透彻理解 Java synchronized 对象锁和类锁的区别

synchronized 加到 static 方法前面是给class 加锁,即类锁;而synchronized 加到非静态方法前面是给对象上锁。这两者的区别我用代码来演示下:

  1. 对象锁和类锁是不同的锁,所以多个线程同时执行这2个不同锁的方法时,是异步的。

在Task2 中定义三个方法 doLongTimeTaskA和doLongTimeTaskB是类锁,而doLongTimeTaskC是对象锁。

public class Task2 {

    public synchronized static void doLongTimeTaskA() {
        System.out.println("name = " + Thread.currentThread().getName() + ", begain");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("name = " + Thread.currentThread().getName() + ", end");
    }

    public synchronized static void doLongTimeTaskB() {
        System.out.println("name = " + Thread.currentThread().getName() + ", begain");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("name = " + Thread.currentThread().getName() + ", end");
    }

    public synchronized void doLongTimeTaskC() {

        System.out.println("name = " + Thread.currentThread().getName() + ", begain");
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println("name = " + Thread.currentThread().getName() + ", end");

    }

三个线程的代码如下:

class ThreadA extends Thread{

    private Task2 mTask2;

    public ThreadA(Task2 tk){
        mTask2 = tk;
    }

    public void run() {
        mTask2.doLongTimeTaskA();
    }
}

class ThreadB extends Thread{

    private Task2 mTask2;

    public ThreadB(Task2 tk){
        mTask2 = tk;
    }

    public void run() {
        mTask2.doLongTimeTaskB();
    }
}

class ThreadC extends Thread{

    private Task2 mTask2;

    public ThreadC(Task2 tk){
        mTask2 = tk;
    }

    public void run() {
        mTask2.doLongTimeTaskC();
    }
}

main函数中执行代码如下:

        Task2 mTask2 = new Task2();
        ThreadA ta = new ThreadA(mTask2);
        ThreadB tb = new ThreadB(mTask2);
        ThreadC tc = new ThreadC(mTask2);

        ta.setName("A");
        tb.setName("B");
        tc.setName("C");

        ta.start();
        tb.start();
        tc.start();

执行的结果如下:

name = A, begain, time = 1487311199783
name = C, begain, time = 1487311199783
name = C, end, time = 1487311200784
name = A, end, time = 1487311200784
name = B, begain, time = 1487311200784
name = B, end, time = 1487311201784

可以看出由于 doLongTimeTaskA和doLongTimeTaskB都是类锁,即同一个锁,所以 A和B是按顺序执行,即同步的。而C是对象锁,和A/B不是同一种锁,所以C和A、B是 异步执行的。(A、B、C代指上面的3中方法)。

我们知道对象锁要想保持同步执行,那么锁住的必须是同一个对象。下面就修改下上面的来证明:

Task2.java不变,修改ThreadA 和 ThreadB 如下:

class ThreadA extends Thread{

    private Task2 mTask2;

    public ThreadA(Task2 tk){
        mTask2 = tk;
    }

    public void run() {
        mTask2.doLongTimeTaskC();
    }
}

class ThreadB extends Thread{

    private Task2 mTask2;

    public ThreadB(Task2 tk){
        mTask2 = tk;
    }

    public void run() {
        mTask2.doLongTimeTaskC();
    }
}

main方法如下:

 Task2 mTaska = new Task2();
 Task2 mTaskb = new Task2();
 ThreadA ta = new ThreadA(mTaska );
 ThreadB tb = new ThreadB(mTaskb );


 ta.setName("A");
 tb.setName("B");

 ta.start();
 tb.start();

结果如下:

name = A, begain, time = 1487311905775
name = B, begain, time = 1487311905775
name = B, end, time = 1487311906775
name = A, end, time = 1487311906775

从结果看来,对象锁锁的对象不一样,分别是mTaska , mTaskb,所以线程A和线程B调用 doLongTimeTaskC 是异步执行的。

但是,类锁可以对类的所有对象的实例起作用。只需修改ThradA
和 ThreadB,main 方法不做改变,修改如下:

class ThreadA extends Thread{

    private Task2 mTask2;

    public ThreadA(Task2 tk){
        mTask2 = tk;
    }

    public void run() {
        //mTask2.doLongTimeTaskC();
        mTask2.doLongTimeTaskA();
    }
}

class ThreadB extends Thread{

    private Task2 mTask2;

    public ThreadB(Task2 tk){
        mTask2 = tk;
    }

    public void run() {
       //mTask2.doLongTimeTaskC();
        mTask2.doLongTimeTaskA();
    }
}

结果如下:

name = A, begain, time = 1487312239674
name = A, end, time = 1487312240674
name = B, begain, time = 1487312240674
name = B, end, time = 1487312241674

可以看出 在线程A执行完doLongTimeTaskA方法后,线程B才会获得该类锁接着去执行doLongTimeTaskA。也就是说,类锁对所有的该类对象都能起作用。

总结:
1. 如果多线程同时访问同一类的 类锁(synchronized 修饰的静态方法)以及对象锁(synchronized 修饰的非静态方法)这两个方法执行是异步的,原因:类锁和对象锁是2中不同的锁。
2. 类锁对该类的所有对象都能起作用,而对象锁不能。

    原文作者:java锁
    原文地址: https://blog.csdn.net/zhujiangtaotaise/article/details/55509939
    本文转自网络文章,转载此文章仅为分享知识,如有侵权,请联系博主进行删除。
点赞