What is the difference between strong reference, soft reference, weak reference and virtual reference?

  Strong citation 、 Soft citation 、 Weak reference 、 Virtual reference · Language sparrow (yuque.com)

since JDK1.2 Start ,Java Provides 4 Kinds of reference relationships , To represent the relationship between the reference and the instance object .

Strong citation “ Live forever ”

Strong citation , Is our most common common common common object reference .

As long as strong quotes still exist , The garbage collector will never recycle the referenced instance object .

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Object o = new Object();

     

      
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Soft citation “ Yes n A second chance to live ”

Before the system is about to run out of memory , The garbage collector will recycle instance objects that are only associated with soft references , If there is not enough memory for this collection , An out-of-memory exception is thrown （OutOfMemoryError）.

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Soft references are often used to implement memory sensitive caching ：

• If you have free memory , You can keep the cache temporarily ;
• If there is not enough memory , Then clean up the cache ;

This ensures that the cache is used at the same time , Does not run out of memory .

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SoftReference<Object> softO = new SoftReference<>(new Object());

     

      
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Weak reference “ Recycle and die ”

Instance objects associated with weak references can only survive until the next garbage collection , When the garbage collector is working , Whether the current memory is enough or not , Will recycle the instance objects that are only weakly referenced .

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Weak references can be used to build a relationship without specific constraints , such as , Maintain a non mandatory mapping relationship , If the object is still there when you try to get it , Just use it , Otherwise, it will be realistic and exemplary . Weak references are also the choice of many cache implementations .

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WeakReference<Object> weakO = new WeakReference(new Object());

     

      
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Virtual reference “ It can be recycled at any time ”

Virtual references are also called “ Ghost quotes ” perhaps “ Phantom reference ”, Virtual reference is the weakest kind of reference relationship .

• stay Java 8 And in previous versions , After virtual reference recycling , The object pointed to by the virtual reference will be recycled .
• stay Java 9 And the latest version , Virtual references have no effect on the lifetime of objects .

Cannot get an instance object by virtual reference .

Virtual references are mainly used to track the garbage collection activities of objects , Reclaim resources associated with objects .

stay Java 8 And in previous versions , When the garbage collector is ready to recycle an instance object , If it's found to have virtual references , Before recycling the instance object , Add this virtual reference to the reference queue associated with it . If the program finds that a virtual reference has been added to the reference queue , Then you can take the necessary action before the memory of the referenced object is recycled , Then disconnect the reference of the virtual reference object , Virtual references are recycled or unreachable , The object pointed to by the virtual reference will be recycled .

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Object counter = new Object();
ReferenceQueue refQueue = new ReferenceQueue<>();
PhantomReference<Object> p = new PhantomReference<>(counter, refQueue);
counter = null;
System.gc();
try {
    // Remove  It's a blocking method , You can specify  timeout, Or choose to block all the time 
    Reference<Object> ref = refQueue.remove(1000L);
    if (ref != null) {
        // do something
        
        ref = null;
    }
} catch (InterruptedException e) {
    // Handle it
}

     

      
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Reference material

  The first 4 speak | Strong citation 、 Soft citation 、 Weak reference 、 What's the difference between phantom quotes ？<br />

 ThreadLocal Principle analysis and memory leak demonstration