Understanding Java’s Garbage Collection

9 min readNov 2, 2023

Introduction

Garbage collection (GC) is a form of automatic memory management. In languages without garbage collection, programmers are responsible for manually allocating and deallocating memory. This can lead to various issues like memory leaks, where memory is allocated but never freed, or dangling pointers, where memory is freed while still being referenced. The garbage collector in Java automates this process by identifying and reclaiming memory that is no longer in use, thereby ensuring efficient memory utilization.

Introduction to Garbage Collection

Garbage collection (GC) is an integral component of many modern programming languages, serving as an automated memory management system. It’s a feature that allows developers to focus on their application’s logic without being burdened by the meticulous task of handling memory allocation and deallocation.

The need for garbage collection arises from the challenges posed by manual memory management. In traditional systems where developers were tasked with both allocating and deallocating memory, it was quite common to encounter problems:

Memory Leaks: This happens when a programmer allocates memory but forgets to free it. Over time, these unfreed chunks of memory accumulate, leading to a gradual reduction in the available memory, which could eventually crash the application or system.
Dangling Pointers: Sometimes, memory that’s still in use or will be used later gets deallocated prematurely. Accessing such memory spaces can result in undefined behaviors, including application crashes or unpredictable results.
Double Freeing: It’s an issue where a programmer tries to deallocate an already freed space. This can corrupt the memory and lead to erratic behavior.

With the challenges of manual memory management evident, the concept of automated garbage collection was introduced. In this system, programmers are only responsible for allocating memory. The responsibility of determining when and what memory to free is entrusted to the garbage collector. This decision is typically based on reachability. If a piece of memory (or an object in languages like Java) is no longer accessible or referenced by any part of the application, the garbage collector identifies it as “garbage” and frees up that memory.

Java’s introduction to the world of programming brought with it a robust garbage collection system. In Java’s environment, the Java Virtual Machine (JVM) oversees the process of garbage collection. As developers code and create objects, the JVM keeps an eye on the memory landscape. When it detects that certain objects are no longer in use, it activates the garbage collection mechanism to reclaim the memory, ensuring optimal utilization of resources.

Understanding the role and significance of garbage collection, especially in a language as widespread as Java, is essential for every developer. Not only does it free developers from the intricacies of memory management, but it also ensures more stable and efficient applications.

How Garbage Collection Works in Java

In Java, garbage collection is orchestrated by the Java Virtual Machine (JVM). The process ensures that memory is efficiently used and reclaimed when no longer necessary. To understand how garbage collection works in Java, it’s essential to first understand the structure of Java’s memory model, primarily the heap.

Java Memory Structure

Java’s memory can be broadly categorized into two areas: the stack and the heap. While the stack is responsible for primitive data types and method calls, the heap is where objects are stored.

Heap Structure: The heap is further divided into:
Young Generation: This is where new objects are initially created. It’s split into three parts: Eden, and two survivor spaces (S0 and S1).
Old Generation (Tenured): Objects that have survived several garbage collection cycles in the Young Generation are moved here.
Permanent Generation (or Metaspace in newer JVMs): This is where the JVM stores metadata related to the classes and methods.

Now, let’s dive into the mechanics of the garbage collection process.

Object Creation

When an object is created, it’s initially placed in the Eden space of the Young Generation. As the Eden space fills up, a minor garbage collection event (often just referred to as “minor GC”) is triggered.

Minor Garbage Collection

This process involves cleaning up the Eden space by removing objects that are no longer in use and moving the surviving objects to one of the survivor spaces (S0 or S1). With each subsequent minor GC event, objects switch between survivor spaces, and those that survive several such cycles are moved to the Old Generation.

Major Garbage Collection

Also referred to as “full GC”, this process deals with the Old Generation. It’s a more intensive process as it reclaims memory from objects that have lived longer and is generally slower than minor GC. The goal is to identify long-lived objects that are no longer in use and reclaim their memory.

Reachability and Mark-Sweep

The fundamental concept behind identifying objects for garbage collection is reachability. An object is considered “reachable” if it can still be accessed directly or indirectly from any active thread. During the “mark” phase, the garbage collector traverses through the object graph, starting from root objects (like active threads or static fields) and marks all the reachable objects. During the “sweep” phase, it goes through the heap and clears out all the objects that weren’t marked, freeing up memory.

Compaction

Over time, as objects are allocated and deallocated, the heap can become fragmented. Fragmentation can slow down object allocation as the JVM might struggle to find large contiguous memory spaces for new objects. Compaction is the process of rearranging the memory, moving the objects closer together to ensure a compacted heap, making object allocation faster and more efficient.

Java’s garbage collection mechanism is an intricate dance of memory management, ensuring that applications run efficiently without memory wastage. By handling memory deallocation and ensuring optimal utilization of the heap, the JVM provides developers with the peace of mind to focus on their application logic rather than the nuances of memory management. However, understanding the underlying principles of garbage collection helps in writing performance-optimized Java applications and aids in diagnosing and resolving memory-related issues.

Types of Java Garbage Collectors

Java’s garbage collection capabilities have evolved over the years, resulting in multiple garbage collectors tailored to various application needs and workloads. Each garbage collector has its unique strengths, catering to specific scenarios. Here’s an in-depth look at the most widely used garbage collectors in the Java ecosystem:

Serial Garbage Collector

Mechanism: The Serial Garbage Collector uses a single-threaded approach for both minor and major garbage collection events.
Best Used: Due to its single-threaded nature, it’s most suitable for single-threaded applications or applications with small heaps. It’s often the default choice for client-style applications running on Java Standard Edition (Java SE).
Pros and Cons: While it’s resource-efficient and introduces minimal overhead, it can introduce noticeable pauses, especially in multi-threaded applications or those requiring low-latency.

Parallel (Throughput) Garbage Collector

Mechanism: Also known as the Throughput Collector, it employs multiple threads for young generation garbage collection. This parallelism significantly speeds up the garbage collection process compared to the Serial Garbage Collector.
Best Used: It’s an excellent choice for medium to large-sized heap applications running in a multi-threaded environment, like many server applications.
Pros and Cons: While it offers a considerable boost in throughput by leveraging parallelism, it can still introduce significant pauses during full garbage collection events in the Old Generation.

CMS (Concurrent Mark-Sweep) Collector

Mechanism: The CMS Collector aims to minimize application pause times. It operates by concurrently marking reachable objects while the application is running. The “sweep” phase, which cleans up the unreachable objects, is also done concurrently, reducing pause times.
Best Used: This collector is well-suited for applications where low-latency is a priority over maximum throughput, such as interactive applications.
Pros and Cons: Although it achieves reduced pause times, it can introduce overhead due to concurrent operations. Additionally, the CMS collector can face fragmentation issues, which might necessitate occasional full garbage collections.

G1 (Garbage-First) Garbage Collector

Mechanism: The G1 Collector is a more modern approach to garbage collection in Java. It divides the heap into regions and, as the name suggests, prioritizes the collection of regions with the most garbage, hence “Garbage-First”. The G1 Collector aims to provide high throughput and predictable response times.
Best Used: G1 is designed with large heap applications in mind, where both throughput and low-latency are essential. It’s particularly beneficial for server applications running on multi-core processors.
Pros and Cons: G1 offers more predictable pause times compared to CMS and manages memory fragmentation efficiently. However, it might require more fine-tuning in terms of JVM flags and configurations to achieve optimal performance.

ZGC (Z Garbage Collector) and Shenandoah

Mechanism: Both ZGC and Shenandoah are newer garbage collectors introduced in recent Java versions. They aim to provide low-latency with pause times not exceeding a few milliseconds, regardless of the heap size.
Best Used: Ideal for applications where ultra-low latency is crucial, such as real-time trading systems or augmented reality applications.
Pros and Cons: They achieve impressively low pause times even with large heaps. However, being relatively new, they might not be as extensively tested in diverse production environments as older garbage collectors.

Choosing the right garbage collector for a Java application often depends on the specific requirements and characteristics of the application. Whether it’s throughput, low-latency, or a balance between the two, Java offers a range of garbage collectors to cater to varying needs. Understanding the nuances of each can help developers and system administrators make informed decisions, ensuring smooth application performance.

Best Practices for Java Memory Management

Effective memory management is critical for ensuring optimal performance, stability, and scalability of Java applications. By following best practices, developers can avoid common pitfalls and memory-related issues that can plague applications. Here are some recommended best practices for memory management in Java:

Object Lifecycle Awareness

Understand the lifecycle of objects in your application. Create objects only when necessary and allow them to be garbage collected as soon as they’re no longer needed.
Using local variables (with limited scope) can be advantageous as they get collected soon after they go out of scope.

Optimize Data Structures

Use appropriate data structures for the task at hand. For instance, an ArrayList might be inefficient for frequent insert/delete operations compared to a LinkedList.
Be cautious with static collections (like static List or Map) as they can lead to memory leaks if not handled correctly.

Use Soft, Weak, and Phantom References

Java provides special reference types (SoftReference, WeakReference, and PhantomReference) that allow developers more control over object retention.
Objects referenced by a WeakReference are cleared by the garbage collector as soon as they are not strongly referenced. SoftReference is similar but can keep the object longer (useful for caches). PhantomReference can be beneficial for scheduling pre-finalization actions.

Close Resources Promptly

Always close resources like database connections, I/O streams, and sockets once they are no longer required. Using try-with-resources (introduced in Java 7) can help automatically manage these resources.

Monitor and Profile Regularly

Use tools like Java VisualVM, JConsole, or third-party solutions to monitor the heap usage, garbage collection cycles, and memory leaks.
Regularly profiling your application can help identify memory hotspots and potential leak sources.

Fine-Tune Garbage Collection

Understand the garbage collectors provided by the JVM and choose the one most appropriate for your application’s requirements.
Tune JVM flags to optimize garbage collection behavior. For instance, setting the initial (-Xms) and maximum (-Xmx) heap sizes can aid in better memory utilization.

Handle OutOfMemoryError Gracefully

It’s good practice to catch OutOfMemoryError in your application, even if the only action is to log and gracefully shut down. This ensures data integrity and aids in debugging.

Pooling of Objects

For objects that are expensive to create or have limited availability (like database connections), use pooling techniques. Libraries like Apache Commons Pool can help in managing object pools.

Reduce Object Immutability

Immutable objects cannot be changed once created. This quality makes them thread-safe and can reduce overhead, especially in concurrent applications. Commonly used immutable classes in Java include String, BigInteger, and BigDecimal.

Avoid Finalizers

Finalizers (finalize() method) can introduce unpredictability in garbage collection. Their execution is not guaranteed, and they can cause unnecessary delays in object reclamation. Prefer alternatives like AutoCloseable and the try-with-resources statement.

Effective memory management in Java is a blend of understanding the language’s features, regular monitoring, and adhering to best practices. While the JVM offers robust garbage collection, developers have a pivotal role in ensuring memory is used efficiently. By adhering to the above practices and continuously educating oneself about Java’s evolving memory management landscape, developers can build resilient and high-performing applications.

Conclusion

Java’s garbage collection mechanism is a powerful tool that ensures efficient memory management and safeguards against common memory-related issues. While the JVM does much of the heavy lifting, understanding the basics of how garbage collection works and the different types of collectors available can help developers write more efficient code and choose the best collector for their specific application needs. As with any system, regular monitoring and profiling are essential to ensure optimal performance.