Synchronized常用三种使用方式
1、修饰普通方法:锁对象即为当前对象
2、修饰静态方法:锁对象为当前Class对象
3、修饰代码块:锁对象为synchronized紧接着的小括号内的对象
一、验证修饰普通方法时锁对象
package com.demo; public class ThreadTest { public static void main(String[] args) { Thread t1 = new MyThread1(); Thread t2 = new MyThread2(t1); t1.start();// 默认priority=5 t2.start(); t2.setPriority(9);
System.out.println("线程1:" + t1.getState()); System.out.println("线程2:" + t2.getState()); } } class MyThread1 extends Thread { @Override public void run() { dosome(); } public synchronized void dosome() { System.out.println("mythread1"); try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("sleep end"); } } class MyThread2 extends Thread { private Object lock; public MyThread2(Object lock) { this.lock = lock; } @Override public void run() { synchronized (lock) { System.out.println("mythread2"); } } }
现象:先逐行输出mythread1,线程1:RUNNABLE,线程2:BLOCKED,之后暂停五分钟,逐行输出sleep end,mythread2
分析:线程2与线程1拥有相同的锁,线程1优先级高于线程2,线程1优先执行,获取到锁,执行sleep,未释放锁,线程2未获取到锁,处于阻塞状态,线程1sleep结束,输出sleep end,并释放锁,线程2获取到锁,执行输出mythread2
结论:当synchronized修饰普通方法时,锁对象即为当前对象
二、验证修饰静态方法时锁对象
对上述代码稍作修改,在dosome方法上添加static关键字,main方法中,线程2对象初始化时,传入MyThread1.class
现象:与上述现象一致
分析:同上
结论:当synchronized修饰静态方法时,锁对象为当前Class对象
三、验证修饰代码块时锁对象
package com.demo; public class ThreadTest { public static void main(String[] args) { Object o = new Object(); Thread t1 = new MyThread1(o); Thread t2 = new MyThread2(o); t1.start(); t2.start(); t2.setPriority(9); System.out.println("线程1:" + t1.getState()); System.out.println("线程2:" + t2.getState()); } } class MyThread1 extends Thread { private Object lock; public MyThread1(Object lock) { this.lock = lock; } @Override public void run() { synchronized (lock) { System.out.println("mythread1"); try { Thread.sleep(5000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("sleep end"); } } } class MyThread2 extends Thread { private Object lock; public MyThread2(Object lock) { this.lock = lock; } @Override public void run() { synchronized (lock) { System.out.println("mythread2"); } } }
现象:同上
分析:同上
结论:当使用synchronized修改代码块时,其后小括号中的对象即为对象锁。