如何實現一個執行緒安全的map?
阿新 • • 發佈:2019-02-13
我們都知道,map是執行緒不安全的,那麼我們如何才能實現一個執行緒安全的map呢?
這裡介紹4種實現方式:
1、使用synchronized來進行約束:
synchronized(obj){
value = map.get(key);
}
2、使用JDK1.5版本所提供的lock機制,java.util.concurrent.locks.Lock:
lock.lock();
value = map.get(key);
lock.unlock();
3、使用JDK提供的讀寫鎖java.util.concurrent.locks.ReadWriteLock:
rwlock.readLock().lock();
value= map.get(key);
rwlock.readLock().unlock();
這樣兩個讀操作可以同時進行,理論上效率比lock要高;
4、使用JDK1.5提供的java.util.concurrent.ConcurrentHashMap,該類將Map的儲存空間分為若干塊,每塊擁有自己的鎖,減少了多個執行緒爭奪同一個鎖的情況:
value=map.get(key);其中,同步機制已經在get方法中實現。
以下是程式碼體現:
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(); // 建立執行緒 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(); } // 等待其它執行緒執行若干時間 Thread.sleep(EXECUTION_MILLES); // 統計 get 操作的次數 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; } } }
文章來自於轉載,請多多指教。