Garbage collector in JVM

Serial collector: A new generation single-threaded collector that uses a copy algorithm for recycling. While it's garbage collecting, all threads from other users will be paused.

Serial Old Collector: The old generation version of Serial uses the mark-clear algorithm and is also a single-threaded collector.

ParNew collector: ParNew collector is actually a multi-threaded version of Serial. Except for multi-threaded garbage collection, the rest including collection algorithm (copy algorithm), object allocation rules, recycling strategy, etc. are exactly the same as Serial collector.

Parallel Scavenge collector: It is a new generation collector that uses a copy algorithm and is a parallel multi-threaded collector. This collector is designed to improve the throughput of the system (time to run user code/(running time) User code time + garbage collection time)), so the Parallel Scavenger collector is also called the "throughput first" collector.

Parallel Old Collector: It is the old generation version of Parallel Scavenge Collector, using the ‘mark-clear’ algorithm.

CMS Collector: Pays special attention to the response speed of user services, and hopes that the system pause time will be the shortest to provide users with a better experience. Concurrent collection, low pause. Instead, a ‘mark-and-sweep’ algorithm is used.

G1 collector: one of the most cutting-edge technologies in the development of current collector technology

Parallel and concurrent methods;

The generational collection used can manage the entire GC heap independently without cooperating with other collectors;

Spatial integration: On the whole, G1 uses a 'mark-organize' algorithm, but partially it is based on a 'copy' algorithm. But in any case, both algorithms mean that the G1 algorithm will not generate memory space fragmentation, and can provide regular available memory after collection.

Operation steps: initial marking; concurrent marking; final marking; screening and recycling.

Remarks:

Concurrency: The garbage collection thread can be executed at the same time as the user thread

Parallel: There can be multiple garbage collection threads, but the user thread at this time is still in a waiting state.