Java SoftReference保证失败

JDK 7文档中有关于 SoftReference 的说法:



但是，在我的测试程序中，我始终看到OutOfMemoryError(下面的“陌生人行为”部分除外):

// RefObjectTest.java

import java.util.*;
import java.lang.ref.*;

public class RefObjectTest {

    public static void main(String[] args) {

        ArrayList<byte[]> leaks = new ArrayList<>();

        byte[] obj = new byte[10 * 1024 * 1024];

        SoftReference<byte[]> ref = new SoftReference<>(obj);

        // WeakReference is supposed to be eagerly GC'ed, but I see no
        // difference in terms of program behavior: still get OOME.
        //
        // WeakReference<byte[]> ref = new WeakReference<>(obj);

        obj = null;

        while(true) {
            byte[] x = ref.get();
            if(x == null) {
                System.out.println("Referent stands garbage collected!!");
                break;
            } else {
                System.out.println("Referent still alive.");
            }

            // Leak memory in small, 10k increments. If soft reference
            // worked the way it's advertized, then just before the OOME, the
            // 10MB referent should have been GC'ed to accomodate this small
            // 10K new memory request. But it doesn't appear to work that way!

//          try {
                leaks.add(new byte[10 * 1024]);
//          } catch(OutOfMemoryError e) {
//              System.out.println(ref.get() == null ? "Referent cleared" :
//                  "Referent still alive");
//          }

            // VERY STRANGE: If I re-instate the above try-catch block, then no OOME is
            // thrown, and the program keeps printing "Referent still alive" lines
            // forever until I finally kill it with a Ctrl+C.

            // Uncommenting gc() tends to delay the OOME in terms of time,
            // but OOME eventually does happen, and after the same number of
            // iterations of this loop.
            //
            // System.gc();
        }
    }
}

这是输出:

$ java -Xmx15m RefObjectTest
Referent still alive.
  ...
Referent still alive.
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at RefObjectTest.main(RefObjectTest.java:38)

陌生人行为

非常奇怪的是，如果我重新设置try-catch块，则该程序似乎永远运行良好，并显示“Referent still live”。直到我累了并杀死它。

$ java -version
java version "1.7.0_45"
Java(TM) SE Runtime Environment (build 1.7.0_45-b18)
Java HotSpot(TM) 64-Bit Server VM (build 24.45-b08, mixed mode)
$
$ uname -a
Linux ida 3.10.11-100.fc18.x86_64 #1 SMP Mon Sep 9 13:06:31 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

所有这些我缺少什么？

最佳答案

一点也不奇怪，分配失败了，这引发了异常。通过捕获异常，您可以说程序可以继续；通过捕获异常，您可以说无法处理，程序退出。

在while循环中，您正在执行以下操作:

        byte[] x = ref.get();

这是根据您的“软引用”创建一个新的“强”引用。一旦您这样做，它就不再符合收集条件。在继续进行新分配之前，您不会清除强引用。

完成测试后，请执行x = null。

关于Java SoftReference保证失败，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/20901300/