Spring—ConcurrentReferenceHashMap

ConcurrentReferenceHashMap与ConcurrentHashMap的区别是ConcurrentReferenceHashMap能指定所存放对象的引用级别，适用于并发下Map的数据缓存。

注：Java四种对象引用级别：强引用、软引用、弱引用、虚引用

测试代码：

注：用弱引用来及时查看效果。

public class TestConcurrentReferenceHashMap {    public static void main(String[] args) {        ConcurrentReferenceHashMap map = new ConcurrentReferenceHashMap(16, ConcurrentReferenceHashMap.ReferenceType.WEAK);        map.put("key","val");        System.out.println(map);        System.gc();        try {            Thread.currentThread().sleep(5000);        } catch (InterruptedException e) {            e.printStackTrace();        }        System.out.println(map);    }}

结果：

{key=val}{}Process finished with exit code 0

注：从下面ConcurrentReferenceHashMap的部分源码我们可以知道ConcurrentReferenceHashMap是基于Segment分段锁实现的，与JDK1.7的ConcurrentHashMap是一样的，与JDK1.8的ConcurrentHashMap是不一样的。

protected final class Segment extends ReentrantLock {        private final ConcurrentReferenceHashMap<K, V>.ReferenceManager referenceManager = ConcurrentReferenceHashMap.this.createReferenceManager();        private final int initialSize;        private volatile ConcurrentReferenceHashMap.Reference<K, V>[] references;        private volatile int count = 0;        private int resizeThreshold;}