我的原則:先會用再說,內部慢慢來
Thread.sleep 與 Thread.yield 的區別
一、 作用
- Thread.sleep 的作用是啥:當前thread睡覺,讓出資源。(不釋放鎖)
- Thread.yield 作用是啥:當前 thread睡覺,讓出資源。(不釋放鎖)
二、注意點
- 記住鎖是鎖對象,不是鎖住線程
三、區別
- Thread.sleep 有一個方法: Thread.sleep(long millis),需要傳遞參數。 millis > 0 的時候,可以被打斷。
- Thread.yield 有一個方法: Thread.yield() ,不需要傳遞參數。睡覺過程不允許被打斷。
- Thread.sleep(0) = Thread.yield() 【效果一樣】,兩者的意思都是不釋放鎖的前提下,讓出資源,等到別的線程搞完再來弄我自己的。
四、代碼
public class _05_01_YieldTest implements Runnable {
@Override
public void run(){
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
for (int i = 0; i < 3; i++) {
System.out.println(Thread.currentThread() + ": " + i);
// Thread.yield();
try {
Thread.sleep(0);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) throws Exception {
_05_01_YieldTest runn = new _05_01_YieldTest();
Thread t1 = new Thread(runn, "t1");
Thread t2 = new Thread(runn, "t2");
t1.start();
t2.start();
Thread.sleep(100);
for (int i = 0; i < 3; i++) {
System.out.println("Main " + Thread.currentThread() + ": " + i);
// Thread.yield();
try {
Thread.sleep(0);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
輸出:
Thread[t1,5,main]: 0
Main Thread[main,5,main]: 0
Thread[t2,5,main]: 0
Main Thread[main,5,main]: 1
Thread[t1,5,main]: 1
Main Thread[main,5,main]: 2
Thread[t2,5,main]: 1
Thread[t1,5,main]: 2
Thread[t2,5,main]: 2
很明顯, Thread.sleep(0) 效果跟Thread.yield()效果都是一樣的。但是如果 Thread.sleep(long millis)的參數 millis大於0,那麼JVM底層調用的是 os::sleep,等於0調用的是 os::yield();
五、 JVM源碼
- 先看 Thread.Yeild 底層
JVM_ENTRY(void, JVM_Yield(JNIEnv *env, jclass threadClass))
JVMWrapper("JVM_Yield");
//檢查是否設置了DontYieldALot參數,默認爲fasle
//如果設置爲true,直接返回
if (os::dont_yield()) return;
//如果ConvertYieldToSleep=true(默認爲false),調用os::sleep,否則調用os::yield
if (ConvertYieldToSleep) {
os::sleep(thread, MinSleepInterval, false);//sleep 1ms
} else {
os::yield();
}
JVM_END
所以實際上調用的是 os::yield()
//sched_yield是linux kernel提供的API,它會使調用線程放棄CPU使用權,加入到同等優先級隊列的末尾;
//如果調用線程是優先級最高的唯一線程,yield方法返回後,調用線程會繼續運行;
//因此可以知道,對於和調用線程相同或更高優先級的線程來說,yield方法會給予了它們一次運行的機會;
void os::yield() {
sched_yield();
}
- 再看 Thread.sleep 底層
JVM_ENTRY(void, JVM_Sleep(JNIEnv* env, jclass threadClass, jlong millis))
JVMWrapper("JVM_Sleep");
if (millis < 0) {//參數校驗
THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
}
//如果線程已經中斷,拋出中斷異常,關於中斷的實現,在另一篇文章中會講解
if (Thread::is_interrupted (THREAD, true) && !HAS_PENDING_EXCEPTION) {
THROW_MSG(vmSymbols::java_lang_InterruptedException(), "sleep interrupted");
}
//設置線程狀態爲SLEEPING
JavaThreadSleepState jtss(thread);
EventThreadSleep event;
if (millis == 0) {
//如果設置了ConvertSleepToYield(默認爲true),和yield效果相同
if (ConvertSleepToYield) {
os::yield(); // ==================== 這裏是重點 !!!!!====================
} else {//否則調用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 {//參數大於0
//保存初始狀態,返回時恢復原狀態
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);
//調用os::sleep方法,如果發生中斷,拋出異常
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);//恢復osThread狀態
}
if (event.should_commit()) {
event.set_time(millis);
event.commit();
}
JVM_END
看到沒,millis = 0 的時候, sleep 底層就是 yield。