How can we simulate OutOfMemory: Metaspace?

I have a task to simulate OutOfMemory: Metaspace error intentionally. I tried different ways, but none gave me needed result. Can someone share good example for this purpose?

My first attempt (using javassist):

static ClassPool classPool = ClassPool.getDefault();

@SneakyThrows
public void task() {

    try {
        for (int i = 0; ; i++) {
            Class cl = classPool.makeClass("task1.Test" + i).toClass();
        }
    } catch (Error er) {
        Runtime.getRuntime().gc();
        log.error(er.getMessage());
    }
}

and settings in gradle.properties file:

org.gradle.jvmargs=-XX:MaxMetaspaceSize=70M

but I’ve got an error:

Exception: java.lang.OutOfMemoryError thrown from the UncaughtExceptionHandler in thread “main”

which I couldn’t catch.

My second attempt:

public void task() {
        try {
            URL url = new File("D:/classes").toURI().toURL();
            URL[] urls = {url};

            ClassLoadingMXBean loadingMXBean = ManagementFactory.getClassLoadingMXBean();

            List<ClassLoader> classLoaders = new ArrayList<>();

            while (true) {
                ClassLoader classLoader = new URLClassLoader(urls);
                classLoaders.add(classLoader);
                classLoader.loadClass("test1.Test1");

                log.info("Total: " + loadingMXBean.getTotalLoadedClassCount());
                log.info("Active: " + loadingMXBean.getLoadedClassCount());
                log.info("Unloaded: " + loadingMXBean.getUnloadedClassCount());
            }
        } catch (Exception ex) {
            log.error(ex + ex.getMessage());
        }
    }

and it didn’t work, the IDEA notified me there’s law memory.

Answer

You are forgetting how classloading works in the JVM. A classloader will first try to let its parent load the class you are looking for. In order for your test to work, you need a custom class loader that will not delegate to its parent for that one class that you are testing with.

Here is an example that works in Java 8

DummyClass.java (default package)

public class DummyClass {
  static String padding = "1234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890";
}

Test.java (default package)

import java.lang.management.ClassLoadingMXBean;
import java.lang.management.ManagementFactory;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.List;

public class Test {
    
    public static void main(String[] args) {

        try {
            
            URL url = Test.class.getResource("").toURI().toURL();
            System.out.println("URL = " + url);
            URL[] urls = {url};
    
            ClassLoadingMXBean loadingMXBean = ManagementFactory.getClassLoadingMXBean();
    
            List<ClassLoader> classLoaders = new ArrayList<>();

            System.out.println("Total: " + loadingMXBean.getTotalLoadedClassCount());
            System.out.println("Active: " + loadingMXBean.getLoadedClassCount());
            System.out.println("Unloaded: " + loadingMXBean.getUnloadedClassCount());

            int i = 0;
            while ( true ) {
                i++;
                System.out.println("### Iteration " + i + " ###");
                ClassLoader classLoader = new URLClassLoader(urls) {
                    @Override
                    public Class<?> loadClass(String name) throws ClassNotFoundException {
                        final Class<?> loadedClass;
                        if ( "DummyClass".equals(name) ) {
                            System.out.println (this + " - finding class " + name);
                            loadedClass = findClass(name);
                            System.out.println (this + " - loading class " + loadedClass);
                        } else {
                            // delegate to parent
                            loadedClass = this.getParent().loadClass(name);
                        }
                        return loadedClass;
                    }
                };
                
                classLoaders.add(classLoader);
                classLoader.loadClass("DummyClass");
    
                System.out.println("Total: " + loadingMXBean.getTotalLoadedClassCount());
                System.out.println("Active: " + loadingMXBean.getLoadedClassCount());
                System.out.println("Unloaded: " + loadingMXBean.getUnloadedClassCount());
            }
        } catch ( Exception ex ) {
            ex.printStackTrace();
        }
        
    }
}

Running with -XX:MaxMetaspaceSize=100m, I eventually get this:

> ### Iteration 16318 ### 
> Test$1@531ed68e - finding class DummyClass
> Test$1@531ed68e - loading class class DummyClass
> Total: 16747 Active:
> 16747 Unloaded: 0
> ### Iteration 16319 ### 
> Test$1@6bbd4048 - finding class DummyClass 
> Exception in thread "main" java.lang.OutOfMemoryError: Metaspace  at
> java.lang.ClassLoader.defineClass1(Native Method)     at
> java.lang.ClassLoader.defineClass(ClassLoader.java:756)   at
> java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
>   at java.net.URLClassLoader.defineClass(URLClassLoader.java:468)     at
> java.net.URLClassLoader.access$100(URLClassLoader.java:74)    at
> java.net.URLClassLoader$1.run(URLClassLoader.java:369)    at
> java.net.URLClassLoader$1.run(URLClassLoader.java:363)    at
> java.security.AccessController.doPrivileged(Native Method)    at
> java.net.URLClassLoader.findClass(URLClassLoader.java:362)    at
> Test$1.loadClass(Test.java:42)    at Test.main(Test.java:53)