并发下的集合

Posted 2021-01-02 bockpecehhe

ArrayList是一个线程不安全的容器 如下列程序

package com.longfor.dragonshard.service.cost.standard.impl;

import java.util.ArrayList;

public class ArrayListMultiThread {

    private static ArrayList arrayList = new ArrayList();

    public static class AddThread implements Runnable{
        @Override
        public void run() {
            for(int i=0;i<10000;i++){
                arrayList.add(i);
            }
        }
    }

    public static void main(String[] args) throws InterruptedException{
        Thread t1 = new Thread(new AddThread());
        Thread t2 = new Thread(new AddThread());
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println(arrayList.size());
    }
}

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可能出现3中情况：

　　①  程序正常结束  所以说未必每次错误未必会表现出来

　　②  扩容过程中没有锁的保护 从而导致访问数据出错

　　③  大小被不正常的访问 从而赋值出错

HashMap同样也是线程不安全的  问题比ArrayList更加诡异

package com.longfor.dragonshard.service.cost.standard.impl;

import java.util.HashMap;
import java.util.Map;

public class HashMapMutiThread {
    private static Map map = new HashMap();


    public static class AddThread implements Runnable{

        public int start = 0;
        public AddThread(int start){
            this.start=start;
        }

        @Override
        public void run() {
            for (int i=0;i<10000;i++){
                map.put(Integer.toString(i),Integer.toBinaryString(i));
            }
        }
    }

    public static void main(String[] args) throws InterruptedException{
        Thread t1 = new Thread(new HashMapMutiThread.AddThread(0));
        Thread t2 = new Thread(new HashMapMutiThread.AddThread(1));
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println(map.size());
    }
}

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错误的加锁  

package com.longfor.dragonshard.service.cost.standard.impl;

public class BadLockOnInteger implements Runnable{
    public static Integer i = 0;
    public static BadLockOnInteger instance = new BadLockOnInteger();

    @Override
    public void run() {
        for (int k=0;k<100000;k++){
            synchronized (i){
                i++;
            }
        }
    }

    public static void main(String[] args) throws InterruptedException{
        Thread t1 = new Thread(instance);
        Thread t2 = new Thread(instance);
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println(i);
    }
}

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Integer属于不变的对象 所以每次新增的时候都是新建一个新的Integer对象 重新赋值   i = Integer.valueOf(i.intValue()+1);

进一步查看valueOf

public static Integer valueOf(int i) {
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}
创建新的Integer对象 所以我们不能对Integer进行同步  因为Integer对象一直在改变