OutOfMemoryError: Metaspace

Why This Happens

Metaspace stores class definitions and metadata. This error occurs when too many classes are loaded, often from dynamically generating classes, having many classloaders, or redeploying web applications without proper cleanup. It replaced PermGen space in Java 8+.

The Problem

// Repeatedly creating classloaders without cleanup
while (true) {
    URLClassLoader loader = new URLClassLoader(new URL[]{jarUrl});
    loader.loadClass("com.example.Plugin");
    // Never closing the classloader
}

The Fix

// Close classloaders when done
try (URLClassLoader loader = new URLClassLoader(new URL[]{jarUrl})) {
    Class<?> clazz = loader.loadClass("com.example.Plugin");
    // Use the class
} // Automatically closed

Step-by-Step Fix

1. 1

   Identify the class loading

   Use -verbose:class JVM flag to see which classes are being loaded. Look for repeated loading of the same classes.

2. 2

   Check for classloader leaks

   In web apps, check for ThreadLocal leaks, JDBC driver registration, or static references that prevent classloader garbage collection.

3. 3

   Fix the leak or increase metaspace

   Close classloaders properly, fix classloader leaks, or increase metaspace with -XX:MaxMetaspaceSize=256m.