JVM源码分析之Attach机制实现完全解读

Attach是什么

在讲这个之前，我们先来点大家都知道的东西，当我们感觉线程一直卡在某个地方，想知道卡在哪里，首先想到的是进行线程dump，而常用的命令是jstack ，我们就可以看到如下线程栈了

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

大家是否注意过上面圈起来的两个线程，”Attach Listener”和“Signal Dispatcher”，这两个线程是我们这次要讲的Attach机制的关键，先偷偷告诉各位，其实Attach Listener这个线程在jvm起来的时候可能并没有的，后面会细说。

那Attach机制是什么？说简单点就是jvm提供一种jvm进程间通信的能力，能让一个进程传命令给另外一个进程，并让它执行内部的一些操作，比如说我们为了让另外一个jvm进程把线程dump出来，那么我们跑了一个jstack的进程，然后传了个pid的参数，告诉它要哪个进程进行线程dump，既然是两个进程，那肯定涉及到进程间通信，以及传输协议的定义，比如要执行什么操作，传了什么参数等

Attach能做些什么

  总结起来说，比如内存dump，线程dump，类信息统计(比如加载的类及大小以及实例个数等)，动态加载agent(使用过btrace的应该不陌生)，动态设置vm flag(但是并不是所有的flag都可以设置的，因为有些flag是在jvm启动过程中使用的，是一次性的)，打印vm flag，获取系统属性等，这些对应的源码(AttachListener.cpp)如下

static AttachOperationFunctionInfo funcs[] = {
{ "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 }
};

后面是命令对应的处理函数。

Attach在jvm里如何实现的

Attach Listener线程的创建

前面也提到了，jvm在启动过程中可能并没有启动Attach Listener这个线程，可以通过jvm参数来启动，代码 （Threads::create_vm）如下：

if (!DisableAttachMechanism) {
if (StartAttachListener || AttachListener::init_at_startup()) {
AttachListener::init();
}
}
bool AttachListener::init_at_startup() {
if (ReduceSignalUsage) {
return true;
} else {
return false;
}
}

其中DisableAttachMechanism，StartAttachListener ，ReduceSignalUsage均默认是false(globals.hpp)

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”)

因此AttachListener::init()并不会被执行，而Attach Listener线程正是在此方法里创建的

// 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);
}
}

既然在启动的时候不会创建这个线程，那么我们在上面看到的那个线程是怎么创建的呢，这个就要关注另外一个线程“Signal Dispatcher”了，顾名思义是处理信号的，这个线程是在jvm启动的时候就会创建的，具体代码就不说了。

下面以jstack的实现来说明触发Attach这一机制进行的过程，jstack命令的实现其实是一个叫做JStack.java的类，查看jstack代码后会走到下面的方法里

private static void runThreadDump(String pid, String args[]) throws Exception {
VirtualMachine vm = null;
try {
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.
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();
}

请注意VirtualMachine.Attach(pid);这行代码，触发Attach pid的关键，如果是在linux下会走到下面的构造函数

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);
}
}

这里要解释下代码了，首先看到调用了createAttachFile方法在目标进程的cwd目录下创建了一个文件/proc//cwd/.Attach_pid，这个在后面的信号处理过程中会取出来做判断(为了安全)，另外我们知道在linux下线程是用进程实现的，在jvm启动过程中会创建很多线程，比如我们上面的信号线程，也就是会看到很多的pid(应该是LWP)，那么如何找到这个信号处理线程呢，从上面实现来看是找到我们传进去的pid的父进程，然后给它的所有子进程都发送一个SIGQUIT信号，而jvm里除了信号线程，其他线程都设置了对此信号的屏蔽，因此收不到该信号，于是该信号就传给了“Signal
Dispatcher”，在传完之后作轮询等待看目标进程是否创建了某个文件，AttachTimeout默认超时时间是5000ms，可通过设置系统变量sun.tools.Attach.AttachTimeout来指定，下面是Signal Dispatcher线程的entry实现

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.
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;
}
….
}
}
}
}

当信号是SIGBREAK(在jvm里做了#define，其实就是SIGQUIT)的时候，就会触发 

AttachListener::is_init_trigger()的执行，

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()) {
init();
return true;
}
}
return false;
}

一开始会判断当前进程目录下是否有个.Attach_pid文件（前面提到了），如果没有就会在/tmp下创建一个/tmp/.Attach_pid，当那个文件的uid和自己的uid是一致的情况下（为了安全）再调用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);
}
}

此时水落石出了，看到创建了一个线程，并且取名为Attach Listener。再看看其子类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;
}

看到其创建了一个监听套接字，并创建了一个文件/tmp/.java_pid，这个文件就是客户端之前一直在轮询等待的文件，随着这个文件的生成，意味着Attach的过程圆满结束了。

Attach listener接收请求

  看看它的entry实现Attach_listener_thread_entry

static void Attach_listener_thread_entry(JavaThread* thread, TRAPS) {
os::set_priority(thread, NearMaxPriority);

thread->record_stack_base_and_size();

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);
}
}

从代码来看就是从队列里不断取AttachOperation，然后找到请求命令对应的方法进行执行，比如我们一开始说的jstack命令，找到 { “threaddump”, thread_dump }的映射关系，然后执行thread_dump方法

再来看看其要调用的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;
}

最终调用的是LinuxAttachListener::dequeue()，

LinuxAttachOperation* LinuxAttachListener::dequeue() {
for (;;) {
int s;

// wait for client to connect
struct sockaddr addr;
socklen_t len = sizeof(addr);
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;
}
}
}

我们看到如果没有请求的话，会一直accept在那里，当来了请求，然后就会创建一个套接字，并读取数据，构建出LinuxAttachOperation返回并执行。

整个过程就这样了，从Attach线程创建到接收请求，处理请求。

PS:此文写了很久了，http://lovestblog.cn/blog/2014/06/18/jvm-attach/，还迁移过一次blog，最近询问attach的人比较多，所以将此文迁移过来