通过LockSupport的park/unpark实现同步？
import java.util.concurrent.locks.LockSupport;
class MyThread extends Thread {
private Object object;
public MyThread(Object object) {
this.object = object;
}
public void run() {
System.out.println("before unpark");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
// 获取blocker
System.out.println("Blocker info " + LockSupport.getBlocker((Thread) object));
// 释放许可
LockSupport.unpark((Thread) object);
// 休眠500ms，保证先执行park中的setBlocker(t, null);
try {
Thread.sleep(500);
} catch (InterruptedException e) {
e.printStackTrace();
}
// 再次获取blocker
System.out.println("Blocker info " + LockSupport.getBlocker((Thread) object));
System.out.println("after unpark");
}
}
public class test {
public static void main(String[] args) {
MyThread myThread = new MyThread(Thread.currentThread());
myThread.start();
System.out.println("before park");
// 获取许可
LockSupport.park("ParkAndUnparkDemo");
System.out.println("after park");
}
}
运行结果:
before park
before unpark
Blocker info ParkAndUnparkDemo
after park
Blocker info null
after unpark
说明: 本程序先执行park，然后在执行unpark，进行同步，并且在unpark的前后都调用了getBlocker，可以看到两次的结果不一样，并且第二次调用的结果为null，这是因为在调用unpark之后，执行了Lock.park(Object blocker)函数中的setBlocker(t, null)函数，所以第二次调用getBlocker时为null。
上例是先调用park，然后调用unpark，现在修改程序，先调用unpark，然后调用park，看能不能正确同步。具体代码如下
import java.util.concurrent.locks.LockSupport;
class MyThread extends Thread {
private Object object;
public MyThread(Object object) {
this.object = object;
}
public void run() {
System.out.println("before unpark");
// 释放许可
LockSupport.unpark((Thread) object);
System.out.println("after unpark");
}
}
public class ParkAndUnparkDemo {
public static void main(String[] args) {
MyThread myThread = new MyThread(Thread.currentThread());
myThread.start();
try {
// 主线程睡眠3s
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("before park");
// 获取许可
LockSupport.park("ParkAndUnparkDemo");
System.out.println("after park");
}
}运行结果:
before unpark
after unpark
before park
after park
说明: 可以看到，在先调用unpark，再调用park时，仍能够正确实现同步，不会造成由wait/notify调用顺序不当所引起的阻塞。因此park/unpark相比wait/notify更加的灵活。
THE END
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