Reference 博客分类: java.lang.ref Reference
程序员文章站
2024-02-05 18:03:16
...
Reference
一、总结
1.jdk 1.8.0
2.引用级别
- FinalReference,不对外提供使用,类的访问权限为默认 protect,即使抛出 out of memory 异常也不会回收所占内存
- SoftReference,在内存不够用时,在抛出 out of memory 前回收内存空间
- WeakReference,第一次GC不回收,当第二次GC时回收,有限期到第二次GC前都在内存中
- PhantomReference
3.引用对比
- | 强引用 | 软引用 | 弱引用 | 虚引用 |
定义 | 强引用指的是,程序中有直接可达的引用,而不需要通过任何引用对象,如Object obj = new Object();中,obj为强引用 | 软引用,非强引用,但是可以通过软引用对象来访问。软引用的对象,只有在内存不足的时候(抛出OOM异常前),垃圾收集器会决定回收该软引用所指向的对象。软引用通常用于实现内存敏感的缓存。 | 弱引用,非强引用和软引用,但是可以通过弱引用对象来访问。弱引用的对象,不管内存是否足够,只要被垃圾收集器发现,该引用的对象就会被回收。实际的应用见WeakHashMap | 虚引用,该引用必须和引用队列(ReferenceQueue)一起使用,一般用于实现追踪垃圾收集器的回收动作,比如在对象被回收的时候,会调用该对象的finalize方法,在使用虚引用可以实现该动作,也更加安全 |
4.其他
- 子类不可直接继承;处于安全考虑,Reference 类与GC交互;实现方式:构造方法的访问权限为默认,即包外的类不可见
- 对象封装了其它对象的引用,可以和普通的对象一样操作,在一定的限制条件下,支持和垃圾收集器的交互。即可以使用Reference对象来引用其它对象,但是最后还是会被垃圾收集器回收。程序有时候也需要在对象回收后被通知,以告知对象的可达性发生变更
5.流程
二、源码分析
/** * Abstract base class for reference objects. This class defines the * operations common to all reference objects. Because reference objects are * implemented in close cooperation with the garbage collector, this class may * not be subclassed directly. * * @author Mark Reinhold * @since 1.2 */ public abstract class Reference<T> {
类的注释:
- Reference 类是引用对象的抽象基类
- Reference 类中定义了引用对象的常用操作
- 由于引用对象是通过与垃圾回收器密切合作来实现的,因此,不能直接为此类创建子类
Reference 类不能被包外的类继承的实现:
- Reference 类的构造方法的访问权限为默认,即同包内的类可见
- 子类的构造方法默认通过 super() 调用父类的构造方法,访问不到父类的构造方法
// 构造方法,两个 // 区别:是否带有 ReferenceQueue 参数 // 不带有 ReferenceQueue 参数的构造方法,当 Reference 被 GC 回收后直接由 Active 状态变为 InActive 状态 Reference(T referent) { this(referent, null); } Reference(T referent, ReferenceQueue<? super T> queue) { this.referent = referent; this.queue = (queue == null) ? ReferenceQueue.NULL : queue; } // 带有 ReferenceQueue 参数的构造方法,当 Reference 被 GC 回收后会被加入到 GC 自动加入到 pending-Reference list 中,即类中的属性 pengding private static Reference<Object> pending = null; // 对应构造方法传入的两个参数 private T referent; /* Treated specially by GC */ volatile ReferenceQueue<? super T> queue;
// 不同状态队列的next元素 /* When active: NULL * pending: this * Enqueued: next reference in queue (or this if last) * Inactive: this */ @SuppressWarnings("rawtypes") Reference next;
- 使用:访问权限为默认,当前类中未使用到该属性;为同包中其他类中调用使用,如:ReferenceQueue 中使用到了
- 作用:Reference 数据结构为链表,用于连接后续的对象
- Reference next , 在 ReferenceQueue 中指代构造方法传入的参数中的 ReferenceQueue
Reference状态 | ReferenceQueue中的next的指向 |
Active | NULL |
Pending | 本身 |
Enqueued | 下一个元素或本身(尾部的元素指向本身) |
Inactive | NULL |
/* 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. */ // 用于保存对象的引用,GC会根据不同Reference来特别对待,构造方法的参数 private T referent; /* Treated specially by GC */ // 简述:如果需要通知机制,则保存的对对应的队列 // 使用:构造方法的参数 // 作用:创建Reference时,将Queue注册到Reference中,当该Reference所引用的对象 // 被垃圾收集器回收时,会将该Reference放到该队列中,相当于一种通知机制 volatile ReferenceQueue<? super T> queue; /* When active: next element in a discovered reference list maintained by GC (or this if last) * pending: next element in the pending list (or null if last) * otherwise: NULL */ // 指向队列中的下一个对象;不同于next,为当前类中使用 transient private Reference<T> discovered; /* used by VM */ /* Object used to synchronize with the garbage collector. The collector * must acquire this lock at the beginning of each collection cycle. It is * therefore critical that any code holding this lock complete as quickly * as possible, allocate no new objects, and avoid calling user code. */ static private class Lock { } private static Lock lock = new Lock(); /* List of References waiting to be enqueued. The collector adds * References to this list, while the Reference-handler thread removes * them. This list is protected by the above lock object. The * list uses the discovered field to link its elements. */ // 等待进行 enqueued 操作的对象集合; // 当 Reference-handler 线程删除元素后,GC 将删除的元素加入此队列中; // 此集合通过上述的 lock 锁实现线程安全 // 此集合通过 discovered 属性链接本身的元素 // pending队列中的元素由GC自动加入(对象回收后放入此队列中) private static Reference<Object> pending = null;
由类中的属性得知:内部的数据结果是单链表
ReferenceQueue<? super T> queue 的作用
- queue 通过构造方法传入,若无默认为 null,用于存入注册到队列上的引用对象
- queue 区分不同状态的 Reference ,不同的状态对应不同的queue
Reference有4种状态
- Active:Active状态的Reference会受到GC的特别关注,当GC察觉到引用的可达性变化为其它的状态之后,它的状态将变化为Pending或Inactive,到底转化为Pending状态还是Inactive状态取决于此Reference对象创建时是否注册了queue.如果注册了queue,则将添加此实例到pending-Reference list中。 新创建的Reference实例的状态是Active。
- Pending:在pending-Reference list中等待着被Reference-handler 线程入队列queue中的元素就处于这个状态。没有注册queue的实例是永远不可能到达这一状态
- Enqueued:队列中的对象的状态,当实例被移动到ReferenceQueue外时,Reference的状态为Inactive。没有注册ReferenceQueue的不可能到达这一状态的
- Inactive:一旦一个实例变为Inactive,则这个状态永远都不会再被改变
Reference四种状态对应的队列
- Active queue:刚创建的队列或队列中没有对象注入;next = null ;由构造方法可知,新创建的实例都处于此状态
- Pending queue:
- Enqueued queue:
- Inactive queue:
/* High-priority thread to enqueue pending References */ private static class ReferenceHandler extends Thread { private static void ensureClassInitialized(Class<?> clazz) { try { Class.forName(clazz.getName(), true, clazz.getClassLoader()); } catch (ClassNotFoundException e) { throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e); } } static { // pre-load and initialize InterruptedException and Cleaner classes // so that we don't get into trouble later in the run loop if there's // memory shortage while loading/initializing them lazily. ensureClassInitialized(InterruptedException.class); ensureClassInitialized(Cleaner.class); } ReferenceHandler(ThreadGroup g, String name) { super(g, name); } public void run() { while (true) { tryHandlePending(true); } } }
/** * Try handle pending {@link Reference} if there is one.<p> * Return {@code true} as a hint that there might be another * {@link Reference} pending or {@code false} when there are no more pending * {@link Reference}s at the moment and the program can do some other * useful work instead of looping. * * @param waitForNotify if {@code true} and there was no pending * {@link Reference}, wait until notified from VM * or interrupted; if {@code false}, return immediately * when there is no pending {@link Reference}. * @return {@code true} if there was a {@link Reference} pending and it * was processed, or we waited for notification and either got it * or thread was interrupted before being notified; * {@code false} otherwise. */ static boolean tryHandlePending(boolean waitForNotify) { Reference<Object> r; Cleaner c; // PhantomReference 的子类 try { synchronized (lock) { // pengding Reference Queue 不为空 if (pending != null) { r = pending; // 将 pending 赋值给 r // 'instanceof' might throw OutOfMemoryError sometimes // so do this before un-linking 'r' from the 'pending' chain... // r 是否是 Cleaner 的实例对象 c = r instanceof Cleaner ? (Cleaner) r : null; // unlink 'r' from 'pending' chain // pengding 指向下一个元素 pending = r.discovered; r.discovered = null; } else { // pending Reference Queue 为空 // The waiting on the lock may cause an OutOfMemoryError // because it may try to allocate exception objects. if (waitForNotify) { lock.wait(); } // retry if waited return waitForNotify; } } } catch (OutOfMemoryError x) { // Give other threads CPU time so they hopefully drop some live references // and GC reclaims some space. // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above // persistently throws OOME for some time... Thread.yield(); // retry return true; } catch (InterruptedException x) { // retry return true; } // Fast path for cleaners if (c != null) { c.clean(); return true; } ReferenceQueue<? super Object> q = r.queue; if (q != ReferenceQueue.NULL) q.enqueue(r); // 入队列 return true; }
static { ThreadGroup tg = Thread.currentThread().getThreadGroup(); for (ThreadGroup tgn = tg; tgn != null; tg = tgn, tgn = tg.getParent()); Thread handler = new ReferenceHandler(tg, "Reference Handler"); /* If there were a special system-only priority greater than * MAX_PRIORITY, it would be used here */ handler.setPriority(Thread.MAX_PRIORITY); handler.setDaemon(true); handler.start(); // provide access in SharedSecrets SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() { @Override public boolean tryHandlePendingReference() { return tryHandlePending(false); } }); }
从源码中可以看出,这个线程在Reference类的static构造块中启动,并且被设置为最高优先级和daemon状态。此线程要做的事情就是不断的的检查pending是否为null,如果pending不为null,则将pending进行enqueue,否则线程进行wait状态。
/** * Returns this reference object's referent. If this reference object has * been cleared, either by the program or by the garbage collector, then * this method returns <code>null</code>. * * @return The object to which this reference refers, or * <code>null</code> if this reference object has been cleared */ // 返回当前引用对象的指向的对象;如果对象已经被清除或被GC回收,返回NULL public T get() { return this.referent; } /** * Clears this reference object. Invoking this method will not cause this * object to be enqueued. * * <p> This method is invoked only by Java code; when the garbage collector * clears references it does so directly, without invoking this method. */ // 触发这个方法则不会进行入队的操作 public void clear() { this.referent = null; } /* -- Queue operations -- */ /** * Tells whether or not this reference object has been enqueued, either by * the program or by the garbage collector. If this reference object was * not registered with a queue when it was created, then this method will * always return <code>false</code>. * * @return <code>true</code> if and only if this reference object has * been enqueued */ // 判断对象是否已经入队 public boolean isEnqueued() { return (this.queue == ReferenceQueue.ENQUEUED); } /** * Adds this reference object to the queue with which it is registered, * if any. * * <p> This method is invoked only by Java code; when the garbage collector * enqueues references it does so directly, without invoking this method. * * @return <code>true</code> if this reference object was successfully * enqueued; <code>false</code> if it was already enqueued or if * it was not registered with a queue when it was created */ // 入队操作 public boolean enqueue() { return this.queue.enqueue(this); }
博文参考:
《Java源码分析》:ReferenceQueue、Reference及其子类
Java Reference 源码分析