关于Map线程安全的几种实现方案

如果使Map线程安全,有四种方法:

1.使用synchronized关键字;

synchronized(anObject){

value = map.get(key);

}

2.使用JDK提供的锁,java.util.concurrent.locks.Lock;

lock.lock();

value = map.get(key);

lock.unlock():

3.使用JDK提供的读写锁java.util.concurrent.locks.ReadWriteLock.

rwlock.readLock().lock();

value= map.get(key);

rwlock.readLock().unlock();

这样两个读操作可以同时进行,理论上效率比lock要高;

4.使用JDK1.5提供的java.util.concurrent.ConcurrentHashMap,该类将Map的存储空间分为若干块,每块拥有自己的锁,减少了多个线程争夺同一个锁的情况.

value= map.get(key);//同步机制内置在get方法中

package com.imooc.concurrentmap;  
  
import java.util.HashMap;  
import java.util.Map;  
import java.util.Random;  
import java.util.concurrent.ConcurrentHashMap;  
import java.util.concurrent.locks.Lock;  
import java.util.concurrent.locks.ReentrantLock;  
  
/** 
 * Map线程安全的测试 
 * 总结: 
 */  
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 InterruptedException, IllegalArgumentException, IllegalAccessException, NoSuchFieldException, SecurityException {  
  
        //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();//执行get次数和时间 : 37116922  (1004) ms  
//          threads[i] = new LockThread();//执行get次数和时间 : 43899435  (1003) ms  
//          threads[i] = new ConcurrentThread();//执行get次数和时间 : 90580822  (1004) ms  
            threads[i].start();  
        }  
          
        //等待其他线程执行若干时间  
        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 timeCost = System.currentTimeMillis() - start;  
        System.out.println(sum + "  (" + timeCost + ") ms");  
          
        System.exit(0);  
          
    }  
      
    /** 
     * 填充map 
     * @throws InterruptedException  
     * @throws SecurityException  
     * @throws NoSuchFieldException  
     * @throws IllegalAccessException  
     * @throws IllegalArgumentException  
     */  
    public static void fillMap(Map<Integer,Integer> map) {  
        Random random = new Random();  
        for (int i = 0; i < MAP_SIZE; i++) {  
            map.put(random.nextInt(),random.nextInt());  
        }  
          
          
    }  
      
      
}  
  
class ConcurrentThread extends Thread {  
    private static Map<Integer,Integer> map = new ConcurrentHashMap<Integer, Integer>();  
    public long count = 0;  
    static {  
        MapTest.fillMap(map);  
    }  
      
    @Override  
    public void run() {  
          
  
        for(;;count++) {  
            int index = (int) (count % MapTest.KEYS.length);  
            map.get(MapTest.KEYS[index]);  
        }  
              
          
    }  
      
}  
  
/** 
 * 同步代码块实现线程安全的MAP 
 */  
class SynchronizedThread extends Thread {  
    private static Map<Integer,Integer> map = new HashMap<Integer, Integer>();  
    public long count = 0;  
    static {  
        MapTest.fillMap(map);  
    }  
      
      
    @Override  
    public void run() {  
          
        for(;;count++) {  
            int index = (int) (count% MapTest.KEYS.length);  
            synchronized(SynchronizedThread.class) {  
                map.get(MapTest.KEYS[index]);  
            }  
        }  
          
    }  
      
}  
  
/** 
 * Lock实现线程安全的map 
 */  
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);  
    }  
      
    @Override  
    public void run() {  
          
        for(;;count++) {  
              
            int index = (int) (count % MapTest.KEYS.length);  
            lock.lock();  
            map.get(MapTest.KEYS[index]);  
            lock.unlock();  
        }  
          
    }  
      
      
}