如何使用 PhantomReference 作为 Finalize() 替代

Javadoc 8 的虚拟参考 http://docs.oracle.com/javase/8/docs/api/java/lang/ref/PhantomReference.html状态：

虚拟引用最常用于调度验尸前与 Java 终结机制相比，清理操作的方式更加灵活。

所以我尝试创建一个调用的线程close()符合垃圾回收条件的测试对象的方法。这run()尝试获取所有测试对象验尸前.

实际上检索到的测试对象都是null。预期的行为是检索测试对象并且close方法被调用。

无论您创建多少个测试对象，都没有一个测试对象可以被捕获验尸前（你必须增加超时并多次调用GC）。

我究竟做错了什么？这是 Java 错误吗？

可运行的测试代码：

我尝试创建一个最小、完整且可验证的示例，但还是很长。我用java version "1.8.0_121"Windows 7 64 位上为 32 位。

public class TestPhantomReference {

    public static void main(String[] args) throws InterruptedException {
        // Create AutoClose Thread and start it
        AutoCloseThread thread = new AutoCloseThread();
        thread.start();

        // Add 10 Test Objects to the AutoClose Thread
        // Test Objects are directly eligible for GC
        for (int i = 0; i < 2; i++) {
            thread.addObject(new Test());
        }

        // Sleep 1 Second, run GC, sleep 1 Second, interrupt AutoCLose Thread
        Thread.sleep(1000);
        System.out.println("System.gc()");
        System.gc();
        Thread.sleep(1000);
        thread.interrupt();
    }

    public static class Test {
        public void close() {
            System.out.println("close()");
        }
    }

    public static class AutoCloseThread extends Thread {
        private ReferenceQueue<Test> mReferenceQueue = new ReferenceQueue<>();
        private Stack<PhantomReference<Test>> mPhantomStack = new Stack<>();

        public void addObject(Test pTest) {
            // Create PhantomReference for Test Object with Reference Queue, add Reference to Stack
            mPhantomStack.push(new PhantomReference<Test>(pTest, mReferenceQueue));
        }

        @Override
        public void run() {
            try {
                while (true) {
                    // Get PhantomReference from ReferenceQueue and get the Test Object inside
                    Test testObj = mReferenceQueue.remove().get();
                    if (null != testObj) {
                        System.out.println("Test Obj call close()");
                        testObj.close();
                    } else {
                        System.out.println("Test Obj is null");
                    }
                }
            } catch (InterruptedException e) {
                System.out.println("Thread Interrupted");
            }
        }
    }
}

预期输出：

System.gc()
Test Obj call close()
close()
Test Obj call close()
close()
Thread Interrupted

实际输出：

System.gc()
Test Obj is null
Test Obj is null
Thread Interrupted

这是设计使然。不像finalize()，这使得对象再次可访问，可通过 a 引用的对象Reference只能使对象无法再次访问。因此，当您要通过它管理资源时，您必须将必要的信息存储到另一个对象中。这并不罕见，使用Reference对象本身。

考虑对您的测试程序进行以下修改：

public class TestPhantomReference {

    public static void main(String[] args) throws InterruptedException {
        // create two Test Objects without closing them
        for (int i = 0; i < 2; i++) {
            new Test(i);
        }
        // create two Test Objects with proper resource management
        try(Test t2=new Test(2); Test t3=new Test(3)) {
            System.out.println("using Test 2 and 3");
        }

        // Sleep 1 Second, run GC, sleep 1 Second
        Thread.sleep(1000);
        System.out.println("System.gc()");
        System.gc();
        Thread.sleep(1000);
    }

    static class TestResource extends PhantomReference<Test> {
        private int id;
        private TestResource(int id, Test referent, ReferenceQueue<Test> queue) {
            super(referent, queue);
            this.id = id;
        }
        private void close() {
            System.out.println("closed "+id);
        }
    }    
    public static class Test implements AutoCloseable {
        static AutoCloseThread thread = new AutoCloseThread();
        static { thread.start(); }
        private final TestResource resource;
        Test(int id) {
            resource = thread.addObject(this, id);
        }
        public void close() {
            resource.close();
            thread.remove(resource);
        }
    }

    public static class AutoCloseThread extends Thread {
        private ReferenceQueue<Test> mReferenceQueue = new ReferenceQueue<>();
        private Set<TestResource> mPhantomStack = new HashSet<>();

        public AutoCloseThread() {
            setDaemon(true);
        }
        TestResource addObject(Test pTest, int id) {
            final TestResource rs = new TestResource(id, pTest, mReferenceQueue);
            mPhantomStack.add(rs);
            return rs;
        }
        void remove(TestResource rs) {
            mPhantomStack.remove(rs);
        }

        @Override
        public void run() {
            try {
                while (true) {
                    TestResource rs = (TestResource)mReferenceQueue.remove();
                    System.out.println(rs.id+" not properly closed, doing it now");
                    mPhantomStack.remove(rs);
                    rs.close();
                }
            } catch (InterruptedException e) {
                System.out.println("Thread Interrupted");
            }
        }
    }
}

这将打印：

using Test 2 and 3
closed 3
closed 2
System.gc()
0 not properly closed, doing it now
closed 0
1 not properly closed, doing it now
closed 1

展示如何使用正确的习惯用法确保资源及时关闭，并且与finalize()，对象可以选择退出事后清理，这使得使用正确的习惯用法更加有效，因为在这种情况下，不需要额外的 GC 周期来在终结后回收对象。