Java中的引用类型

在Java中，有四种类型的引用在它们被垃圾收集的方式上有所不同。

1. 强引用
2. 弱引用
3. 软引用
4. 幻影参考

先决条件：垃圾收集

• 强引用：这是引用对象的默认类型/类。任何具有活动强引用的对象都没有资格进行垃圾回收。只有当被强引用的变量指向 null 时，对象才会被垃圾回收。

  MyClass obj = new MyClass ();          
  

  这里的 'obj' 对象是对新创建的 MyClass 实例的强引用，当前 obj 是活动对象，因此不能被垃圾收集。

  obj = null;
  //'obj' object is no longer referencing to the instance. 
  So the 'MyClass type object is now available for garbage collection.          
  

  

  // Java program to illustrate Strong reference
  class Gfg
  {
      //Code..
  }
  public class Example
  {
      public static void main(String[] args)
      {
           //Strong Reference - by default
          Gfg g = new Gfg();    
            
          //Now, object to which 'g' was pointing earlier is 
          //eligible for garbage collection.
          g = null; 
      }
  } 
  

• 弱引用：弱引用对象不是引用对象的默认类型/类，在使用它们时应明确指定。
  • 这种类型的引用在 WeakHashMap 中用于引用条目对象。
  • 如果 JVM 检测到一个只有弱引用的对象（即没有强或软引用链接到任何对象对象），该对象将被标记为垃圾回收。
  • 要创建此类引用，使用Java.lang.ref.WeakReference 类。
  • 这些引用在实时应用程序中使用，同时建立一个 DBConnection，当使用数据库的应用程序关闭时，垃圾收集器可能会清理该 DBConnection。

  

  //Java Code to illustrate Weak reference
  import java.lang.ref.WeakReference;
  class Gfg
  {
      //code
      public void x()
      {
          System.out.println("GeeksforGeeks");
      }
  }
    
  public class Example
  {
      public static void main(String[] args)
      {
          // Strong Reference
          Gfg g = new Gfg();     
          g.x();
            
          // Creating Weak Reference to Gfg-type object to which 'g' 
          // is also pointing.
          WeakReference weakref = new WeakReference(g);
            
          //Now, Gfg-type object to which 'g' was pointing earlier
          //is available for garbage collection.
          //But, it will be garbage collected only when JVM needs memory.
          g = null; 
            
          // You can retrieve back the object which
          // has been weakly referenced.
          // It successfully calls the method.
          g = weakref.get(); 
            
          g.x();
      }
  }
  

  输出：

  GeeksforGeeks
  GeeksforGeeks
  

可以招募两种不同级别的弱点：软弱点和幻影弱点

• 软引用：在软引用中，即使对象可以进行垃圾回收，也不会被垃圾回收，直到 JVM 严重需要内存。当 JVM 内存不足时，对象会从内存中清除。创建这样的引用使用Java.lang.ref.SoftReference 类。
  

  //Code to illustrate Soft reference
  import java.lang.ref.SoftReference;
  class Gfg
  {
      //code..
      public void x()
      {
          System.out.println("GeeksforGeeks");
      }
  }
    
  public class Example
  {
      public static void main(String[] args)
      {
          // Strong Reference
          Gfg g = new Gfg();     
          g.x();
            
          // Creating Soft Reference to Gfg-type object to which 'g' 
          // is also pointing.
          SoftReference softref = new SoftReference(g);
            
          // Now, Gfg-type object to which 'g' was pointing
          // earlier is available for garbage collection.
          g = null; 
            
          // You can retrieve back the object which
          // has been weakly referenced.
          // It successfully calls the method.
          g = softref.get(); 
            
          g.x();
      }
  }
  

  输出：

  GeeksforGeeks
  GeeksforGeeks
  

• 幻影引用：幻影引用所引用的对象符合垃圾回收条件。但是，在将它们从内存中删除之前，JVM 会将它们放入一个名为 'reference queue' 的队列中。在对它们调用 finalize() 方法后将它们放入引用队列中。使用Java.lang.ref.PhantomReference 类创建此类引用。

  //Code to illustrate Phantom reference
  import java.lang.ref.*;
  class Gfg
  {
      //code
      public void x()
      {
          System.out.println("GeeksforGeeks");
      }
  }
    
  public class Example
  {
      public static void main(String[] args)
      {
          //Strong Reference
          Gfg g = new Gfg();     
          g.x();
            
          //Creating reference queue
          ReferenceQueue refQueue = new ReferenceQueue();
    
          //Creating Phantom Reference to Gfg-type object to which 'g' 
          //is also pointing.
          PhantomReference phantomRef = null;
            
          phantomRef = new PhantomReference(g,refQueue);
            
          //Now, Gfg-type object to which 'g' was pointing
          //earlier is available for garbage collection.
          //But, this object is kept in 'refQueue' before 
          //removing it from the memory.
          g = null; 
            
          //It always returns null. 
          g = phantomRef.get(); 
            
          //It shows NullPointerException.
          g.x();
      }
  }
  

  运行时错误：

  Exception in thread "main" java.lang.NullPointerException
      at Example.main(Example.java:31)

  输出：

  GeeksforGeeks