How Garbage Collection works in Java

Garbage Collection (GC) in Java is an automated memory management process that aims to reclaim memory occupied by objects that are no longer in use. The main goal is to prevent memory leaks and relieve developers of the responsibility of manually managing memory allocation and deallocation.

Here are the main concepts and how Garbage Collection works in Java.

Object Generation

The JVM (Java Virtual Machine) organizes objects into different generations based on their characteristics and expected lifetime.

- Young Generation: This is where objects are initially allocated. Many objects have a short lifespan and are collected quickly.

- Old Generation or Tenured Generation: Objects that survive several collections in the young generation are promoted to the old generation.

Garbage Collection Algorithms

Different garbage collection algorithms are used to manage different generations. Some common algorithms include:

- Young Garbage Collection: Uses algorithms such as "Paralel Collector" or "G1 Garbage Collector".

- Old Garbage Collection: Uses algorithms such as "CMS Collector" (Concurrent Mark-Sweep) or "G1 Garbage Collector".

Garbage Collection Process

The Garbage Collection process involves the following steps:

- Mark: Identify objects that are being referenced and are considered alive.

- Sweep: Remove unmarked objects (considered dead) and free the memory associated with them.

- Compression (Optional): In some algorithms, there may be a compression step to reduce memory fragmentation.

Collection Strategies

There are different garbage collection strategies that can be configured in the JVM based on performance requirements and application characteristics.

- Stop-the-World Garbage Collection: At times, application execution is suspended to perform garbage collection. This is known as a "Stop-the-World" event.

- Concurrent Garbage Collection: Some algorithms, such as CMS, attempt to minimize application downtime by performing much of the garbage collection while the application is running.

Method Call "Finalize" 

Before an object is permanently removed, the "finalize" method can be called (if the object has implemented this method). This gives the object one last chance to perform cleanup actions before being deallocated.

Configuration and Monitoring

The JVM can be configured with parameters related to Garbage Collection, such as algorithm choice, generation size, collection frequency, among others. Monitoring tools such as VisualVM can be used to analyze Garbage Collection behavior and optimize its configuration.

Garbage Collection in Java is a fundamental feature that helps developers write safer and more efficient code by reducing workload related to memory management. However, understanding how Garbage Collection works and adjusting settings as necessary is important for optimizing the performance of Java applications.