Strong references to Java objects, soft references, weak references, and virtual references

1. Strong references

Most of the references we used before are actually strong references, which are the most common references. 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 references (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 determine whether a virtual reference has been added to the reference queue to understand

Whether the referenced object will be garbage collected. 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.

It is very important to note that weak references and virtual references are seldom used in program design, because soft references can speed up the JVM's recovery of garbage memory, maintain the security of the system, and prevent the generation of memory overflow (OutOfMemory) problems.

Strong references to Java objects, soft references, weak references, and virtual references