Reference和Reference Queue

起因

看guaua cache的时候看到源码里面有用到softReference、weakReference等作为key和value，突然想要深入理解一下引用的原理，看了源码和不少文章，终于有点感觉了，主要是gc控制了一些参数的设置，所以感觉有点难理解。关于基本的知识，我就不赘述了，反正我也是看的大家的文章?

源码和流程

大部分网上的文章都是差不多贴的源码注释：

/* A Reference instance is in one of four possible internal states:
     *
     *     Active: Subject to special treatment by the garbage collector.  Some
     *     time after the collector detects that the reachability of the
     *     referent has changed to the appropriate state, it changes the
     *     instance's state to either Pending or Inactive, depending upon
     *     whether or not the instance was registered with a queue when it was
     *     created.  In the former case it also adds the instance to the
     *     pending-Reference list.  Newly-created instances are Active.
     *
     *     Pending: An element of the pending-Reference list, waiting to be
     *     enqueued by the Reference-handler thread.  Unregistered instances
     *     are never in this state.
     *
     *     Enqueued: An element of the queue with which the instance was
     *     registered when it was created.  When an instance is removed from
     *     its ReferenceQueue, it is made Inactive.  Unregistered instances are
     *     never in this state.
     *
     *     Inactive: Nothing more to do.  Once an instance becomes Inactive its
     *     state will never change again.
     *
     * The state is encoded in the queue and next fields as follows:
     *
     *     Active: queue = ReferenceQueue with which instance is registered, or
     *     ReferenceQueue.NULL if it was not registered with a queue; next =
     *     null.
     *
     *     Pending: queue = ReferenceQueue with which instance is registered;
     *     next = this
     *
     *     Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
     *     in queue, or this if at end of list.
     *
     *     Inactive: queue = ReferenceQueue.NULL; next = this.
     *
     * With this scheme the collector need only examine the next field in order
     * to determine whether a Reference instance requires special treatment: If
     * the next field is null then the instance is active; if it is non-null,
     * then the collector should treat the instance normally.
     *
     * To ensure that a concurrent collector can discover active Reference
     * objects without interfering with application threads that may apply
     * the enqueue() method to those objects, collectors should link
     * discovered objects through the discovered field. The discovered
     * field is also used for linking Reference objects in the pending list.
     */

注释讲的算比较清楚了，虽然是英文，大家应该可以看懂啦！有个概念就行。网上有个状态转换图，比较清晰：


上面的图大概描述了一个对象从创建被回收的过程，看了这些好像有点了解了，但是还是有点蒙，我们再看看Reference的成员变量：

referent：被封装的对象
queue：注册的引用队列，只有注册了队列的Reference，才会有pending和enqueued的状态，引用队列的使用可以让我们知道对象什么时候被gc，gc的时候引用会被enqueue到队列里面，我们可以从队列中取出来，或者做一些监控（资源清除等），没有注册队列的reference，我们就一脸懵逼了。
next：在引用队列里面形成一个链表，也就是指向下一个被gc的引用对象，既然next指向引用队列的gc对象，那注释里面说”next != null，说明就是被gc了，收集器应该处理掉他了”就不难理解了。
discovered：这个我觉得是待加入到引用队列里面的一个对象，JVM帮我们设置的，和pending对象联合使用形成一个链表，相当于pending队列的next吧。
pending：static类型的，说明只有一个，也就是pending链表的head了。
ReferenceHandler：一个Thead，而且在static初始化代码里面设置了Thread.MAX_PRIORITY最高优先级，run方法里面就是个死循环

public void run() {
            while (true) {
                tryHandlePending(true);
            }
       }

tryHandlePending这个方法里面主要就是不断取出pending列表里面的引用，并且加入到ReferenceQueue里面去，也就是上图中从pending队列放到了ReferenceQueue队列中了。

reference里面的pending队列和referenceQueue队列都有lock进行控制，不会出现并发问题。

分析完成员变量，不知道大家理解一点了没有，我的表达水平有限，所以想多写写，练练手?。上面两张图还是画的很清楚的?，大家可以结合源码再看看，有什么问题可以交流，帮助我打怪升级。

具体的引用

软引用和弱引用：

软引用是在内存不足的情况下进行回收，那什么时候算内存不足呢？具体的jvm源码也不贴了，大致的计算方式就是：

interval > get_heap_free_at_last_gc * SoftRefLRUPolicyMSPerMB

SoftRefLRUPolicyMSPerMB：每1M空闲空间可保持的SoftReference对象生存的时长（单位毫秒），默认值是1000

interval= timestamp - clock（软引用的两个变量）

意思就是当这个引用被get（访问）的时间足够长，就可以被回收了，而get_heap_free_at_last_gc越大（内存越大，越不会内存不足），或者SoftRefLRUPolicyMSPerMB越大，软引用存活的时间就越长。

软引用会有一定的问题，由于要在内存不足的时候被回收，那如果一直不回收，就会被放到老年代，而老年代会越来越大，导致不停地full gc。

相对软引用，弱引用用的就比较多了，只要gc就会被回收，对JVM没有副作用，对于一般的缓存还挺有效的。

幻引用主要的实现就是Cleaner了，实现资源的回收和释放。

今天就这样了。以后有新发现再说。

参考：

https://www.jianshu.com/p/439a8f738153
https://coldwalker.com/2019/02//gc_intro/