JVM Heap

What Is Java Heap Memory?

Java heap memory is a vital component of the Java Virtual Machine (JVM) responsible for dynamically allocating and managing objects during program execution. It acts as a runtime data area where objects are stored and accessed by the Java application. Upon program launch, the JVM allocates a fixed amount of memory to the heap, which can be adjusted using command-line options.

Additional JVM Heap and Garbage Collection Reading

We have been working with the JVM, tuning the heap, garbage collection, and everything else that has a tuning knob in a JVM for many-many years and have shared a lot of our knowledge about the JVM heap and garbage collection tuning over the years. Here are additional JVM heap and garbage collection tuning how-tos and tutorials you may want to check out:

• JVM Tuning: How to Prepare Your Environment for Performance Tuning
• What Is Garbage Collection in Java & How It works: Tutorials & Best Practices
• A Step-by-Step Guide to Java Garbage Collection Tuning
• Understanding Memory Leaks in Java: Common Causes & How to Detect Them
• How to Handle Java Lang OutOfMemoryError Exceptions
• Key JVM Metrics to Monitor for Peak Java Application Performance
• Understanding Java Garbage Collection Logging: What Are GC Logs and How to Analyze Them
• Best Java GC Log Analyzers: Top Analysis Tools You Need to Know in 2023

Heap Space and Stack Memory in Java

In Java, memory is divided into two main regions: Heap space and Stack memory. Each region serves a specific purpose and has different characteristics, making them crucial for managing memory during program execution.

Heap Space:

• Purpose: Heap space is used for dynamic memory allocation and storage of objects. It is a shared memory area accessible to all threads in the Java application.
• Object Storage: Objects created at runtime, such as instances of classes, arrays, and collections, are stored in the heap. The JVM automatically manages memory allocation and deallocation for objects in this region.
• Lifetime: Objects in the heap have a more extended lifetime, and they persist until they are no longer referenced or explicitly garbage-collected by the JVM.
• Size: The heap size can be adjusted using command-line options, and its memory is typically larger than the stack memory.

Stack Memory:

• Purpose: Stack memory is used for managing method call frames and local variables. Each time a method is called, a new frame is pushed onto the stack, containing method-specific data like local variables, parameters, and return addresses.
• Primitive Data and References: The stack holds primitive data types and references to objects, not the actual objects themselves.
• Lifetime: The lifetime of data in the stack is short-lived and tied to the method’s execution. When the method completes, its frame is popped off the stack, and the associated data is automatically deallocated.
• Size: The stack size is typically much smaller than the heap and is usually fixed.

What Is Java Heap Memory Used For?

Some key purposes of Java Heap Memory are:

Object Allocation: All objects created during program execution, including instances of classes, arrays, and collections, are allocated memory in the heap.

Automatic Memory Management: Java uses automatic memory management through a process known as garbage collection. The JVM automatically identifies and deallocates objects that are no longer reachable, freeing up memory space for new object allocations.

Memory Efficiency: Heap memory allows for flexible memory allocation, enabling the JVM to manage memory efficiently and avoid manual memory management pitfalls like memory leaks.

Shared Memory: The heap is a shared memory area accessible to all threads in a Java application. It allows multiple threads to interact and share data through objects stored in the heap.

How Java Uses Heap Memory

The Java heap is divided into several generations, each serving different purposes to optimize memory management and garbage collection.

Young Generation

The Young Generation is the first part of the Java heap. It is further divided into two survivor spaces and an Eden space. When objects are created, they are initially allocated in the Eden space. As the Young Generation fills up, a minor garbage collection, known as a “Minor GC,” is triggered. During this process, the JVM identifies and clears short-lived objects (garbage) from the Young Generation, promoting surviving objects to the survivor spaces. Objects that survive multiple Minor GC cycles are eventually moved to the Old Generation.

Old Generation

The Old Generation, also known as the Tenured Generation, is the part of the heap that stores long-lived objects. Objects that persist for a longer time or have survived several Minor GC cycles get promoted to the Old Generation. Major garbage collections, called “Full GC,” are less frequent and occur in the Old Generation. The Full GC reclaims memory by identifying and collecting long-lived objects that are no longer in use.

Permanent Generation (Prior to Java 8)

In older versions of Java (prior to Java 8), the Permanent Generation was a separate region within the heap. It was used to store metadata related to classes, methods, and other JVM internals. This region held information like class definitions, method bytecode, and interned strings. However, the Permanent Generation was often a source of issues like OutOfMemoryErrors in applications with a large number of dynamically loaded classes. In Java 8 and later, the concept of the Permanent Generation was replaced by the “Metaspace,” which allows class metadata to be stored in native memory outside the heap, making it more flexible and manageable.

Java Heap Size

The Java Heap Size refers to the amount of memory allocated to the heap, where Java objects are dynamically created and stored during the execution of Java programs. The heap size is a critical parameter in Java applications, as it directly affects memory management and performance. When a Java program starts, the JVM allocates a specific initial heap size, and it can expand up to a maximum heap size as needed.

The heap size is specified using command-line options when launching a Java application. The two commonly used options are:

• -Xms: Sets the initial heap size.
• -Xmx: Sets the maximum heap size.

If the initial heap size is too small, the JVM may need to resize the heap frequently, resulting in performance overhead due to increased garbage collection frequency. On the other hand, if the maximum heap size is set too high, it might lead to inefficient memory usage, as the JVM reserves more memory than required.

What Is a Good Heap Size in Java?

Determining a good heap size in Java involves finding the right balance between allocating enough memory to avoid frequent garbage collections and not wasting memory by setting an excessively large heap. When setting up a heap size, there are different implications for choosing a smaller or larger heap size.

Smaller Heap Size

Advantages:

• Faster Startup: A smaller heap size requires less memory to allocate, leading to faster startup times for the Java application.
• Lower Overhead: With less memory to manage, garbage collection overhead is reduced, resulting in shorter pauses and improved application responsiveness.

Disadvantages:

• Increased Garbage Collections: Smaller heap sizes may lead to more frequent garbage collections as the available memory gets exhausted quickly. Frequent garbage collections can cause performance degradation and longer application pauses.
• OutOfMemoryError Risk: Setting a heap size too small might lead to an OutOfMemoryError if the application demands more memory than is available in the heap.

Larger Heap Size

Advantages:

• Reduced Garbage Collections: A larger heap size allows more objects to be retained, reducing the frequency of garbage collections. This results in better application performance and reduced CPU overhead.
• Improved Scalability: Applications dealing with large datasets or serving many concurrent users can benefit from a larger heap size to efficiently handle increased memory demands.

Disadvantages:

• Slower Startup: A larger heap size requires more memory allocation during startup, leading to slightly longer startup times for the Java application.
• Increased Garbage Collection Times: With a larger heap, garbage collection pauses might become longer, impacting application responsiveness. While Full GC cycles might be less frequent, they can be more time-consuming.

Finding the Right Heap Size

The ideal approach is to start with a conservative initial heap size and monitor the application’s memory usage and behavior. If the heap size is too small, frequent garbage collections and OutOfMemoryErrors may occur. In contrast, if it is too large, the application might experience longer garbage collection pauses and waste valuable memory.

By profiling and analyzing the application’s memory usage and performance, developers can fine-tune the heap size, gradually increasing it until an optimal balance is achieved. Regular monitoring and performance testing are essential to ensure the Java application runs smoothly and efficiently without encountering memory-related issues.

What Happens When Heap Memory Is Full in Java?

When the heap memory in Java becomes full, it indicates that the Java Virtual Machine (JVM) can no longer allocate additional objects, and there is no more free space available to accommodate new requests for memory allocation. When the heap is exhausted, the JVM cannot create new objects, and attempting to do so results in an OutOfMemoryError.

Here’s what happens when the heap memory is full:

OutOfMemoryError

When the heap becomes full and the JVM cannot allocate more memory for new objects, it throws an OutOfMemoryError. This error is an indication that the application has exhausted its allocated heap space and cannot fulfill the memory requirements of the running code.

Application Failure

Once an OutOfMemoryError is thrown, the JVM typically terminates the application’s execution. This is because the application can no longer continue functioning without enough memory to create new objects or execute its code.

Possible Root Causes:

Several factors can lead to heap memory exhaustion and trigger the OutOfMemoryError, such as:

• Memory Leak: If objects are not properly dereferenced and removed from memory when they are no longer needed, they can accumulate and consume all available heap space, leading to a memory leak.
• Insufficient Heap Size: If the initial or maximum heap size is set too small for the application’s memory requirements, the heap can quickly fill up, causing an OutOfMemoryError.
• Overloaded Data Processing: Applications dealing with large datasets or handling high concurrent user traffic may demand more memory than what the heap size can accommodate.
• Handling OutOfMemoryError: Handling an OutOfMemoryError can be challenging since it indicates a fundamental issue with memory management in the application. To address this error, developers need to analyze and optimize the application’s memory usage, check for memory leaks, and consider increasing the heap size if necessary.

To avoid OutOfMemoryErrors, it is essential to carefully manage memory in Java applications, ensure proper object dereferencing, and set an appropriate heap size that meets the application’s memory requirements. Regularly monitoring memory usage and using profiling tools can help identify potential memory issues before they lead to heap memory exhaustion.

Monitor Java Heap Usage with Sematext

With a good JVM monitoring tool you can get notifications from various channels easily when you start having spikes or anomalies with your JVM Heap usage. Dig deeper into the root cause of the problem by checking metrics and logs, figure out if your servers need some tuning or detect memory leaks in your JVM processes.

Sematext Cloud is an all-in-one JVM Monitoring platform where you can see your JVM metrics and JVM garbage collection logs at the same time without switching context. This gives you the ability to correlate unusual spikes with logs and find interdependencies between them. The view of the memory usage, JVM pool size, JVM pool utilization, and heap memory helps you understand the patterns. When looking at these JVM metrics over a longer period of time you can easily spot the memory growing over time, which is a potential sign of the memory leak happening inside your application. The JVM monitoring integration also gives the view of the basic garbage collector metrics, which is also extremely useful. Combined with Garbage Collector logs you gain full sight on what is happening inside your JVM in real time.