Deep understanding of JVM (IV) - garbage collection (I)

In depth understanding of JVM（ Four ）—— Garbage collection （ One ）

1、 How to judge whether an object can be recycled

Reference counting

disadvantages ： When quoting circularly , The count of both objects is 1, As a result, both objects cannot be released

Reachability analysis algorithm

• JVM The garbage collector in passes through Accessibility analysis To explore all living objects
• Scan objects in the heap , See if you can follow GC Root Object as the starting point of the reference chain to find the object , If Can't find , It means that... Can be recycled
• It can be used as GC Root The object of
  • Virtual machine stack （ Local variables in stack frames ） Object referenced in .
  • Object referenced by a class static property in a method area
  • The object referenced by a constant in the method area
  • Local method stack JNI（ In other words Native Method ） Referenced object

Five quotations

Strong citation

Only GC Root Don't quote When the object , To recycle Strong citation object

• Pictured above B、C Objects do not reference A1 Object time ,A1 The object will be recycled

Soft citation

When GC Root When pointing to a soft reference object , stay When there is not enough memory , Meeting Reclaim the objects referenced by soft references

• As shown in the figure above, if B Object no longer references A2 Object and out of memory , Soft reference refers to A2 The object will be recycled

Use of soft references

public class Demo1 {
	public static void main(String[] args) {
		final int _4M = 4*1024*1024;
		// Use soft reference objects  list and SoftReference Is a strong quote , and SoftReference and byte Arrays are soft references 
		List<SoftReference<byte[]>> list = new ArrayList<>();
		SoftReference<byte[]> ref= new SoftReference<>(new byte[_4M]);
	}
}

If insufficient memory is found during garbage collection , When reclaiming the object pointed to by the soft reference , Soft references themselves are not cleaned up

If you want to Clean up soft references , Need to make Use reference queue

public class Demo1 {
	public static void main(String[] args) {
		final int _4M = 4*1024*1024;
		// Use reference queue , Used to remove soft reference objects with empty references 
		ReferenceQueue<byte[]> queue = new ReferenceQueue<>();
		// Use soft reference objects  list and SoftReference Is a strong quote , and SoftReference and byte Arrays are soft references 
		List<SoftReference<byte[]>> list = new ArrayList<>();
		SoftReference<byte[]> ref= new SoftReference<>(new byte[_4M]);

		// Traversing the reference queue , If there are elements , The remove 
		Reference<? extends byte[]> poll = queue.poll();
		while(poll != null) {
			// The reference queue is not empty , Then remove the element from the collection 
			list.remove(poll);
			// Move to the next element in the reference queue 
			poll = queue.poll();
		}
	}
}

** The general idea is ：** Check whether there are soft references in the reference queue , If there is , Remove the soft reference from the collection where it is stored （ Here is a list aggregate ）

Weak reference

Only weak references refer to this object , During garbage collection , Whether the memory is sufficient or not , Will recycle the objects referenced by weak references

• As shown in the figure above, if B Object no longer references A3 object , be A3 Objects will be recycled

The use of weak references is similar to that of soft references , Just to SoftReference In exchange for WeakReference

Virtual reference

When the object referenced by the virtual reference object is recycled , The virtual reference object will be put into the reference queue , Call the method of virtual reference

• An embodiment of virtual reference is Free the memory allocated by direct memory , When the referenced object ByteBuffer After being recycled , Virtual reference object Cleaner Will be placed in the reference queue , And then call Cleaner Of clean Method to free direct memory
• Pictured above ,B Object no longer references ByteBuffer object ,ByteBuffer It will be recycled . However, the memory in direct memory has not been reclaimed . In this case, the virtual reference object Cleaner Put in the reference queue , And then call it. clean Method to free direct memory

Terminator references

All classes inherit from Object class ,Object Class has one. finalize Method . When an object is no longer referenced by other objects , The finalizer reference object will be put into the reference queue first , Then find the object it refers to according to the finalizer reference object , Then the object is called. finalize Method . After calling , The object can be garbage collected

• Pictured above ,B Object no longer references A4 object . This is the finalizer object, which will be put into the reference queue , The reference queue will be based on it , Find the object it references . And then invoke the referenced object finalize Method . After calling , The object can be garbage collected

Reference queue

• Soft quotes and weak quotes Can cooperate with Reference queue
  • stay Weak reference and Virtual reference After the referenced object is recycled , These references are placed in the reference queue , It is convenient to recycle these soft / Weak references
• Virtual references and finalizer references Must cooperate Reference queue
  • Virtual references and finalizer references are associated with a reference queue