Java garbage collection (GC) is a mechanism by which JVM automatically manages memory, avoiding manual memory management by identifying and freeing objects that are no longer in use. 1. GC uses accessibility analysis to judge the survival of objects, with the heap as the main recycling area; 2. Java heap is divided into the Chronicle and the Elderly. The Chronicle includes the Eden area and the two Survivor areas. Most objects are allocated in the Eden area. Minor GC cleans up the area and copy the surviving objects to the Survivor area. Long-term surviving objects enter the old age; 3. Mainstream GC algorithms include mark-clearing, copying (for the Chronicle), mark-tissue (for the elderly); 4. Commonly used collectors include Serial GC, Parallel Scavenge, CMS, G1 and ZGC/Shenandoah, which are suitable for different scenarios respectively; 5. GC performance can be monitored through jstat, GC logs, and visualization tools, and tuned by adjusting the heap size, selecting a suitable collector, reducing temporary object creation, and controlling large objects to enter the old age. Mastering the GC mechanism can help solve problems such as memory leaks, frequent Full GC and long pauses, thereby improving the performance and stability of Java applications.

Java's Garbage Collection (GC) mechanism is the core of its automatic memory management. Many developers know that Java does not need to manually release memory, but they don’t have a deep understanding of how GC works, when it triggers, and how it affects program performance. If you encounter memory leaks, frequent Full GC or long pauses during development, it becomes very critical to understand the GC process.

What is Java garbage collection?

Simply put, Java garbage collection is the JVM automatically recognizes objects that are no longer used and frees up the memory space they occupy. This process mainly occurs on the heap. JVM uses Reachability Analysis to determine which objects are "living" and which are "dead". The so-called "dead" object is an object that is not referenced by any active thread or GC Roots.

Common GC Roots include:

• Objects referenced in the virtual machine stack
• Class static attribute references in the method area
• Constant references
• Object referenced by JNI (Native method) in the local method stack

Brief description of Java heap memory structure

Java heaps are usually divided into two main regions: Young Generation and Old Generation.
The Ceponymous generation is divided into Eden area and two Survivor areas (S0 and S1). Most newly created objects are assigned to the Eden area. When the Eden area is full, Minor GC will be triggered once. The surviving object is copied to one of the Survivor areas. Those who survive after multiple GCs will be promoted to the old age.

This generational design is designed to improve GC efficiency. Because most objects have a short life cycle, Minor GC frequency is high but fast; while the survival rate of elderly objects is high, GC frequency is low but takes a long time.

Common garbage collection algorithms and collectors

Currently there are three mainstream GC algorithms:

• Mark-Sweep : First mark all objects that need to be recycled, and then clean them up uniformly. The disadvantage is that it will generate memory fragmentation.
• Copying : Divide the memory into two pieces, using only one piece at a time. When GC, copy the surviving object to another block and clear the original area. Suitable for the new generation.
• Mark-Compact : combines the advantages of the first two. After marking, instead of clearing it directly, it moves the surviving object to one end and then cleans up memory outside the boundary. Suitable for the elderly.

Commonly used GC collectors for modern JVMs include:

• Serial GC: Single threaded, suitable for client mode
• Parallel Scavenge: multi-threading, focusing on throughput
• CMS (Concurrent Mark Sweep): Low latency, suitable for response time-sensitive applications
• G1 (Garbage First): Partition management, taking into account both throughput and latency
• ZGC/Shenandoah: A new generation of GC with ultra-low latency

How to monitor and tune GC performance?

If you find that the application has lag and the response is slow, you can check the GC status in the following ways:

• Use jstat -gc <pid></pid> command to view GC frequency and time consumption
• Add JVM parameters -XX: PrintGCDetails -XX: PrintGCDateStamps output GC log
• Monitor with visualization tools such as VisualVM, JConsole, or Prometheus Grafana

Tuning suggestions:

• Set the heap size reasonably to avoid frequent GCs due to too small. Too large may increase the time-consuming of Full GCs.
• Select the appropriate GC collector according to the application characteristics
• Avoid frequent creation of temporary objects and reduce Minor GC pressure
• Pay attention to the problem of large objects entering the old age directly, and adjust the threshold appropriately ( -XX:PretenureSizeThreshold )
• If you frequently have Full GC, it may be a memory leak. Use MAT and other tools to analyze the heap dump (heap dump)

Basically that's it. Although the GC mechanism is complex, as long as you master the basic principles and common problem detection methods, you can effectively improve the stability and performance of Java applications.

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