自定义CopyOnWriteHashMap

    在系统设计的过程中经常使用本地缓存（ConcurrentHashMap实现），由于ConcurrentHashMap的特性，可以保证线程安全。通常缓存中的数据往往是读多写少的，ConcurrentHashMap是完完全全线程安全类，虽然相比较HashTable做了降低锁粒度的优化，但对于读请求是没有必要加锁。Doug Lea大神在设计concurrent包的时候为我们提供了一个CopyOnWriteArrayList，这个类通过读写分离的方式来实现线程安全，即读请求不加锁，写请求才加锁（典型的空间换时间实现）。这对于缓存来说是个很好的容器，不过很可惜JDK并未提供CopyOnWriteMap。这里自己简单模仿写了一个，希望在今后能有所帮助，类的实现如下：

import java.io.Serializable;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.locks.ReentrantLock;

/**
 * @author zhangyining on 18/2/27 027.
 */
public class CopyOnWriteHashMap<K,V> implements Serializable, Cloneable {

    private static final long serialVersionUID = 5209276986986262310L;

    /**
     * lock
     */
    private final transient ReentrantLock lock = new ReentrantLock();

    /**
     * internal collection
     */
    private transient volatile HashMap<K,V> map;

    /**
     * init capacity
     */
    private static final int DEFAULT_INITIAL_CAPACITY = 1 << 4;

    /**
     * max capacity
     */
    private static final int MAXIMUM_CAPACITY = 1 << 30;

    public CopyOnWriteHashMap() {
        setMap(new HashMap<>(DEFAULT_INITIAL_CAPACITY));
    }

    public CopyOnWriteHashMap(int initialCapacity) {
        if (initialCapacity < 0){
            throw new IllegalArgumentException("Illegal initial capacity: " +
                    initialCapacity);
        }
        if (initialCapacity > MAXIMUM_CAPACITY){

            initialCapacity = MAXIMUM_CAPACITY;
        }
        setMap(new HashMap<>(initialCapacity));
    }

    public CopyOnWriteHashMap(HashMap<? extends K, ? extends V> m) {
        HashMap<K,V> map = new HashMap<>(m);
        setMap(map);
    }

    final HashMap<K,V> getMap() {
        return map;
    }

    final void setMap(HashMap<K,V> map) {
        this.map = map;
    }

    public int size() {
        return this.map.size();
    }

    public boolean isEmpty() {
        return size() == 0;
    }

    public boolean containsKey(K key) {
        return map.containsKey(key);
    }

    public void clear() {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            HashMap<K,V> newMap = new HashMap<>(map);
            newMap.clear();
            map = newMap;
        } finally {
            lock.unlock();
        }
    }

    public boolean containsValue(V value) {
        return map.containsValue(value);
    }

    public Set<K> keySet() {
        return map.keySet();
    }

    public Set<Map.Entry<K, V>> entrySet() {
        return map.entrySet();
    }

    public Collection<V> values() {
        return map.values();
    }

    public V replace(K key, V value) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            HashMap<K,V> newMap = new HashMap<>(map);
            V val = newMap.replace(key, value);
            map = newMap;
            return val;
        } finally {
            lock.unlock();
        }
    }

    public boolean replace(K key, V oldValue, V newValue) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            HashMap<K,V> newMap = new HashMap<>(map);
            boolean flag = newMap.replace(key, oldValue, newValue);
            map = newMap;
            return flag;
        } finally {
            lock.unlock();
        }
    }

    public V put(K key, V value) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            HashMap<K,V> newMap = new HashMap<>(map);
            V val = newMap.put(key, value);
            map = newMap;
            return val;
        } finally {
            lock.unlock();
        }
    }

    public void putAll(Map<? extends K, ? extends V> m) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            HashMap<K,V> newMap = new HashMap<>(map);
            newMap.putAll(m);
            map = newMap;
        } finally {
            lock.unlock();
        }
    }

    public V get(K key) {
        return map.get(key);
    }

    public V remove(K key) {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            HashMap<K,V> newMap = new HashMap<>(map);
            V val = newMap.remove(key);
            map = newMap;
            return val;
        } finally {
            lock.unlock();
        }
    }
}

该类并没有按照JDK集合的方式来创建，后续有时间结合Collection补齐接口和抽象类

二者对比

类	一致性	效率	内存占用	适用情况
ConcurrentHashMap	强一致性	较高	一般	无要求
CopyOnWriteHashMap	最终一致性	高	双倍内存	读多写少

这里强调一点，由于读请求不加锁，所以会有可能读取到脏数据，此情况发生在读写并发高的情况，通常都是毫秒级别的误差，对于数据时效性严格要求的场景不适用，且对于写数据较多的场景会经常触发GC，使用前请慎重考虑具体场景！