java - 为什么子线程中的synchronized方法会持有主线程的锁

Question

我有一个同步方法。然后我将启动一个长时间运行的线程，子线程也有一个同步方法，但是子线程中的同步方法将持有同步方法的锁，这将导致我的应用程序中出现错误。

我的代码是:

import java.util.Date;

public class ThreadTest {

    static MQTTThread mThread;

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            System.out.println("the  " + i + " -  restart time =  "
                    + new Date());
            restart(i);
        }
    }

    private static synchronized void restart(int i) {
        System.out.println("the " + i + " -  restart excute " + new Date());
        if (null != mThread) {
            if (!mThread.isAlive()) {
                try {
                    System.out
                    .println("Action:restartConnectThread in mThread.runFlag)");

                    mThread = new MQTTThread();
                    mThread.setName("MQTTThread");
                    mThread.start();
                    // mqttExecutor.execute(mThread);
                } catch (Exception e) {
                    System.out.println("!mThread.runFlag");
                }
            } else {
                System.out.println("Action:restartConnectThread - CONNECTING");
            }
        } else {
            try {
                System.out
                .println("Action:restartConnectThread in null thread");
                mThread = new MQTTThread();
                mThread.setName("MQTTThread");
                mThread.start();
            } catch (Exception e) {
                System.out.println("null mThread");
            }
        }
    }

    private static class MQTTThread extends Thread {
        public void run() {

            connectToServer();
            System.out.println("connected");
        }
    }

    public static synchronized void connectToServer() {
        try {
            System.out.println("Thread.sleep " + new Date());
            Thread.sleep(20000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

Answer 1

这是同步方法的标准行为。

看看http://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html

it is not possible for two invocations of synchronized methods on the same object to interleave. When one thread is executing a synchronized method for an object, all other threads that invoke synchronized methods for the same object block (suspend execution) until the first thread is done with the object.

所以代码

public synchronized void method() { 
} 

相当于

public void method() { 
    synchronized (this) {
    }
}

出于您的目的，您应该为 connectToServer 和 restart 方法使用不同的锁定对象

UPD。抱歉，我错过了，您的方法是静态的。在这种情况下specification 8.4.3.6

A synchronized method acquires a monitor before it executes. For a class (static) method, the monitor associated with the Class object for the method's class is used.

所以你不能同时运行一个类的两个同步方法，即使它们是静态的