synchronized 使用方法

概述

在java中synchronized是一个非常重要的关键字，它主要用来控制线程同步的，在多线程环境下能实现临界区资源的互斥访问。

synchronized关键字最主要有以下3种应用方式，下面分别介绍

修饰实例方法，作用于当前实例加锁，进入同步代码前要获得当前实例的锁。
修饰静态方法，作用于当前类对象加锁，进入同步代码前要获得当前类对象的锁。
修饰代码块，指定加锁对象，对给定对象加锁，进入同步代码库前要获得给定对象的锁。
修饰代码块，加锁对象为对象，则该类的所有的对象共享同一把锁。

1. 修饰实例方法

class SafeAdd implements Runnable {
    public static int num = 0;

    public synchronized void add() {
        num ++;
    }

    @Override
    public void run() {
        for (int i = 0; i < 10000; ++i) {
            add();
        }
    }
}


public class SyncMethod {

    public static void main(String[] args) throws InterruptedException {
        SafeAdd safeAdd = new SafeAdd();
        Thread t1 = new Thread(safeAdd);
        Thread t2 = new Thread(safeAdd);

        t1.start();
        t2.start();

        t1.join();
        t2.join();
        System.out.println("SafeAdd: num = " + SafeAdd.num);
    }
}

运行结果：

SafeAdd: num = 20000

Process finished with exit code 0

2.修饰静态方法

class SafeAdd implements Runnable {
    public static int num = 0;

    public static synchronized void add() {
        num ++;
    }

    @Override
    public void run() {
        for (int i = 0; i < 10000; ++i) {
            add();
        }
    }
}


public class SyncMethod {

    public static void main(String[] args) throws InterruptedException {
        SafeAdd safeAdd1 = new SafeAdd();
        SafeAdd safeAdd2 = new SafeAdd();
        Thread t1 = new Thread(safeAdd1);
        Thread t2 = new Thread(safeAdd2);

        t1.start();
        t2.start();

        t1.join();
        t2.join();
        System.out.println("SafeAdd: num = " + SafeAdd.num);
    }
}

运行结果：

SafeAdd: num = 20000

Process finished with exit code 0

3. 同步代码块

class SafeAdd implements Runnable {
    public static int num = 0;

    @Override
    public void run() {
        synchronized (this) {
            for (int i = 0; i < 10000; ++i) {
                num ++;
            }
        }
    }
}


public class SyncMethod {

    public static void main(String[] args) throws InterruptedException {
        SafeAdd safeAdd = new SafeAdd();
        Thread t1 = new Thread(safeAdd);
        Thread t2 = new Thread(safeAdd);

        t1.start();
        t2.start();

        t1.join();
        t2.join();
        System.out.println("SafeAdd: num = " + SafeAdd.num);
    }
}

运行结果：

SafeAdd: num = 20000

Process finished with exit code 0

4. 同步代码块，加锁对象为Class对象

class SafeAdd implements Runnable {
    public static int num = 0;

    public static void add() {
        num ++;
    }

    @Override
    public void run() {
        synchronized (SafeAdd.class) {
            for (int i = 0; i < 10000; ++i) {
                add();
            }
        }
    }
}

public class SyncMethod {
    public static void main(String[] args) throws InterruptedException {
        SafeAdd safeAdd1 = new SafeAdd();
        SafeAdd safeAdd2 = new SafeAdd();
        Thread t1 = new Thread(safeAdd1);
        Thread t2 = new Thread(safeAdd2);

        t1.start();
        t2.start();

        t1.join();
        t2.join();
        System.out.println("SafeAdd: num = " + SafeAdd.num);
    }
}

运行结果：

SafeAdd: num = 20000

Process finished with exit code 0

总结

synchronized的作用对象是实例方法和普通对象时，则多个线程仅共同争抢该对象的锁。
synchronized的作用对象是静态方法或一个类时，则该类所有的对象共享一个锁。

来源：oschina

链接：https://my.oschina.net/u/4418236/blog/3233450

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