HashMap Collections.synchronized ConcurrentHashMap

在javadoc中找到这样一段话：

代码

* <p>The iterators returned by all of this class's "collection view methods"
* are <i>fail-fast</i>: if the map is structurally modified at any time after
* the iterator is created, in any way except through the iterator's own
* <tt>remove</tt> or <tt>add</tt> methods, the iterator will throw a
* <tt>ConcurrentModificationException</tt>.  Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than risking
* arbitrary, non-deterministic behavior at an undetermined time in the
* future.

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如果hashmap在迭代的同时，被其他线程修改，则会抛出一个ConcurrentModificationException异常，如以下这段程序：

代码

package hashmap;

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;
import java.util.Map.Entry;

public class HashMapTest {
    private static final Map<Integer, Integer> map = new HashMap<Integer, Integer>();

    public static void main(String[] args) {
        try {
            for (int i = 0; i < 10; i++) {
                map.put(i, i);
            }
            new Thread() {
                @Override
                public void run() {
                    Set<Entry<Integer, Integer>> set = map.entrySet();
                    synchronized (map) {
                        Iterator<Entry<Integer, Integer>> it = set.iterator();
                        while (it.hasNext()) {
                            Entry<Integer, Integer> en = it.next();
                            System.out.println(en.getKey());
                        }
                    }
                }
            }.start();
            for (int i = 0; i < 10; i++) {
                map.put(i, i);
            }
        } catch (Throwable e) {
            e.printStackTrace();
        }

    }
}

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为了解决这个问题，jdk5.0以前的版本提供了Collections.SynchronizedXX();方法，对已有容器进行同步实现，这样容器在迭代时其他线程就不能同时进行修改了，但是由于Collections.SynchronizedXX()生成的容器把所有方法都进行了同步，其他线程如果只是读取数据也必须等待迭代结束。于是JDK5.0中新加了ConcurrentHashMap类，这个类通过内部独立锁的机制对写操作和读操作分别进行了同步，当一个线程在进行迭代操作时，其他线程也可以同步的读写，Iterator返回的只是某一时点上的，不会抛ConcurrentModificationException异常，比起hashmap效率也不会有太大损失

 

 

参考资料:Java 理论与实践: 构建一个更好的 HashMap

               Java Map 集合类简介