在JDK 1.2以前的版本中，若一个对象不被任何变量引用，那么程序就无法再使用这个对象。也就是说，只有对象处于可触及（reachable）状态，程序才能使用它。从JDK 1.2版本开始，把对象的引用分为4种级别，从而使程序能更加灵活地控制对象的生命周期。这4种级别由高到低依次为：强引用、软引用、弱引用和虚引用

StrongReference

SoftReference

public class SoftTest{    
    public static void main(String[] args) {    
      Object ref = new Object();//ref是Object对象的强引用    
      //将一个软引用指向对象，此时Object对象有两个引用    
      SoftReference<Object> sf = new SoftReference<Object>(ref);       
      ref = null;//去除对象的强引用    
      System.gc();//gc只有在内存不足是才会回收软引用对象    
    }    
} 

WeakReference

public class WeakTest{    
    public static void main(String[] args) {    
        Object ref = new Object();//ref是Object对象的强引用       
        //将一个弱引用指向对象，此时Object对象有两个引用    
       WeakReference<Object> wf = new WeakReference<Object>(ref);    
        ref = null;//去除对象的强引用    
       System.gc();//gc对弱引用对象进行回收    
    }    
}    

PhantomReference

public class PhantomTest{    
     public static void main(String[] args) {    
        Object ref = new Object();//ref是Object对象的强引用      
        //将一个幽灵引用指向对象，PhantomReference必须与ReferenceQueue一同使用    
        PhantomReference<Object> pf = new PhantomReference<Object>(ref, new ReferenceQueue<Object>());  
    System.out.println(pf.get());    
     }    
}  

android中的应用

public abstract class AsyncQueryHandler extends Handler {  
    private static final String TAG = "AsyncQuery";  
    private static final boolean localLOGV = false;  
  
    private static final int EVENT_ARG_QUERY = 1;  
    private static final int EVENT_ARG_INSERT = 2;  
    private static final int EVENT_ARG_UPDATE = 3;  
    private static final int EVENT_ARG_DELETE = 4;  
  
    /* package */ final WeakReference<ContentResolver> mResolver;  

public abstract class BaseMemoryCache implements MemoryCache {  
  
    /** Stores not strong references to objects */  
    private final Map<String, Reference<Bitmap>> softMap = Collections.synchronizedMap(new HashMap<String, Reference<Bitmap>>());  
    ......  
  
    @Override  
    protected Reference<Bitmap> createReference(Bitmap value) {  
        return new WeakReference<Bitmap>(value);  
}  

private static class MyHandler extends Handler {  
    private final WeakReference<SampleActivity> mActivity;  
  
    public MyHandler(SampleActivity activity) {  
      mActivity = new WeakReference<SampleActivity>(activity);  
    }  
  
    @Override  
    public void handleMessage(Message msg) {  
      SampleActivity activity = mActivity.get();  
      if (activity != null) {  
        // ...  
      }  
    }  
 }  

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有效避免OOM--合理使用软引用和弱引用

原文：http://blog.csdn.net/robertcpp/article/details/51570823