Memory Management


Introduction
In the previous lesson, we explored Streams and Lambda Expressions, introducing functional programming for efficient data processing. This week, we dive into Memory Management in Java - a critical topic that helps you write performant and leak-free applications.
Stack vs. Heap Memory
Java memory is divided into two main areas:
Stack Memory:
Stores method calls, local variables, and references to objects in the heap. It operates in a Last-In-First-Out (LIFO) manner.
Heap Memory: Stores objects and class instances. The garbage collector automatically manages memory allocation and deallocation here.
Example: Stack and Heap Memory
class MemoryExample {
int instanceVariable; // Stored in Heap
void method() {
int localVariable = 10; // Stored in Stack
MemoryExample obj = new MemoryExample(); // obj reference is in Stack, object is in Heap
}
}
Garbage Collection Basics
Garbage collection (GC) in Java automatically frees up memory by removing unused objects from the heap. It prevents memory leaks and enhances application performance.
How Garbage Collection Works
Mark and Sweep: Identifies unused objects and clears them from memory.
Generational Garbage Collection: Java divides heap memory into Young Generation, Old Generation, and Permanent Generation for efficient management.
Example: Forcing Garbage Collection (Not Recommended for Production)
public class GCDemo {
public static void main(String[] args) {
GCDemo obj = new GCDemo();
obj = null; // Object is now eligible for garbage collection
System.gc(); // Suggests JVM to run Garbage Collector
System.out.println("Garbage Collection requested");
}
}
Best Practices to Avoid Memory Leaks in Java
Release References When No Longer Needed
Set object references to
null
if they won’t be used again — especially in long-lived collections or caches.Avoid Unbounded Object Retention
Be cautious when adding objects to collections like
List
,Map
, orSet
. If you forget to remove them, the JVM won’t garbage collect them.Be Careful with Static Fields
Static variables live for the lifetime of the application. If they hold references to large objects or collections, those won't be garbage collected.
Monitor with Tools
Use profiling tools like VisualVM, JConsole, or Eclipse MAT to monitor memory usage and detect leaks during testing.
Close Resources Properly
Use try-with-resources for streams, JDBC, etc.
try (FileInputStream fis = new FileInputStream("data.txt")) { // Auto-closed }
The Java Memory Model (JMM)
The Java Memory Model defines how threads interact through memory which is crucial for concurrency and thread safety.
It specifies rules about visibility and ordering of variables across threads.
Without it, changes made by one thread may not be seen by another due to CPU caching or compiler optimizations.
Key Concepts
Happens-Before Relationship
Ensures that memory writes by one thread are visible to another under certain conditions (e.g.,synchronized
,volatile
, threadstart()
/join()
).Volatile Variables
Ensure visibility, that is changes made by one thread are always seen by others.
Memory Areas in the JVM
Area | Description |
Heap | Stores objects and class instances |
Stack | Stores method calls and local variables |
Method Area | Stores class metadata (part of the metaspace in modern JVMs) |
Program Counter (PC) | Tracks the current thread instruction |
Native Method Stack | Used for native code execution |
Conclusion
Understanding Java memory management is essential for writing efficient and optimized applications. By knowing how stack and heap memory work and how garbage collection operates, developers can avoid memory leaks and improve performance.
Stay tuned for the next topic in the Java Short Notes series!
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Written by

Emmanuel Enchill
Emmanuel Enchill
I am a passionate software engineer in training at ALX. I have a strong preference for back-end development. I am on course to share my learning journey with you.