/* * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.util.Map; /** * A {@link java.util.Map} providing additional atomic * <tt>putIfAbsent</tt>, <tt>remove</tt>, and <tt>replace</tt> methods. * ConcurrentMap提供了原子的操作putIfAbsent,remove,replace * <p>Memory consistency effects: As with other concurrent * collections, actions in a thread prior to placing an object into a * {@code ConcurrentMap} as a key or value * [url=package-summary.html#MemoryVisibility]<i>happen-before</i>[/url] * actions subsequent to the access or removal of that object from * the {@code ConcurrentMap} in another thread. * 内存一致性:与其他并发集合意向,线程有限执行put操,即put的操作happen-before 其他线程读操作或者移除操作。 * <p>This interface is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values */ public interface ConcurrentMap<K, V> extends Map<K, V> { /** * If the specified key is not already associated * with a value, associate it with the given value. * This is equivalent to * <pre> * if (!map.containsKey(key)) * return map.put(key, value); * else * return map.get(key);</pre> * except that the action is performed atomically. * 如果Map不包含对应的Key则,执行put的操作,否则返回旧值 * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with the specified key, or * <tt>null</tt> if there was no mapping for the key. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with the key, * if the implementation supports null values.) * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by this map * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of the specified key * or value prevents it from being stored in this map * */ V putIfAbsent(K key, V value); /** * Removes the entry for a key only if currently mapped to a given value. * This is equivalent to * <pre> * if (map.containsKey(key) && map.get(key).equals(value)) { * map.remove(key); * return true; * } else return false;</pre> * except that the action is performed atomically. * 如果Map中存在Key和value相等的Entry,则移除 * @param key key with which the specified value is associated * @param value value expected to be associated with the specified key * @return <tt>true</tt> if the value was removed * @throws UnsupportedOperationException if the <tt>remove</tt> operation * is not supported by this map * @throws ClassCastException if the key or value is of an inappropriate * type for this map * ([url=../Collection.html#optional-restrictions]optional[/url]) * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * ([url=../Collection.html#optional-restrictions]optional[/url]) */ boolean remove(Object key, Object value); /** * Replaces the entry for a key only if currently mapped to a given value. * This is equivalent to * <pre> * if (map.containsKey(key) && map.get(key).equals(oldValue)) { * map.put(key, newValue); * return true; * } else return false;</pre> * except that the action is performed atomically. * 如果Map中存在Key和value相等的Entry,put的操作,更新值 * @param key key with which the specified value is associated * @param oldValue value expected to be associated with the specified key * @param newValue value to be associated with the specified key * @return <tt>true</tt> if the value was replaced * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by this map * @throws ClassCastException if the class of a specified key or value * prevents it from being stored in this map * @throws NullPointerException if a specified key or value is null, * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of a specified key * or value prevents it from being stored in this map */ boolean replace(K key, V oldValue, V newValue); 如果Map中存在Key相等的Entry,put的操作,更新值 /** * Replaces the entry for a key only if currently mapped to some value. * This is equivalent to * <pre> * if (map.containsKey(key)) { * return map.put(key, value); * } else return null;</pre> * except that the action is performed atomically. * * @param key key with which the specified value is associated * @param value value to be associated with the specified key * @return the previous value associated with the specified key, or * <tt>null</tt> if there was no mapping for the key. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with the key, * if the implementation supports null values.) * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by this map * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of the specified key * or value prevents it from being stored in this map */ V replace(K key, V value); }
ConcurrentHashMap由于是线程安全,是否可以用ConcurrentHashMap来实现单例模式能,答案是肯定的
来看具体的实例:
package juc.map; import java.util.concurrent.ConcurrentHashMap; import java.util.concurrent.ConcurrentMap; /** * 用ConcurrentHashMap实现单例模式 * @author donald * 2017年3月11日 * 下午6:18:09 */ public class TestInstanceConcurrentMap { private static final ConcurrentMap<String, TestInstanceConcurrentMap> map = new ConcurrentHashMap<String, TestInstanceConcurrentMap>(); private static TestInstanceConcurrentMap instance; /** * * @return */ public static TestInstanceConcurrentMap getInstance() { if (instance == null) { map.putIfAbsent("INSTANCE", new TestInstanceConcurrentMap()); instance = map.get("INSTANCE"); } return instance; } /** * */ private TestInstanceConcurrentMap() { } }