Heap Memory and How It Works in Java

Heap memory is an important concept in Java that every developer should understand. The heap is the runtime data area. Hence heap memory keeps all class instances and arrays that are created at runtime.

In this article, we will dive into how the Java heap works. In addition to that, we will explain how to properly use it when developing applications.

If you don’t have a medium membership, you can use this link to reach the article without a paywall.

What is Heap Memory?

Heap memory in Java is a dynamic storage area where objects are stored at runtime. Unlike the stack memory, the heap caters to objects that have a more extended lifespan. This makes heap memory pivotal for applications dealing with large datasets or numerous objects.

When objects are created in Java using the new keyword, JVM allocates memory on the heap to store those objects. People sometimes refer to the heap as the “garbage-collected heap”. Because the garbage collector manages the heap memory region.

Objects created in the heap will remain there until the garbage collector removes them. The garbage collector removes them when there is no reference to the object. That way, objects become eligible for garbage collection.

Java’s garbage collector operates in the heap, reclaiming memory occupied by objects that are no longer in use. This automatic memory management mechanism simplifies the developer’s task of memory cleanup.

Some key facts about heap memory:

• Stored in computer RAM
• Developers can specify the heap size while running the application. The default heap size varies between JVM versions but is usually between 128MB to 1024MB.
• Heap does not have fix size, and can dynamically expand and shrink
• All class instances and arrays are allocated on the heap at runtime.
• Slower access speeds than stack
• Managed by garbage collector, no manual de-allocation

Heap Memory Structure

Understanding the structure of heap memory is crucial for effective Java programming. The heap consists of two main sections: the Young Generation and the Old Generation. Permanent Generation is not part of the heap.

Young Generation

The Young Generation is where new objects are created. It consists of three areas: the Eden space and two survivor spaces (S0 and S1). Eden space is the first place of the new objects. When you create a new object, Firstly it takes place in the Eden space.

As objects survive garbage collection cycles, they may be promoted to the survivor spaces. The garbage collector employs a mechanism called the Minor Garbage Collection to clear out short-lived objects. Afterward, it moves those cleared objects that persist to the Old Generation.

Old Generation

Objects that survive multiple garbage collection cycles in the Young Generation are eventually moved to the Old Generation. This area is designed for long-lived objects. This area’s garbage collection occurs less frequently but involves a more comprehensive sweep of the heap. So, Major Garbage Collection or Full Garbage Collection is the name of this process.

How Objects Are Allocated on the Heap

When an object is instantiated in Java using the new keyword, the memory is allocated on the heap. A reference variable to that object is also created on the stack. This will allow the code to access the object’s attributes and methods.

Even primitive data types like int or boolean can be created on the heap. This can be done by using wrapper classes like Integer or Boolean.

When you create a new object in Java using a statement like:

MyObject object = new MyObject();

The following happens:

1. Memory is allocated on the heap to store all fields defined by the MyObject class. This includes any inherited fields from parent classes.
2. The constructor for the MyObject class initializes the new object’s fields.
3. The reference variable object in the code will point to the newly allocated and initialized object on the heap. The code can access this object using that reference variable.

Objects on the heap will remain allocated until they are no longer referenced and can be garbage collected. Primitive local variables such as ints and doubles, however, are stored on the stack instead of the heap.

Let’s explore heap memory through a practical example. Consider the following Java code snippet:

public class MemoryExample {
    public static void main(String[] args) {
        // Creating objects in the heap
        String str1 = new String("Hello");
        String str2 = new String("World");
        
        // Concatenating strings
        String result = str1 + str2;
        
        // Creating an array
        int[] numbers = {1, 2, 3, 4, 5};
        
        // Manipulating the array
        for (int i = 0; i < numbers.length; i++) {
            numbers[i] *= 2;
        }
    }
}

In this example, the String objects str1 and str2 are created in the heap memory. When concatenating them, a new String object (result) also takes place in the heap. The array numbers is yet another example of heap memory usage.

The Java Virtual Machine (JVM) manages the allocation and deallocation of memory in the heap. Also, it does those during the execution of this program. Moreover, it ensures that GC appropriately cleans up those objects when they are no longer needed.

Heap Space Management

The key to avoiding out of memory errors is managing your use of heap space properly. Here are some tips:

• Avoid holding on to object references you no longer need. This allows the garbage collector to free that unused memory.
• Be aware of collection classes that can accumulate many objects, such as HashMaps. Consider size limits.
• Use profiling tools like VisualVM to monitor heap usage. Look for memory leaks.
• Set appropriate heap sizes for your application’s use case. Don’t over allocate.
• Avoid unintended object retention via caching or listeners/callbacks.
• Use tools like java.lang.ref.WeakReference to avoid holding hard references to no-longer needed objects.

You can configure size of heap memory and permanent generation with below flags. In that way, you can have a larger or smaller heap size than default.

Large and small heap sizes come with their own advantages and disadvantages.

Large Heap

Pros:

• Can store more data in memory before needing garbage collection. Suitable for data-intensive applications like scientific simulations, big data processing, and enterprise-level software.
• Reduces the frequency of minor garbage collections. This contributes to better application performance, as the system interruption frequency is less
• Larger caches are possible.

Cons:

• Longer garbage collection pause times due to larger old generation.
• Higher memory footprint.
• More data scanned during garbage collections.

Small Heap

Pros:

• Faster and more frequent minor garbage collections.
• Lower memory footprint.
• Less data scanned during garbage collections.

Cons:

• More frequent garbage collection. Hence more application interruption.
• Higher risk of out of memory errors if data exceeds heap size.
• Less data can be cached in memory.

Heap Memory Optimization Techniques

Efficient memory usage is crucial for the performance of Java applications. Here are some optimization techniques related to heap memory:

1. Object Pooling

Object pooling involves reusing objects instead of creating new ones. This can reduce the overhead associated with object creation and garbage collection. Libraries like Apache Commons Pool provide implementations for object pooling in Java.

2. Garbage Collection Tuning

Java provides options to fine-tune garbage collection according to application requirements. Understanding and configuring garbage collection parameters, such as heap size and collection algorithms, can significantly impact the performance of an application.

3. Memory Leak Detection

Identifying and rectifying memory leaks is essential to prevent excessive memory consumption. Tools like Java Flight Recorder (JFR) and profilers can help pinpoint memory leaks by analyzing heap dumps.

Heap memory is a fundamental aspect of Java programming, playing a crucial role in managing objects with varying lifespans. Understanding the structure of the heap, the Young and Old Generations is important to develop robust applications. In addition to that implementing optimization techniques are key for efficient Java applications.

Proper use of the heap space is key to avoiding the dreaded out of memory exceptions in Java applications. There are a few points to boost the stability of your application. Follow best practices, monitor your memory usage, and use things like soft references. Applying those will help you.

👏 Thank You for Reading!

👨‍💼 I appreciate your time and hope you found this story insightful. If you enjoyed it, don’t forget to show your appreciation by clapping 👏 for the hard work!

📰 Keep the Knowledge Flowing by Sharing the Article!

✍ Feel free to share your feedback or opinions about the story. Your input helps me improve and create more valuable content for you.

✌ Stay Connected! 🚀 For more engaging articles, make sure to follow me on social media:

• Twitter
• Instagram

🔍 Explore More! 📖 Dive into a treasure trove of knowledge at Codimis. There’s always more to learn, and we’re here to help you on your journey of discovery.