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();
}

}

}