Java 引用类型及常见应用

引用的类型主要关注的是该引用如何与GC交互。

1.Strong References
java中常见的引用类型：

StringBuffer buf = new StringBuffer();

这里buf就是一个强引用。如果一个对象在一条强引用链上可达，那么它不适合GC，也不会被回收。

2.Weak References
弱引用，是一种较弱的引用，它不会阻止GC回收只被弱引用引用的对象。
当一个对象最强只被弱引用引用时,那么在下个GC周期,它可以被回收.
你可以使用如下的方式构建一个弱引用：

WeakReference<Widget> weakWidget = new WeakReference<Widget>(widget);

在其他地方你可以使用如下的代码来获取原始对象：

weakWidget.get()

但是你可能会得到一个NULL值。
使用到弱引用的常见类是:
ThreadLocal类：

java.util.WeakHashMap

3.Soft References
软引用,与弱引用类似,弱引用在下一个GC周期就会被回收,但是软引用一般会坚持更久.
更常见的,软引用,只要在内存够用的情况下就会一直存在.
那么它非常适合实现缓存.
下图是jdk中的继承关系图,可以看见很多跟缓存相关.

更多可以参考Google guava:
com.google.common.collect.MapMakerInternalMap

4.Phantom References
幻象引用,一种更弱的引用,最弱的,它的get()方法总是返回NULL.
它一般用于在对象GC时得到一个通知,通知你可以做一些别的事情,
在apache的commons-fileupload 和commons-io中很巧妙的使用了这个类,来实现上传时临时文件的清除.
在commons-fileupload中每个文件都指定了一个文件清除的跟踪器.

org.apache.commons.fileupload.disk.DiskFileItemFactory.fileCleaningTracker

而commons-io中实现,最重要的在:Reaper线程的run方法中.(删除了部分代码.)

package org.apache.commons.io;

/** * Keeps track of files awaiting deletion, and deletes them when an associated * marker object is reclaimed by the garbage collector. * <p> * This utility creates a background thread to handle file deletion. * Each file to be deleted is registered with a handler object. * When the handler object is garbage collected, the file is deleted. * <p> * In an environment with multiple class loaders (a servlet container, for * example), you should consider stopping the background thread if it is no * longer needed. This is done by invoking the method */
public class FileCleaningTracker {

    // Note: fields are package protected to allow use by test cases

    /** * Queue of <code>Tracker</code> instances being watched. */
    ReferenceQueue<Object> q = new ReferenceQueue<Object>();
    /** * Collection of <code>Tracker</code> instances in existence. */
    final Collection<Tracker> trackers = Collections.synchronizedSet(new HashSet<Tracker>()); // synchronized
    /** * Whether to terminate the thread when the tracking is complete. */
    volatile boolean exitWhenFinished = false;
    /** * The thread that will clean up registered files. */
    Thread reaper;

    /** * Track the specified file, using the provided marker, deleting the file * when the marker instance is garbage collected. * The speified deletion strategy is used. * * @param path the full path to the file to be tracked, not null * @param marker the marker object used to track the file, not null * @param deleteStrategy the strategy to delete the file, null means normal * @throws NullPointerException if the path is null */
    public void track(final String path, final Object marker, final FileDeleteStrategy deleteStrategy) {
        if (path == null) {
            throw new NullPointerException("The path must not be null");
        }
        addTracker(path, marker, deleteStrategy);
    }

    /** * Adds a tracker to the list of trackers. * * @param path the full path to the file to be tracked, not null * @param marker the marker object used to track the file, not null * @param deleteStrategy the strategy to delete the file, null means normal */
    private synchronized void addTracker(final String path, final Object marker, final FileDeleteStrategy
            deleteStrategy) {
        // synchronized block protects reaper
        if (exitWhenFinished) {
            throw new IllegalStateException("No new trackers can be added once exitWhenFinished() is called");
        }
        if (reaper == null) {
            reaper = new Reaper();
            reaper.start();
        }
        trackers.add(new Tracker(path, deleteStrategy, marker, q));
    }



    //-----------------------------------------------------------------------
    /** * The reaper thread. */
    private final class Reaper extends Thread {
        /** Construct a new Reaper */
        Reaper() {
            super("File Reaper");
            setPriority(Thread.MAX_PRIORITY);
            setDaemon(true);
        }

        /** * Run the reaper thread that will delete files as their associated * marker objects are reclaimed by the garbage collector. */
        @Override
        public void run() {
            // thread exits when exitWhenFinished is true and there are no more tracked objects
            while (exitWhenFinished == false || trackers.size() > 0) {
                try {
                    // Wait for a tracker to remove.
                    final Tracker tracker = (Tracker) q.remove(); // cannot return null
                    trackers.remove(tracker);
                    if (!tracker.delete()) {
                        deleteFailures.add(tracker.getPath());
                    }
                    tracker.clear();
                } catch (final InterruptedException e) {
                    continue;
                }
            }
        }
    }

    //-----------------------------------------------------------------------
    /** * Inner class which acts as the reference for a file pending deletion. */
    private static final class Tracker extends PhantomReference<Object> {

        /** * The full path to the file being tracked. */
        private final String path;
        /** * The strategy for deleting files. */
        private final FileDeleteStrategy deleteStrategy;

        /** * Constructs an instance of this class from the supplied parameters. * * @param path the full path to the file to be tracked, not null * @param deleteStrategy the strategy to delete the file, null means normal * @param marker the marker object used to track the file, not null * @param queue the queue on to which the tracker will be pushed, not null */
        Tracker(final String path, final FileDeleteStrategy deleteStrategy, final Object marker,
                final ReferenceQueue<? super Object> queue) {
            super(marker, queue);
            this.path = path;
            this.deleteStrategy = deleteStrategy == null ? FileDeleteStrategy.NORMAL : deleteStrategy;
        }

        /** * Deletes the file associated with this tracker instance. * * @return {@code true} if the file was deleted successfully; * {@code false} otherwise. */
        public boolean delete() {
            return deleteStrategy.deleteQuietly(new File(path));
        }
    }

}