Java~源码分析为什么ArrayList是线程不安全的, 如何将其变为线程安全的

ArrayList不是线程安全的,使用一个案例演示

• 任务类

import java.util.List;

/**
 * Created with IntelliJ IDEA.
 * Description: If you don't work hard, you will a loser.
 * User: Listen-Y.
 * Date: 2020-09-27
 * Time: 15:42
 */
//线程任务类
public class CollectionTask implements Runnable {
    
    

    private List<String> list;

    public CollectionTask(List<String> list) {
    
    
        this.list = list;
    }


    @Override
    public void run() {
    
    
        //在任务中插入名字
        try {
    
    
            Thread.sleep(100);
        } catch (InterruptedException e) {
    
    
            e.printStackTrace();
        }
        list.add(Thread.currentThread().getName());

    }
}

• 测试类

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

/**
 * Created with IntelliJ IDEA.
 * Description: If you don't work hard, you will a loser.
 * User: Listen-Y.
 * Date: 2020-09-27
 * Time: 15:41
 */
//测试类
public class ArrayListSafetyTest {
    
    

    public static void main(String[] args) {
    
    
        List<String> list = new ArrayList<>();
        //list = Collections.synchronizedList(list);
        CollectionTask task = new CollectionTask(list);
        for (int i = 0; i < 50; i++) {
    
    
            new Thread(task).start();
        }
        //等待线程执行
        try {
    
    
            Thread.sleep(1000);
        } catch (InterruptedException e) {
    
    
            e.printStackTrace();
        }
        //遍历链表
        for (String str : list
             ) {
    
    
            System.out.println(str);
        }


    }
}

结果分析

• 问题就是遍历链表出现null值, 我们看看源码分析一下这是为什么

public boolean add(E e) {
    
    
        ensureCapacityInternal(size + 1);  // Increments modCount!!
        //添加一次元素就会size++一次
        elementData[size++] = e;
        return true;
    }

• 问题就出在size++上, 多个线程同时给一个位置添加元素, 导致size++了, 但是数据覆盖了, 所以就会出现空的位置

需要线程安全怎么办?

• 使用Collections.synchronizedList(list)或者Vector<>()

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

/**
 * Created with IntelliJ IDEA.
 * Description: If you don't work hard, you will a loser.
 * User: Listen-Y.
 * Date: 2020-09-27
 * Time: 15:41
 */
//测试类
public class ArrayListSafetyTest {
    
    

    public static void main(String[] args) {
    
    
        List<String> list = new ArrayList<>();
        //重点
        list = Collections.synchronizedList(list);
        CollectionTask task = new CollectionTask(list);
        for (int i = 0; i < 50; i++) {
    
    
            new Thread(task).start();
        }
        //等待线程执行
        try {
    
    
            Thread.sleep(1000);
        } catch (InterruptedException e) {
    
    
            e.printStackTrace();
        }
        //遍历链表
        for (String str : list
             ) {
    
    
            System.out.println(str);
        }


    }
}

源码分析其为何安全?

• 很简单就是给加个一把锁