Back to Home

Understanding Java Memory Management

January 6, 2024

Understanding Java Memory Management

Java's automatic memory management is one of its most powerful features, but understanding how it works is crucial for writing efficient applications.

Memory Areas in Java

Heap Memory

The heap is where all objects are allocated. It's divided into:

  1. Young Generation
    • Eden Space
    • Survivor Spaces (S0 and S1)
  2. Old Generation
  3. Metaspace (replaced PermGen in Java 8+)
// Object allocation in heap
Object obj = new Object(); // Goes to Eden space

Stack Memory

Each thread has its own stack that contains:

  • Method calls
  • Local variables
  • Primitive types
  • Object references
public void method() {
    int x = 5; // Stack variable
    String str = "Hello"; // Reference on stack, actual string in heap
}

Garbage Collection

How GC Works

  1. Mark: Identify live objects
  2. Sweep: Remove dead objects
  3. Compact: Defragment memory (optional)

GC Algorithms

  1. Serial GC
  2. Parallel GC
  3. CMS (Concurrent Mark Sweep)
  4. G1 (Garbage First)
  5. ZGC (Z Garbage Collector)
System.gc(); // Suggestion to run GC (not recommended in production)

Memory Leaks

Common causes of memory leaks:

  1. Static Fields
public class Cache {
    private static Map<Key, Value> cache = new HashMap<>(); // Potential memory leak
}
  1. Unclosed Resources
// Bad practice
FileInputStream fis = new FileInputStream("file.txt");
// ... use fis ...
// Never closed!

// Good practice
try (FileInputStream fis = new FileInputStream("file.txt")) {
    // ... use fis ...
} // Automatically closed
  1. Inner Class References
public class Outer {
    private byte[] data = new byte[1000000];
    
    public class Inner {
        // Holds implicit reference to Outer
    }
}

Best Practices

  1. Use try-with-resources
  2. Avoid finalizers
  3. Clear collections when no longer needed
  4. Use weak references when appropriate
  5. Profile your application
// Using WeakHashMap for caching
Map<Key, Value> cache = new WeakHashMap<>();

Monitoring Tools

  1. JVisualVM
  2. JConsole
  3. Java Mission Control
  4. Memory Analyzer (MAT)

Understanding memory management helps you:

  • Write more efficient code
  • Prevent memory leaks
  • Optimize garbage collection
  • Improve application performance

Comments