什么是脏读

什么是脏读

• 本次事务访问到了其他事务未提交的数据

代码

service.java

package com.service;

import java.util.ArrayList;
import java.util.List;

/**
 * 脏读
 */
public class Service {

    private List<Object> lists = new ArrayList<Object>();

    public void print() throws InterruptedException {
        lists.add("abc");
        System.out.println("休息5秒钟，制造脏读场景大小是:" + lists.size());
        Thread.sleep(5000);
        lists.remove("abc");
        System.out.println("回退操作:" + lists.size());

    }

    public int getSize() {
        return lists.size();
    }
}

ThreadA.java

package com.service;

public class ThreadA extends Thread {

    private Service service;

    public ThreadA(Service service) {
        this.service = service;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(200);
            int size = service.getSize();
            System.out.println(Thread.currentThread().getName()+ "->读取到的大小是:"+size);
        } catch (InterruptedException e) {
            e.printStackTrace();

        }
    }
}

ThreadB.java

package com.service;

public class ThreadB extends Thread {

    private Service service;

    public ThreadB(Service service){
        this.service = service;
    }

    @Override
    public void run() {
        System.out.println("开始增加");
        try {
            service.print();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("结束增加");
        try {
            Thread.sleep(50);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
}

Run.java

package com.service;

public class Run {
    public static void main(String[] args) {
        Service service = new Service();
        ThreadA a = new ThreadA(service);
        a.setName("A");
        a.start();

        ThreadB b = new ThreadB(service);
        b.setName("B");
        b.start();
    }
}

运行结果是：

开始增加
休息5秒钟，制造脏读场景大小是:1
A->读取到的大小是:1
回退操作:0
结束增加

Process finished with exit code 0

解决方法

• 增加锁synchronized

service.java

public class Service {
    //线程不安全的类
    private List<Object> lists = new ArrayList<Object>();

    public void print() throws InterruptedException {
        synchronized (Service.class){
            lists.add("abc");
            System.out.println("休息5秒钟，制造脏读场景大小是:" + lists.size());
            Thread.sleep(5000);
            lists.remove("abc");
            System.out.println("回退操作:" + lists.size());
        }
    }

    public int getSize(){
        synchronized (Service.class){
            return lists.size();
        }
    }
}