ArrayList的线程安全问题

　　ArrayList是不安全的。多个线程对ArrayList操作，会出现问题。

实例1

　　下面这个demo，主要测试内容：三个线程，一个线程向集合中add数据，一个线程从集合中remove数据，一个线程判断如果集合中有数据，从其中获取数据。运行程序，会报错：越界异常。
　　以下是异常图片：

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

public class ArrayListThreadTest {
//    private static List<String> TEST_LIST = new CopyOnWriteArrayList<String>();
    private static List<String> TEST_LIST = Collections.synchronizedList(new ArrayList<String>());
//    private static List<String> TEST_LIST = new ArrayList<>();

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    TEST_LIST.add("11");
                    System.out.println("Thread1 running");
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    TEST_LIST.clear();
                    System.out.println("Thread2 running");
                }
            }
        }).start();

        new Thread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }

                    if (TEST_LIST.size() > 0) {
                        int size = TEST_LIST.size();
                        System.out.println(size + "====");
                        System.out.println(size + "===" + TEST_LIST.get(size-1));
                    }
//                    System.out.println("Thread3 running");
                }
            }
        }).start();
    }
}

实例2

　　演示内容：多个线程向集合arraylist、线程安全的arraylist中添加数据，集合长度不同。
　　下面例子则来源于：http://www.oschina.net/code/snippet_70229_2425

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

// Java语言: ArrayList的线程安全与不安全对比演示
public class ThreadSafeDemo {
    public static int demo(final List list, final int testCount) throws InterruptedException {
        ThreadGroup group = new ThreadGroup(list.getClass().getName() + "@" + list.hashCode());
        final Random rand = new Random();

        Runnable listAppender = new Runnable() {
            public void run() {
                try {
                    Thread.sleep(rand.nextInt(2));
                } catch (InterruptedException e) {
                    return;
                }
                list.add("0");
            }
        };

        for (int i = 0; i < testCount; i++) {
            new Thread(group, listAppender, "InsertList-" + i).start();
        }

        while (group.activeCount() > 0) {
            Thread.sleep(10);
        }

        return list.size();
    }
    public static void main(String[] args) throws InterruptedException {
        List unsafeList = new ArrayList();
        List safeList = Collections.synchronizedList(new ArrayList());
        final int N = 10000;
        for (int i = 0; i < 10; i++) {
            unsafeList.clear();
            safeList.clear();
            int unsafeSize = demo(unsafeList, N);
            int safeSize = demo(safeList, N);
            System.out.println("unsafe/safe: " + unsafeSize + "/" + safeSize);
        }
    }
}

总结

　　ArrayList是线程不安全的，多线程中，我们可以使用Collections.synchronizedList(list);方法使其转换成安全的。或者我们也可以调用自己写的get、set、remove方法方法，自己方法内部加锁，调用list的get、set、remove方法

public class TestList {
    public static List<String> list = new ArrayList<>();

    public static void addMessage(String message){
        synchronized (TestList.class) {
            list.add(message);
        }
    }

    public static void getMessage(String message){
        synchronized (TestList.class) {
            if (list.size() > 0) {
                list.remove(0);    
            }
        }
    }
}