Types of Garbage Collectors in Java

Garbage collection (GC) is one of the most important features of Java that helps manage memory automatically. It frees up memory by identifying and reclaiming objects that are no longer in use, allowing the program to run efficiently without memory leaks. Java provides several types of garbage collectors (GCs), each with its own unique characteristics, use cases, and trade-offs. Understanding these garbage collectors is crucial for optimizing Java applications, especially when dealing with performance-sensitive or memory-intensive applications.

In this blog post, we will explore the different types of garbage collectors available in Java, their features, and when to use them.

What is Garbage Collection in Java?

Before diving into the types of garbage collectors, let’s understand the concept of garbage collection in Java.

Garbage collection is the process of automatically identifying and removing objects from memory that are no longer reachable or needed by the program. Java’s automatic garbage collector helps manage heap memory, which is where objects are stored, by reclaiming memory from unused or unreachable objects, reducing the chances of memory leaks and out-of-memory errors.

Types of Garbage Collectors in Java

Java provides several garbage collection algorithms, each suited for different scenarios. The garbage collectors are primarily designed to balance between throughput (how much work is done by the application) and latency (how much time the application spends on garbage collection).

Here are the most commonly used garbage collectors in Java:

1. Serial Garbage Collector (SerialGC)

• Full Name: Serial Garbage Collector
• Enabled By: -XX:+UseSerialGC
• Description: The Serial Garbage Collector is a simple and single-threaded collector that performs all garbage collection tasks using one thread. It is designed for environments with small memory requirements or where low memory usage is preferred.
• Use Case: This GC is ideal for single-threaded environments or systems with small heap sizes, such as embedded systems or applications with limited resources.
• Advantages:
  • Simple and easy to use.
  • Low memory overhead.
• Disadvantages:
  • It is slow in multi-threaded or multi-core environments since it uses a single thread.
  • Not suitable for applications with large heaps or those that require high throughput or low latency.

2. Parallel Garbage Collector (ParallelGC)

• Full Name: Parallel Garbage Collector (also known as Throughput Collector)
• Enabled By: -XX:+UseParallelGC
• Description: The Parallel Garbage Collector is a multi-threaded garbage collector that performs both minor and major garbage collections in parallel. It is designed to maximize throughput by utilizing multiple processor cores during garbage collection.
• Use Case: This GC is well-suited for server-class machines and applications with multi-core processors and large heaps where performance and throughput are critical, such as scientific computing or batch processing applications.
• Advantages:
  • Improves application throughput by utilizing multiple threads.
  • Suitable for applications with large memory requirements and multi-core CPUs.
• Disadvantages:
  • Can lead to higher pause times during major GC events.
  • Might not be suitable for latency-sensitive applications.

3. Concurrent Mark-Sweep Garbage Collector (CMS)

• Full Name: Concurrent Mark-Sweep Collector
• Enabled By: -XX:+UseConcMarkSweepGC
• Description: The Concurrent Mark-Sweep (CMS) garbage collector is designed for low-latency applications. It performs most of the garbage collection work concurrently with the application’s threads, thus minimizing pause times. It uses multiple threads to mark and sweep through the heap, identifying unreachable objects and reclaiming memory.
• Use Case: CMS is ideal for applications that require low pause times and high responsiveness, such as real-time systems or large-scale web applications with stringent latency requirements.
• Advantages:
  • Reduces pause times by performing most GC work concurrently.
  • Good for low-latency applications.
• Disadvantages:
  • Higher CPU usage due to concurrent threads.
  • Full GC (compaction) can still cause pauses.
  • CMS is no longer actively maintained, and Java developers are encouraged to switch to G1 GC.

4. G1 Garbage Collector (G1GC)

• Full Name: Garbage-First Collector (G1GC)
• Enabled By: -XX:+UseG1GC
• Description: The Garbage-First (G1) Collector is a low-pause, server-style garbage collector designed for applications with large heap sizes and multi-core processors. It aims to achieve high throughput while maintaining acceptable pause times. G1GC divides the heap into regions and performs garbage collection in parallel and concurrently, allowing it to achieve predictable and lower pause times.
• Use Case: G1GC is ideal for applications that require a balance between high throughput and low-latency, especially those with large heap sizes, such as large enterprise applications or cloud-based applications.
• Advantages:
  • Predictable and low pause times.
  • Suitable for applications with large heaps (greater than 2GB).
  • Can perform concurrent and parallel GC phases to minimize application pauses.
• Disadvantages:
  • More complex to tune compared to Parallel GC.
  • Can require significant memory overhead in certain situations.

5. Z Garbage Collector (ZGC)

• Full Name: Z Garbage Collector (ZGC)
• Enabled By: -XX:+UseZGC
• Description: The Z Garbage Collector (ZGC) is a relatively new low-latency garbage collector introduced in Java 11. It is designed to provide low-pause times (in the range of milliseconds) even with very large heap sizes (multi-terabyte heaps). ZGC performs all garbage collection operations concurrently with the application threads, ensuring minimal interruption to the application’s execution.
• Use Case: ZGC is ideal for latency-sensitive applications and large-scale data-intensive applications, such as big data platforms or real-time data processing systems, where even small pauses can have a significant impact.
• Advantages:
  • Extremely low pause times, even for very large heaps.
  • Excellent for real-time applications and large-scale systems.
• Disadvantages:
  • Requires JDK 11 or later.
  • Still a relatively new garbage collector, which may require additional configuration and testing.

6. Shenandoah Garbage Collector

• Full Name: Shenandoah Garbage Collector
• Enabled By: -XX:+UseShenandoahGC
• Description: Shenandoah GC is another low-latency garbage collector introduced in Java 12. It is designed to provide very short pause times and can handle large heap sizes efficiently. Shenandoah aims to perform garbage collection in parallel with application threads to minimize pauses, similar to ZGC.
• Use Case: Shenandoah is particularly beneficial for applications that require low pause times, such as high-performance computing and large enterprise applications.
• Advantages:
  • Low pause times even for large heaps.
  • Performs GC work concurrently with application threads.
• Disadvantages:
  • Available only in certain JVM versions (Java 12 and later).
  • May not be as mature or widely adopted as other GCs like G1.

Choosing the Right Garbage Collector

The choice of a garbage collector depends on the specific requirements of your application. Here are some guidelines:

• Use SerialGC if your application is simple, has a small heap size, and runs on a single-threaded environment.
• Use ParallelGC if you have a multi-core machine and want to maximize throughput without much concern for pause times.
• Use CMS if your application needs low-latency and high throughput but can tolerate some occasional long GC pauses.
• Use G1GC if you have a large heap and need to balance low pause times with high throughput.
• Use ZGC or Shenandoah if your application requires ultra-low pause times and can benefit from concurrent garbage collection, especially with large heaps.

Conclusion

Java offers a variety of garbage collectors to suit different application needs. From the simple and efficient SerialGC to the advanced and low-latency collectors like ZGC and Shenandoah, understanding each garbage collector’s characteristics and trade-offs will help you optimize your Java application’s performance. By carefully selecting the appropriate GC for your use case, you can ensure that your application runs smoothly, efficiently, and with minimal interruptions.

Happy coding, and may your Java applications run without memory concerns!