JVM notes

How to determine which objects need to be recycled ？
Root reachability algorithm , If the root unreachable object is not referenced by any method, then the useless object can be recycled
What are the garbage collection algorithms ？

1. Mark removal will clear the garbage objects , But it will lead to discontinuous memory space .
2. Tag compression not only removes garbage objects, but also compresses useful objects into one piece , Free up contiguous memory space , So it is better than mark removal .
3. Copy is to copy useful objects to a new area after clearing , such as JVM
   youngGC All the new generation of useful objects will be copied to an empty survive Area and then eden Area and another survivor All objects in the zone are cleared ,survivor0 and survivor1 Two zones rotate, and one zone is always empty , Every time it happens youngGC Will copy all the surviving objects in one area to another survive District , It is also to solve the problem of memory fragmentation .
   

What are the garbage collectors ？
Serial Serial garbage collector , Single thread garbage collection ,GC Time meeting STW, All business threads stop, etc GC Only when it's over , Small memory is OK , Large memory garbage collection takes a long time
CMS concurrent mark sweep Concurrent tag scan , In order to solve STW And was born , You can recycle and execute business threads at the same time
G1

Tri-color Marking Algorithm
Objects found by both yourself and the objects you reference are marked black
Objects found by themselves but not found by the objects they reference are marked as gray
Objects that are not found by themselves are marked white