[英]WeakReference in Java inside Thread
i am trying to create an background thread that updates a Runnable at a given interval. 我正在尝试创建一个后台线程,以给定的间隔更新Runnable。
It should also not prevent the "parent" from beeing garbage collected. 它也不应该阻止“父母”收集垃圾。
My problem is as follows. 我的问题如下。 My WeakReference seems to act as a "strong" Reference, It doesn't stop my thread form accessing the runnable that i am supposed to have made available for gc.
我的WeakReference似乎充当了“强大的”参考,它并没有阻止我的线程形式访问我应该为gc提供的runnable。
Why is my Weakreference preventing gc? 为什么我的Weakreference会阻止gc?
Below is my full implementation 以下是我的全面实施
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");
}
}
}
In addition to making ThreadRunnable static you also need to set object to null before you Thread.sleep() . 除了使ThreadRunnable静态之外,还需要在Thread.sleep()之前将object设置为null。 The garbage collector cannot reclaim the object unless that reference is cleared out.
除非清除该引用,否则垃圾收集器无法回收该对象。 Just move the Thread.sleep() code down below the object = null;
只需将Thread.sleep()代码向下移动到object = null; assignment and that should give the garbage collector a chance.
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");
}
Not sure if thats the cause, but ThreadRunnable
is a non-static inner class, this gets passed a reference to the containing class in its generated contructor. 不确定是否是原因,但
ThreadRunnable
是一个非静态内部类,这会在其生成的构造ThreadRunnable
中传递对包含类的引用。 You could try making it a static inner class and pass the neccessary parameters manually in the constructor like this (untested): 你可以尝试使它成为一个静态内部类,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() { ... }
}
I advise you not to count on nulling the local variable in your main to avail the runnable to gc. 我建议你不要指望对你的main中的局部变量进行归零以利用runnable到gc。 Can you split out some or all of the if-then blocks to their own methods and make the runnable only a local var in one of those methods.
你能否将部分或全部if-then块拆分为自己的方法,并使runnable只能在其中一种方法中使用局部变量。
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