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This article introduces the relevant knowledge of "what are the references to java objects?". In the operation of actual cases, many people will encounter such a dilemma. Next, let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

The reference reference of java object is divided into several forms: strong reference, soft reference, weak reference and virtual reference.

1. Soft Reference (soft reference)

The objects referred to will not be cleared even if there is no Direct Reference, and will not be cleared until the jvm is out of memory and there is no Direct Reference.

Application scenarios: commonly used to design caches, not only can cache objects, but also will not easily cause insufficient memory.

2. Weak Reference (weak reference)

As soon as GC discovers a weak reference relationship, it begins to collect objects.

Application scenario: want to view some object information without affecting the collection of objects by GC.

Interaction with GC:

1. Set referent as a null reference

two。 Set the target object in the heap to "finalizable"

3. Finalize () is executed first to free up memory, and the soft reference is added to the RefernceQueue.

ReferenceQueue can be null in soft renference and weak reference, but it must be in virtual applications!

3. PhantomReference (virtual reference)

Interaction with GC:

1. Do not set referent as a null reference; instead, set the target object in the heap to "finalizable"

two。 The soft reference is added to the RefernceQueue first. Then collect objects and free memory

Note: ReferenceQueue can be null in soft renference and weak reference, but it must be in a virtual reference!

4. Abstract Reference analysis

There are four classes in the java.lang.ref package, and their relationship is that PhantomReference,WeakReference,SoftReference inherits the abstract class Reference. In the Reference class, there is a thread subclass (ReferenceHandler), which is opened in the static area to monitor the pending field in real time.

The pending is assigned by jvm, and when the reachable state of the referent object within the Reference changes, jvm puts the Reference object into the pending linked list.

Once relevent= null, garbage collection marks WeakReference's referent as unreachable get () returns null, and triggers the ReferenceHandler thread to process pending by assigning a value to pending.

What the ReferenceHandler thread needs to do is to enqueue the pending object, but by default the queue we provide, that is, what is passed in from the constructor is null. In fact, we use the ReferenceQueue.NULL,Handler thread to judge that queue is ReferenceQueue.NULL and do not operate. Only non-ReferenceQueue.NULL queue will enqueue the Reference.

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