線程安全:

線程安全的方法一定是排隊運行的。

public class SyncObject {
    synchronized public void methodA() {
        try {
            System.out.println("begin methodA threadName = " +
                    Thread.currentThread().getName());
            Thread.sleep(5000);
            System.out.println("end");
        } 
 
catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

}

public class SyncThreadA extends Thread{
    private SyncObject object;

    public SyncThreadA(SyncObject object) {
        this.object = object;
    }

    @Override
    public void run() {
        
 
super.run();
        object.methodA();
    }
}

public class SyncThreadB extends Thread{
    private SyncObject object;

    public SyncThreadB(SyncObject object) {
        this.object = object;
    }

    @Override
    public void run() {
        super.run();
        object.methodA();
    }
}

 
public class ThreadRunMain {
    public static void main(String[] args) {
        testSyncThread();
    }
    public static void testSyncThread(){
        SyncObject object = new SyncObject();
        SyncThreadA a = new SyncThreadA(object);
        a.setName("A");
        SyncThreadB b = new SyncThreadB(object);
        b.setName("B");
        a.start();
        b.start();
    }

}

運行結果：

如果去掉synchronized，運行結果：

Java 學習筆記之 線程安全