4

我正在尝试创建一个以给定间隔更新 Runnable 的后台线程。

它也不应该阻止“父母”收集垃圾。

我的问题如下。我的 WeakReference 似乎充当“强”参考，它不会阻止我的线程表单访问我应该为 gc 提供的可运行文件。

为什么我的弱引用会阻止 gc？

下面是我的完整实现

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.lang.ref.WeakReference;

public final class WeakIntervalUpdater {

    private final long updateFrequencyMs;
    private final WeakReference updateObject;
    private Thread runningThread;

    /**
     * Will keep a thread running fireing the updaterunnable every updateFrequencyMs.
     *
     * the updateRunnable is first fired after updateFrequencyMs ms after startUpdating() is called
     *
     * This thread will require calls to be made to stopUpdating() or that the 
     * updateRunnable is garbage collected to stop updateing and be eligable for
     * garbage collection. 
     * 
     * This class maintains only a weak reference to the updateRunnablein order.
     *
     *
     * @param updateFrequencyMs number of ms between each update
     * @param updateRunnable the update runnable
     */
    public WeakIntervalUpdater(long updateFrequencyMs, Runnable updateRunnable) {
    this.updateFrequencyMs = updateFrequencyMs;
    this.updateObject = new WeakReference(updateRunnable);

    }

    public void startUpdating() {
    if (runningThread != null) {
        if (runningThread.isAlive()) {
        return;
        }
        runningThread.interrupt();
        runningThread = new Thread(createThreadRunnable());
    } else {
        runningThread = new Thread(createThreadRunnable());
    }
    runningThread.setDaemon(true);
    runningThread.start();
    }

    public void stopUpdating() {
    if (runningThread != null) {
        runningThread.interrupt();
        runningThread = null;
    }
    }

    Runnable createThreadRunnable() {
    return new ThreadRunnable();
    }

    private class ThreadRunnable implements Runnable {

    public void run() {
        Object object;
        while ((object = updateObject.get()) != null) {
        System.out.println("object is now: " + object);
        try {
            Thread.sleep(updateFrequencyMs);
        } catch (InterruptedException ex) {
            System.out.println("Thread interrupted, killing thread");
            return;
        }
        ((Runnable) object).run();
        object = null;
        }
        System.out.println("lost object reference: killing thread");
    }
    }

    private static void printTestHelp() {
    System.out.println("\n\n\n\n---------------------");
    System.out.println("Commands:");
    System.out.println("c : create an updater with a reference to an updateRunnable");
    System.out.println("r : release reference to updateRunnable");
    System.out.println("gc: run garbagecollection");
    System.out.println("s : stop updater");
    System.out.println("i : print object references");
    System.out.println("q : quit program");
    System.out.println("\nPlease enter your command");
    }

    public static void main(String[] args) throws IOException {

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    String line;
    WeakIntervalUpdater updater = null;
    Runnable myUpdateRunnable = null;
    printTestHelp();
    while (!(line = br.readLine()).equals("q")) {
        if (line.equals("c")) {
        if (updater != null) {
            updater.stopUpdating();
            System.out.println("\tUpdater stopped");
        }
        myUpdateRunnable = new UpdateTesterRunnable();
        updater = new WeakIntervalUpdater(1000, myUpdateRunnable);
        updater.startUpdating();
        System.out.println("\tcreated updater! updateing every 1000 ms");
        } else if (line.equals("r")) {
        //updater = null;
        myUpdateRunnable = null;
        System.out.println("\tDropped refrence to updater!");
        System.out.println("\tupdateRunnable=" + myUpdateRunnable);
        } else if (line.equals("gc")) {
        System.gc();
        Runtime.getRuntime().runFinalization();
        System.out.println("\tGarbage collection running!");
        } else if (line.equals("s")) {
        if (updater != null) {
            updater.stopUpdating();
            System.out.println("\tUpdater stopped");
        } else {
            System.out.println("\tNo updater running");
        }
        } else if (line.equals("i")) {
        System.out.println("\tupdater = " + updater);
        System.out.println("\tupdateRunnable = " + myUpdateRunnable);
        } else {
        printTestHelp();
        }
    }
    System.out.println("Goodbye");
    }

    private static class UpdateTesterRunnable implements Runnable {

    public void run() {
        System.out.println("\t\t\t(updating)");
    }

    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        System.out.println("finalize");
    }
    }
}
4

3 回答 3

1

除了使 ThreadRunnable 成为静态之外，您还需要在Thread.sleep()之前将object设置为 null 。除非清除该引用，否则垃圾收集器无法回收该对象。只需将Thread.sleep()代码移到object = null下方；分配，这应该给垃圾收集器一个机会。

public void run() {
    Object object;
    while ((object = updateObject.get()) != null) {
        System.out.println("object is now: " + object);
        ((Runnable) object).run();
        object = null;
        try {
            Thread.sleep(updateFrequencyMs);
        } catch (InterruptedException ex) {
            System.out.println("Thread interrupted, killing thread");
            return;
        }
    }
    System.out.println("lost object reference: killing thread");
}
于 2010-07-08T14:53:26.287 回答
0

我建议您不要指望将 main 中的局部变量归零以利用可运行的 gc。您能否将部分或全部 if-then 块拆分为它们自己的方法，并在其中一种方法中使可运行对象仅成为本地变量。

于 2010-07-08T15:50:50.600 回答
0

不确定这是否是原因，但ThreadRunnable它是一个非静态内部类，它会在其生成的构造函数中传递对包含类的引用。您可以尝试将其设为静态内部类并在构造函数中手动传递必要的参数，如下所示（未经测试）：

static class ThreadRunnable implements Runnable {

    private WeakReference updateObject;

    ThreadRunnable(WeakReference updateObject) {
        this.updateObject = updateObject;
    }

     public void run() { ... }
}
于 2010-07-08T14:10:44.670 回答