多线程下的计数器（线程安全）

需求：最近做一个项目需要用多线程文件落地，文件名末尾按批次号命名，file1..file2 3 4 ....

开3个固定大小的线程池，不断往里面加任务，每个线程共用一个计数器，争抢计数，从1开始递增。

package com.demo.thread;

import java.util.concurrent.atomic.AtomicInteger;

public class MyCounter {
    private AtomicInteger batchNum = new AtomicInteger(0);

    public AtomicInteger getBatchNum() {
        return batchNum;
    }

    public void setBatchNum(AtomicInteger batchNum) {
        this.batchNum = batchNum;
    }
}

public class TestA extends Thread {
    private MyCounter myCounter;
    public TestA(MyCounter myCounter){
        this.myCounter = myCounter;
    }

    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName()+"---"+myCounter.getBatchNum().incrementAndGet());
    }
}

import com.demo.thread.MyCounter;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Test {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        long start = System.currentTimeMillis();
        MyCounter counter  = new MyCounter();
        while (System.currentTimeMillis() - start <10) {
            TestA t = new TestA(counter);
            executorService.execute(t);
        }
    }
}

可以看到线程池里的不同线程共享计数器 

创建一个数据库表，把打印出来的数据插入到主键字段，可以看到，正常插入，没有重复。 

 例子分析

下面例子能够使多个线程共同争抢一个计数器，不会出现两个线程之间出现重复数字。

package com.demo.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class ThreadTest {
    private static AtomicInteger atomicInteger = new AtomicInteger(0);

    public static void main(String[] args) {
        int i = 0;
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        while (i < 3) {
            TA tt = new TA(atomicInteger);
            executorService.execute(tt);
            i++;
        }
    }
}

class TA implements Runnable {
    AtomicInteger atomicInteger;
    long start = System.currentTimeMillis();
    public TA(AtomicInteger atomicInteger) {
        this.atomicInteger = atomicInteger;
    }
    @Override
    public void run() {
        while(System.currentTimeMillis() -start<20) {
            System.out.println(atomicInteger.incrementAndGet());
        }
    }
}

但是，如果你要用 AtomicInteger.get()呢，因为会有很多的方法，又不想取出atomicInteger.incrementAndGet()然后一个个方法参数传进去，这样也太麻烦了。

用get()用注意，用的不好，有可能出现重复情况。上面线程TA改成如下代码：

 public void run() {
        while(System.currentTimeMillis() -start<20) {
            atomicInteger.incrementAndGet();
            System.out.println(atomicInteger.get());
        }
    }

 将打印出来的数字放入oracle主键字段，发现重复数据：

脏读的几种写法：

1. 先incrementAndGet，再get

package com.demo.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class ThreadTest {
    private static AtomicInteger atomicInteger = new AtomicInteger(0);

    public static void main(String[] args) {
        int i = 0;
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        while (i < 10) {
            TA tt = new TA(atomicInteger);
            executorService.execute(tt);
            i++;
        }
    }
}

class TA implements Runnable {
    AtomicInteger atomicInteger;
    long start = System.currentTimeMillis();
    public TA(AtomicInteger atomicInteger) {
        this.atomicInteger = atomicInteger;
    }
    @Override
    public void run() {
        atomicInteger.incrementAndGet();
        try {
            //模拟程序执行耗时
            Thread.sleep(10);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(atomicInteger.get());

    }
}

 

2.先incrementAndGet，再赋值get，再使用。

网上有人说先赋值再使用就不会脏读，事实证明，还是会的。

   int cnt = atomicInteger.get();
        System.out.println(cnt);

 

3. 线程里面while(true)自增incrementAndGet，再逻辑耗时，再get

package com.demo.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class ThreadTest {
    private static AtomicInteger atomicInteger = new AtomicInteger(0);

    public static void main(String[] args) {
        int i = 0;
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        while (i < 10) {
            TA tt = new TA(atomicInteger);
            executorService.execute(tt);
            i++;
        }
    }
}

class TA implements Runnable {
    AtomicInteger atomicInteger;
    long start = System.currentTimeMillis();
    public TA(AtomicInteger atomicInteger) {
        this.atomicInteger = atomicInteger;
    }
    @Override
    public void run() {
       while(System.currentTimeMillis() -start<20) {
           atomicInteger.incrementAndGet();
           try {
               //模拟程序执行耗时
               Thread.sleep(10);
           } catch (InterruptedException e) {
               e.printStackTrace();
           }
            System.out.println( atomicInteger.get());
        }
    }
}

4. 线程while(true),先incrementAndGet，马上get，再逻辑耗时

package com.demo.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class ThreadTest {
    private static AtomicInteger atomicInteger = new AtomicInteger(0);

    public static void main(String[] args) {
        int i = 0;
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        while (i < 10) {
            TA tt = new TA(atomicInteger);
            executorService.execute(tt);
            i++;
        }
    }
}

class TA implements Runnable {
    AtomicInteger atomicInteger;
    long start = System.currentTimeMillis();
    public TA(AtomicInteger atomicInteger) {
        this.atomicInteger = atomicInteger;
    }
    @Override
    public void run() {
       while(System.currentTimeMillis() -start<30) {
           atomicInteger.incrementAndGet();
           int cnt = atomicInteger.get();
           try {
               //模拟程序执行耗时
               Thread.sleep(10);
           } catch (InterruptedException e) {
               e.printStackTrace();
           }
            System.out.println( cnt);
        }
    }
}

执行一次，发现减少脏读概率，但是多执行几次还是出现脏读了。 但是概率小很多，大概执行10次，出现一次重复。

 

分享一个不会出现脏读的写法：

package com.demo.thread;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

public class ThreadTest {
    private static AtomicInteger atomicInteger = new AtomicInteger(0);

    public static void main(String[] args) {
        int i = 0;
        ExecutorService executorService = Executors.newFixedThreadPool(3);
        while (i < 3) {//3改成100效果一样，还是不会出现脏读，3其实相当于Thread,没用到线程池的重复利用。
            TA tt = new TA(atomicInteger);
            executorService.execute(tt);
            i++;
        }
    }
}

class TA implements Runnable {
    AtomicInteger atomicInteger;
    long start = System.currentTimeMillis();
    public TA(AtomicInteger atomicInteger) {
        this.atomicInteger = atomicInteger;
    }
    private int cnt;//线程自己的变量
    @Override
    public void run() {
       while(System.currentTimeMillis() -start<3000) {
           cnt =  atomicInteger.incrementAndGet();
           try {
               //模拟程序执行耗时
               Thread.sleep(10);
           } catch (InterruptedException e) {
               e.printStackTrace();
           }
            System.out.println( cnt);
        }
    }
}

 总结

1.线程池加任务的时候，new Thread一定要放在循环里面，否则，线程里面的成员变量就编程线程不安全了，就要用局部变量取。

2.new Thread 放在循环里添加任务，线程里面就可以用成员变量。

3.用incrementAndGet()取值，别用get。

4.incrementAndGet可以保证不重复，get会出现重复，即使自增完马上取，还是会出现重复。