Question

我认为WeakReference引用对象将在调用System.gc（）之后完成，但是我错了。

这里有两个测试用例，唯一的区别是WeakReference构造函数，第一个是新对象，而第二个使用引用，它们具有不同的性能，我不知道为什么...

弱引用对象，不会阻止其引用对象进行定型，定型，然后回收。参考文献最弱通常用于实现规范化的映射。

假设垃圾收集器在某个时间点确定一个物体是弱的可达的。到那时，它将原子清除所有弱引用。该对象以及对其他任何弱可达对象的所有弱引用通过一连串的强壮而柔软的物体可以从该物体到达参考。同时它将声明所有以前的弱可到达对象将被终结。同时或在某个时候稍后它将排队那些新近清除的弱引用在参考队列中注册。

package com.zeng.javaReference;

import org.junit.Test;

import java.lang.ref.WeakReference;

/**
 * @author zeng
 * @version 1.0.0
 * @date 2020-05-11
 */
public class WeakReferenceTest {

    @Test
    public void weakRefRemoved() {
        WeakReference<Apple> weakReference = new WeakReference<>(new Apple("green-apple"));

        System.gc();

        if (weakReference.get() == null) {
            System.out.println("GC remove weakReference!");
        } else {
            System.out.println("weakReference still alive");
        }
    }

    @Test
    public void weakRefNotRemoved() {
        Apple apple = new Apple("green-apple");
        WeakReference<Apple> weakReference = new WeakReference<>(apple);

        System.gc();

        if (weakReference.get() == null) {
            System.out.println("GC remove weakReference!");
        } else {
            System.out.println("weakReference still alive");
        }
    }

    public static class Apple {

        private String name;

        public Apple(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        @Override
        protected void finalize() throws Throwable {
            super.finalize();
            System.out.println("Apple： " + name + " finalized。");
        }

        @Override
        public String toString() {
            return "Apple{" +
                    "name='" + name + '\'' +
                    '}' + ", hashCode:" + this.hashCode();
        }
    }

}

Answer 1

Unable to connect to Chrome debug port: 51656 FormatException: Unexpected character (at character 1) <html> ^ #0 throwToolExit (package:flutter_tools/src/base/common.dart:14:3) #1 ChromeLauncher._connect (package:flutter_tools/src/web/chrome.dart:251:9) <asynchronous suspension> #2 ChromeLauncher.launch (package:flutter_tools/src/web/chrome.dart:183:12) <asynchronous suspension> #3 ChromeDevice.startApp (package:flutter_tools/src/web/web_device.dart:142:46) #4 _ResidentWebRunner.run.<anonymous closure> (package:flutter_tools/src/build_runner/resident_web_runner.dart:443:29) #5 _rootRunUnary (dart:async/zone.dart:1192:38) #6 _CustomZone.runUnary (dart:async/zone.dart:1085:19) #7 _FutureListener.handleValue (dart:async/future_impl.dart:141:18) #8 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:682:45) #9 Future._propagateToListeners (dart:async/future_impl.dart:711:32) #10 Future._completeWithValue (dart:async/future_impl.dart:526:5) #11 _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:36:15) #12 _completeOnAsyncReturn (dart:async-patch/async_patch.dart:298:13) #13 _ResidentWebRunner._updateDevFS (package:flutter_tools/src/build_runner/resident_web_runner.dart) #14 _rootRunUnary (dart:async/zone.dart:1192:38) #15 _CustomZone.runUnary (dart:async/zone.dart:1085:19) #16 _FutureListener.handleValue (dart:async/future_impl.dart:141:18) #17 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:682:45) #18 Future._propagateToListeners (dart:async/future_impl.dart:711:32) #19 Future._completeWithValue (dart:async/future_impl.dart:526:5) #20 _AsyncAwaitCompleter.complete (dart:async-patch/async_patch.dart:36:15) #21 _completeOnAsyncReturn (dart:async-patch/async_patch.dart:298:13) #22 WebDevFS.update (package:flutter_tools/src/build_runner/devfs_web.dart) #23 _rootRunUnary (dart:async/zone.dart:1192:38) #24 _CustomZone.runUnary (dart:async/zone.dart:1085:19) #25 _FutureListener.handleValue (dart:async/future_impl.dart:141:18) #26 Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:682:45) #27 Future._propagateToListeners (dart:async/future_impl.dart:711:32) #28 Future._completeWithValue (dart:async/future_impl.dart:526:5) #29 Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:556:7) #30 _rootRun (dart:async/zone.dart:1184:13) #31 _CustomZone.run (dart:async/zone.dart:1077:19) #32 _CustomZone.runGuarded (dart:async/zone.dart:979:7) #33 _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:1019:23) #34 _microtaskLoop (dart:async/schedule_microtask.dart:43:21) #35 _startMicrotaskLoop (dart:async/schedule_microtask.dart:52:5) #36 _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:118:13) #37 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:169:5)可能会也可能不会触发垃圾回收。

通过手动调用它，您只是在向JVM提供提示，但是天气会尊重它，这完全取决于Java运行时。 JVM是非常复杂的系统，不允许直接访问资源。

关于文档中的System.gc()

弱引用是指所引用的引用不足以使要保留在内存中的对象。因此，弱引用可以让垃圾收集器确定对象的可及性以及是否有问题的对象应该保留在内存中。

再次决定收集物体的天气完全取决于收集器。

Answer 2

这确实很有趣。

假设您使用的GC收集器在调用System.gc()时实际上起到了某些作用，例如G1，并且您没有禁用通过-XX:+DisableExplicitGC对GC的显式调用，则有一个解释这，以某种方式。

weakRefNotRemoved在理论上应与weakRefRemoved相同。这与scope和reachability有关，有关此here的更多详细信息。

问题在于，在weakRefNotRemoved中，apple引用之后，任何人都没有使用调用System.gc()，因此在那时可以到达因此，从理论上讲，GC可以回收它，但是无论出于何种原因它都不会回收。我只能假定只有在存在实际内存压力时才触发此特殊路径。

为证明这一点，我们可以将代码稍作更改为：

 public static void weakRefNotRemoved() {
    Apple apple = new Apple("green-apple");
    WeakReference<Apple> weakReference = new WeakReference<>(apple);

    // explicitly set to null
    apple = null;
    System.gc();

    if (weakReference.get() == null) {
        System.out.println("GC remove weakReference!");
    } else {
        System.out.println("weakReference still alive");
    }
}

现在，GC确实会清理它。通常，如果您试图证明一个观点，则最好在while循环中调用该观点，因为无法保证该循环将“抓住” that reference。同样是因为清除发生在asynchronous fashion中。像这样：

 public static void weakRefNotRemoved() {
    Apple apple = new Apple("green-apple");
    WeakReference<Apple> weakReference = new WeakReference<>(apple);

    apple = null;
    while (weakReference.get() != null) {
        System.out.println("weakReference still alive");
        System.gc();
        LockSupport.parkNanos(TimeUnit.SECONDS.toNanos(1));
    }

    System.out.println("GC remove weakReference!");

}

获得这样的输出是完全合法的，例如：

weakReference still alive  // zero or more of these messages
GC remove weakReference!

Java WeakReference GC已完成，尚未完成？

2 个答案: