为什么子线程中的同步方法持有主线程的锁

Question

I have a synchronized method. then I will start a Thread with long time operation, the sub-thread also have a synchronized method, But the synchronized method in sub thread will hold the lock of the synchronized method which will cause anr in my appliction.

My code is:

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

It is standard behavior of synchronized methods.

Look at 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.

So code

public synchronized void method() { 
} 

is equivalent to

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

For your purposes you should use different lock-objects for connectToServer and restart methods

UPD. Sorry, I missed, that your methods are static. In this case 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.

So you can not run two synchronized methods of a class simultaneously, even if they are static

Answer 2

Blocking the main UI thread for 5 secods or more causes the ANR.

from Anr Message with synchronized method

Maybe that's happening because of:

Thread.sleep(20000); 

Avoid to lock the main UI thread.