What are the differences between the four types of references in Java?

#What are the differences between the four types of references in Java?

There are four types of references in Java, they are: strong references, soft references, weak references, and virtual references. These four citation strengths gradually weaken in sequence. [Recommended learning: java course]

Strong Reference

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.

Soft Reference(SoftReference)

If an object only has a soft reference, then the memory space is enough, the garbage collector will not reclaim it; if there is insufficient memory space , the memory of these objects will be reclaimed. 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 caches.

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 reference queue associated with it. .

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, since 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.

Virtual Reference (PhantomReference)

"Phantom Reference", as the name suggests, is in name only. Unlike other references, a 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.

Java provides these four reference types for two main purposes:

• allows programmers to determine certain objects through code The life cycle;

• is conducive to JVM garbage collection.

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