry, and entries can be deleted only during GC, // so we cannot allow GC to occur while we're holding this entry. // We're using a No_Safepoint_Verifier to catch any place where we // might potentially do a GC at all. // Dictionary::do_unloading() asserts that classes in SD are only // unloaded at a safepoint. Anonymous classes are not in SD. No_Safepoint_Verifier nosafepoint; if (dictionary()->is_valid_protection_domain(d_index, d_hash, name, loader_data, protection_domain)) { return k(); } } // Verify protection domain. If it fails an exception is thrown validate_protection_domain(k, class_loader, protection_domain, CHECK_NULL); return k(); }
有點複雜，空了細看吧。另外可以提一下的就是，每一次class的載入，都會附帶一個鎖的操作
{ MutexLocker mu(SystemDictionary_lock, THREAD); kk = find_class(d_index, d_hash, name, ik->class_loader_data()); }
這種鎖超出作用域後，就會呼叫析構方法，然後就會自動進行鎖釋放。這和很多的鎖需要 lock() -> unlock() 到是省了一些事。
2.5. 類例項的返回
JNIHandles::make_local(), 大概意思是將前面解析出來的 Klass 轉換對應的 jclass , 而這其中又有很多彎彎繞。
// share/vm/runtime/jniHandles.cpp jobject JNIHandles::make_local(JNIEnv* env, oop obj) { if (obj == NULL) { return NULL; // ignore null handles } else { JavaThread* thread = JavaThread::thread_from_jni_environment(env); assert(Universe::heap()->is_in_reserved(obj), "sanity check"); return thread->active_handles()->allocate_handle(obj); } } jobject JNIHandleBlock::allocate_handle(oop obj) { assert(Universe::heap()->is_in_reserved(obj), "sanity check"); if (_top == 0) { // This is the first allocation or the initial block got zapped when // entering a native function. If we have any following blocks they are // not valid anymore. for (JNIHandleBlock* current = _next; current != NULL; current = current->_next) { assert(current->_last == NULL, "only first block should have _last set"); assert(current->_free_list == NULL, "only first block should have _free_list set"); current->_top = 0; if (ZapJNIHandleArea) current->zap(); } // Clear initial block _free_list = NULL; _allocate_before_rebuild = 0; _last = this; if (ZapJNIHandleArea) zap(); } // Try last block if (_last->_top < block_size_in_oops) { oop* handle = &(_last->_handles)[_last->_top++]; *handle = obj; // 出口1 return (jobject) handle; } // Try free list if (_free_list != NULL) { oop* handle = _free_list; _free_list = (oop*) *_free_list; *handle = obj; // 出口2 return (jobject) handle; } // Check if unused block follow last if (_last->_next != NULL) { // update last and retry _last = _last->_next; return allocate_handle(obj); } // No space available, we have to rebuild free list or expand if (_allocate_before_rebuild == 0) { rebuild_free_list(); // updates _allocate_before_rebuild counter } else { // Append new block Thread* thread = Thread::current(); Handle obj_handle(thread, obj); // This can block, so we need to preserve obj accross call. _last->_next = JNIHandleBlock::allocate_block(thread); _last = _last->_next; _allocate_before_rebuild--; obj = obj_handle(); } return allocate_handle(obj); // retry }
主要就是一個型別的轉換，或者包裝Kclass 以便可以操作更多，細節自行閱讀。
也3. 一點閒話
本文著重講解了jvm對java類的查詢，以及對類方法的查詢實現。而且看起來，實現得挺複雜挺難的樣子。
然而，我們單就對一個類的查詢方法的查詢而言，應該是很簡單的。而且兩場景相似度也很高，只是一個入參是類名，另一個是方法簽名。比如，對類的查詢，無外乎一個hash資料結構的存取實現而已。只是在對類的初始過程，需要保證執行緒安全而已。而對於方法的查詢，則可能更簡單，因為方法畢竟有限，不如類來得多。甚至可能就是一個連結串列搞定，通過遍歷簽名即可得到方法id。
實際上，當我們提出一個問題時，往往就已經將事情簡單化了，或許已關係場景本身的初衷。因為，像jvm這種高難度玩意，需要極高的理論基礎，設計能力，極廣的知識面，以及超高的實現能力。因為，它本身的場景，就是提供各種不確定性。我等，只是做個吃瓜群眾罷了。