测试弱参考 [英] Testing WeakReference

问题描述

在 Java 中测试弱引用的正确方法是什么?

What is the proper approach to testing a weak reference in Java?

我最初的想法是做以下事情:

My initial idea is to do the following:

public class WeakReferenceTest {

    public class Target{
        private String value;    

        public Target(String value){
            this.value = value;
        }    
        public String toString(){
            return value;
        }
    }

    public class UsesWeakReference{    
        WeakReference<Target> reference;   

        public UsesWeakReference(Target test){
            reference = new WeakReference<Target>(test);
        }    
        public String call(){
            Target test = reference.get();
            if(test != null){
                return test.toString();
            }
            return "empty";
        }
    }

    @Test
    public void testWeakReference(){    
        Target target = new Target("42");

        UsesWeakReference usesWeakReference = new UsesWeakReference(target);    
        WeakReference<Target> triggerReference = new WeakReference<Target>(target);    
        assertEquals("42", usesWeakReference.call());

        target = null;    
        while(triggerReference.get() != null){
            System.gc();
        }

        assertEquals("empty", usesWeakReference.call());    
    }    
}

我对该方法的保留意见是使用 System.gc()，据我所知，它在不同的 JVM 上可能表现不同.

The reservation I have about the approach is using System.gc(), as I understand that it can behave differently on different JVMs.

推荐答案

没有 100% 防弹的方法来测试使用引用类型的代码.Reference 对象的行为取决于 GC 何时运行，并且没有 100% 可靠的方法来强制 GC 运行.

There's no 100% bombproof way of testing code that uses the Reference types. The behaviour of Reference objects depends on when the GC runs, and there is no 100% reliable way of forcing the GC to run.

你能做的最好的是:

• 在运行测试时检查您是否设置了正确的 JVM 选项，并且
• 编写您的测试，使其在 System.gc() 是空操作的情况下不会失败 OR 愿意禁用或跳过测试，或忽略测试失败.

• check that you have the right JVM options set when running the tests, and
• write your test so that it doesn't fail in the event that System.gc() is a no-op OR be willing to disable or skip the test, or ignore the test failure.

(您应该能够通过查看调用前后使用的内存量来检测 System.gc() 是否被忽略；例如，通过调用 Runtime.totalMemory())

(You should be able to detect that System.gc() is being ignored by looking at how much memory is in use before and after the call; e.g. by calling Runtime.totalMemory())

实际上，还有另一个解决方案".让你的单元测试产生大量的垃圾……足以保证你会触发垃圾收集.(IMO，这不是个好主意.)

Actually, there is another "solution". Have your unit test generate a huge amount of garbage ... enough to guarantee that you will trigger garbage collection. (Not a good idea, IMO.)

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