我们都知道Thread.yield()会释放CPU资源，让优先级更高(至少是相同)的线程获得执行机会；sleep当传入参数为0时，和yield相同，当传入参数大于0时，也是释放CPU资源，只不过此时可以让任何其它优先级的线程获得执行机会。为什么会这样？
Thread.yield()源码分析
Thread.yield底层是通过JVM_Yield方法实现的（见jvm.cpp）:
JVM_ENTRY(void, JVM_Yield(JNIEnv *env, jclass threadClass))
JVMWrapper("JVM_Yield");
//检查是否设置了DontYieldALot参数,默认为fasle
//如果设置为true,直接返回
if (os::dont_yield()) return;
//如果ConvertYieldToSleep=true（默认为false）,调用os::sleep,否则调用os::yield
if (ConvertYieldToSleep) {
os::sleep(thread, MinSleepInterval, false);//sleep 1ms
} else {
os::yield();
}
JVM_END
从上面知道，实际上调用的是os::yield:
//sched_yield是linux kernel提供的API,它会使调用线程放弃CPU使用权，加入到同等优先级队列的末尾；
//如果调用线程是优先级最高的唯一线程，yield方法返回后，调用线程会继续运行；
//因此可以知道，对于和调用线程相同或更高优先级的线程来说，yield方法会给予了它们一次运行的机会；
void os::yield() {
sched_yield();
}
Thread.sleep()源码分析
Thread.sleep最终调用JVM_Sleep方法：
JVM_ENTRY(void, JVM_Sleep(JNIEnv* env, jclass threadClass, jlong millis))
JVMWrapper("JVM_Sleep");
if (millis < 0) {//参数校验
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
//如果线程已经中断，抛出中断异常
if (Thread::is_interrupted (THREAD, true) && !HAS_PENDING_EXCEPTION) {
THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted");
}
//设置线程状态为SLEEPING
JavaThreadSleepState jtss(thread);
EventThreadSleep event;
if (millis == 0) {
//如果设置了ConvertSleepToYield(默认为true),和yield效果相同
if (ConvertSleepToYield) {
os::yield();
} else {//否则调用os::sleep方法
ThreadState old_state = thread->osthread()->get_state();
thread->osthread()->set_state(SLEEPING);
os::sleep(thread, MinSleepInterval, false);//sleep 1ms
thread->osthread()->set_state(old_state);
}
} else {//参数大于0
//保存初始状态，返回时恢复原状态
ThreadState old_state = thread->osthread()->get_state();
//osthread->thread status mapping:
// NEW->NEW
//RUNNABLE->RUNNABLE
//BLOCKED_ON_MONITOR_ENTER->BLOCKED
//IN_OBJECT_WAIT,PARKED->WAITING
//SLEEPING,IN_OBJECT_WAIT_TIMED,PARKED_TIMED->TIMED_WAITING
//TERMINATED->TERMINATED
thread->osthread()->set_state(SLEEPING);
//调用os::sleep方法，如果发生中断，抛出异常
if (os::sleep(thread, millis, true) == OS_INTRPT) {
if (!HAS_PENDING_EXCEPTION) {
if (event.should_commit()) {
event.set_time(millis);
event.commit();
}
THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted");
}
}
thread->osthread()->set_state(old_state);//恢复osThread状态
}
if (event.should_commit()) {
event.set_time(millis);
event.commit();
}
JVM_END
从中我们可以看到当sleep()方法中传入参数为0时，默认调用的其实也是os::yield()方法，与yield()方法完全相同；当参数大于0时，才会调用真正的os::sleep方法进行睡眠。
os::sleep的源码如下：
int os::sleep(Thread* thread, jlong millis, bool interruptible) {
assert(thread == Thread::current(), "thread consistency check");
//线程有如下几个成员变量:
//ParkEvent * _ParkEvent ; // for synchronized()
//ParkEvent * _SleepEvent ; // for Thread.sleep
//ParkEvent * _MutexEvent ; // for native internal Mutex/Monitor
//ParkEvent * _MuxEvent ; // for low-level muxAcquire-muxRelease
ParkEvent * const slp = thread->_SleepEvent ;
slp->reset() ;
OrderAccess::fence() ;
//如果millis>0,传入interruptible＝true,否则为false
if (interruptible) {
jlong prevtime = javaTimeNanos();
for (;;) {
if (os::is_interrupted(thread, true)) {//判断是否中断
return OS_INTRPT;
}
jlong newtime = javaTimeNanos();//获取当前时间
//如果linux不支持monotonic lock,有可能出现newtime<prevtime
if (newtime - prevtime < 0) {
assert(!Linux::supports_monotonic_clock(), "time moving backwards");
} else {
millis -= (newtime - prevtime) / NANOSECS_PER_MILLISEC;
}
if(millis <= 0) {
return OS_OK;
}
prevtime = newtime;
{
assert(thread->is_Java_thread(), "sanity check");
JavaThread *jt = (JavaThread *) thread;
ThreadBlockInVM tbivm(jt);
OSThreadWaitState osts(jt->osthread(), false );
jt->set_suspend_equivalent();
slp->park(millis);
jt->check_and_wait_while_suspended();
}
}
} else {//如果interruptible=false
//设置osthread的状态为CONDVAR_WAIT
OSThreadWaitState osts(thread->osthread(), false );
jlong prevtime = javaTimeNanos();
for (;;) {
jlong newtime = javaTimeNanos();
if (newtime - prevtime < 0) {
assert(!Linux::supports_monotonic_clock(), "time moving backwards");
} else {
millis -= (newtime - prevtime) / NANOSECS_PER_MILLISEC;
}
if(millis <= 0) break ;
prevtime = newtime;
slp->park(millis);//底层调用pthread_cond_timedwait实现
}
return OS_OK ;
}
}
(完）
