Understanding JVM Memory Architecture

Optimizing Code in Java

Pavlos Kosmetatos

Lead Engineer @Wealthyhood

What is the JVM?

Platform-independent execution environment
Converts bytecode (.class) to machine code
Memory management and garbage collection
Just-In-Time compilation

JVM memory architecture overview

The JVM memory architecture

Stack memory

Memory area for method execution and local variables
LIFO (Last-In-First-Out) structure
Stores:
- Local variables
- Primitive data types
- Method call information
StackOverflowError if size is exceeded

Heap memory

Stores all objects and arrays
new creates an object and allocates it in the heap
Managed by garbage collector
Dynamic size
Divided into generations

Memory in action

public void processData() {
    int count = 0;                  // Stack

    String name = "Java";           // Heap

    List<String> items =            // Reference on Stack
        new ArrayList<>();          // Object on Heap

    items.add(new String("Item"));  // Object on Heap
}

Memory-efficient coding patterns

// Inefficient
List<String> list1 = new ArrayList<>();
for (int i = 0; i < 1000; i++) {
    String s = new String("value"); // 1000 objects
    list1.add(s);
}

// Better
List<String> list2 = new ArrayList<>();
String s = "value";  // Single object
for (int i = 0; i < 1000; i++) {
    list2.add(s);
}

Summary

JVM manages memory through (mainly) stack & heap areas
Stack memory -> fast, stores primitives and references
Heap memory -> Stores objects

Let's practice!

Optimizing Code in Java

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