Java strong references, soft references, weak references, virtual references

1. Strong references
Usually when we program, for example: Object Object=new object (); That object is a strong reference. If an object has a strong reference, it is similar to essential necessities, and the garbage collector will never recycle it. When there is not enough memory space, the Java virtual Machine prefers to throw a outofmemoryerror error, which causes the program to terminate abnormally, and does not rely on random recycling of strongly referenced objects to resolve out-of-memory issues.

2. Soft Reference (SoftReference)
If an object has only soft references, it is similar to a living thing that can be used. If the memory space is sufficient, the garbage collector does not reclaim it, and if the memory space is insufficient, the memory of those objects is reclaimed. The object can be used by the program as long as it is not reclaimed by the garbage collector. Soft references can be used to implement memory-sensitive caches. A soft reference can be used in conjunction with a reference queue (Referencequeue), and if the object referenced by the soft reference is garbage collected, the Java Virtual machine will add the soft reference to the reference queue associated with it.

3. Weak references (WeakReference)

If an object has only weak references, it is similar to a living thing that can be used. The difference between a weak reference and a soft reference is that an object with only a weak reference has a shorter life cycle. As the garbage collector thread scans the area of memory it governs, once an object with only a weak reference is found, its memory is reclaimed, regardless of whether the current memory space is sufficient or not.  However, because the garbage collector is a low-priority thread, it is not necessarily quick to discover objects that have only weak references. A weak reference can be used in conjunction with a reference queue (Referencequeue), and if the object referenced by the weak reference is garbage collected, the Java virtual machine adds the weak reference to the reference queue associated with it.

4. Virtual reference (phantomreference)

"Virtual Reference", as the name implies, is a dummy, unlike several other references, a virtual reference does not determine the object's life cycle. If an object holds only virtual references, it can be garbage collected at any time, just as there are no references. Virtual references are primarily used to track the activities of objects that are garbage collected. One difference between a virtual reference and a soft reference and a weak reference is that the virtual reference must be used in conjunction with the reference queue (Referencequeue). When the garbage collector prepares to reclaim an object, if it finds that it has a virtual reference, it will add the virtual reference to the reference queue associated with it before reclaiming the object's memory. The program can see if the referenced object is going to be garbage collected by judging whether the reference queue has been added to the virtual reference. If the program finds that a virtual reference has been added to the reference queue, it can take the necessary action before the memory of the referenced object is recycled.

Java strong references, soft references, weak references, virtual references