1 创建线程
1.1 两种创建方法
我们可以通过继承Thread类来创建一个线程：
Thread thread = new Thread() {
@Override
public void run() {
System.out.println("run()");
}
};
thread.start();
System.out.println("main()");
也可以通过实现Runnable接口来创建一个线程：
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("run()");
}
});
thread.start();
System.out.println("main()");
1.2 异同点
实际上，从执行流程上来看，这两种方式并没有太大的区别：
- 通过
java.lang.Thread#start()方法，调用本地方法启动线程。 - 线程会执行
java.lang.Thread#run()方法中的代码。
java.lang.Thread#run()源码如下：
public void run() {
if (target != null) {
target.run();
}
}
对于继承Threa类方式，我们重写了run()方法中的逻辑。
对于实现Runnable接口方式，Thread#run()方法会调用target.run()方法，转而执行我们重写的逻辑。这实际上是对代理模式的应用：
![[Runnable.png]]
从它们的执行逻辑来看，Thread代表的是线程，而Runnable代表的是任务。
由于线程只能启动一次（多次启动会抛异常），而任务可以被多个线程执行。因此，更加推荐使用实现Runnable接口的方式创建线程。
例如：
Runnable task = new Runnable() {
@Override
public void run() {
System.out.println("run()");
}
};
Thread thread1 = new Thread(task);
thread1.start();
Thread thread2 = new Thread(task);
thread2.start();
2 Thread核心源码
2.1 成员变量
![[Thread.png]]
核心成员变量如下：
name：线程名group：归属的线程组daemon：是否是守护线程priority：优先级contextClassLoader：上下文类加载器inheritedAccessControlContext：继承的访问控制上下文，用于检查对系统资源的访问权限target：目标任务inheritableThreadLocals：存储线程本地变量stackSize：当前线程的栈大小tid：线程idthreadStatus：线程状态，默认为0，映射为State.NEW。
2.2 线程状态
Java中的线程包括以下状态：
NEW：线程对象还没有执行start()方法。RUNNABLE：线程正在虚拟机中执行。BLOCKED：线程由于在等待锁而处于阻塞状态。WAITING：线程无限期地等待其他线程执行特定操作（notify()等）。TIMED_WAITING：线程有限期地等待其他线程执行特定操作（notify()等）。TERMINATED：线程已经终止退出。
java.lang.Thread.State：
public enum State {
/**
* Thread state for a thread which has not yet started.
*/
NEW,
/**
* Thread state for a runnable thread. A thread in the runnable
* state is executing in the Java virtual machine but it may
* be waiting for other resources from the operating system
* such as processor.
*/
RUNNABLE,
/**
* Thread state for a thread blocked waiting for a monitor lock.
* A thread in the blocked state is waiting for a monitor lock
* to enter a synchronized block/method or
* reenter a synchronized block/method after calling
* {@link Object#wait() Object.wait}.
*/
BLOCKED,
/**
* Thread state for a waiting thread.
* A thread is in the waiting state due to calling one of the
* following methods:
* <ul>
* <li>{@link Object#wait() Object.wait} with no timeout</li>
* <li>{@link #join() Thread.join} with no timeout</li>
* <li>{@link LockSupport#park() LockSupport.park}</li>
* </ul>
*
* <p>A thread in the waiting state is waiting for another thread to
* perform a particular action.
*
* For example, a thread that has called <tt>Object.wait()</tt>
* on an object is waiting for another thread to call
* <tt>Object.notify()</tt> or <tt>Object.notifyAll()</tt> on
* that object. A thread that has called <tt>Thread.join()</tt>
* is waiting for a specified thread to terminate.
*/
WAITING,
/**
* Thread state for a waiting thread with a specified waiting time.
* A thread is in the timed waiting state due to calling one of
* the following methods with a specified positive waiting time:
* <ul>
* <li>{@link #sleep Thread.sleep}</li>
* <li>{@link Object#wait(long) Object.wait} with timeout</li>
* <li>{@link #join(long) Thread.join} with timeout</li>
* <li>{@link LockSupport#parkNanos LockSupport.parkNanos}</li>
* <li>{@link LockSupport#parkUntil LockSupport.parkUntil}</li>
* </ul>
*/
TIMED_WAITING,
/**
* Thread state for a terminated thread.
* The thread has completed execution.
*/
TERMINATED;
}
获取当前线程的状态java.lang.Thread#getState()：
public State getState() {
// get current thread state
return sun.misc.VM.toThreadState(threadStatus);
}
Java会将操作系统的线程状态映射到统一的State中，例如sun.misc.VM#toThreadState()：
public static State toThreadState(int var0) {
if ((var0 & 4) != 0) {
return State.RUNNABLE;
} else if ((var0 & 1024) != 0) {
return State.BLOCKED;
} else if ((var0 & 16) != 0) {
return State.WAITING;
} else if ((var0 & 32) != 0) {
return State.TIMED_WAITING;
} else if ((var0 & 2) != 0) {
return State.TERMINATED;
} else {
return (var0 & 1) == 0 ? State.NEW : State.RUNNABLE;
}
}
2.3 静态方法
Thread在类加载时会执行registerNatives()方法，用来注册本地方法：
private static native void registerNatives();
static {
registerNatives();
}
在Thread.c中，定义了需要注册的本地方法：
// 本地方法数组
static JNINativeMethod methods[] = {
{"start0", "()V", (void *)&JVM_StartThread},
{"stop0", "(" OBJ ")V", (void *)&JVM_StopThread},
{"isAlive", "()Z", (void *)&JVM_IsThreadAlive},
{"suspend0", "()V", (void *)&JVM_SuspendThread},
{"resume0", "()V", (void *)&JVM_ResumeThread},
{"setPriority0", "(I)V", (void *)&JVM_SetThreadPriority},
{"yield", "()V", (void *)&JVM_Yield},
{"sleep", "(J)V", (void *)&JVM_Sleep},
{"currentThread", "()" THD, (void *)&JVM_CurrentThread},
{"countStackFrames", "()I", (void *)&JVM_CountStackFrames},
{"interrupt0", "()V", (void *)&JVM_Interrupt},
{"isInterrupted", "(Z)Z", (void *)&JVM_IsInterrupted},
{"holdsLock", "(" OBJ ")Z", (void *)&JVM_HoldsLock},
{"getThreads", "()[" THD, (void *)&JVM_GetAllThreads},
{"dumpThreads", "([" THD ")[[" STE, (void *)&JVM_DumpThreads},
{"setNativeName", "(" STR ")V", (void *)&JVM_SetNativeThreadName},
};
// 注册本地方法
JNIEXPORT void JNICALL
Java_java_lang_Thread_registerNatives(JNIEnv *env, jclass cls)
{
(*env)->RegisterNatives(env, cls, methods, ARRAY_LENGTH(methods));
}
2.4 初始化方法
我们可以通过构造函数创建线程对象，常用的两个构造函数如下：
public Thread() {
init(null, null, "Thread-" + nextThreadNum(), 0);
}
public Thread(Runnable target) {
init(null, target, "Thread-" + nextThreadNum(), 0);
}
可以看到，实际初始化方法是java.lang.Thread#init()，它会对核心成员变量进行初始化：
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
// 设置线程名
if (name == null) {
throw new NullPointerException("name cannot be null");
}
this.name = name;
Thread parent = currentThread();
SecurityManager security = System.getSecurityManager();
if (g == null) {
/* Determine if it's an applet or not */
/* If there is a security manager, ask the security manager
what to do. */
if (security != null) {
g = security.getThreadGroup();
}
/* If the security doesn't have a strong opinion of the matter
use the parent thread group. */
if (g == null) {
g = parent.getThreadGroup();
}
}
/* checkAccess regardless of whether or not threadgroup is
explicitly passed in. */
g.checkAccess();
/*
* Do we have the required permissions?
*/
if (security != null) {
if (isCCLOverridden(getClass())) {
security.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
}
}
g.addUnstarted();
this.group = g;
this.daemon = parent.isDaemon();
this.priority = parent.getPriority();
if (security == null || isCCLOverridden(parent.getClass()))
this.contextClassLoader = parent.getContextClassLoader();
else
this.contextClassLoader = parent.contextClassLoader;
this.inheritedAccessControlContext =
acc != null ? acc : AccessController.getContext();
this.target = target;
setPriority(priority);
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
线程对象初始化后，它的threadStatus=0，处于State.NEW状态。
2.5 启动线程方法
通过java.lang.Thread#start()方法启动线程：
public synchronized void start() {
/**
* A zero status value corresponds to state "NEW".
*/
if (threadStatus != 0)
throw new IllegalThreadStateException();
/* Notify the group that this thread is about to be started
* so that it can be added to the group's list of threads
* and the group's unstarted count can be decremented.
*/
group.add(this);
boolean started = false;
try {
start0();
started = true;
} finally {
try {
if (!started) {
group.threadStartFailed(this);
}
} catch (Throwable ignore) {
/* do nothing. If start0 threw a Throwable then
it will be passed up the call stack */
}
}
}
启动线程时会校验当前线程的状态，结合synchronized关键字，保证每个线程最多只能启动一次。
实际启动线程的方法位于java.lang.Thread#start0()，它是一个本地方法。因此会执行之前注册的本地方法：
{"start0", "()V", (void *)&JVM_StartThread}
JVM_StartThread是执行虚拟机的内部方法：
JVM_ENTRY(void, JVM_StartThread(JNIEnv* env, jobject jthread))
JVMWrapper("JVM_StartThread");
JavaThread *native_thread = NULL;
// 是否抛出异常标识
// We cannot hold the Threads_lock when we throw an exception,
// due to rank ordering issues. Example: we might need to grab the
// Heap_lock while we construct the exception.
bool throw_illegal_thread_state = false;
// We must release the Threads_lock before we can post a jvmti event
// in Thread::start.
{
// Ensure that the C++ Thread and OSThread structures aren't freed before
// we operate.
MutexLocker mu(Threads_lock);
// Since JDK 5 the java.lang.Thread threadStatus is used to prevent
// re-starting an already started thread, so we should usually find
// that the JavaThread is null. However for a JNI attached thread
// there is a small window between the Thread object being created
// (with its JavaThread set) and the update to its threadStatus, so we
// have to check for this
if (java_lang_Thread::thread(JNIHandles::resolve_non_null(jthread)) != NULL) {
// 当前线程已启动，设置抛异常标识
throw_illegal_thread_state = true;
} else {
// We could also check the stillborn flag to see if this thread was already stopped, but
// for historical reasons we let the thread detect that itself when it starts running
jlong size =
java_lang_Thread::stackSize(JNIHandles::resolve_non_null(jthread));
// Allocate the C++ Thread structure and create the native thread. The
// stack size retrieved from java is signed, but the constructor takes
// size_t (an unsigned type), so avoid passing negative values which would
// result in really large stacks.
// 创建线程
size_t sz = size > 0 ? (size_t) size : 0;
native_thread = new JavaThread(&thread_entry, sz);
// At this point it may be possible that no osthread was created for the
// JavaThread due to lack of memory. Check for this situation and throw
// an exception if necessary. Eventually we may want to change this so
// that we only grab the lock if the thread was created successfully -
// then we can also do this check and throw the exception in the
// JavaThread constructor.
if (native_thread->osthread() != NULL) {
// Note: the current thread is not being used within "prepare".
native_thread->prepare(jthread);
}
}
}
if (throw_illegal_thread_state) {
THROW(vmSymbols::java_lang_IllegalThreadStateException());
}
assert(native_thread != NULL, "Starting null thread?");
if (native_thread->osthread() == NULL) {
// No one should hold a reference to the 'native_thread'.
delete native_thread;
if (JvmtiExport::should_post_resource_exhausted()) {
JvmtiExport::post_resource_exhausted(
JVMTI_RESOURCE_EXHAUSTED_OOM_ERROR | JVMTI_RESOURCE_EXHAUSTED_THREADS,
"unable to create new native thread");
}
THROW_MSG(vmSymbols::java_lang_OutOfMemoryError(),
"unable to create new native thread");
}
#if INCLUDE_JFR
if (JfrRecorder::is_recording() && EventThreadStart::is_enabled() &&
EventThreadStart::is_stacktrace_enabled()) {
JfrThreadLocal* tl = native_thread->jfr_thread_local();
// skip Thread.start() and Thread.start0()
tl->set_cached_stack_trace_id(JfrStackTraceRepository::record(thread, 2));
}
#endif
// 启动线程
Thread::start(native_thread);
JVM_END
线程对象启动后，它会处于State.RUNNABLE状态。
2.6 阻塞线程方法
2.6.1 获取锁失败
如果当前线程在执行时间片中尝试获取锁，但是发现锁被其他线程持有了，那么当前线程就会放弃执行，进入阻塞状态。等到下次执行时，再重新尝试获取锁。
例如，我们定义一个任务：
static class MyThread implements Runnable {
private volatile int index = 0;
@Override
public void run() {
print();
}
private synchronized void print() {
for (; index < 3; index++) {
if (index == 2) {
System.exit(-1);
}
System.out.println(Thread.currentThread().getName() + ":" + index);
}
}
}
使用两个线程去执行：
Runnable task = new MyThread();
Thread thread1 = new Thread(task, "thread1");
Thread thread2 = new Thread(task, "thread2");
System.out.println("before start(), thread1:" + thread1.getState() + ", thread2:" + thread2.getState());
thread1.start();
thread2.start();
while (!MyThread.exit) {
System.out.println("in while(), thread1:" + thread1.getState() + ", thread2:" + thread2.getState());
}
for (int i = 0; i < 3; i++) {
System.out.println("in for(), thread1:" + thread1.getState() + ", thread2:" + thread2.getState());
}
输出结果可能如下：
before start(), thread1:NEW, thread2:NEW
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:BLOCKED
in while(), thread1:RUNNABLE, thread2:BLOCKED
thread1:RUNNABLE:0
in while(), thread1:RUNNABLE, thread2:BLOCKED
in while(), thread1:BLOCKED, thread2:BLOCKED
thread1:RUNNABLE:1
in while(), thread1:BLOCKED, thread2:BLOCKED
thread1:RUNNABLE:2
in for(), thread1:RUNNABLE, thread2:BLOCKED
in for(), thread1:TERMINATED, thread2:TERMINATED
in for(), thread1:TERMINATED, thread2:TERMINATED
在启动线程前，线程状态都是NEW。
执行任务前，线程状态都是RUNNABLE。
由于thread1抢到了锁，所以thread2在试图获取锁时失败，进入BLOCKED状态。
任务执行完，线程状态都变成了TERMINATED。
需要注意的是，尽管thread1全程持有锁，但是它在某些时候也会处理BLOCKED状态（不知道为什么，可能是时间片用完后会进入阻塞状态）。
2.6.2 从WAITING或TIMED_WAITING状态苏醒
当线程从WAITING状态苏醒后，会先进入到BLOCKED状态。例如，我们修改上述任务逻辑：
for (; index < 3; index++) {
if (index == 2) {
exit = true;
}
System.out.println(Thread.currentThread().getName() + ":" + Thread.currentThread().getState() + ":" + index);
if (index == 0) {
index++;
try {
this.wait();
System.out.println(Thread.currentThread().getName() + ":after wait():" + Thread.currentThread().getState() + ":" + index);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
if (index == 1) {
this.notifyAll();
}
}
执行的可能结果如下：
before start(), thread1:NEW, thread2:NEW
in while(), thread1:RUNNABLE, thread2:RUNNABLE
in while(), thread1:RUNNABLE, thread2:BLOCKED
thread1:RUNNABLE:0
in while(), thread1:RUNNABLE, thread2:BLOCKED
thread2:RUNNABLE:1
in while(), thread1:WAITING, thread2:RUNNABLE
thread2:RUNNABLE:2
in for(), thread1:BLOCKED, thread2:RUNNABLE
thread1:after wait():RUNNABLE:3
in for(), thread1:BLOCKED, thread2:TERMINATED
in for(), thread1:TERMINATED, thread2:TERMINATED
在启动线程前，线程状态都是NEW。
执行任务前，线程状态都是RUNNABLE。
由于thread1抢到了锁，所以thread2在试图获取锁时失败，进入BLOCKED状态。
thread1打印后，调用Object#wait()后释放锁，并进入WAITING状态。
thread2获取锁，并执行。
thread2通知thread1从WAITING状态苏醒，thread1进入BLOCKED状态。
thread2执行完，释放锁。
thread1获取锁，从Object#wait()方法开始执行。
任务执行完，线程状态都变成了TERMINATED。
需要注意的是，Thread.sleep()方法并不会释放锁。如果我们将任务逻辑修改如下，又会执行完全不同的逻辑：
for (; index < 3; index++) {
if (index == 2) {
exit = true;
}
System.out.println(Thread.currentThread().getName() + ":" + Thread.currentThread().getState() + ":" + index);
if (index == 0) {
index++;
try {
Thread.sleep(1);
System.out.println(Thread.currentThread().getName() + ":after sleep():" + Thread.currentThread().getState() + ":" + index);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
if (index == 1) {
this.notifyAll();
}
}
执行的可能结果如下：
before start(), thread1:NEW, thread2:NEW
in while(), thread1:RUNNABLE, thread2:RUNNABLE
thread1:RUNNABLE:0
in while(), thread1:RUNNABLE, thread2:BLOCKED
in while(), thread1:TIMED_WAITING, thread2:BLOCKED
in while(), thread1:BLOCKED, thread2:BLOCKED
in while(), thread1:BLOCKED, thread2:BLOCKED
in while(), thread1:BLOCKED, thread2:BLOCKED
thread1:after sleep():RUNNABLE:1
thread1:RUNNABLE:2
in while(), thread1:BLOCKED, thread2:BLOCKED
in for(), thread1:TERMINATED, thread2:TERMINATED
in for(), thread1:TERMINATED, thread2:TERMINATED
in for(), thread1:TERMINATED, thread2:TERMINATED
在启动线程前，线程状态都是NEW。
执行任务前，线程状态都是RUNNABLE。
由于thread1抢到了锁，所以thread2在试图获取锁时失败，进入BLOCKED状态。
thread1打印后，调用Thread#sleep()后不会释放锁，并进入TIMED_WAITING状态。
thread2获取始终获取不到锁，处于BLOCKED状态。
thread1从TIMED_WAITING状态苏醒，进入BLOCKED状态。
thread1获取执行时间片，进入RUNNABLE状态，从Thread#sleep()方法开始执行。
任务执行完，线程状态都变成了TERMINATED。
2.7 （时间）等待线程方法
使用Object#wait()、Thread#join()和LockSupport#park()方法，可以让线程进入WAITING状态。
使用Thread#sleep()、Object#wait(long)、Thread#join(long)、LockSupport#parkNanos()和LockSupport#parkUntil()方法，可以让线程进入TIMED_WAITING状态。
2.7.1 Object的方法
Object#wait()方法有两个要求：
- 当前对象是锁对象。
Object#wait()在同步代码块中执行。
使用Object#wait()方法，可以让当前线程进入WAITING状态，并释放锁：
public final void wait() throws InterruptedException {
wait(0);
}
使用Object#wait(long)和Object#wait(long,int)方法，可以让当前线程进入TIMED_WAITING状态，并释放锁。
它们底层是同一个方法：
public final native void wait(long timeout) throws InterruptedException;
在Object.c中可以查看对应的虚拟机方法：
static JNINativeMethod methods[] = {
{"hashCode", "()I", (void *)&JVM_IHashCode},
{"wait", "(J)V", (void *)&JVM_MonitorWait},
{"notify", "()V", (void *)&JVM_MonitorNotify},
{"notifyAll", "()V", (void *)&JVM_MonitorNotifyAll},
{"clone", "()Ljava/lang/Object;", (void *)&JVM_Clone},
};
JVM_MonitorWait方法如下：
JVM_ENTRY(void, JVM_MonitorWait(JNIEnv* env, jobject handle, jlong ms))
JVMWrapper("JVM_MonitorWait");
Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
JavaThreadInObjectWaitState jtiows(thread, ms != 0);
if (JvmtiExport::should_post_monitor_wait()) {
JvmtiExport::post_monitor_wait((JavaThread *)THREAD, (oop)obj(), ms);
// The current thread already owns the monitor and it has not yet
// been added to the wait queue so the current thread cannot be
// made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
// event handler cannot accidentally consume an unpark() meant for
// the ParkEvent associated with this ObjectMonitor.
}
// 调用ObjectSynchronizer的wait()方法
ObjectSynchronizer::wait(obj, ms, CHECK);
JVM_END
ObjectSynchronizer::wait()方法如下：
void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
if (millis < 0) {
TEVENT (wait - throw IAX) ;
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_wait);
DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
// 调用监视器的wait()方法
monitor->wait(millis, true, THREAD);
/* This dummy call is in place to get around dtrace bug 6254741. Once
that's fixed we can uncomment the following line and remove the call */
// DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
dtrace_waited_probe(monitor, obj, THREAD);
}
ObjectMonitor::wait()方法如下：
void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
Thread * const Self = THREAD ;
assert(Self->is_Java_thread(), "Must be Java thread!");
JavaThread *jt = (JavaThread *)THREAD;
DeferredInitialize () ;
// Throw IMSX or IEX.
CHECK_OWNER();
EventJavaMonitorWait event;
// check for a pending interrupt
if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
// post monitor waited event. Note that this is past-tense, we are done waiting.
if (JvmtiExport::should_post_monitor_waited()) {
// Note: 'false' parameter is passed here because the
// wait was not timed out due to thread interrupt.
JvmtiExport::post_monitor_waited(jt, this, false);
// In this short circuit of the monitor wait protocol, the
// current thread never drops ownership of the monitor and
// never gets added to the wait queue so the current thread
// cannot be made the successor. This means that the
// JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
// consume an unpark() meant for the ParkEvent associated with
// this ObjectMonitor.
}
if (event.should_commit()) {
post_monitor_wait_event(&event, this, 0, millis, false);
}
TEVENT (Wait - Throw IEX) ;
THROW(vmSymbols::java_lang_InterruptedException());
return ;
}
TEVENT (Wait) ;
assert (Self->_Stalled == 0, "invariant") ;
Self->_Stalled = intptr_t(this) ;
jt->set_current_waiting_monitor(this);
// create a node to be put into the queue
// Critically, after we reset() the event but prior to park(), we must check
// for a pending interrupt.
ObjectWaiter node(Self);
node.TState = ObjectWaiter::TS_WAIT ;
Self->_ParkEvent->reset() ;
OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
// Enter the waiting queue, which is a circular doubly linked list in this case
// but it could be a priority queue or any data structure.
// _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
// by the the owner of the monitor *except* in the case where park()
// returns because of a timeout of interrupt. Contention is exceptionally rare
// so we use a simple spin-lock instead of a heavier-weight blocking lock.
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
AddWaiter (&node) ;
Thread::SpinRelease (&_WaitSetLock) ;
if ((SyncFlags & 4) == 0) {
_Responsible = NULL ;
}
intptr_t save = _recursions; // record the old recursion count
_waiters++; // increment the number of waiters
_recursions = 0; // set the recursion level to be 1
exit (true, Self) ; // exit the monitor
guarantee (_owner != Self, "invariant") ;
// The thread is on the WaitSet list - now park() it.
// On MP systems it's conceivable that a brief spin before we park
// could be profitable.
//
// TODO-FIXME: change the following logic to a loop of the form
// while (!timeout && !interrupted && _notified == 0) park()
int ret = OS_OK ;
int WasNotified = 0 ;
{ // State transition wrappers
OSThread* osthread = Self->osthread();
OSThreadWaitState osts(osthread, true);
{
ThreadBlockInVM tbivm(jt);
// Thread is in thread_blocked state and oop access is unsafe.
jt->set_suspend_equivalent();
if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
// Intentionally empty
} else
if (node._notified == 0) {
if (millis <= 0) {
Self->_ParkEvent->park () ;
} else {
ret = Self->_ParkEvent->park (millis) ;
}
}
// were we externally suspended while we were waiting?
if (ExitSuspendEquivalent (jt)) {
// TODO-FIXME: add -- if succ == Self then succ = null.
jt->java_suspend_self();
}
} // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
// Node may be on the WaitSet, the EntryList (or cxq), or in transition
// from the WaitSet to the EntryList.
// See if we need to remove Node from the WaitSet.
// We use double-checked locking to avoid grabbing _WaitSetLock
// if the thread is not on the wait queue.
//
// Note that we don't need a fence before the fetch of TState.
// In the worst case we'll fetch a old-stale value of TS_WAIT previously
// written by the is thread. (perhaps the fetch might even be satisfied
// by a look-aside into the processor's own store buffer, although given
// the length of the code path between the prior ST and this load that's
// highly unlikely). If the following LD fetches a stale TS_WAIT value
// then we'll acquire the lock and then re-fetch a fresh TState value.
// That is, we fail toward safety.
if (node.TState == ObjectWaiter::TS_WAIT) {
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
if (node.TState == ObjectWaiter::TS_WAIT) {
DequeueSpecificWaiter (&node) ; // unlink from WaitSet
assert(node._notified == 0, "invariant");
node.TState = ObjectWaiter::TS_RUN ;
}
Thread::SpinRelease (&_WaitSetLock) ;
}
// The thread is now either on off-list (TS_RUN),
// on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
// The Node's TState variable is stable from the perspective of this thread.
// No other threads will asynchronously modify TState.
guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
OrderAccess::loadload() ;
if (_succ == Self) _succ = NULL ;
WasNotified = node._notified ;
// Reentry phase -- reacquire the monitor.
// re-enter contended monitor after object.wait().
// retain OBJECT_WAIT state until re-enter successfully completes
// Thread state is thread_in_vm and oop access is again safe,
// although the raw address of the object may have changed.
// (Don't cache naked oops over safepoints, of course).
// post monitor waited event. Note that this is past-tense, we are done waiting.
if (JvmtiExport::should_post_monitor_waited()) {
JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
if (node._notified != 0 && _succ == Self) {
// In this part of the monitor wait-notify-reenter protocol it
// is possible (and normal) for another thread to do a fastpath
// monitor enter-exit while this thread is still trying to get
// to the reenter portion of the protocol.
//
// The ObjectMonitor was notified and the current thread is
// the successor which also means that an unpark() has already
// been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
// consume the unpark() that was done when the successor was
// set because the same ParkEvent is shared between Java
// monitors and JVM/TI RawMonitors (for now).
//
// We redo the unpark() to ensure forward progress, i.e., we
// don't want all pending threads hanging (parked) with none
// entering the unlocked monitor.
node._event->unpark();
}
}
if (event.should_commit()) {
post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
}
OrderAccess::fence() ;
assert (Self->_Stalled != 0, "invariant") ;
Self->_Stalled = 0 ;
assert (_owner != Self, "invariant") ;
ObjectWaiter::TStates v = node.TState ;
if (v == ObjectWaiter::TS_RUN) {
enter (Self) ;
} else {
guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
ReenterI (Self, &node) ;
node.wait_reenter_end(this);
}
// Self has reacquired the lock.
// Lifecycle - the node representing Self must not appear on any queues.
// Node is about to go out-of-scope, but even if it were immortal we wouldn't
// want residual elements associated with this thread left on any lists.
guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
assert (_owner == Self, "invariant") ;
assert (_succ != Self , "invariant") ;
} // OSThreadWaitState()
jt->set_current_waiting_monitor(NULL);
guarantee (_recursions == 0, "invariant") ;
_recursions = save; // restore the old recursion count
_waiters--; // decrement the number of waiters
// Verify a few postconditions
assert (_owner == Self , "invariant") ;
assert (_succ != Self , "invariant") ;
assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
if (SyncFlags & 32) {
OrderAccess::fence() ;
}
// check if the notification happened
if (!WasNotified) {
// no, it could be timeout or Thread.interrupt() or both
// check for interrupt event, otherwise it is timeout
if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
TEVENT (Wait - throw IEX from epilog) ;
THROW(vmSymbols::java_lang_InterruptedException());
}
}
// NOTE: Spurious wake up will be consider as timeout.
// Monitor notify has precedence over thread interrupt.}
如果线程A通过调用Object#wait()方法进入了WAITING状态，那么需要其他线程调用同一个锁对象的Object#notify()或Object#notifyAll()方法进行唤醒。
Object#notify()和Object#notifyAll()都是本地方法：
public final native void notify();
public final native void notifyAll();
Object#notify()对应的虚拟机方法是JVM_MonitorNotify：
JVM_ENTRY(void, JVM_MonitorNotify(JNIEnv* env, jobject handle))
JVMWrapper("JVM_MonitorNotify");
Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
ObjectSynchronizer::notify(obj, CHECK);
JVM_END
ObjectSynchronizer::notify()：
void ObjectSynchronizer::notify(Handle obj, TRAPS) {
if (UseBiasedLocking) {
BiasedLocking::revoke_and_rebias(obj, false, THREAD);
assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
}
markOop mark = obj->mark();
if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
return;
}
ObjectSynchronizer::inflate(THREAD,
obj(),
inflate_cause_notify)->notify(THREAD);
}
ObjectMonitor::notify()：
void ObjectMonitor::notify(TRAPS) {
CHECK_OWNER();
if (_WaitSet == NULL) {
TEVENT (Empty-Notify) ;
return ;
}
DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
int Policy = Knob_MoveNotifyee ;
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
ObjectWaiter * iterator = DequeueWaiter() ;
if (iterator != NULL) {
TEVENT (Notify1 - Transfer) ;
guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
guarantee (iterator->_notified == 0, "invariant") ;
if (Policy != 4) {
iterator->TState = ObjectWaiter::TS_ENTER ;
}
iterator->_notified = 1 ;
Thread * Self = THREAD;
iterator->_notifier_tid = JFR_THREAD_ID(Self);
ObjectWaiter * List = _EntryList ;
if (List != NULL) {
assert (List->_prev == NULL, "invariant") ;
assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
assert (List != iterator, "invariant") ;
}
if (Policy == 0) { // prepend to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
List->_prev = iterator ;
iterator->_next = List ;
iterator->_prev = NULL ;
_EntryList = iterator ;
}
} else
if (Policy == 1) { // append to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
// CONSIDER: finding the tail currently requires a linear-time walk of
// the EntryList. We can make tail access constant-time by converting to
// a CDLL instead of using our current DLL.
ObjectWaiter * Tail ;
for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
}
} else
if (Policy == 2) { // prepend to cxq
// prepend to cxq
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Front = _cxq ;
iterator->_next = Front ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
break ;
}
}
}
} else
if (Policy == 3) { // append to cxq
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Tail ;
Tail = _cxq ;
if (Tail == NULL) {
iterator->_next = NULL ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
break ;
}
} else {
while (Tail->_next != NULL) Tail = Tail->_next ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
break ;
}
}
} else {
ParkEvent * ev = iterator->_event ;
iterator->TState = ObjectWaiter::TS_RUN ;
OrderAccess::fence() ;
ev->unpark() ;
}
if (Policy < 4) {
iterator->wait_reenter_begin(this);
}
// _WaitSetLock protects the wait queue, not the EntryList. We could
// move the add-to-EntryList operation, above, outside the critical section
// protected by _WaitSetLock. In practice that's not useful. With the
// exception of wait() timeouts and interrupts the monitor owner
// is the only thread that grabs _WaitSetLock. There's almost no contention
// on _WaitSetLock so it's not profitable to reduce the length of the
// critical section.
}
Thread::SpinRelease (&_WaitSetLock) ;
if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
ObjectMonitor::_sync_Notifications->inc() ;
}
}
Object#notifyAll()对应的虚拟机方法是JVM_MonitorNotifyAll：
JVM_ENTRY(void, JVM_MonitorNotifyAll(JNIEnv* env, jobject handle))
JVMWrapper("JVM_MonitorNotifyAll");
Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
ObjectSynchronizer::notifyall(obj, CHECK);
JVM_END
ObjectSynchronizer::notifyall()：
void ObjectMonitor::notifyAll(TRAPS) {
CHECK_OWNER();
ObjectWaiter* iterator;
if (_WaitSet == NULL) {
TEVENT (Empty-NotifyAll) ;
return ;
}
DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
int Policy = Knob_MoveNotifyee ;
int Tally = 0 ;
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
for (;;) {
iterator = DequeueWaiter () ;
if (iterator == NULL) break ;
TEVENT (NotifyAll - Transfer1) ;
++Tally ;
// Disposition - what might we do with iterator ?
// a. add it directly to the EntryList - either tail or head.
// b. push it onto the front of the _cxq.
// For now we use (a).
guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
guarantee (iterator->_notified == 0, "invariant") ;
iterator->_notified = 1 ;
Thread * Self = THREAD;
iterator->_notifier_tid = JFR_THREAD_ID(Self);
if (Policy != 4) {
iterator->TState = ObjectWaiter::TS_ENTER ;
}
ObjectWaiter * List = _EntryList ;
if (List != NULL) {
assert (List->_prev == NULL, "invariant") ;
assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
assert (List != iterator, "invariant") ;
}
if (Policy == 0) { // prepend to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
List->_prev = iterator ;
iterator->_next = List ;
iterator->_prev = NULL ;
_EntryList = iterator ;
}
} else
if (Policy == 1) { // append to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
// CONSIDER: finding the tail currently requires a linear-time walk of
// the EntryList. We can make tail access constant-time by converting to
// a CDLL instead of using our current DLL.
ObjectWaiter * Tail ;
for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
}
} else
if (Policy == 2) { // prepend to cxq
// prepend to cxq
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Front = _cxq ;
iterator->_next = Front ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
break ;
}
}
} else
if (Policy == 3) { // append to cxq
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Tail ;
Tail = _cxq ;
if (Tail == NULL) {
iterator->_next = NULL ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
break ;
}
} else {
while (Tail->_next != NULL) Tail = Tail->_next ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
break ;
}
}
} else {
ParkEvent * ev = iterator->_event ;
iterator->TState = ObjectWaiter::TS_RUN ;
OrderAccess::fence() ;
ev->unpark() ;
}
if (Policy < 4) {
iterator->wait_reenter_begin(this);
}
// _WaitSetLock protects the wait queue, not the EntryList. We could
// move the add-to-EntryList operation, above, outside the critical section
// protected by _WaitSetLock. In practice that's not useful. With the
// exception of wait() timeouts and interrupts the monitor owner
// is the only thread that grabs _WaitSetLock. There's almost no contention
// on _WaitSetLock so it's not profitable to reduce the length of the
// critical section.
}
Thread::SpinRelease (&_WaitSetLock) ;
if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
ObjectMonitor::_sync_Notifications->inc(Tally) ;
}
}
2.7.2 Thread的方法
使用Thread#join()方法，可以让当前线程进入WAITING状态。
使用Thread#join(long)和Thread#sleep()方法，可以让当前线程进入TIMED_WAITING状态。
Thread#join()、Thread#join(long)和Thread#join(long,int)底层是同一个方法，它们会让当前线程在指定线程执行完成后再执行：
public final synchronized void join(long millis)
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
// Thread#join()或Thread#join(0)
if (millis == 0) {
// 如果线程对象仍然存活
while (isAlive()) {
// 当前线程wait()，
wait(0);
}
}
// Thread#join(long)
else {
// 如果线程对象仍然存活
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
// 当前线程wait(long)
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
需要注意区分isAlive()和wait()的作用对象：
isAlive()的作用对象是线程对象，用于校验指定线程的状态。wait()的作用对象是当前join()方法所在锁对象，会阻塞调用该方法的线程。
Thread#sleep()方法会让当前线程进入TIMED_WAITING状态，它和Object#wait(long)方法的区别是，它不会释放锁：
public static native void sleep(long millis) throws InterruptedException;
JVM_Sleep方法如下：
JVM_ENTRY(void, JVM_Sleep(JNIEnv* env, jclass threadClass, jlong millis))
JVMWrapper("JVM_Sleep");
if (millis < 0) {
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
if (Thread::is_interrupted (THREAD, true) && !HAS_PENDING_EXCEPTION) {
THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted");
}
// Save current thread state and restore it at the end of this block.
// And set new thread state to SLEEPING. JavaThreadSleepState jtss(thread);
#ifndef USDT2
HS_DTRACE_PROBE1(hotspot, thread__sleep__begin, millis);
#else /* USDT2 */
HOTSPOT_THREAD_SLEEP_BEGIN(
millis);
#endif /* USDT2 */
EventThreadSleep event;
if (millis == 0) {
// When ConvertSleepToYield is on, this matches the classic VM implementation of
// JVM_Sleep. Critical for similar threading behaviour (Win32)
// It appears that in certain GUI contexts, it may be beneficial to do a short sleep
// for SOLARIS
if (ConvertSleepToYield) {
os::yield();
} else {
ThreadState old_state = thread->osthread()->get_state();
thread->osthread()->set_state(SLEEPING);
os::sleep(thread, MinSleepInterval, false);
thread->osthread()->set_state(old_state);
}
} else {
ThreadState old_state = thread->osthread()->get_state();
thread->osthread()->set_state(SLEEPING);
if (os::sleep(thread, millis, true) == OS_INTRPT) {
// An asynchronous exception (e.g., ThreadDeathException) could have been thrown on
// us while we were sleeping. We do not overwrite those.
if (!HAS_PENDING_EXCEPTION) {
if (event.should_commit()) {
post_thread_sleep_event(&event, millis);
}
#ifndef USDT2
HS_DTRACE_PROBE1(hotspot, thread__sleep__end,1);
#else /* USDT2 */
HOTSPOT_THREAD_SLEEP_END(
1);
#endif /* USDT2 */
// TODO-FIXME: THROW_MSG returns which means we will not call set_state()
// to properly restore the thread state. That's likely wrong.
THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted");
}
}
thread->osthread()->set_state(old_state);
}
if (event.should_commit()) {
post_thread_sleep_event(&event, millis);
}
#ifndef USDT2
HS_DTRACE_PROBE1(hotspot, thread__sleep__end,0);
#else /* USDT2 */
HOTSPOT_THREAD_SLEEP_END(
0);
#endif /* USDT2 */
JVM_END
2.7.3 LockSupport
使用LockSupport#park()方法，可以让线程进入WAITING状态：
public static void park() {
UNSAFE.park(false, 0L);
}
使用LockSupport#parkNanos()和LockSupport#parkUntil()方法，可以让线程进入TIMED_WAITING状态：
public static void parkNanos(long nanos) {
if (nanos > 0)
UNSAFE.park(false, nanos);
}
public static void parkUntil(long deadline) {
UNSAFE.park(true, deadline);
}
2.8 其他方法
2.8.1 yield
Thread#yield()会使当前线程让出时间片，进入RUNNABLE状态，但是不会释放锁，也没办法保证下次是哪个线程抢到时间片。
2.8.2 interrupt
每个线程都会持有一个interrupted状态。
使用Thread.interrupted()和thread.isInterrupted()方法可以获取该状态。
使用thread.interrupt()可以在线程外部设置该状态。
通常会使用Thread.interrupted()和thread.interrupt()方法来安全的停止线程，例如：
Runnable task = new Runnable() {
@Override
public void run() {
while (!Thread.interrupted()) {
System.out.println(Thread.currentThread().getName() + "interrupted: " + Thread.interrupted());
}
}
};
Thread thread = new Thread(task);
thread.start();
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("before interrupt(): " + thread.isInterrupted());
thread.interrupt();
System.out.println("after interrupt()" + thread.isInterrupted());
2.8.3 exit
使用Thread#exit方法可以在线程退出时进行回调处理，我们可以重写该方法，用于回收资源等。默认方法如下：
private void exit() {
if (group != null) {
group.threadTerminated(this);
group = null;
}
/* Aggressively null out all reference fields: see bug 4006245 */
target = null;
/* Speed the release of some of these resources */
threadLocals = null;
inheritableThreadLocals = null;
inheritedAccessControlContext = null;
blocker = null;
uncaughtExceptionHandler = null;
}