0 前言
前面文章，我們已講述了《基於JVMTI的Agent實現》和《基於Java Instrument的Agent實現》兩種Agent的實現方式，其中每種方式都會分為：啟動時Agent、執行時Agent。
對於 啟動時Agent的觸發機制，在上一節《JVMTI Agent 工作原理及核心原始碼分析》中，已經在原始碼級進行了分析，具體如下：
但是對於 執行時Agent的觸發機制，卻沒有進行詳細說明，本節的主要目標就是在原始碼級分析下JVMTI Attach 工作機制。
1 Attach是什麼
Attach機制是JVM提供一種JVM程式間通訊的能力，能讓一個程式傳命令給另外一個程式，並讓它執行內部的一些操作。
比如：為了讓另外一個JVM程式把執行緒dump出來，那麼首先跑了一個jstack的程式，然後傳了個pid的引數，告訴它要哪個程式進行執行緒dump，既然是兩個程式，那肯定涉及到程式間通訊，以及傳輸協議的定義，比如：要執行什麼操作，傳了什麼引數等。
有時當我們感覺執行緒一直卡在某個地方，想知道卡在哪裡，首先想到的是進行 執行緒dump，而常用的命令是jstack，我們就可以看到如下執行緒棧：
2014-06-18 12:56:14 Full thread dump Java HotSpot(TM) 64-Bit Server VM (24.51-b03 mixed mode):
"Attach Listener" daemon prio=5 tid=0x00007fb0c6800800 nid=0x440b waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Service Thread" daemon prio=5 tid=0x00007fb0c584d800 nid=0x5303 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread1" daemon prio=5 tid=0x00007fb0c482e000 nid=0x5103 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"C2 CompilerThread0" daemon prio=5 tid=0x00007fb0c482c800 nid=0x4f03 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Signal Dispatcher" daemon prio=5 tid=0x00007fb0c4815800 nid=0x4d03 runnable [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Finalizer" daemon prio=5 tid=0x00007fb0c4813800 nid=0x3903 in Object.wait() [0x00000001187d2000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000007aaa85568> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:135)
- locked <0x00000007aaa85568> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:151)
at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:189)
"Reference Handler" daemon prio=5 tid=0x00007fb0c4800000 nid=0x3703 in Object.wait() [0x00000001186cf000]
java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
- waiting on <0x00000007aaa850f0> (a java.lang.ref.Reference$Lock)
at java.lang.Object.wait(Object.java:503)
at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:133)
- locked <0x00000007aaa850f0> (a java.lang.ref.Reference$Lock)
"main" prio=5 tid=0x00007fb0c5800800 nid=0x1903 waiting on condition [0x0000000107962000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test.main(Test.java:5)
"VM Thread" prio=5 tid=0x00007fb0c583d800 nid=0x3503 runnable
"GC task thread#0 (ParallelGC)" prio=5 tid=0x00007fb0c401e000 nid=0x2503 runnable
"GC task thread#1 (ParallelGC)" prio=5 tid=0x00007fb0c401e800 nid=0x2703 runnable
"GC task thread#2 (ParallelGC)" prio=5 tid=0x00007fb0c401f800 nid=0x2903 runnable
"GC task thread#3 (ParallelGC)" prio=5 tid=0x00007fb0c4020000 nid=0x2b03 runnable
"GC task thread#4 (ParallelGC)" prio=5 tid=0x00007fb0c4020800 nid=0x2d03 runnable
"GC task thread#5 (ParallelGC)" prio=5 tid=0x00007fb0c4021000 nid=0x2f03 runnable
"GC task thread#6 (ParallelGC)" prio=5 tid=0x00007fb0c4022000 nid=0x3103 runnable
"GC task thread#7 (ParallelGC)" prio=5 tid=0x00007fb0c4022800 nid=0x3303 runnable
"VM Periodic Task Thread" prio=5 tid=0x00007fb0c5845000 nid=0x5503 waiting on condition
複製程式碼在上面的Thread Dump日誌中，出現了兩個執行緒：“Attach Listener” 和 “Signal Dispatcher”，這兩個執行緒便是Attach機制的關鍵。
那麼JVM是如何啟動這兩個執行緒呢？JVM有很多執行緒主要在thread.cpp裡的create_vm方法體裡實現：
JvmtiExport::enter_live_phase();
// 1. Signal Dispatcher 需要在釋出VMInit事件之前啟動
os::signal_init();
// 2. Start Attach Listener 如果配置 +StartAttachListener; 否則會延遲啟動
if (!DisableAttachMechanism) {
if (StartAttachListener || AttachListener::init_at_startup()) {
AttachListener::init();
}
}
複製程式碼其中JVM相關引數：DisableAttachMechanism，StartAttachListener ，ReduceSignalUsage 均預設是 false：
product(bool, DisableAttachMechanism, false, "Disable mechanism that allows tools to Attach to this VM”);
product(bool, StartAttachListener, false, "Always start Attach Listener at VM startup");
product(bool, ReduceSignalUsage, false, "Reduce the use of OS signals in Java and/or the VM”);
複製程式碼如上面create_vm原始碼所示，在啟動的時候有可能不會建立AttachListener執行緒，那麼 在上面Thread Stack日誌中看到的AttachListener執行緒是怎麼建立的呢，這個就要關注另外一個執行緒“Signal Dispatcher”了，顧名思義是處理訊號的，這個執行緒是在JVM啟動的時候肯定會建立的。
1.1 Signal Dispatcher 執行緒
在os.cpp中的 signal_init() 函式中，啟動了signal dispatcher 執行緒，對signal dispather執行緒主要是用於處理訊號，等待訊號並且分發處理，可以詳細看 signal_thread_entry 的方法：
// 該方法用於Signal Dispatcher執行緒處理接受到的訊號
static void signal_thread_entry(JavaThread* thread, TRAPS) {
os::set_priority(thread, NearMaxPriority);
while (true) {
int sig;
{
// FIXME : Currently we have not decieded what should be the status
// for this java thread blocked here. Once we decide about
// that we should fix this. 等待訊號
sig = os::signal_wait();
}
if (sig == os::sigexitnum_pd()) {
// Terminate the signal thread
return;
}
switch (sig) {
case SIGBREAK: {
// Check if the signal is a trigger to start the Attach Listener - in that
// case don't print stack traces.
if (!DisableAttachMechanism && AttachListener::is_init_trigger()) {
continue;
}
// Print stack traces
// Any SIGBREAK operations added here should make sure to flush
// the output stream (e.g. tty->flush()) after output. See 4803766.
// Each module also prints an extra carriage return after its output.
VM_PrintThreads op;
VMThread::execute(&op);
VM_PrintJNI jni_op;
VMThread::execute(&jni_op);
VM_FindDeadlocks op1(tty);
VMThread::execute(&op1);
Universe::print_heap_at_SIGBREAK();
if (PrintClassHistogram) {
VM_GC_HeapInspection op1(gclog_or_tty, true /* force full GC before heap inspection */,
true /* need_prologue */);
VMThread::execute(&op1);
}
if (JvmtiExport::should_post_data_dump()) {
JvmtiExport::post_data_dump();
}
break;
}
default: {
// Dispatch the signal to java
HandleMark hm(THREAD);
klassOop k = SystemDictionary::resolve_or_null(vmSymbolHandles::sun_misc_Signal(), THREAD);
KlassHandle klass (THREAD, k);
if (klass.not_null()) {
JavaValue result(T_VOID);
JavaCallArguments args;
args.push_int(sig);
JavaCalls::call_static(
&result,
klass,
vmSymbolHandles::dispatch_name(),
vmSymbolHandles::int_void_signature(),
&args,
THREAD
);
}
if (HAS_PENDING_EXCEPTION) {
// tty is initialized early so we don't expect it to be null, but
// if it is we can't risk doing an initialization that might
// trigger additional out-of-memory conditions
if (tty != NULL) {
char klass_name[256];
char tmp_sig_name[16];
const char* sig_name = "UNKNOWN";
instanceKlass::cast(PENDING_EXCEPTION->klass())->
name()->as_klass_external_name(klass_name, 256);
if (os::exception_name(sig, tmp_sig_name, 16) != NULL)
sig_name = tmp_sig_name;
warning("Exception %s occurred dispatching signal %s to handler"
"- the VM may need to be forcibly terminated",
klass_name, sig_name );
}
CLEAR_PENDING_EXCEPTION;
}
}
}
}
}
複製程式碼可以看到通過 os::signal_wait(); 等待訊號，而在Linux裡是通過 sem_wait() 來實現，當接受到訊號是SIGBREAK(在JVM裡做了#define，其實就是SIGQUIT)的時候，就會觸發 AttachListener::is_init_trigger()的執行初始化attach listener執行緒。
- 第一次收到訊號，會開始初始化，當初始化成功，將會直接返回，而且 不返回任何執行緒stack的資訊（通過socket file的操作返回），並且第二次將不在需要初始化。如果初始化不成功，將直接在控制檯的outputstream中列印執行緒棧資訊；
- 第二次收到訊號，如果已經初始化過，將直接在控制檯中列印執行緒的棧資訊。如果沒有初始化，繼續初始化，走和第一次相同的流程；
比如：我們經常會 使用 kill -3 pid的操作列印出執行緒棧資訊，可以看到具體的實現是在Signal Dispatcher 執行緒中完成的，因為kill -3 pid 並不會建立.attach_pid#pid檔案，所以一直初始化不成功，從而執行緒的棧資訊被列印到控制檯中。
1.2 Attach Listener 執行緒
Attach Listener 執行緒是負責接收到外部的命令，而對該命令進行執行的並且把結果返回給傳送者。在JVM啟動的時候，如果沒有指定 +StartAttachListener，該Attach Listener執行緒是不會啟動的。
在接受到 quit 訊號之後，會呼叫 AttachListener::is_init_trigger() 方法， AttachListener::is_init_trigger() 內會呼叫AttachListener::init() 啟動了Attach Listener 執行緒，在不同的作業系統下初始化實現是不同的，在linux中是在attachListener_Linux.cpp檔案中實現的。
AttachListener::is_init_trigger() 程式碼如下：
bool AttachListener::is_init_trigger() {
if (init_at_startup() || is_initialized()) {
return false; // initialized at startup or already initialized
}
char fn[PATH_MAX+1];
sprintf(fn, ".Attach_pid%d", os::current_process_id());
int ret;
struct stat64 st;
RESTARTABLE(::stat64(fn, &st), ret);
if (ret == -1) {
snprintf(fn, sizeof(fn), "%s/.Attach_pid%d",
os::get_temp_directory(), os::current_process_id());
RESTARTABLE(::stat64(fn, &st), ret);
}
if (ret == 0) {
// simple check to avoid starting the Attach mechanism when
// a bogus user creates the file
if (st.st_uid == geteuid()) {
// 建立AttachListener執行緒
init();
return true;
}
}
return false;
}
複製程式碼一開始會 判斷當前程式目錄下是否有個.Attach_pid檔案，如果沒有就會在/tmp下建立一個/tmp/.Attach_pid，當那個檔案的uid和自己的uid是一致的情況下（為了安全）再呼叫init方法。
// Starts the Attach Listener thread
void AttachListener::init() {
EXCEPTION_MARK;
klassOop k = SystemDictionary::resolve_or_fail(vmSymbols::java_lang_Thread(), true, CHECK);
instanceKlassHandle klass (THREAD, k);
instanceHandle thread_oop = klass->allocate_instance_handle(CHECK);
const char thread_name[] = "Attach Listener";
Handle string = java_lang_String::create_from_str(thread_name, CHECK);
// Initialize thread_oop to put it into the system threadGroup
Handle thread_group (THREAD, Universe::system_thread_group());
JavaValue result(T_VOID);
JavaCalls::call_special(&result, thread_oop,
klass,
vmSymbols::object_initializer_name(),
vmSymbols::threadgroup_string_void_signature(),
thread_group,
string,
CHECK);
KlassHandle group(THREAD, SystemDictionary::ThreadGroup_klass());
JavaCalls::call_special(&result,
thread_group,
group,
vmSymbols::add_method_name(),
vmSymbols::thread_void_signature(),
thread_oop, // ARG 1
CHECK);
{ MutexLocker mu(Threads_lock);
JavaThread* listener_thread = new JavaThread(&Attach_listener_thread_entry);
// Check that thread and osthread were created
if (listener_thread == NULL || listener_thread->osthread() == NULL) {
vm_exit_during_initialization("java.lang.OutOfMemoryError",
"unable to create new native thread");
}
java_lang_Thread::set_thread(thread_oop(), listener_thread);
java_lang_Thread::set_daemon(thread_oop());
listener_thread->set_threadObj(thread_oop());
Threads::add(listener_thread);
Thread::start(listener_thread);
}
}
複製程式碼此時水落石出了，看到建立了一個執行緒，並且取名為Attach Listener。再看看Linux系統下其子類LinuxAttachListener的init方法：
int LinuxAttachListener::init() {
char path[UNIX_PATH_MAX]; // socket file
char initial_path[UNIX_PATH_MAX]; // socket file during setup
int listener; // listener socket (file descriptor)
// register function to cleanup
::atexit(listener_cleanup);
int n = snprintf(path, UNIX_PATH_MAX, "%s/.java_pid%d",
os::get_temp_directory(), os::current_process_id());
if (n < (int)UNIX_PATH_MAX) {
n = snprintf(initial_path, UNIX_PATH_MAX, "%s.tmp", path);
}
if (n >= (int)UNIX_PATH_MAX) {
return -1;
}
// create the listener socket
listener = ::socket(PF_UNIX, SOCK_STREAM, 0);
if (listener == -1) {
return -1;
}
// bind socket
struct sockaddr_un addr;
addr.sun_family = AF_UNIX;
strcpy(addr.sun_path, initial_path);
::unlink(initial_path);
int res = ::bind(listener, (struct sockaddr*)&addr, sizeof(addr));
if (res == -1) {
RESTARTABLE(::close(listener), res);
return -1;
}
// put in listen mode, set permissions, and rename into place
res = ::listen(listener, 5);
if (res == 0) {
RESTARTABLE(::chmod(initial_path, S_IREAD|S_IWRITE), res);
if (res == 0) {
res = ::rename(initial_path, path);
}
}
if (res == -1) {
RESTARTABLE(::close(listener), res);
::unlink(initial_path);
return -1;
}
set_path(path);
set_listener(listener);
return 0;
}
複製程式碼看到其建立了一個監聽套接字，並建立了一個檔案/tmp/.java_pid，這個檔案就是客戶端之前一直在輪詢等待的檔案，隨著這個檔案的生成，意味著Attach的建立過程圓滿結束了。
Attach Listener執行緒接收到請求時，具體的請求處理在 attach_listener_thread_entry 方法體中實現：
static void attach_listener_thread_entry(JavaThread* thread, TRAPS) {
os::set_priority(thread, NearMaxPriority);
if (AttachListener::pd_init() != 0) {
return;
}
AttachListener::set_initialized();
for (;;) {
AttachOperation* op = AttachListener::dequeue();
if (op == NULL) {
return; // dequeue failed or shutdown
}
ResourceMark rm;
bufferedStream st;
jint res = JNI_OK;
// handle special detachall operation
if (strcmp(op->name(), AttachOperation::detachall_operation_name()) == 0) {
AttachListener::detachall();
} else {
// find the function to dispatch too
AttachOperationFunctionInfo* info = NULL;
for (int i=0; funcs[i].name != NULL; i++) {
const char* name = funcs[i].name;
assert(strlen(name) <= AttachOperation::name_length_max, "operation <= name_length_max");
if (strcmp(op->name(), name) == 0) {
info = &(funcs[i]);
break;
}
}
// check for platform dependent attach operation
if (info == NULL) {
info = AttachListener::pd_find_operation(op->name());
}
if (info != NULL) {
// dispatch to the function that implements this operation
res = (info->func)(op, &st);
} else {
st.print("Operation %s not recognized!", op->name());
res = JNI_ERR;
}
}
// operation complete - send result and output to client
op->complete(res, &st);
}
}
複製程式碼從程式碼來看就是 從佇列裡不斷取AttachOperation，然後找到請求命令對應的方法進行執行，比如一開始說的jstack命令，找到 { “threaddump”, thread_dump }的對映關係，然後執行thread_dump方法。
AttachOperation有很多種類，比如：記憶體dump，執行緒dump，類資訊統計(比如載入的類及大小以及例項個數等)，動態載入agent，動態設定vm flag(但是並不是所有的flag都可以設定的，因為有些flag是在jvm啟動過程中使用的，是一次性的)，列印vm flag，獲取系統屬性等，這些對應的原始碼（AttachListener.cpp）如下：
static AttachOperationFunctionInfo funcs[] = {
// 第二個引數是命令對應的處理函式
{ "agentProperties", get_agent_properties },
{ "datadump", data_dump },
{ "dumpheap", dump_heap },
{ "load", JvmtiExport::load_agent_library },
{ "properties", get_system_properties },
{ "threaddump", thread_dump },
{ "inspectheap", heap_inspection },
{ "setflag", set_flag },
{ "printflag", print_flag },
{ "jcmd", jcmd },
{ NULL, NULL }
};
複製程式碼再來看看其要呼叫的 AttachListener::dequeue();：
AttachOperation* AttachListener::dequeue() {
JavaThread* thread = JavaThread::current();
ThreadBlockInVM tbivm(thread);
thread->set_suspend_equivalent();
// cleared by handle_special_suspend_equivalent_condition() or
// java_suspend_self() via check_and_wait_while_suspended()
AttachOperation* op = LinuxAttachListener::dequeue();
// were we externally suspended while we were waiting?
thread->check_and_wait_while_suspended();
return op;
}
複製程式碼最終會呼叫的是 LinuxAttachListener::dequeue()：
LinuxAttachOperation* LinuxAttachListener::dequeue() {
for (;;) {
int s;
// wait for client to connect
struct sockaddr addr;
socklen_t len = sizeof(addr);
// 如果沒有請求的話，會一直accept在那裡
RESTARTABLE(::accept(listener(), &addr, &len), s);
if (s == -1) {
return NULL; // log a warning?
}
// get the credentials of the peer and check the effective uid/guid
// - check with jeff on this.
struct ucred cred_info;
socklen_t optlen = sizeof(cred_info);
if (::getsockopt(s, SOL_SOCKET, SO_PEERCRED, (void*)&cred_info, &optlen) == -1) {
int res;
RESTARTABLE(::close(s), res);
continue;
}
uid_t euid = geteuid();
gid_t egid = getegid();
if (cred_info.uid != euid || cred_info.gid != egid) {
int res;
RESTARTABLE(::close(s), res);
continue;
}
// peer credential look okay so we read the request
LinuxAttachOperation* op = read_request(s);
if (op == NULL) {
int res;
RESTARTABLE(::close(s), res);
continue;
} else {
return op;
}
}
}
複製程式碼如上程式碼中可以看到，如果沒有請求的話，會一直accept在那裡，當來了請求，然後就會建立一個套接字，並讀取資料，構建出LinuxAttachOperation返回，找到請求對應的操作，呼叫操作得到結果並把結果寫到這個socket的檔案，如果你把socket的檔案刪除，jstack/jmap會出現錯誤資訊 unable to open socket file:........
1.3 jstack/jmap命令流程圖
以jstack的實現來說明觸發Attach這一機制進行的過程，jstack命令的實現其實是一個叫做JStack.java的類，jstack命令首先會attach到目標JVM程式，產生VirtualMachine類；Linux系統下，其實現類為LinuxVirtualMachine，呼叫其remoteDataDump方法，列印堆疊資訊；檢視JStack.java程式碼後會走到下面的方法裡：
private static void runThreadDump(String pid, String args[]) throws Exception {
VirtualMachine vm = null;
try {
// jstack命令首先會attach到目標JVM程式
vm = VirtualMachine.Attach(pid);
} catch (Exception x) {
String msg = x.getMessage();
if (msg != null) {
System.err.println(pid + ": " + msg);
} else {
x.printStackTrace();
}
if ((x instanceof AttachNotSupportedException) &&
(loadSAClass() != null)) {
System.err.println("The -F option can be used when the target " +
"process is not responding");
}
System.exit(1);
}
// Cast to HotSpotVirtualMachine as this is implementation specific
// method.
// 輸出堆疊資訊
InputStream in = ((HotSpotVirtualMachine)vm).remoteDataDump((Object[])args);
// read to EOF and just print output
byte b[] = new byte[256];
int n;
do {
n = in.read(b);
if (n > 0) {
String s = new String(b, 0, n, "UTF-8");
System.out.print(s);
}
} while (n > 0);
in.close();
vm.detach();
}
複製程式碼那麼VirtualMachine是如何連線到目標JVM程式的呢？請注意 VirtualMachine.Attach(pid); 這行程式碼，觸發Attach pid的關鍵，如果是在Linux下具體的實現邏輯在 sun.tools.attach.LinuxVirtualMachine 的建構函式：
LinuxVirtualMachine(AttachProvider provider, String vmid) throws AttachNotSupportedException, IOException
{
super(provider, vmid);
// This provider only understands pids
int pid;
try {
pid = Integer.parseInt(vmid);
} catch (NumberFormatException x) {
throw new AttachNotSupportedException("Invalid process identifier");
}
// Find the socket file. If not found then we attempt to start the
// Attach mechanism in the target VM by sending it a QUIT signal.
// Then we attempt to find the socket file again.
path = findSocketFile(pid);
if (path == null) {
File f = createAttachFile(pid);
try {
// On LinuxThreads each thread is a process and we don't have the
// pid of the VMThread which has SIGQUIT unblocked. To workaround
// this we get the pid of the "manager thread" that is created
// by the first call to pthread_create. This is parent of all
// threads (except the initial thread).
if (isLinuxThreads) {
int mpid;
try {
mpid = getLinuxThreadsManager(pid);
} catch (IOException x) {
throw new AttachNotSupportedException(x.getMessage());
}
assert(mpid >= 1);
sendQuitToChildrenOf(mpid);
} else {
sendQuitTo(pid);
}
// give the target VM time to start the Attach mechanism
int i = 0;
long delay = 200;
int retries = (int)(AttachTimeout() / delay);
do {
try {
Thread.sleep(delay);
} catch (InterruptedException x) { }
path = findSocketFile(pid);
i++;
} while (i <= retries && path == null);
if (path == null) {
throw new AttachNotSupportedException(
"Unable to open socket file: target process not responding " +
"or HotSpot VM not loaded");
}
} finally {
f.delete();
}
}
// Check that the file owner/permission to avoid Attaching to
// bogus process
checkPermissions(path);
// Check that we can connect to the process
// - this ensures we throw the permission denied error now rather than
// later when we attempt to enqueue a command.
int s = socket();
try {
connect(s, path);
} finally {
close(s);
}
}
複製程式碼
- 查詢/tmp目錄下是否存在
".java_pid"+pid檔案；- 如果檔案不存在，則首先建立
"/proc/" + pid + "/cwd/" + ".attach_pid" + pid檔案；- 通過
kill命令傳送SIGQUIT訊號給目標JVM程式，由於JVM裡除了訊號執行緒，其他執行緒都設定了對此訊號的遮蔽，因此收不到該訊號，於是該訊號就傳給了“Signal Dispatcher”；- 目標JVM程式接收到訊號之後，會在
/tmp目錄下建立".java_pid"+pid檔案；- 當發現
/tmp目錄下存在".java_pid"+pid檔案，LinuxVirtualMachine會通過connect系統呼叫連線到該檔案描述符，後續通過該fd進行雙方的通訊；
JVM接受SIGQUIT訊號的相關邏輯處理，則是在前面 signal_thread_entry 方法中進行實現。
前面JStack.java原始碼中，輸出堆疊資訊是通過呼叫remoteDataDump方法實現的，該方法就是通過往前面提到的fd中寫入threaddump指令，讀取返回結果，從而得到目標JVM的堆疊資訊。
2 Java 程式碼實現動態 attach Agent
Java動態attach Agent與上面所講到的JStack.java實現基本類似，在 attach 的java程式碼中，使用sun自用的tool.jar中的VirtualMachine的attach的方式：
VirtualMachine vm = VirtualMachine.attach(processid);
vm.loadAgent(agentpath, args)
複製程式碼在HotSpotVirtualMachine.java中，loadAgent 方法原始碼如下：
public void loadAgent(String agent, String options) throws AgentLoadException, AgentInitializationException, IOException
{
String args = agent;
if (options != null) {
args = args + "=" + options;
}
try {
loadAgentLibrary("instrument", args);
} .....
}
private void loadAgentLibrary(String agentLibrary, boolean isAbsolute, String options) throws AgentLoadException, AgentInitializationException, IOException
{
InputStream in = execute("load", agentLibrary, isAbsolute ? "true" : "false", options);
try {
int result = readInt(in);
if (result != 0) {
throw new AgentInitializationException("Agent_OnAttach failed", result);
}
} finally {
in.close();
}
}
複製程式碼在LinuxVirtualMachine.java中的execute方法：
InputStream execute(String cmd, Object ... args) throws AgentLoadException, IOException {
assert args.length <= 3; // includes null
// did we detach?
String p;
synchronized (this) {
if (this.path == null) {
throw new IOException("Detached from target VM");
}
p = this.path;
}
// create UNIX socket
int s = socket();
// connect to target VM
try {
connect(s, p);
} catch (IOException x) {
close(s);
throw x;
}
IOException ioe = null;
// connected - write request
// <ver> <cmd> <args...>
try {
writeString(s, PROTOCOL_VERSION);
writeString(s, cmd);
for (int i=0; i<3; i++) {
if (i < args.length && args[i] != null) {
writeString(s, (String)args[i]);
} else {
writeString(s, "");
}
}
} catch (IOException x) {
ioe = x;
}
// Create an input stream to read reply
SocketInputStream sis = new SocketInputStream(s);
// Read the command completion status
int completionStatus;
try {
completionStatus = readInt(sis);
} catch (IOException x) {
sis.close();
if (ioe != null) {
throw ioe;
} else {
throw x;
}
}
....
}
複製程式碼也就是向socket的中寫入了，格式為：
<ver> <cmd> <args...>
複製程式碼具體內容為：
1 load instrument agentPath=path.jar
複製程式碼既然Load Agent 往socket裡發了load指令，匹配到JVM的操作：
static AttachOperationFunctionInfo funcs[] = {
{ "agentProperties", get_agent_properties },
{ "datadump", data_dump },
#ifndef SERVICES_KERNEL
{ "dumpheap", dump_heap },
#endif // SERVICES_KERNEL
{ "load", JvmtiExport::load_agent_library },
{ "properties", get_system_properties },
{ "threaddump", thread_dump },
{ "inspectheap", heap_inspection },
{ "setflag", set_flag },
{ "printflag", print_flag },
{ NULL, NULL }
};
複製程式碼"load", JvmtiExport::load_agent_library，具體原始碼如下：
jint JvmtiExport::load_agent_library(AttachOperation* op, outputStream* st) {
char ebuf[1024];
char buffer[JVM_MAXPATHLEN];
void* library;
jint result = JNI_ERR;
const char* agent = op->arg(0);
const char* absParam = op->arg(1);
const char* options = op->arg(2);
bool is_absolute_path = (absParam != NULL) && (strcmp(absParam,"true")==0);
if (is_absolute_path) {
library = os::dll_load(agent, ebuf, sizeof ebuf);
} else {
// Try to load the agent from the standard dll directory
os::dll_build_name(buffer, sizeof(buffer), Arguments::get_dll_dir(), agent);
library = os::dll_load(buffer, ebuf, sizeof ebuf);
if (library == NULL) {
// not found - try local path
char ns[1] = {0};
os::dll_build_name(buffer, sizeof(buffer), ns, agent);
library = os::dll_load(buffer, ebuf, sizeof ebuf);
}
}
if (library != NULL) {
// Lookup the Agent_OnAttach function
OnAttachEntry_t on_attach_entry = NULL;
const char *on_attach_symbols[] = AGENT_ONATTACH_SYMBOLS;
for (uint symbol_index = 0; symbol_index < ARRAY_SIZE(on_attach_symbols); symbol_index++) {
on_attach_entry =
CAST_TO_FN_PTR(OnAttachEntry_t, os::dll_lookup(library, on_attach_symbols[symbol_index]));
if (on_attach_entry != NULL) break;
}
if (on_attach_entry == NULL) {
// Agent_OnAttach missing - unload library
os::dll_unload(library);
} else {
// Invoke the Agent_OnAttach function
JavaThread* THREAD = JavaThread::current();
{
extern struct JavaVM_ main_vm;
JvmtiThreadEventMark jem(THREAD);
JvmtiJavaThreadEventTransition jet(THREAD);
result = (*on_attach_entry)(&main_vm, (char*)options, NULL);
}
if (HAS_PENDING_EXCEPTION) {
CLEAR_PENDING_EXCEPTION;
}
if (result == JNI_OK) {
Arguments::add_loaded_agent(agent, (char*)options, is_absolute_path, library);
}
// Agent_OnAttach executed so completion status is JNI_OK
st->print_cr("%d", result);
result = JNI_OK;
}
}
return result;
}
#define AGENT_ONATTACH_SYMBOLS {"Agent_OnAttach"}
複製程式碼3 執行 Instrument 的 Agent on attach
載入instrument的動態庫，並且呼叫方法instrument動態庫中的Agent_OnAttach方法：
JNIEXPORT jint JNICALL Agent_OnAttach(JavaVM* vm, char *args, void * reserved) {
.....
initerror = createNewJPLISAgent(vm, &agent);
if ( initerror == JPLIS_INIT_ERROR_NONE ) {
......
if (parseArgumentTail(args, &jarfile, &options) != 0) {
return JNI_ENOMEM;
}
attributes = readAttributes( jarfile );
if (attributes == NULL) {
fprintf(stderr, "Error opening zip file or JAR manifest missing: %s\n", jarfile);
free(jarfile);
if (options != NULL) free(options);
return AGENT_ERROR_BADJAR;
}
agentClass = getAttribute(attributes, "Agent-Class");
if (agentClass == NULL) {
fprintf(stderr, "Failed to find Agent-Class manifest attribute from %s\n",
jarfile);
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return AGENT_ERROR_BADJAR;
}
if (appendClassPath(agent, jarfile)) {
fprintf(stderr, "Unable to add %s to system class path "
"- not supported by system class loader or configuration error!\n",
jarfile);
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return AGENT_ERROR_NOTONCP;
}
oldLen = strlen(agentClass);
newLen = modifiedUtf8LengthOfUtf8(agentClass, oldLen);
if (newLen == oldLen) {
agentClass = strdup(agentClass);
} else {
char* str = (char*)malloc( newLen+1 );
if (str != NULL) {
convertUtf8ToModifiedUtf8(agentClass, oldLen, str, newLen);
}
agentClass = str;
}
if (agentClass == NULL) {
free(jarfile);
if (options != NULL) free(options);
freeAttributes(attributes);
return JNI_ENOMEM;
}
bootClassPath = getAttribute(attributes, "Boot-Class-Path");
if (bootClassPath != NULL) {
appendBootClassPath(agent, jarfile, bootClassPath);
}
convertCapabilityAtrributes(attributes, agent);
success = createInstrumentationImpl(jni_env, agent);
jplis_assert(success);
/*
* Turn on the ClassFileLoadHook.
*/
if (success) {
success = setLivePhaseEventHandlers(agent);
jplis_assert(success);
}
if (success) {
success = startJavaAgent(agent,
jni_env,
agentClass,
options,
agent->mAgentmainCaller);
}
if (!success) {
fprintf(stderr, "Agent failed to start!\n");
result = AGENT_ERROR_STARTFAIL;
}
if (options != NULL) free(options);
free(agentClass);
freeAttributes(attributes);
}
return result;
}
複製程式碼上面程式碼裡一開始的createNewJPLISAgent和on_load是一樣的註冊了一些鉤子函式，具體詳情可參考：《JVMTI Agent 工作原理及核心原始碼分析》。
在上面的Agent_OnAttach程式碼中我們也看到了，讀取載入的jar中MANIFEST Agent-Class的配置：
agentClass = getAttribute(attributes, "Agent-Class");
複製程式碼建立生成sun.instrument.InstrumentationImpl物件：
success = createInstrumentationImpl(jni_env, agent);
複製程式碼通過InstrumentationImpl物件中的loadClassAndCallAgentmain方法去初始化在Agent-Class中的類，並呼叫class裡的agentmain的方法：
success = startJavaAgent(agent, jni_env, agentClass, options, agent->mAgentmainCaller);
複製程式碼也就是說定義的on_attach的class裡需要有agentmain的方法實現：
public class MyTransformer {
public static void agentmain(String agentArgs, Instrumentation inst) throws ClassNotFoundException, UnmodifiableClassException, NotFoundException, CannotCompileException, IOException{
....
}
}複製程式碼