1.中断后LockSupport.park()直接返回
public class MultInterruptParkDemo {
public static volatile boolean flag = true;
public static void main(String[] args) {
ThreadDemo04 t4 = new ThreadDemo04();
t4.start();
t4.interrupt();
flag = false;
}
public static class ThreadDemo04 extends Thread {
@Override
public void run() {
while (flag) {
}
LockSupport.park();
System.out.println("本打印出现在第一个park()之后");
LockSupport.park();
System.out.println("本打印出现在第二个park()之后");
}
}
}
结果：
本打印出现在第一个park()之后
本打印出现在第二个park()之后
1.1 分析
Thread中断方法分析（包括HotSpot源码）中interrupt源码：
void os::interrupt(Thread* thread) {
assert(Thread::current() == thread || Threads_lock->owned_by_self(),
"possibility of dangling Thread pointer");
OSThread* osthread = thread->osthread();
if (!osthread->interrupted()) {
osthread->set_interrupted(true);
如果线程未设置中断标志位，会进行设置。
LockSupport及HotSpot层Parker::park/unpark分析中park源码：
void Parker::park(bool isAbsolute, jlong time) {
// Ideally we'd do something useful while spinning, such
// as calling unpackTime().
// Optional fast-path check:
// Return immediately if a permit is available.
// We depend on Atomic::xchg() having full barrier semantics
// since we are doing a lock-free update to _counter.
if (Atomic::xchg(0, &_counter) > 0) return;
Thread* thread = Thread::current();
assert(thread->is_Java_thread(), "Must be JavaThread");
JavaThread *jt = (JavaThread *)thread;
// Optional optimization -- avoid state transitions if there's an interrupt pending.
// Check interrupt before trying to wait
if (Thread::is_interrupted(thread, false)) {
return;
}
只要线程设置了中断标志位，就直接返回，且这里的Thread::is_interrupted(thread, false)因为是false，所以不会清除中断标志位。
1.2 总结
- t4.interrupt()会设置线程的中断标志位
- LockSupport.park()会检查线程是否设置了中断标志位，如果设置了，则返回（这里并不会清除中断标志位）
2.unpark不会累积许可（最多为1）
public class MultInterruptParkDemo2 {
public static volatile boolean flag = true;
public static void main(String[] args) {
ThreadDemo04 t4 = new ThreadDemo04();
t4.start();
LockSupport.unpark(t4);
LockSupport.unpark(t4);
LockSupport.unpark(t4);
flag = false;
}
public static class ThreadDemo04 extends Thread {
@Override
public void run() {
while (flag) {
}
LockSupport.park();
System.out.println("本打印出现在第一个park()之后");
LockSupport.park();
System.out.println("本打印出现在第二个park()之后");
}
}
}
结果：
本打印出现在第一个park()之后
2.1 分析
LockSupport及HotSpot层Parker::park/unpark分析中park源码：
void Parker::park(bool isAbsolute, jlong time) {
// Ideally we'd do something useful while spinning, such
// as calling unpackTime().
// Optional fast-path check:
// Return immediately if a permit is available.
// We depend on Atomic::xchg() having full barrier semantics
// since we are doing a lock-free update to _counter.
if (Atomic::xchg(0, &_counter) > 0) return;
每次调用park都会将_counter直接置为0。
void Parker::unpark() {
int s, status ;
status = pthread_mutex_lock(_mutex);
assert (status == 0, "invariant") ;
s = _counter;
_counter = 1;
每次调用unpark都会将_counter直接置为1.
2.2 总结
- 因此，总的许可数总是保持在1，无论调用多少次unpark，都只会将_counter置为1。
- 每次park都会将_counter置为0，如果之前为1，则直接返回。后面的park调用就会阻塞。
3.Thread.interrupted()会清除中断标志
public class MultInterruptParkDemo3 {
public static volatile boolean flag = true;
public static void main(String[] args) {
ThreadDemo04 t4 = new ThreadDemo04();
t4.start();
t4.interrupt();
flag = false;
}
public static class ThreadDemo04 extends Thread {
@Override
public void run() {
while (flag) {
}
LockSupport.park();
System.out.println("本打印出现在第一个park()之后");
System.out.println(Thread.interrupted());
System.out.println(Thread.interrupted());
LockSupport.park();
System.out.println("本打印出现在第二个park()之后");
}
}
}
结果：
本打印出现在第一个park()之后
true
false
3.1 分析
Thread中断方法分析（包括HotSpot源码）中Thread.interrupted()源码：
public static boolean interrupted() {
return currentThread().isInterrupted(true);
}
bool os::is_interrupted(Thread* thread, bool clear_interrupted) {
assert(Thread::current() == thread || Threads_lock->owned_by_self(),
"possibility of dangling Thread pointer");
OSThread* osthread = thread->osthread();
bool interrupted = osthread->interrupted();
if (interrupted && clear_interrupted) {
osthread->set_interrupted(false);
// consider thread->_SleepEvent->reset() ... optional optimization
}
return interrupted;
}
可以看到如果中断了并且clear_interrupted为true，则会清除中断标志位。
3.2 总结
Thread.interrupted()清除中断标志位后，第二个LockSupport.park()就不会直接返回，而是正常地阻塞。
4.sleep清除中断标志位但不会更改_counter
public class MultInterruptParkDemo4 {
public static volatile boolean flag = true;
public static void main(String[] args) {
ThreadDemo04 t4 = new ThreadDemo04();
t4.start();
t4.interrupt();
flag = false;
}
public static class ThreadDemo04 extends Thread {
@Override
public void run() {
while (flag) {
}
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("本打印出现在第一个sleep()之后");
System.out.println(Thread.interrupted());
System.out.println(Thread.interrupted());
LockSupport.park();
System.out.println("本打印出现在第二个park()之后");
}
}
}
结果
java.lang.InterruptedException: sleep interrupted
at java.lang.Thread.sleep(Native Method)
at wz.interrupt.MultInterruptParkDemo4$ThreadDemo04.run(MultInterruptParkDemo4.java:19)
本打印出现在第一个sleep()之后
false
false
本打印出现在第二个park()之后
4.1 分析
Thread中断方法分析（包括HotSpot源码）中interrupt源码：
void os::interrupt(Thread* thread) {
assert(Thread::current() == thread || Threads_lock->owned_by_self(),
"possibility of dangling Thread pointer");
OSThread* osthread = thread->osthread();
if (!osthread->interrupted()) {
osthread->set_interrupted(true);
// More than one thread can get here with the same value of osthread,
// resulting in multiple notifications. We do, however, want the store
// to interrupted() to be visible to other threads before we execute unpark().
OrderAccess::fence();
ParkEvent * const slp = thread->_SleepEvent ;
if (slp != NULL) slp->unpark() ;
}
// For JSR166. Unpark even if interrupt status already was set
if (thread->is_Java_thread())
((JavaThread*)thread)->parker()->unpark();
ParkEvent * ev = thread->_ParkEvent ;
if (ev != NULL) ev->unpark() ;
}
会调用Parker::unpark()，那么_counter会设置为1.
参考阻塞线程的相关方法分析（包括HotSpot层源码）中关于sleep源码：
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");
}
如果线程已中断，则清除中断标记并抛出中断异常，直接返回，但是并没有更改_counter。
4.2 总结
- interrupt会调用((JavaThread*)thread)->parker()->unpark()，将_counter设置为1，也即后面调用park不会阻塞
- sleep如果检测到中断会直接清除中断标志，并抛出异常。
- 因此两个Thread.interrupted()都返回false，且LockSupport.park()不会阻塞。
5.对于LockSupport.park()阻塞总结
如下两个条件任何一个成立，park()都不会阻塞：
- 中断标志位存在（wait、join、sleep或Thread.interrupted()都会清除中断标志位）
- _counter为1（之前调用了unpark或者interrupt）
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