ConcurrentMap介绍

/*
 * 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
 * putIfAbsent, remove, and replace methods.
 *
 ConcurrentMap提供了原子的操作putIfAbsent，remove，replace
 * 
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]happen-before[/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。
 * 
This interface is a member of the
 * 
 * Java Collections Framework.
 *
 * @since 1.5
 * @author Doug Lea
 * @param  the type of keys maintained by this map
 * @param  the type of mapped values
 */
public interface ConcurrentMap extends Map {
    /**
     * If the specified key is not already associated
     * with a value, associate it with the given value.
     * This is equivalent to
     * 

     *   if (!map.containsKey(key))
     *       return map.put(key, value);
     *   else
     *       return map.get(key);
     * 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
     *         null if there was no mapping for the key.
     *         (A null return can also indicate that the map
     *         previously associated null with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the put 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
     * 

     *   if (map.containsKey(key) && map.get(key).equals(value)) {
     *       map.remove(key);
     *       return true;
     *   } else return false;
     * 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 true if the value was removed
     * @throws UnsupportedOperationException if the remove 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
     * 

     *   if (map.containsKey(key) && map.get(key).equals(oldValue)) {
     *       map.put(key, newValue);
     *       return true;
     *   } else return false;
     * 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 true if the value was replaced
     * @throws UnsupportedOperationException if the put 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
     * 

     *   if (map.containsKey(key)) {
     *       return map.put(key, value);
     *   } else return null;
     * 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
     *         null if there was no mapping for the key.
     *         (A null return can also indicate that the map
     *         previously associated null with the key,
     *         if the implementation supports null values.)
     * @throws UnsupportedOperationException if the put 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 map = new ConcurrentHashMap();
		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() {
		}
}