What is Method Area / Metaspace?

Direct Answer

The method area is the JVM specification's shared logical storage for per-class structures, runtime constant pools, field and method data, and method code. Metaspace is HotSpot's native-memory area for class metadata; it is an implementation detail related to, but not identical to, the entire specified method area.

Deep Dive & Architecture

Method area and Metaspace

The method area is the JVM specification's shared logical home for per-class structures. Metaspace is HotSpot's native-memory storage for class metadata. They are related, but treating them as identical hides that the specification allows different physical designs and HotSpot stores related runtime data in more than one place.

A class-loader redeployment leak

An application server loads version 1 of a web application with loader A. After deployment of version 2, loader B defines the new classes. If a timer thread or static registry still references loader A, that loader and the classes it defined cannot complete their intended lifecycle. Repeating the deployment can grow class metadata until HotSpot reports OutOfMemoryError: Metaspace.

Classes are not normally unloaded one arbitrary class at a time while their defining loader remains live. Dynamic proxies, bytecode generation, template engines, and repeated plugin loading can all produce legitimate metadata growth before reaching a plateau. A leak means old loaders or generated classes remain beyond their intended lifetime, not merely that the number rose once.

Evidence for Metaspace and heap usage

A heap dump is useful for heap objects and may reveal paths retaining class loaders, but it does not fully describe VM-native allocation. Class-loading counters, jcmd, GC/class-unloading logs, and HotSpot Native Memory Tracking can add the missing view. NMT must be enabled and has scope and overhead limits. Raising -Xmx targets the Java heap; -XX:MaxMetaspaceSize targets a different boundary.

Code Example

import java.lang.management.ClassLoadingMXBean;
import java.lang.management.ManagementFactory;

class MethodAreaAndMetaspace {
    public static void main(String[] args) {
        ClassLoadingMXBean classes = ManagementFactory.getClassLoadingMXBean();
        System.out.println("currently loaded: " + classes.getLoadedClassCount());
        System.out.println("total loaded: " + classes.getTotalLoadedClassCount());
        System.out.println("total unloaded: " + classes.getUnloadedClassCount());
    }
}