从几段简短的代码探讨WeakReference和SoftReference

看过很多讲java里引用的文章，但大多都是告诉你，弱引用是当java gc发生的时候，就会进行回收，软引用是内存不足的时候进行回收等等。但这样总感觉有点抽象。前几天看到一篇文章，里面的代码引起了我的兴趣。我在原文代码的基础上做了一些改动，这样或许可以对软引用和弱引用有更好的理解。文章原文可以点击这里查看

java.png

public class WeakestRef {
    //coding starts from here
    public void something()
    {
        System.out.println("This is printed out on the screen");
    }
}

public static void main(String[] args) {
      // Strong Reference 
        WeakestRef obj1 = new WeakestRef ();    
        obj1.something(); 
          
        // Creating Weak Reference to WeakestRef -type object to which 'obj1' is also pointing. 
        WeakReference< WeakestRef > weakref = new WeakReference< WeakestRef >(obj1); 
          
        //Now, WeakestRef -type object to which 'obj1' was pointing earlier is not available for garbage  collection. But will be only be garbage collected when JVM needs memory. 
        obj1= null;  
          
        // Note: You can also retrieve back the object which has been weakly referenced. It succesfully  calls the method. 
        obj1= weakref.get();  
          
        obj1.something();
    }

上面这两段是原文里的代码，打印出来的结果是

This is printed out on the screen
This is printed out on the screen

作者的意思可能是当一个对象是弱引用的时候，你把他置空，但他不会立即被垃圾回收器回收，只有JVM需要内存，需要进行垃圾清理的时候才会被回收。如果你没觉得这段代码没有什么问题，那么请继续往下看

现在，让我对这段代码进行一些改动，改动成下面这样：

public static void main(String[] args) {
      WeakestRef obj1 = new WeakestRef();
        obj1.something();
        WeakReference<WeakestRef> weakref = new WeakReference<WeakestRef>(obj1);
        obj1 = null;
        //手动调用gc
        System.gc();
        obj1 = weakref.get();
        obj1.something();
    }

我在把对象置空之后，加了一段手动调用gc的代码，那么打印出来的结果就变了，

This is printed out on the screen
Exception in thread "main" java.lang.NullPointerException
    at Main.main(Main.java:21)

这或许证实了WeakReference只要发生gc，那么就会被回收的说法。那么如果把WeakReference换成SoftReference，会发生什么呢？

public static void main(String[] args) {
     WeakestRef obj1 = new WeakestRef();
        obj1.something();
        //WeakReference变为SoftReference
        SoftReference<WeakestRef> weakref = new SoftReference<WeakestRef>(obj1);
        obj1 = null;
        //手动调用gc
        System.gc();
        obj1 = weakref.get();
        obj1.something();
    }

我把WeakReference变为了SoftReference，那么打印的结果就变成了：

This is printed out on the screen
This is printed out on the screen

这或许也印证了，软引用只有在内存不足的时候才会被垃圾回收期回收。我编译的环境是基于openjdk11。这两段代码的结果和大多数讲weakReference和softReference的文章所表达的是一样的，只是更加直观一些。希望对其他不是很理解weakReference和softReference的人有所帮助