What is Java Garbage Collection: Everything You Need to Know

Imagine building a high-performance Java application that runs smoothly at first but eventually slows down, crashes, or throws out-of-memory errors as it grows. Manually tracking and freeing up unused objects in a large, complex system is nearly impossible. This is where Java Garbage Collection (GC) comes in.

Java Garbage Collection is a process where the JVM automatically identifies and removes unused objects, freeing memory and ensuring efficient resource utilization. With the latest Java 23 update, performance improvements in Garbage Collection are more significant than ever.

In this blog, we’ll explore how garbage collection works, its benefits, and the different types of garbage collectors in Java. We’ll also cover what triggers GC, its impact on performance, and best practices our Java experts follow to optimize it. Let’s dive in!

What is Java Garbage Collection?

Java Garbage Collection (GC) is like a built-in cleanup crew for your Java programs. When you create objects in Java, they take up memory. But when those objects are no longer needed, they just sit there, wasting space. Manually removing them is going to take a lot of time and is error-prone. That’s where Java Garbage Collection steps in.

Garbage Collection is a process where the Java Virtual Machine (JVM) automatically finds and removes unused objects from memory. It keeps your application running smoothly by freeing up memory that’s no longer needed, so you don’t have to worry about it.

Benefits of Java Garbage Collection

There are many benefits of automatic garbage collection like:

• Automatic Memory Management: You don’t have to manually delete objects or worry about freeing up memory. The JVM handles it for you, saving time and effort.
• Prevents Memory Leaks: Without garbage collection, unused objects can pile up and consume memory, causing your application to slow down or crash. GC ensures these objects are removed, preventing memory leaks.
• Improves Application Performance: By efficiently managing memory, garbage collection helps your application run faster and more reliably.
• Reduces Developer Stress: Developers can focus on writing code instead of managing memory, making the development process smoother and less error-prone.
• Supports Large-Scale Applications: Garbage collection is especially useful for large, complex applications where manual memory management would be impractical.

In short, Java Garbage Collection makes memory management effortless, efficient, and reliable, allowing you to build better applications with less hassle.

Important Concepts in Java Garbage Collection

You need to know some concepts in order to understand how garbage collection works in Java:

• Object Reference: An object reference is like a pointer that tells the program where to find an object in memory. If no part of the program is using that reference anymore, the object is considered “unused” and can be garbage collected.
• Reachability: If an object can be accessed by any part of the program (through a reference), it’s called “reachable.” Unreachable objects, which can no longer be accessed, are eligible for garbage collection.
• Finalization: Before an object is removed from memory, it can run some cleanup code through a method called finalize(). This is like a last chance to clean up resources before the object is deleted. However, it’s rarely used in modern Java.
• Heap: The heap is where objects are stored in memory. Garbage collection mainly works on cleaning up the heap by removing objects that are no longer needed.
• GC Roots: GC roots are objects that are always accessible by the program, like the main thread or active variables. Anything connected directly or indirectly to GC roots is considered reachable and won’t be collected.
• Generational Garbage Collection: Java divides objects into different generations (Young, Old, Permanent) based on how long they live. Objects that live longer are moved to the Old Generation, and this helps the garbage collector manage memory more efficiently.

Understanding these concepts helps you get a better idea of how garbage collection keeps your program’s memory clean and running smoothly.

Java Garbage Collection Types and Strategies

Java Garbage Collection isn’t a one-size-fits-all process. Depending on your application’s needs, the JVM uses different types of garbage collectors and strategies to manage memory efficiently. Let’s break it down in simple terms:

1. Types of Garbage Collectors

Java has different types of garbage collectors, each designed to handle memory cleaning in a way that’s best for certain situations:

• Serial Garbage Collector: This one works by doing all the garbage collection tasks one at a time. It’s simple and best for small applications or those running on computers with limited resources.
• Parallel Garbage Collector: This collector does things faster than the Serial Garbage Collector by using multiple threads to clean up memory at the same time. It’s better for applications that need to process a lot of data quickly.
• G1 Garbage Collector (Garbage-First): G1 aims to balance speed and efficiency. It tries to clean up memory without causing long pauses, making it ideal for applications that need to run smoothly with minimal interruptions.
• Z Garbage Collector (ZGC): ZGC is designed for applications that handle a huge amount of memory. It works by keeping the pauses for garbage collection very short, so your application can keep running without major delays.
• Shenandoah Garbage Collector: Shenandoah also focuses on minimizing pauses. Like ZGC, it’s for large applications where even small delays can cause problems. It helps keep your app running without noticeable interruptions.

Each garbage collector has its own strengths and is used depending on what your application needs

2. Generational Garbage Collection Strategy

Java uses a strategy called Generational Garbage Collection, which divides memory into different parts to make cleaning up faster. The idea is that most objects don’t live long, so we separate them based on their age. There are three main parts:

• Young Generation: This is where new objects are created. Most objects in this area don’t live for long, so garbage collection happens more often here to quickly clean up the ones that are no longer needed.
• Old Generation: Objects that live longer are moved here. These objects stay around longer, so garbage collection happens less often in this area. But when it does happen, it can take more time because there are usually more objects to check.
• Permanent Generation: This area holds things like classes and methods that are used by the application. These objects are needed for the whole life of the program, so garbage collection doesn’t really affect this area.

By organizing memory this way, Java can clean up objects more efficiently, focusing on the ones that don’t stick around for long.

3. Other Strategies

In addition to the basic garbage collection methods, there are some other strategies Java uses to make memory management even better:

• Concurrent Mark-Sweep (CMS): CMS is designed to reduce the time your application pauses for garbage collection. It works by marking which objects are still needed while the application is still running, instead of stopping everything to do the cleanup. This helps to keep things running smoothly, especially in applications that can’t afford long pauses.
• Incremental Garbage Collection: This method breaks the garbage collection process into smaller steps. Instead of doing everything at once, it does little bits of work while the application is running, which helps avoid long pauses. This can be helpful in situations where you need to keep the application responsive but still need to clean up memory regularly.

These strategies are used to improve performance and make sure that memory is cleaned up in a way that causes as little disruption to the application as possible.

Simply put, by understanding these types and strategies, you can optimize your application’s performance and memory usage without getting bogged down in manual memory management.

What Triggers Java Garbage Collection?

Garbage collection happens automatically in Java, but certain things can trigger it to start. Here’s what can cause it to happen:

• Allocation Failure: Garbage collection happens when there’s not enough memory to create a new object. If the JVM can’t allocate space for a new object, it will trigger garbage collection to free up memory.
• Heap Size: If the memory (called the heap) gets too full, garbage collection is triggered to clean up space and make room for more objects.
• System.gc(): You can ask for garbage collection to happen by calling System.gc(). However, this is just a request, and the JVM may or may not do it right away.
• Time-Based: In some cases, the JVM may trigger garbage collection based on certain time intervals to keep the memory in check, even if there’s no immediate need for it.

In short, while garbage collection runs automatically, understanding what triggers it can help you better manage memory and ensure your application runs smoothly without unnecessary interruptions.

How Does Java Garbage Collection Work?

Java garbage collection works automatically to clean up memory by removing objects that are no longer needed. Here’s how it works, step by step:

• Marking: The garbage collector first checks all the objects in memory and marks the ones that are still being used by the program. If an object is no longer needed (because nothing in the program is using it), it’s marked for removal.
• Sweeping: Once the objects are marked, the garbage collector removes all the objects that are not in use and frees up the memory. This process is called “sweeping.”
• Compacting: After the garbage collector has removed the unused objects, it sometimes moves the remaining objects around to make sure there’s enough space in memory. This helps avoid memory gaps and makes the program run more efficiently.
• Reclaiming Space: The space taken up by unused objects is now free and can be used by new objects the program needs to create.

Java’s garbage collector does all this work automatically, so you don’t have to worry about cleaning up memory yourself. It helps keep your program running smoothly without taking up too much space.

Ways to Request JVM to Run Garbage Collector

Normally, the Java Virtual Machine (JVM) runs garbage collection automatically. However, there are a few ways you can ask it to run manually if needed:

• Using System.gc(): You can call System.gc() in your code to suggest that the JVM run garbage collection. This is just a request, and the JVM may or may not do it right away. It’s not guaranteed.
• Using Runtime.getRuntime().gc(): This is another way to request garbage collection, similar to System.gc(). It tells the JVM to start garbage collection, but again, it’s only a suggestion, not a command.
• JVM Options: You can also use certain JVM options when starting your Java program to influence how and when garbage collection happens. For example, you can set a maximum memory limit, which can trigger garbage collection when the memory is close to being full.

While you can request garbage collection, it’s usually better to let the JVM handle it automatically since it knows the best time to clean up memory for your program.

Why Is It Not Recommended to Request Garbage Collection?

While you can request garbage collection using methods like System.gc(), it’s generally not recommended to do so. Here’s why:

• Performance Issues: Manually triggering garbage collection can slow down your program, as it forces the JVM to pause and clean memory.
• JVM Knows Best: The JVM is good at deciding when to run garbage collection. Requesting it yourself might interrupt its optimized process and harm performance.
• Unpredictable Results: Forcing garbage collection can cause unpredictable behavior and inefficient memory use.
• Wasting Resources: Running garbage collection unnecessarily uses up system resources, slowing down your program.

In short, you can request for Java garbage collection but it is not recommended to do so as it may cause performance issues, give unpredictable results and waste resources.

How to Ensure a Java Object is Removed During Garbage Collection?

You can’t directly force garbage collection on a specific object, but you can make the object inaccessible to the rest of the program, which signals to the garbage collector that it can be removed. Here’s how to do that:

• Create Objects Inside a Method: If you create an object inside a method, once the method finishes executing, the object becomes unreachable, and it can be collected by the garbage collector.
• Nullify the Reference Variable: You can set the reference variable of an object to null. When there are no active references to the object, it becomes unreachable and eligible for garbage collection.
• Reassign the Reference Variable: Instead of setting a reference variable to null, you can reassign it to another object. This also removes the link to the original object, making it eligible for garbage collection.
• Create Anonymous Objects: Anonymous objects, which don’t have a reference variable, can be collected as soon as they are no longer in use because there’s no reference pointing to them.

By making objects inaccessible in these ways, you help the garbage collector know which objects can be safely removed.

Java Garbage Collection Best Practices

To make Java garbage collection work efficiently and keep your program running smoothly, you should follow some best practices. They are:

• Choose the Right Garbage Collector: Different garbage collectors are better for different types of applications. For example, use the G1 Garbage Collector for large applications that need low pause times, or the Serial Garbage Collector for simpler, smaller applications with limited memory.
• Monitor and Analyze Garbage Collection Logs: Regularly check garbage collection logs to understand how it’s performing. Monitoring helps you spot potential problems like frequent or long pauses, allowing you to make improvements. Java Profiling Tools like VisualVM and JProfiler can help you analyze GC behavior and identify performance bottlenecks.
• Optimize Heap Size: Set the heap size based on your application’s needs. Too much or too little memory can affect garbage collection performance. Finding the right heap size can help minimize unnecessary garbage collection pauses.
• Tune Garbage Collection Parameters: Adjust JVM parameters for garbage collection to fit your application’s requirements. Fine-tuning these settings can help balance memory usage and performance, ensuring efficient garbage collection.
• Minimize Object Creation: Create objects only when necessary. Excessive object creation, especially in loops, can lead to frequent garbage collection cycles, slowing down your program. Reuse objects where possible to reduce the load on the garbage collector.
• Use Parallelism and Concurrency: Take advantage of parallel garbage collection or concurrency when needed. Using multiple threads to handle garbage collection tasks can speed up the process and reduce the impact on your program’s performance.

To sum up, choosing the right garbage collector along with memory analysis and heap size optimization will make garbage collection work efficiently and will help your program run smoothly.

FAQs on Java Garbage Collection

Let’s Conclude

Java Garbage Collection is an essential part of memory management, automatically handling the cleanup of unused objects to keep your applications running smoothly. It eliminates the need for manual memory management, preventing issues like memory leaks and improving performance. 

By understanding the different types of garbage collectors, strategies, and best practices, you can optimize memory usage in your Java applications. Following best practices like choosing the right garbage collector and monitoring GC logs will help your application run efficiently and scale better.

If you need help in building high-performance, scalable, and memory-efficient Java projects, our Java development company is here to assist you.