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:

线程安全-002-多个线程多把锁&类锁_第1张图片

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。

线程安全-002-多个线程多把锁&类锁_第2张图片

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();
    }

线程安全-002-多个线程多把锁&类锁_第3张图片

 

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:

线程安全-002-多个线程多把锁&类锁_第4张图片

是按照t1、t2 顺序执行的。

线程安全-002-多个线程多把锁&类锁_第5张图片

 

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