java中的reference(三): FinalReference和Finalizer的源码分析

/**
 * Final references, used to implement finalization
 */
class FinalReference<T> extends Reference<T> {

    public FinalReference(T referent, ReferenceQueue<? super T> q) {
        super(referent, q);
    }
}

//final修饰的类不可被继承
final class Finalizer extends FinalReference<Object> { /* Package-private; must be in
                                                          same package as the Reference
                                                          class */

   //重写了queue属性，Finalizer必须使用ReferenceQueue,因此一开始就对queue进行了实例化
    private static ReferenceQueue<Object> queue = new ReferenceQueue<>();
    private static Finalizer unfinalized = null;
    //重载了锁
    private static final Object lock = new Object();
    
    //链表指针，Finalizer是个双向链表
    private Finalizer
        next = null,
        prev = null;
    //判断链表是否有下一个节点 
    private boolean hasBeenFinalized() {
        return (next == this);
    }

    //链表中增加元素 注意此方法是private
    private void add(){ ... }
    //链表中删除元素 注意此方法是private
    private void remove(){ ... }
    
    //构造器  private
    private Finalizer(Object finalizee) {
        super(finalizee, queue);
        add();
    }
    
    //register是非private的 
    /* Invoked by VM */
    static void register(Object finalizee) {
        new Finalizer(finalizee);
    }
    
    /* Called by Runtime.runFinalization() */
    static void runFinalization() {
        ...
    }
    
    /* Invoked by java.lang.Shutdown */
    static void runAllFinalizers(){
        ...
    }

...

}

 /**
     * Called by the garbage collector on an object when garbage collection
     * determines that there are no more references to the object.
     * A subclass overrides the {@code finalize} method to dispose of
     * system resources or to perform other cleanup.
     * <p>
     * The general contract of {@code finalize} is that it is invoked
     * if and when the Java&trade; virtual
     * machine has determined that there is no longer any
     * means by which this object can be accessed by any thread that has
     * not yet died, except as a result of an action taken by the
     * finalization of some other object or class which is ready to be
     * finalized. The {@code finalize} method may take any action, including
     * making this object available again to other threads; the usual purpose
     * of {@code finalize}, however, is to perform cleanup actions before
     * the object is irrevocably discarded. For example, the finalize method
     * for an object that represents an input/output connection might perform
     * explicit I/O transactions to break the connection before the object is
     * permanently discarded.
     * <p>
     * The {@code finalize} method of class {@code Object} performs no
     * special action; it simply returns normally. Subclasses of
     * {@code Object} may override this definition.
     * <p>
     * The Java programming language does not guarantee which thread will
     * invoke the {@code finalize} method for any given object. It is
     * guaranteed, however, that the thread that invokes finalize will not
     * be holding any user-visible synchronization locks when finalize is
     * invoked. If an uncaught exception is thrown by the finalize method,
     * the exception is ignored and finalization of that object terminates.
     * <p>
     * After the {@code finalize} method has been invoked for an object, no
     * further action is taken until the Java virtual machine has again
     * determined that there is no longer any means by which this object can
     * be accessed by any thread that has not yet died, including possible
     * actions by other objects or classes which are ready to be finalized,
     * at which point the object may be discarded.
     * <p>
     * The {@code finalize} method is never invoked more than once by a Java
     * virtual machine for any given object.
     * <p>
     * Any exception thrown by the {@code finalize} method causes
     * the finalization of this object to be halted, but is otherwise
     * ignored.
     *
     * @throws Throwable the {@code Exception} raised by this method
     * @see java.lang.ref.WeakReference
     * @see java.lang.ref.PhantomReference
     * @jls 12.6 Finalization of Class Instances
     */
    protected void finalize() throws Throwable { }

 private static class FinalizerThread extends Thread {
        private volatile boolean running;
        FinalizerThread(ThreadGroup g) {
            super(g, "Finalizer");
        }
        public void run() {
            if (running)
                return;

            // Finalizer thread starts before System.initializeSystemClass
            // is called.  Wait until JavaLangAccess is available
            while (!VM.isBooted()) {
                // delay until VM completes initialization
                try {
                    VM.awaitBooted();
                } catch (InterruptedException x) {
                    // ignore and continue
                }
            }
            final JavaLangAccess jla = SharedSecrets.getJavaLangAccess();
            running = true;
            for (;;) {
                try {
                    Finalizer f = (Finalizer)queue.remove();
                    f.runFinalizer(jla);
                } catch (InterruptedException x) {
                    // ignore and continue
                }
            }
        }
    }

  private void runFinalizer(JavaLangAccess jla) {
        synchronized (this) {
            if (hasBeenFinalized()) return;
            remove();
        }
        try {
            Object finalizee = this.get();
            if (finalizee != null && !(finalizee instanceof java.lang.Enum)) {
                jla.invokeFinalize(finalizee);

                /* Clear stack slot containing this variable, to decrease
                   the chances of false retention with a conservative GC */
                finalizee = null;
            }
        } catch (Throwable x) { }
        super.clear();
    }

    static {
        ThreadGroup tg = Thread.currentThread().getThreadGroup();
        for (ThreadGroup tgn = tg;
             tgn != null;
             tg = tgn, tgn = tg.getParent());
        Thread finalizer = new FinalizerThread(tg);
        finalizer.setPriority(Thread.MAX_PRIORITY - 2);
        finalizer.setDaemon(true);
        finalizer.start();
    }