JVM Memory Architecture: Heap, Metaspace & Thread Stacks

1Concept

JVM memory is partitioned into: 1. Heap (shared across threads, housing all objects); 2. Metaspace (native OS RAM housing class metadata, method bytecode, and interned strings); 3. Thread Stacks (per-thread memory holding local frames, operand stacks, and method call chains); 4. PC Registers; 5. Native Method Stacks.

2Architecture Diagram

+-------------------------------------------------------------------------+
|                         JVM RUNTIME MEMORY MODEL                        |
|  +------------------------------------+  +---------------------------+  |
|  | HEAP MEMORY (Shared by all threads)|  | METASPACE (Native RAM)    |  |
|  |  +----------------+ +------------+ |  | - Class Metadata          |  |
|  |  | Young Gen      | | Old Gen    | |  | - Method Bytecode         |  |
|  |  | (Eden, S0, S1) | | (Tenured)  | |  | - Constant Pool           |  |
|  |  +----------------+ +------------+ |  +---------------------------+  |
|  +------------------------------------+                                 |
|  +-------------------------------------------------------------------+  |
|  | PER-THREAD MEMORY: Thread Stack Frames | PC Register | Native Stack|  |
|  +-------------------------------------------------------------------+  |
+-------------------------------------------------------------------------+

3Code Example

public class JvmMemoryMetricsDemo {
    public static void main(String[] args) {
        Runtime runtime = Runtime.getRuntime();
        long mb = 1024 * 1024;

        System.out.println("=== JVM Runtime Memory Metrics ===");
        System.out.println("Available CPU Processors: " + runtime.availableProcessors());
        System.out.println("Total Memory Allocated:   " + (runtime.totalMemory() / mb) + " MB");
        System.out.println("Free Memory in JVM:       " + (runtime.freeMemory() / mb) + " MB");
        System.out.println("Max Memory Limit (-Xmx):  " + (runtime.maxMemory() / mb) + " MB");
    }
}

4Expected Output

=== JVM Runtime Memory Metrics ===
Available CPU Processors: 8
Total Memory Allocated:   256 MB
Free Memory in JVM:       248 MB
Max Memory Limit (-Xmx):  4096 MB

5Key Takeaways

• ✓Metaspace replaced PermGen in Java 8 and grows dynamically in native OS RAM by default.
• ✓StackOverflowError occurs when thread recursion exceeds stack frame depth (`-Xss`).
• ✓OutOfMemoryError: Java heap space occurs when active objects exceed `-Xmx`.