Java's garbage collection (GC) is a mechanism by which the JVM automatically manages memory. Its core role is to identify and clear objects that are no longer in use to free memory. GC judges whether an object is useless through accessibility analysis. Common GC Roots include local variables in threads, static properties of classes, constant references and JNI references. JVM provides a variety of collectors, such as Serial GC, Parallel GC, CMS GC, G1 GC, ZGC and Shenandoah GC, which are suitable for different scenarios. The garbage collection process usually includes three stages: marking, cleaning and sorting. Some collectors such as G1 divide the heap into multiple areas for flexible recycling. Developers can optimize GC performance by avoiding frequent creation of temporary objects, reasonably setting heap size, selecting the right collector, and using monitoring tools. In addition, you need to pay attention to avoid memory leakage and prevent OutOfMemoryError. Understanding GC principles can help write more efficient and stable Java programs.

Java's Garbage Collection (GC) is a mechanism for Java virtual machines (JVMs) to automatically manage memory. Its main function is to identify and clear objects that are no longer used in the program, thereby freeing up memory space and avoiding problems caused by memory leaks and manual memory management.

The core idea of ??GC is to automatically track which objects are being used and which are no longer needed, and recycle the memory they occupy at the right time. Developers do not need to manually call free() methods to free memory, but the JVM automatically completes this process.

The basic principles of Java garbage collection

To understand Java's garbage collection mechanism, you must first know how the JVM determines whether an object is "useless".

The most common way of judgment is Reachability Analysis : Starting from a series of root nodes called "GC Roots", searching downward along the reference chain. If an object cannot reach GC Roots through any reference chain, it is considered a recyclable object.

Common GC Roots include:

• Local variables in running threads
• Static properties of a class
• Constant references
• References in JNI (local method)

Once it is determined which objects are unreachable, GC will reclaim the memory space where these objects are located at appropriate times.

Common garbage collectors in Java

JVM provides a variety of garbage collectors, each suitable for different scenarios and performance requirements. Here are some common collectors:

• Serial GC : Single-threaded collector, suitable for single-core CPUs and small applications.
• Parallel GC (also known as Throughput GC) : a multi-threaded collector that focuses on throughput and is suitable for background computing applications.
• CMS (Concurrent Mark Sweep) GC : aims at the shortest pause time, suitable for applications that are sensitive to response time.
• G1 (Garbage-First) GC : Modern mainstream collector, can predict pause times, suitable for large piles of memory.
• ZGC and Shenandoah GC : A new generation of low-latency collectors that support TB-level heap memory while maintaining millisecond pauses.

The default GCs used by different versions of JVMs are also different, such as JDK 9 and later use G1 by default.

The garbage collection process is roughly divided into several stages

Although the specific implementations of different collectors vary slightly, most garbage collection processes include the following basic stages:

1. Mark : Find all surviving objects.
2. Clear (Sweep) : Recycle the space occupied by unmarked objects.
3. Compact (Optional) : Moves the surviving object to one end of the memory to free up continuous space and reduce fragmentation.

Some collectors (such as G1) divide the heap memory into multiple small regions and recycle them separately, which allows more flexible control of the recycling time and range.

How to affect the performance of garbage collection?

Although garbage collection is automatic, the developer's writing and configuration will affect its efficiency. Here are some suggestions:

• Avoid frequent creation of temporary objects, especially in methods called within loops or in high-frequency methods.
• Reasonably set the heap size (-Xms and -Xmx). Too large may increase the GC time, and too small may lead to frequent GC.
• Choose the right garbage collector based on the application type, such as web applications that are more suitable for low latency CMS or G1.
• Use tools to monitor GC behavior, such as VisualVM, JConsole, jstat, etc., to observe GC frequency and time-consuming.

In addition, be careful to avoid "memory leak", such as cache not being properly cleaned, listener not logged out, etc. These will cause the object to be recycled and eventually raise OutOfMemoryError.

Basically that's it. Garbage collection is the core of Java memory management. Understanding its basic principles and behaviors will help write more efficient and stable programs.

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