- Reference提供了一些引用的基本方法以及静态代码块最高优先级启动ReferenceHandler线程
Reference提供了一些引用的基本方法以及静态代码块最高优先级启动ReferenceHandler线程
- package xxx;
- /**
- * 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.
- * 引用对象的抽象基类。这个类定义了所有引用对象的通用行为。
- * 因为引用对象是通过与垃圾回收期密切合作来实现的,所以不能直接为此类创建子类.
- *
- *
- * @version 1.43, 04/10/06
- * @author Mark Reinhold
- * @since 1.2
- */
- public abstract class Reference<T> {
- /* A Reference instance is in one of four possible internal states:
- * 一种引用实例是可能是四种内部状态之一:
- * 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.
- * 激活:垃圾回收器特别处理的主题。有时候在回收器检测到被引用(对象)的可达性被改变成适当
- * 的状态,它会把实例的状态改变成等待状态或者未激活状态,这取决于实例是否被一个队列注册当
- * 它被创建。在前一种情况下(等待状态),它也往等待-引用集合增加实例。
- *
- *
- * Pending: An element of the pending-Reference list, waiting to be
- * enqueued by the Reference-handler thread. Unregistered instances
- * are never in this state.
- * 等待:一个等待-引用集合里的元素,等待被引用处理线程放入队列中。
- * 未注册的实例永远不会在这个状态
- *
- * 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.
- * 入队:实例被创建的时候被登记注册成一个队列的元素。当一个实例从引用队列中删除,它变成非激活状态。
- * 未注册的实例永远不会在这个状态。
- *
- * Inactive: Nothing more to do. Once an instance becomes Inactive its
- * state will never change again.
- * 非激活:不会再做什么。一旦一个实例成为非激活的,它的状态永远不会被改变。
- *
- *
- * The state is encoded in the queue and next fields as follows:
- * 状态在队列里被处理并且每个状态所表现的属性如下:
- *
- *
- * Active: queue = ReferenceQueue with which instance is registered, or
- * ReferenceQueue.NULL if it was not registered with a queue; next =
- * null.
- * 激活:queue=引用队列时候,实例被它注册,
- * 或者实例不被注册,当queue=ReferenceQueue.NULL时候;
- * next=null.
- *
- * Pending: queue = ReferenceQueue with which instance is registered;
- * next = Following instance in queue, or this if at end of list.
- * 等待:queue=引用队列时候,实例被它注册,
- * next=剩下的queue队列里面的实例,或者=this,如果this是队列的最后一个。
- *
- * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
- * in queue, or this if at end of list.
- * 入队:queue=ReferenceQueue.ENQUEUED
- * next=剩下的queue队列里面的实例,或者=this,如果this是队列的最后一个
- *
- *
- * Inactive: queue = ReferenceQueue.NULL; next = this.
- * 终止:队列=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 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.
- */
- private T referent; /* Treated specially by GC */ //被GC引用的对象
- ReferenceQueue<? super T> queue;//引用队列
- Reference next;//下个引用
- transient private Reference<T> discovered; /* used by VM *///被VM引用的瞬态对象
- /* 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.
- * GC线程在回收的时候的锁
- * 对象被用来和GC同步。GC必须获得锁在每个回收的生命周期。
- * 更关键的是任何持有锁的代码尽可能快的执行完,没有分配新的对象,并避免使用使用者的代码。
- *
- */
- 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.
- * 排队的引用集合。当处理引用的线程删除引用时候,收集器添加引用到这个集合。
- * 这个集合受上面的对象锁保护。
- */
- private static Reference pending = null;
- /* High-priority thread to enqueue pending References
- * 处理排队等待的引用的高优先的线程
- */
- private static class ReferenceHandler extends Thread {
- ReferenceHandler(ThreadGroup g, String name) {
- super(g, name);
- }
- public void run() {
- for (;;) {
- /*
- * 给pending赋值
- * 如果pending.next=pending,pending=null;否则pending=pengding.next,最后把pending.next=pending
- * 下次执行线程里的代码时候pending=null了,再下次执行同步代码块就线程阻塞了
- *
- * 如果pending属性为空,释放锁的对象监视器,阻塞当前线程
- * */
- Reference r;
- synchronized (lock) {
- if (pending != null) {
- r = pending;
- Reference rn = r.next;
- pending = (rn == r) ? null : rn;
- r.next = r;
- } else {
- try {
- lock.wait();
- } catch (InterruptedException x) { }
- continue;
- }
- }
- // Fast path for cleaners
- if (r instanceof Cleaner) {
- ((Cleaner)r).clean();
- continue;
- }
- /*
- * ReferenceQueue.NULL的实现:
- * private static class Null extends ReferenceQueue {
- * boolean enqueue(Reference r) {
- * return false;
- }
- }
- * 如果Q不为空,把引用放入Queue
- * 在刚创建一个引用,第二个参数没放Queue时候,为空。
- * */
- ReferenceQueue q = r.queue;
- if (q != ReferenceQueue.NULL) q.enqueue(r);
- }
- }
- }
- static {
- //取得当前线程组
- ThreadGroup tg = Thread.currentThread().getThreadGroup();
- //取得最上层的System线程组
- for (ThreadGroup tgn = tg;
- tgn != null;
- tg = tgn, tgn = tg.getParent());
- //创建线程对象
- 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);//设置最高优先级
- handler.setDaemon(true);//标记守护线程或用户线程
- handler.start();//守护线程启动
- }
- /* -- Referent accessor and setters -- */
- /**
- * 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
- * 获得引用对象
- */
- 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.
- * 清除引用对象
- */
- 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
- */
- public boolean isEnqueued() {
- /* In terms of the internal states, this predicate actually tests
- whether the instance is either Pending or Enqueued
- 是否处于等待状态
- 判断条件:队列不为空,
- 它的next属性不为空。
- 刚初始化一个Reference时候,next属性肯定是空的,因此肯定不处于等待状态
- */
- synchronized (this) {
- return (this.queue != ReferenceQueue.NULL) && (this.next != null);
- }
- }
- /**
- * 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
- * 加入等待队列
- */
- public boolean enqueue() {
- return this.queue.enqueue(this);
- }
- /* -- Constructors -- */
- Reference(T referent) {
- this(referent, null);
- }
- Reference(T referent, ReferenceQueue<? super T> queue) {
- this.referent = referent;
- this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
- }
- }