Jdk1.6 JUC源码解析(5)-locks-LockSupport
作者：大飞
功能简介：
- LockSupport是用于构建锁和其他同步机制的基础类，提供了基本的线程阻塞行为。
源码分析：
- 看下LockSupport代码：
public class LockSupport {
private LockSupport() {} // Cannot be instantiated.
// Hotspot implementation via intrinsics API
private static final Unsafe unsafe = Unsafe.getUnsafe();
private static final long parkBlockerOffset;
static {
try {
parkBlockerOffset = unsafe.objectFieldOffset
(java.lang.Thread.class.getDeclaredField("parkBlocker"));
} catch (Exception ex) { throw new Error(ex); }
}
private static void setBlocker(Thread t, Object arg) {
// Even though volatile, hotspot doesn't need a write barrier here.
unsafe.putObject(t, parkBlockerOffset, arg);
}
public static Object getBlocker(Thread t) {
return unsafe.getObjectVolatile(t, parkBlockerOffset);
}LockSupport中使用Unsafe来做一些阻塞相关的操作，如park、unpark；同时也使用Unsafe来支持对Thread中的parkBlocker域的访问。
看一下LockSupport支持的park操作，park相当于获取可用的许可(初始的许可不可用)，调用park()方法会使得当前调用线程阻塞(之前不要调用unpark方法)。
/**
* Disables the current thread for thread scheduling purposes unless the
* permit is available.
*
* <p>If the permit is available then it is consumed and the call returns
* immediately; otherwise
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
*
* <ul>
* <li>Some other thread invokes {@link #unpark unpark} with the
* current thread as the target; or
*
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
* <li>The call spuriously (that is, for no reason) returns.
* </ul>
*
* <p>This method does <em>not</em> report which of these caused the
* method to return. Callers should re-check the conditions which caused
* the thread to park in the first place. Callers may also determine,
* for example, the interrupt status of the thread upon return.
*
* @param blocker the synchronization object responsible for this
* thread parking
* @since 1.6
*/
public static void park(Object blocker) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, 0L);
setBlocker(t, null);
}
public static void parkNanos(Object blocker, long nanos) {
if (nanos > 0) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(false, nanos);
setBlocker(t, null);
}
}
public static void parkUntil(Object blocker, long deadline) {
Thread t = Thread.currentThread();
setBlocker(t, blocker);
unsafe.park(true, deadline);
setBlocker(t, null);
}
public static void park() {
unsafe.park(false, 0L);
}
public static void parkNanos(long nanos) {
if (nanos > 0)
unsafe.park(false, nanos);
}
public static void parkUntil(long deadline) {
unsafe.park(true, deadline);
}park系列方法内部都是调用Unsafe的park方法，找一下Unsafe中park方法的实现。找到hotspot/share/vm/prims/unsafe.cpp，可以看到park方法的实现。
{CC"park", CC"(ZJ)V", FN_PTR(Unsafe_Park)},
...
UNSAFE_ENTRY(void, Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute, jlong time))
UnsafeWrapper("Unsafe_Park");
#ifndef USDT2
HS_DTRACE_PROBE3(hotspot, thread__park__begin, thread->parker(), (int) isAbsolute, time);
#else /* USDT2 */
HOTSPOT_THREAD_PARK_BEGIN(
(uintptr_t) thread->parker(), (int) isAbsolute, time);
#endif /* USDT2 */
JavaThreadParkedState jtps(thread, time != 0);
thread->parker()->park(isAbsolute != 0, time);
#ifndef USDT2
HS_DTRACE_PROBE1(hotspot, thread__park__end, thread->parker());
#else /* USDT2 */
HOTSPOT_THREAD_PARK_END(
(uintptr_t) thread->parker());
#endif /* USDT2 */
UNSAFE_END
内部调用了线程的parker对象的park方法。找到hotspot/share/vm/runtime/thread.hpp，看到如下代码：
// JSR166 per-thread parker
private:
Parker* _parker;
public:
Parker* parker() { return _parker; }继续找到hotspot/share/vm/runtime/park.hpp，看到方法定义：
public:
// For simplicity of interface with Java, all forms of park (indefinite,
// relative, and absolute) are multiplexed into one call.
void park(bool isAbsolute, jlong time);park实现和具体平台相关，找到hotspot/src/os/linux/vm/os_linux.cpp，可以看到方法实现：
void Parker::park(bool isAbsolute, jlong time) {
// Optional fast-path check:
// Return immediately if a permit is available.
if (_counter > 0) {
_counter = 0 ;
OrderAccess::fence();
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;
}
// Next, demultiplex/decode time arguments
timespec absTime;
if (time < 0 || (isAbsolute && time == 0) ) { // don't wait at all
return;
}
if (time > 0) {
unpackTime(&absTime, isAbsolute, time);
}
// Enter safepoint region
// Beware of deadlocks such as 6317397.
// The per-thread Parker:: mutex is a classic leaf-lock.
// In particular a thread must never block on the Threads_lock while
// holding the Parker:: mutex. If safepoints are pending both the
// the ThreadBlockInVM() CTOR and DTOR may grab Threads_lock.
ThreadBlockInVM tbivm(jt);
// Don't wait if cannot get lock since interference arises from
// unblocking. Also. check interrupt before trying wait
if (Thread::is_interrupted(thread, false) || pthread_mutex_trylock(_mutex) != 0) {
return;
}
int status ;
if (_counter > 0) { // no wait needed
_counter = 0;
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
OrderAccess::fence();
return;
}
#ifdef ASSERT
// Don't catch signals while blocked; let the running threads have the signals.
// (This allows a debugger to break into the running thread.)
sigset_t oldsigs;
sigset_t* allowdebug_blocked = os::Linux::allowdebug_blocked_signals();
pthread_sigmask(SIG_BLOCK, allowdebug_blocked, &oldsigs);
#endif
OSThreadWaitState osts(thread->osthread(), false /* not Object.wait() */);
jt->set_suspend_equivalent();
// cleared by handle_special_suspend_equivalent_condition() or java_suspend_self()
if (time == 0) {
status = pthread_cond_wait (_cond, _mutex) ;
} else {
status = os::Linux::safe_cond_timedwait (_cond, _mutex, &absTime) ;
if (status != 0 && WorkAroundNPTLTimedWaitHang) {
pthread_cond_destroy (_cond) ;
pthread_cond_init (_cond, NULL);
}
}
assert_status(status == 0 || status == EINTR ||
status == ETIME || status == ETIMEDOUT,
status, "cond_timedwait");
#ifdef ASSERT
pthread_sigmask(SIG_SETMASK, &oldsigs, NULL);
#endif
_counter = 0 ;
status = pthread_mutex_unlock(_mutex) ;
assert_status(status == 0, status, "invariant") ;
// If externally suspended while waiting, re-suspend
if (jt->handle_special_suspend_equivalent_condition()) {
jt->java_suspend_self();
}
OrderAccess::fence();
} 可见，在linux平台底层用的是POSIX threads API。
接下来看一下unpark系列方法，unpark相当于释放许可(或使许可变为可用)。调用unpark方法会使目标线程在之前阻塞(调用park)地方继续执行，如果目标线程之前没有调用过park，那么在接下来调用park时不会阻塞。
/**
* Makes available the permit for the given thread, if it
* was not already available. If the thread was blocked on
* {@code park} then it will unblock. Otherwise, its next call
* to {@code park} is guaranteed not to block. This operation
* is not guaranteed to have any effect at all if the given
* thread has not been started.
*
* @param thread the thread to unpark, or {@code null}, in which case
* this operation has no effect
*/
public static void unpark(Thread thread) {
if (thread != null)
unsafe.unpark(thread);
}按照上面类似的过程，找到hotspot/share/vm/prims/unsafe.cpp
UNSAFE_ENTRY(void, Unsafe_Unpark(JNIEnv *env, jobject unsafe, jobject jthread))
UnsafeWrapper("Unsafe_Unpark");
Parker* p = NULL;
if (jthread != NULL) {
oop java_thread = JNIHandles::resolve_non_null(jthread);
if (java_thread != NULL) {
jlong lp = java_lang_Thread::park_event(java_thread);
if (lp != 0) {
// This cast is OK even though the jlong might have been read
// non-atomically on 32bit systems, since there, one word will
// always be zero anyway and the value set is always the same
p = (Parker*)addr_from_java(lp);
} else {
// Grab lock if apparently null or using older version of library
MutexLocker mu(Threads_lock);
java_thread = JNIHandles::resolve_non_null(jthread);
if (java_thread != NULL) {
JavaThread* thr = java_lang_Thread::thread(java_thread);
if (thr != NULL) {
p = thr->parker();
if (p != NULL) { // Bind to Java thread for next time.
java_lang_Thread::set_park_event(java_thread, addr_to_java(p));
}
}
}
}
}
}
if (p != NULL) {
#ifndef USDT2
HS_DTRACE_PROBE1(hotspot, thread__unpark, p);
#else /* USDT2 */
HOTSPOT_THREAD_UNPARK(
(uintptr_t) p);
#endif /* USDT2 */
p->unpark();
}
UNSAFE_END
找到hotspot/src/os/linux/vm/os_linux.cpp，可以看到方法实现：
void Parker::unpark() {
int s, status ;
status = pthread_mutex_lock(_mutex);
assert (status == 0, "invariant") ;
s = _counter;
_counter = 1;
if (s < 1) {
if (WorkAroundNPTLTimedWaitHang) {
status = pthread_cond_signal (_cond) ;
assert (status == 0, "invariant") ;
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
} else {
status = pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
status = pthread_cond_signal (_cond) ;
assert (status == 0, "invariant") ;
}
} else {
pthread_mutex_unlock(_mutex);
assert (status == 0, "invariant") ;
}
}
源码就分析到这里，提几点相关知识：
1.Java线程一般与操作系统进程是一对一的关系，比如在linux平台，对应的是linux的轻量级进程(也就是linux的线程)。
2.linux线程的调度和具体调度器有关，比如CFS调度器下，所有待调度的线程按照nice值排列在一棵红黑树中；系统级的休眠与唤醒依赖系统信号，如果一个线程休眠，会被从红黑树移动到一个等待队列中，被唤醒后再移动回来，大体的过程是这样的。
