多线程学习笔记(七)

在使用synchronized时,当一个线程得到一个对象锁后,再次请求此对象锁时是可以再次得到该对象锁的.这也证明在一个synchronized方法/块的内部调用本类的其他synchronized方法/块时,是永远可以得到锁的.

public class ThreadReenter {synchronized public void firstService() {System.out.println("This is the first service!");secondService();}synchronized public void secondService() {System.out.println("This is the second service!");thirdService();}synchronized public void thirdService() {System.out.println("This is the third service!");}
}

public class FirstThread extends Thread {&＃64;Overridepublic void run(){ThreadReenter threadReenter &＃61; new ThreadReenter();threadReenter.firstService();}
}

public class Run {public static void main(String[] args) {FirstThread firstThread &＃61; new FirstThread();firstThread.start();}
}

运行结果:

可重入锁的概念:

自己可以再次获取自己的内部锁.比如一个线程获取了某个对象的锁,此时这个对象的锁还没有释放,当这个线程想要获取这个对象的锁的时候还是可以获取的,如果不可以锁重入的话,就会造成死锁.

注:可重入锁也适用于父子类继承的环境中.

public class ThreadReenter {public int i &＃61; 10;synchronized public void ancestor(){try{i--;System.out.println("the ancestor i &＃61; " &＃43; i);Thread.sleep(200);}catch (Exception e){e.printStackTrace();}}}

public class SonOfThreadReenter extends ThreadReenter{synchronized public void Posterity(){try{while(i > 0){i--;System.out.println("the Posterity i &＃61; " &＃43; i);Thread.sleep(200);this.ancestor();}}catch (Exception e){e.printStackTrace();}}
}

public class FirstThread extends Thread {&＃64;Overridepublic void run(){SonOfThreadReenter sonOfThreadReenter &＃61; new SonOfThreadReenter();sonOfThreadReenter.Posterity();}
}

public class Run {public static void main(String[] args) {FirstThread firstThread &＃61; new FirstThread();firstThread.start();}
}

运行结果:

当存在父子类继承时,子类是完全可以通过可重入锁调用父类的同步方法的.