对象及变量的并发访问-----synchronized同步----- 脏读

Posted 2020-10-10

一、接上一步，如果变量在方法外部，对变量加上关键字synchonzied即可

变量加synchronized

package service;

/**
 * Created by zhc on 2017/10/10
 */
public class HasSelfPrivateNum {
    private  int num = 0;
    synchronized public void addI(String username) {
        try {

            if (username.equals("a")) {
                num = 100;
                System.out.println("a is set over");
            } else {
                num = 200;
                System.out.println("b is set over");
            }
            System.out.println(username + " num = " + num);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

二、出现脏读情况

　　首先给出一个实体类，通俗讲，set方法进行加锁，get没有

package entity;

/**
 * Created by zhc on 2017/10/10
 */
public class PublicVar {
    public String username = "A";
    public String password = "AA";
    synchronized public void  setValue(String username,String password){
        try {
            this.username = username;
            Thread.sleep(5000);
            this.password = password;
            System.out.println(" setValue method thread name "+Thread.currentThread().getName()
            +"username and password ="+username+" "+password);
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }

    public void getValue(){
        System.out.println(" getValue method thread name "+Thread.currentThread().getName()
                +"username and password ="+username+" "+password);
    }
}

PublicVar

　　然后线程进行设值，然后读取。

package extthread;

import entity.PublicVar;

/**
 * Created by zhc on 2017/10/10
 */
public class ThreadC extends Thread {
    private PublicVar publicVar;
    public ThreadC(PublicVar publicVar){
        super();
        this.publicVar = publicVar;
    }

    @Override
    public void run() {
        super.run();
        publicVar.setValue("B","BB");
    }
}

Thread

　　开启线程，进行测验

Run

　　出现结果：

　　getValue method thread name main  username and password =B AA
 　　setValue method thread name Thread-0  username and password =B BB

　　出现脏读情况，发现synchronized某方法（set），可以获得此方法锁，也就是获得该对象锁，但是get方法因为未被synchronized，则被调用时无关是否被锁。

　　但是如果get方法也被synchronized，那么B线程的get方法调用一定要等A线程执行完set先结束才行。

　　我们对get方法加synchronized，

package entity;

/**
 * Created by zhc on 2017/10/10
 */
public class PublicVar {
    public String username = "A";
    public String password = "AA";
    synchronized public void  setValue(String username,String password){
        try {
            this.username = username;
            Thread.sleep(5000);
            this.password = password;
            System.out.println(" setValue method thread name "+Thread.currentThread().getName()
            +"  username and password ="+username+" "+password);
        }catch (InterruptedException e){
            e.printStackTrace();
        }
    }

    synchronized public void getValue(){
        System.out.println(" getValue method thread name "+Thread.currentThread().getName()
                +"  username and password ="+username+" "+password);
    }
}

get方法加synchronized

　　看看结果：

　　setValue method thread name Thread-0  username and password =B BB
 　　getValue method thread name main  username and password =B BB