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Understanding Garbage Collection in Java: A Beginner’s Guide

Understanding Garbage Collection in Java: A Beginner's Guide

Disclaimer: It does not depict the actual architecture or operation of a garbage collector within the Java Virtual Machine (JVM).


Have you ever wondered what happens to those objects in Java code that you no longer use? They don’t just disappear into thin air. This is where garbage collection comes in, acting as a silent housekeeper for your Java Virtual Machine (JVM). Let’s explore this essential process that keeps your applications running smoothly.

Java is known for its robust memory management capabilities, primarily attributed to its automatic garbage collection mechanism. For new and seasoned developers alike, understanding how garbage collection (GC) works can demystify many aspects of Java programming and improve application performance. Let’s dive into the world of garbage collection in Java.

What is Garbage Collection (GC)?

Garbage Collection (GC) in Java is an automatic memory management process. It works by identifying and reclaiming the memory occupied by objects that your program can no longer reach. Think of it as cleaning up your digital workspace before running out of room for new things.

How Garbage Collection Works

  • Young Generation: Newly created objects reside here. It is further divided into one Eden space and two Survivor spaces (S0 and S1). Most objects die young, so collecting garbage from this area is often efficient.
  • Old Generation: Objects that have survived several garbage collection cycles in the Young Generation are moved to the Old Generation. Collecting garbage from this area is more expensive, so it happens less frequently.
  • Permanent Generation/Metaspace (Java 8 and above): This area holds metadata such as class definitions. It’s not part of the heap in Java 8 and beyond, and its size is automatically adjusted by the JVM.

Garbage Collection Process

  • Minor GC: This process cleans up the Young Generation. Objects that are no longer reachable by any thread or by static references are considered garbage and are removed. Live objects are moved between the Eden space and the Survivor spaces or directly to the Old Generation if they survive long enough.
  • Major GC/Full GC: This involves cleaning the Old Generation and is more comprehensive and time-consuming than Minor GC. Full GC also includes the Young Generation, making it a complete cleanup of the heap.

Garbage Collection Algorithms

Various algorithms are used for garbage collection in Java, including:

  • Mark-Sweep: Identifies live objects and clears unreferenced objects, potentially leading to fragmentation.
  • Copy: Used in the Young Generation, it copies live objects to one of the Survivor spaces, clearing out the rest.
  • Mark-Compact: Similar to Mark-Sweep, but it also compacts the memory to solve fragmentation issues.
  • Generational Copying: Combines strategies to optimize garbage collection across generations.

Garbage Collection Invocation

  • Garbage collection can be triggered automatically by the JVM when it detects that memory is running low. Programmers can also suggest garbage collection by calling System.gc(), but it’s only a suggestion, and the JVM decides whether to act on it.

Java’s garbage collection mechanism aims to abstract away memory management tasks from programmers, allowing them to focus on developing application logic. However, understanding how it works is crucial for optimizing application performance, especially in memory-intensive applications.

Tuning Your GC

The JVM offers options to fine-tune the garbage collection process. The best settings depend on your application’s specific needs and memory behavior.

Frequently Asked Questions

Certainly! Below is a list of frequently asked questions (FAQs) about Garbage Collection (GC) in Java, along with concise answers:

1. What is garbage collection in Java?

Answer: Garbage collection in Java is an automatic memory management process that identifies and discards objects that are no longer needed by a program, freeing up memory resources and preventing memory leaks.

2. How does the garbage collector determine that an object is no longer needed?

Answer: The garbage collector uses algorithms to identify objects that are no longer reachable through any references from running threads or static references. These objects are considered eligible for garbage collection.

3. What are the main areas of the heap in Java memory management?

Answer: The Java heap is divided into the Young Generation (for newly created objects), the Old Generation (for objects that have existed for some time), and, in earlier versions of Java, the Permanent Generation (PermGen) or Metaspace (Java 8 and above) for class metadata and statics.

4. What is the difference between Minor GC and Major GC?

Answer: Minor GC clears unused objects from the Young Generation, which is faster and occurs more frequently. Major GC (or Full GC) involves cleaning the entire heap, including the Old Generation, and is more comprehensive and time-consuming.

5. Why is garbage collection in the Young Generation considered efficient?

Answer: It’s efficient because the Young Generation is smaller, and most objects die young. Cleaning this area uses less CPU time and has shorter pause times, improving application performance.

6. Can we force garbage collection in Java?

Answer: While you can suggest garbage collection by calling System.gc(), you cannot force it. The garbage collection process is managed by the JVM, which decides the optimal time to perform it.

7. How does garbage collection affect Java application performance?

Answer: Garbage collection can temporarily pause application threads, affecting performance. However, modern JVMs and garbage collection algorithms are designed to minimize disruption and optimize memory management.

8. What is a garbage collection algorithm, and which ones are commonly used in Java?

Answer: A garbage collection algorithm is a method used to identify and deallocate unused objects. Common algorithms include Mark-and-Sweep, Copying, and Generational Collection, each with its own strategy for managing memory efficiently.

9. How can developers minimize the impact of garbage collection on their applications?

Answer: Developers can optimize their code to reduce the creation of unnecessary objects, manage object lifecycles effectively, and tune JVM settings to adjust garbage collection behavior according to application needs.

10. Are there different types of garbage collectors in the JVM?

Answer: Yes, the JVM offers several garbage collectors, each designed for different types of applications and workloads. These include the Serial GC, Parallel GC, Concurrent Mark Sweep (CMS) GC, Garbage-First (G1) GC, and more recently, the Z Garbage Collector (ZGC) and Shenandoah GC, offering various trade-offs in terms of throughput and pause times.

11. How does the Java garbage collector determine which objects to collect?

Answer: The primary mechanism is reachability. Starting from “GC roots” (like objects in the current thread stack and static variables), the GC traces object references. Objects that can’t be reached through any reference chain are considered garbage.

12. What is generational garbage collection and how does it work?

Answer:  It’s a GC strategy based on the observation that most objects tend to be short-lived. The heap is divided into generations (e.g., Young, Old/Tenured). New objects are in the Young Generation. Objects surviving GC cycles get promoted to older generations. GC algorithms and frequency are often tailored to each generation.

13. What are the benefits of generational garbage collection?

Answer: By focusing GC efforts on the Young Generation where most objects die quickly, generational GC improves overall performance and can reduce the frequency of longer pauses caused by full GC runs.

12. What is generational garbage collection and how does it work?

Answer:  It’s a GC strategy based on the observation that most objects tend to be short-lived. The heap is divided into generations (e.g., Young, Old/Tenured). New objects are in the Young Generation. Objects surviving GC cycles get promoted to older generations. GC algorithms and frequency are often tailored to each generation.

13. What are some common garbage collection algorithms used in Java?


  • Serial Collector: Simple, good for smaller applications.
  • Parallel Collector: Uses multiple threads for throughput.
  • Concurrent Mark Sweep (CMS): Aims for shorter pauses.
  • G1 (Garbage First): Designed for low latency.

14. What are some common JVM options for tuning garbage collection?

Answer: Options vary across JVM implementations, but some common ones include:

  • Heap size settings (-Xms-Xmx)
  • Generation size settings
  • Choice of GC algorithm (e.g., -XX:+UseParallelGC)


About Author

As Neelabh Singh, I am a Senior Software Engineer with 6.6 years of experience, specializing in Java technologies, Microservices, AWS, Algorithms, and Data Structures. I am also a technology blogger and an active participant in several online coding communities.

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