Strong, soft, weak and virtual references of Java objects +ReferenceQueue

Strong, soft, weak and virtual references of Java objects +ReferenceQueue

1. Strong reference (StrongReference)

Strong reference is the most commonly used reference. If an object has a strong reference, the garbage collector will never reclaim it. When there is insufficient memory space, the Java virtual machine would rather throw an OutOfMemoryError error and cause the program to terminate abnormally, rather than arbitrarily recycling objects with strong references to solve the problem of insufficient memory.

2. Soft Reference (SoftReference)

If an object only has a soft reference, the memory space is sufficient, and the garbage collector will not recycle it; if the memory space is insufficient, it will be recycled memory for these objects. As long as the garbage collector does not collect it, the object can be used by the program. Soft references can be used to implement memory-sensitive caching (an example is given below).

Soft references can be used in conjunction with a reference queue (ReferenceQueue). If the object referenced by the soft reference is recycled by the garbage collector, the Java virtual machine will add the soft reference to the associated object. in the reference queue.

3. Weak Reference (WeakReference)

The difference between weak reference and soft reference is that objects with only weak references have a shorter life cycle. During the process of the garbage collector thread scanning the memory area under its jurisdiction, once an object with only weak references is found, its memory will be reclaimed regardless of whether the current memory space is sufficient. However, because the garbage collector is a very low priority thread, it may not necessarily find objects with only weak references quickly.

Weak references can be used in conjunction with a reference queue (ReferenceQueue). If the object referenced by the weak reference is garbage collected, the Java virtual machine will add the weak reference to the reference queue associated with it. middle.

4. Phantom Reference

As the name suggests, "phantom reference" is in name only. Unlike other references, phantom reference does not determine the life cycle of the object. If an object holds only phantom references, then it may be reclaimed by the garbage collector at any time as if it had no references.

Virtual references are mainly used to track the activities of objects being recycled by the garbage collector. One difference between virtual references, soft references and weak references is that virtual references must be used in conjunction with a reference queue (ReferenceQueue). When the garbage collector is preparing to recycle an object, if it finds that it still has a virtual reference, it will add the virtual reference to the reference queue associated with it before recycling the object's memory.

1. ReferenceQueue queue = new ReferenceQueue ();

2. ##PhantomReference pr =

   new PhantomReference (object, queue);

The program can understand the reference by judging whether a virtual reference has been added to the reference queue. Whether the object will be garbage collected. If the program discovers that a virtual reference has been added to the reference queue, it can take necessary actions before the memory of the referenced object is reclaimed.

5. ReferenceQueue

It is a reference queue. If the Reference object itself is saved, if the object pointed to by the Reference reference is recycled by the GC, the Reference is actually invalid

This kind of Reference will be put into the reference queue and can be cleared here to avoid occupying space

6. WeakHashMap

Weak reference map: The Key key is a weak reference key. If If the Key key is recycled, after getting the value in the map, the value will be automatically removed

If the Key key is always strongly referenced, it cannot be recycled;

Note that the Value is Strongly referenced, so do not let Value indirectly reference the Key key, which will cause the key clock to be strongly referenced

This: suitable for caches controlled by the life cycle of the Key

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