在java中,要让线程等待最普通的方法是调用Object.wait()方法,
Causes the current thread to wait until another thread invokes the notify() method for this object.
但是当我阅读JUC(java.util.concurrent)的源码时发现这个包不是这样做的, 我跟踪CountDownLatch.await()调用,最后跟到了LockSupport.park()方法里, 这里调用的是 unsafe.park()方法来block线程。
LockSupport 和 CAS一样是JUC很多控制机制的基础(但他们的底层其实都是在依赖Unsafe),下面就来学习下LockSupport这个类:
// Hotspot implementation via intrinsics API private static final Unsafe unsafe = Unsafe.getUnsafe(); //unsafe 用来实现底层操作 private static final long parkBlockerOffset; //辅助参数,配合unsafe用的 //This object is recorded while // the thread is blocked to permit monitoring and diagnostic tools to // identify the reasons that threads are blocked. //设置一个线程和关联的blocker对象,blocker用来做分析,debug用的 private static void setBlocker(Thread t, Object arg) { // Even though volatile, hotspot doesn't need a write barrier here. unsafe.putObject(t, parkBlockerOffset, arg); } //block当前线程,是否真的block了取决于permit是否available //permit相当于1,0的开关, 默认是0, 调一次unpark就+1变成1了,调一次park会消费这个1又变成0了(park立即返回), //再次调用park会变成block(因为没有1可以拿了,会等在这,直到有1),这时调用unpark会把1给回去(线程解锁返回) //每个线程都有个相关的permit, permit最多一个,调用unpark多次也不会积累 //当为permit available时,方法会立即返回,不会block,反之就会block当前线程直到下面3件事发生 //1. 其他线程调用了unpark(此线程) //2. 其他线程interrupts了此线程 //3. The call spuriously (that is, for no reason) returns. public static void park() { unsafe.park(false, 0L); } //对于给定线程,让permit变得avaliable, public static void unpark(Thread thread) { if (thread != null) unsafe.unpark(thread); } //然后park有2个带限定时间的版本,所以一共有3个park version, 这3个version又有带blocker的debug版本 public static void parkNanos(long nanos) { public static void parkUntil(long deadline) {
下面写些代码试下
System.out.println("start"); LockSupport.parkNanos(1000000000); System.out.println("end"); //一开始会block线程,直到给定时间过去后才往下走 System.out.println("start"); LockSupport.unpark(Thread.currentThread()); LockSupport.parkNanos(1000000000); System.out.println("end"); //不会block,因为一开始给了一个permit System.out.println("start"); LockSupport.unpark(Thread.currentThread()); LockSupport.unpark(Thread.currentThread()); LockSupport.parkNanos(1000000000); System.out.println("inter"); LockSupport.parkNanos(1000000000); System.out.println("end"); //第一个park不会block,第2个会,因为permit不会因为多次调用unpark就积累