线程安全-002-多个线程多把锁&类锁

一、多个对象多把锁

例子代码：

package com.lhy.thread01;

public class MultiThread {

    //static
    private int num = 0;

    //加上static后就是类级别的锁。不加，是对象级别的锁，此时多个线程之间是互不干扰
    public synchronized  void printNum(String tag){
        try{
            if("a".equals(tag)){
                num = 100;
                System.out.println("tag a ,set num over!");
                Thread.sleep(1000);//a会睡1秒，b不会
            }else{
                num = 200;
                System.out.println("tag b ,set num over!");
            }
            System.out.println("tag "+ tag +" , num = "+ num);
        }catch(Exception e){
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        //两个不同的对象
        final MultiThread m1 = new MultiThread();
        final MultiThread m2 = new MultiThread();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                m1.printNum("a");
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                m2.printNum("b");
            }
        });

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

}

执行结果：

关键字synchornized获得的锁是对象锁，哪个线程先执行synchornized关键字的方法，哪个线程就持有该方法所属对象的锁，例子程序中，由于m1和m2是两个不同的对象，t1 线程获得 m1对象的锁，t2线程获得m2对象的锁，所以互不影响。

验证， 如果将main方法改为如下所示，t1 和 t2 线程都执行m1对象的printNum方法，此时两个线程在抢一把锁，所以执行会按顺序来：

public static void main(String[] args) {
        //两个不同的对象
        final MultiThread m1 = new MultiThread();
        //final MultiThread m2 = new MultiThread();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                m1.printNum("a");
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                m1.printNum("b");
            }
        });

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

二、多个对象一把锁（类锁）

在静态方法上加上synchornized关键字，表示锁定.class类，类一级别的锁（独占.class 类）

例子程序：

public class MultiThread {

    //static
    private static int num = 0;

    //加上static后就是类级别的锁。不加，是对象级别的锁，此时多个线程之间是互不干扰
    public static synchronized  void printNum(String tag){
        try{
            if("a".equals(tag)){
                num = 100;
                System.err.println("tag a ,set num over!");
                Thread.sleep(1000);//a会睡1秒，b不会
            }else{
                num = 200;
                System.err.println("tag b ,set num over!");
            }
            System.err.println("tag "+ tag +" , num = "+ num);
        }catch(Exception e){
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        //两个不同的对象
        final MultiThread m1 = new MultiThread();
        final MultiThread m2 = new MultiThread();

        Thread t1 = new Thread(new Runnable() {
            @Override
            public void run() {
                m1.printNum("a");
            }
        });

        Thread t2 = new Thread(new Runnable() {
            @Override
            public void run() {
                m2.printNum("b");
            }
        });

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

}

执行结果：

是按照t1、t2 顺序执行的。