LockSupport类常用方法源码
LockSupport只是一个简单的基础类，位于java.util.concurrent.locks包下，多用于线程的阻塞和唤醒，因此LockSupport也被称为其他线程的工具类。
LockSupport类的源码有标注，LockSupport类无法实例化。LockSupport类的底层是有Unsafe类实现的，LockSupport加载时的初始化也用到了Unsafe获取成员的偏移量，其源码如下：
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// Hotspot implementation via intrinsics API private static final sun.misc.Unsafe UNSAFE; private static final long parkBlockerOffset; private static final long SEED; private static final long PROBE; private static final long SECONDARY; static { try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class<?> tk = Thread. class ; parkBlockerOffset = UNSAFE.objectFieldOffset (tk.getDeclaredField( "parkBlocker" )); SEED = UNSAFE.objectFieldOffset (tk.getDeclaredField( "threadLocalRandomSeed" )); PROBE = UNSAFE.objectFieldOffset (tk.getDeclaredField( "threadLocalRandomProbe" )); SECONDARY = UNSAFE.objectFieldOffset (tk.getDeclaredField( "threadLocalRandomSecondarySeed" )); } catch (Exception ex) { throw new Error(ex); } } |
LockSupport类中有一些核心的线程操作方法，多用于线程的阻塞与唤醒。
调用park()方法使线程阻塞：
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public static void park(Object blocker) { Thread t = Thread.currentThread(); setBlocker(t, blocker); UNSAFE.park( false , 0L); setBlocker(t, null ); } private static void setBlocker(Thread t, Object arg) { // Even though volatile, hotspot doesn't need a write barrier here. UNSAFE.putObject(t, parkBlockerOffset, arg); } |
调用park(Object blocker)对传入的线程进行阻塞
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public static void park(Object blocker) { Thread t = Thread.currentThread(); setBlocker(t, blocker); UNSAFE.park( false , 0L); setBlocker(t, null ); } |
在截止时间之前阻塞传入的某个线程：
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public static void parkUntil(Object blocker, long deadline) { Thread t = Thread.currentThread(); setBlocker(t, blocker); UNSAFE.park( true , deadline); setBlocker(t, null ); } |
在nanos的时间范围内阻塞传入的线程：
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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 ); } } |
唤醒传入的线程：
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public static void unpark(Thread thread) { if (thread != null ) UNSAFE.unpark(thread); } |
wait/notify方法和park/unpark方法区别
LockSupport类中的方法还有很多，在此先列举到这里。当我们看到阻塞和唤醒方法时，我们会联想到另一组唤醒方法wait()和notify()，这两组方法还是有所区别的。
这里直接记录下结论：wait和notify方法只能在同步代码块中使用（即必须与synchronized连用）；必须先执行wait方法，然后再执行notify方法唤醒线程，调换顺序的话线程仍处于阻塞状态。
而park()和unpark()方法与之不同，这里可以通过代码运行结果来看：
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package XIAOWEI; import java.util.concurrent.locks.LockSupport;
public class Xiaowei{ public static void main(String[] args) { Thread A = new Thread(()-> { System.out.println( "线程A已经被阻塞QWQ" ); LockSupport.park(); System.out.println( "线程A被线程B唤醒啦~~~" ); }); A.start(); new Thread(()->{ System.out.println( "线程B在唤醒线程A ing~~~" ); LockSupport.unpark(A); }, "B" ).start(); } } |
那如果我们先通过线程B唤醒线程A，然后再让线程A阻塞呢（让线程A的阻塞休眠两秒）？
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package XIAOWEI; import java.util.concurrent.locks.LockSupport; public class Xiaowei { public static void main(String[] args) { Thread A = new Thread(()-> { try { Thread.sleep( 2000 ); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println( "线程A已经被阻塞QWQ(第二版)" ); LockSupport.park(); System.out.println( "线程A被线程B唤醒啦~~~（第二版）" ); }); A.start(); new Thread(()->{ System.out.println( "线程B在唤醒线程A ing~~~驾驾驾" ); LockSupport.unpark(A); }, "B" ).start(); } } |
由上面输出结果来看，虽然线程B先唤醒了线程A，然后线程A再开始阻塞，但是线程A还是处于唤醒状态，这是为什么呢？
接下来我找了段LockSupport类中的注释，其实有时看看注释也挺有意思的哈哈：
* <p>This class associates, with each thread that uses it, a permit
* (in the sense of the {@link java.util.concurrent.Semaphore
* Semaphore} class). A call to {@code park} will return immediately
* if the permit is available, consuming it in the process; otherwise
* it <em>may</em> block. A call to {@code unpark} makes the permit
* available, if it was not already available. (Unlike with Semaphores
* though, permits do not accumulate. There is at most one.)
这段话大意是说，LockSupport类使用permits这个东西来实现线程的阻塞和唤醒。每一个线程都会使用到（拥有）permit，且permit的值默认为0。接着它又说，这个概念和Semaphore信号量差不多，但是permit的值只有0和1两个值。哦~原来是这样。
对于上面例子，线程B调用unpark方法唤醒A后，会使得线程A的permit值为1，当线程调用park方法使自己阻塞时，发现自己已经有许可（permit）了，就会继续向下执行业务，而不会阻塞不动。
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原文链接：https://blog.csdn.net/qq_53847859/article/details/129110742
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