众所周知, 这两个结构都不是线程安全的.对于ArrayList, 可以通过多个线程向其添加元素, 若它不是线程安全的, 则最后它实际存储的元素数量很可能不等于实际添加的元素数量.HashMap的验证方法也类似

需要注意的是, 这里的线程不安全指的是原子操作, 比如add这种, 得不到预期效果, 而不是add和get这样一组操作. 在原子操作线程安全的情况下, 一组原子操作也是线程不安全的, 需要另外加锁.

证明ArrayList的非线程安全性

package com.ibm.javacore.collections.threadsafe;

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

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); 
        }
    }
}

证明HashMap的非线程安全性

package com.concurrence;

import java.util.HashMap;

public class ThreadNotSafeHashmap {
    public static void main(String args[]) throws InterruptedException {
        final HashMap<String, String> firstHashMap = new HashMap<String, String>();
        Thread t1 = new Thread() {
            public void run() {
                for (int i = 0; i < 2500; i++) {
                    firstHashMap.put(String.valueOf(i), String.valueOf(i));
                }
            }
        };
        Thread t2 = new Thread() {
            public void run() {
                for (int j = 2500; j < 5000; j++) {
                    firstHashMap.put(String.valueOf(j), String.valueOf(j));
                }
            }
        };
        t1.start();
        t2.start();

        Thread.sleep(1000);
        for (int k = 0; k < 5000; k++) {
            if (!String.valueOf(k).equals(firstHashMap.get(String.valueOf(k)))) {
                System.err.println(String.valueOf(k) + ":" + firstHashMap.get(String.valueOf(k)));
            }
        }
    }
}