Four Java references

In jdk1.2, object reference is divided into four levels, so thatProgramIt can control the object lifecycle more flexibly. The four levels are from high to low: strong reference, soft reference, weak reference, and virtual reference.

1. Strong referenceThe references mentioned above in this chapter are actually strong references, which are the most common references. If an object has a strong reference, it is similar to an essential necessities. The Garbage Collector will never recycle it. When the memory space is insufficient, the Java Virtual Machine would rather throw an outofmemoryerror to terminate the program abnormally and does not recycle strongly referenced objects to solve the problem of insufficient memory.

2. Soft reference)

If an object only has soft references, it is similar to the necessities that can be used. If the memory space is sufficient, the garbage collector will not recycle it. If the memory space is insufficient, the memory of these objects will be recycled. The object can be used by programs as long as the garbage collector does not recycle it. Soft references can be used to implement memory-sensitive high-speed cache. Soft references can be used together with a referencequeue, the Java virtual machine adds this soft reference to the reference queue associated with it.

3. weakreference)If an object only has weak references, it is similar to a necessities that can be used. The difference between weak references and soft references is that only objects with weak references have a shorter life cycle. When the Garbage Collector thread scans the memory area under its jurisdiction, its memory will be recycled no matter whether the current memory space is sufficient or not. However, since the garbage collector is a thread with a low priority, it may not soon find objects with weak references. Weak references can be used together with a referencequeue, the Java virtual machine adds this weak reference to the reference queue associated with it.

4. phantomreference "Virtual Reference", as its name implies, is essentially a virtual object. It is different from several other types of references. Virtual Reference does not determine the object lifecycle. If an object only holds a virtual reference, it is the same as no reference and may be recycled at any time. Virtual references are mainly used to track the activity of objects being reclaimed by garbage collection. A difference between virtual and soft references and weak references is that virtual references must be used together with the reference Queue (referencequeue. When the garbage collector is preparing to recycle an object, if it finds that it has a virtual reference, it will add this virtual reference to the reference queue associated with it before it recycles the object's memory. The program can determine whether the referenced object has been added to the reference queue to check whether the referenced object is to be recycled. If the program finds that a virtual reference has been added to the reference queue, it can take necessary action before the memory of the referenced object is recycled.