看一看Dubbo中实现的LRU缓存

最近遇见多次LRUCache，网上看了不少，自己也手写了一个，最近看Dubbo源码也碰到了Dubbo中的LRUCache。

摘出来瞧瞧。

Dubbo的LRUCache位于 dubbo-common的 com.alibaba.dubbo.common.utils 包下，跟我自己手写的LRUCache区别不大，都是继承了LinkedHashMap来实现，

dubbo的LRUCache重写了LinkedHashMap的get、put、remove、size、clear和containsKey方法，将这些方法改为线程安全的实现，采用ReentrantLock加锁实现，

ReentrantLock和synchronized一样，加的是悲观锁，底层实现机制不同，synchronized底层依赖JAVA对象的Monitor锁，ReentrantLock底层是AQS（抽象队列同步器）。

据说ReentrantLock比synchronized性能要好，不过我也没验证过。

package com.alibaba.dubbo.common.utils;

import java.util.LinkedHashMap;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class LRUCache<K, V> extends LinkedHashMap<K, V> {

    private static final long serialVersionUID = -5167631809472116969L;

    private static final float DEFAULT_LOAD_FACTOR = 0.75f;

    private static final int DEFAULT_MAX_CAPACITY = 1000;
    private final Lock lock = new ReentrantLock();
    private volatile int maxCapacity;

    public LRUCache() {
        this(DEFAULT_MAX_CAPACITY);
    }

    public LRUCache(int maxCapacity) {
        super(16, DEFAULT_LOAD_FACTOR, true);
        this.maxCapacity = maxCapacity;
    }

    @Override
    protected boolean removeEldestEntry(java.util.Map.Entry<K, V> eldest) {
        return size() > maxCapacity;
    }

    @Override
    public boolean containsKey(Object key) {
        try {
            lock.lock();
            return super.containsKey(key);
        } finally {
            lock.unlock();
        }
    }

    @Override
    public V get(Object key) {
        try {
            lock.lock();
            return super.get(key);
        } finally {
            lock.unlock();
        }
    }

    @Override
    public V put(K key, V value) {
        try {
            lock.lock();
            return super.put(key, value);
        } finally {
            lock.unlock();
        }
    }

    @Override
    public V remove(Object key) {
        try {
            lock.lock();
            return super.remove(key);
        } finally {
            lock.unlock();
        }
    }

    @Override
    public int size() {
        try {
            lock.lock();
            return super.size();
        } finally {
            lock.unlock();
        }
    }

    @Override
    public void clear() {
        try {
            lock.lock();
            super.clear();
        } finally {
            lock.unlock();
        }
    }

    public int getMaxCapacity() {
        return maxCapacity;
    }

    public void setMaxCapacity(int maxCapacity) {
        this.maxCapacity = maxCapacity;
    }

}