Introduction to Memory

JVM Memory Architecture

The Java Virtual Machine (JVM) manages memory automatically, providing a robust and efficient memory management system. The JVM memory architecture consists of several key components:

• Heap: The runtime data area from which memory for all class instances and arrays is allocated
• Stack: Stores local variables and partial results
• Method Area: Stores class-level data and method code
• Program Counter (PC) Register: Contains the address of the JVM instruction currently being executed
• Native Method Stack: Contains native method information

Memory Management in Java

Automatic Memory Management

Java provides automatic memory management through garbage collection, which:

• Identifies and removes unused objects
• Prevents memory leaks
• Manages memory allocation and deallocation
• Optimizes memory usage

Memory Allocation

// Object allocation
String str = new String("Hello"); // Allocated on heap
int number = 42; // Allocated on stack

// Array allocation
int[] array = new int[1000]; // Allocated on heap
double[][] matrix = new double[10][10]; // Allocated on heap

Memory Types

Stack Memory

• Stores primitive types and object references
• LIFO (Last In First Out) structure
• Thread-specific
• Fixed size

Heap Memory

• Stores objects and arrays
• Shared among all threads
• Dynamic size
• Managed by garbage collector

Method Area

• Stores class-level data
• Contains method code
• Shared among all threads
• Part of the heap in some JVM implementations

Memory Errors

Common Memory Errors

// StackOverflowError
public void recursiveMethod() {
    recursiveMethod(); // Infinite recursion
}

// OutOfMemoryError
public void createLargeArray() {
    int[] array = new int[Integer.MAX_VALUE]; // Too large array
}

// Memory Leak Example
public class MemoryLeak {
    private static List<Object> list = new ArrayList<>();
    
    public void addObject(Object obj) {
        list.add(obj); // Objects never removed
    }
}

Error Prevention

• Use appropriate data structures
• Implement proper resource cleanup
• Monitor memory usage
• Use profiling tools

Memory Monitoring

JVM Parameters

// Set initial heap size
-Xms512m

// Set maximum heap size
-Xmx1024m

// Enable garbage collection logging
-verbose:gc

// Set garbage collector
-XX:+UseG1GC

Monitoring Tools

• JConsole
• VisualVM
• Java Flight Recorder
• Eclipse Memory Analyzer (MAT)

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