Map线程安全几种实现方法

如果需要使 Map 线程安全，大致有这么四种方法：
1、使用 synchronized 关键字，代码如下
synchronized(anObject) {
　　value = map.get(key);
}

2、使用 JDK1.5提供的锁（java.util.concurrent.locks.Lock）。代码如下
lock.lock();
value = map.get(key);
lock.unlock();

3、使用 JDK1.5 提供的读写锁（java.util.concurrent.locks.ReadWriteLock）。代码如下
rwlock.readLock().lock();
value = map.get(key);
rwlock.readLock().unlock();
这样两个读操作可以同时进行，理论上效率会比方法 2 高。

4、使用 JDK1.5 提供的 java.util.concurrent.ConcurrentHashMap 类。该类将 Map 的存储空间分为若干块，每块拥有自己的锁，大大减少了多个线程争夺同一个锁的情况。代码如下
value = map.get(key); //同步机制内置在 get 方法中

比较：
1、不同步确实最快，与预期一致。
2、四种同步方式中，ConcurrentHashMap 是最快的，接近不同步的情况。
3、synchronized 关键字非常慢，比使用锁慢了两个数量级。如果需自己实现同步，则使用 JDK1.5 提供的锁机制，避免使用 synchronized 关键字。

 

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public class MapTest {             public static final int THREAD_COUNT = 1;             public static final int MAP_SIZE = 1000;             public static final int EXECUTION_MILLES = 1000;             public static final int[] KEYS = new int[100];               public static void main(String[] args) throws Exception {                 // 初始化                 Random rand = new Random();                 for (int i = 0; i < KEYS.length; ++i)                     KEYS[i] = rand.nextInt();                 // 创建线程                 long start = System.currentTimeMillis();                 Thread[] threads = new Thread[THREAD_COUNT];                 for (int i = 0; i < THREAD_COUNT; ++i) {                     threads[i] = new SynchronizedThread();                     // threads[i] = new LockThread();                     threads[i].start();                 }                 // 等待其它线程执行若干时间                 Thread.sleep(EXECUTION_MILLES);                 // 统计 get 操作的次数                 long sum = 0;                 for (int i = 0; i < THREAD_COUNT; ++i) {                     sum += threads[i].getClass().getDeclaredField("count")                             .getLong(threads[i]);                 }                 long millisCost = System.currentTimeMillis() - start;                 System.out.println(sum + "(" + (millisCost) + "ms)");                 System.exit(0);             }               public static void fillMap(Map<Integer, Integer> map) {                 Random rand = new Random();                 for (int i = 0; i < MAP_SIZE; ++i) {                     map.put(rand.nextInt(), rand.nextInt());                 }             }         }         class SynchronizedThread extends Thread {             private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();             public long count = 0;             static {                 MapTest.fillMap(map);             }               public void run() {                 for (;;) {                     int index = (int) (count % MapTest.KEYS.length);                     synchronized (SynchronizedThread.class) {                         map.get(MapTest.KEYS[index]);                     }                     ++count;                 }             }         }           class LockThread extends Thread {             private static Map<Integer, Integer> map = new HashMap<Integer, Integer>();             private static Lock lock = new ReentrantLock();             public long count = 0;             static {                 MapTest.fillMap(map);             }               public void run() {                 for (;;) {                     int index = (int) (count % MapTest.KEYS.length);                     lock.lock();                     map.get(MapTest.KEYS[index]);                     lock.unlock();                     ++count;                 }             }         }