Give me an example

As we explained in the previous chapter, Metaspace usage is strongly correlated with the number of classes loaded into the JVM. The following code serves as the most straightforward example:

public class Metaspace {
	static javassist.ClassPool cp = javassist.ClassPool.getDefault();

	public static void main(String[] args) throws Exception{
		for (int i = 0; ; i++) { 
			Class c = cp.makeClass("eu.plumbr.demo.Generated" + i).toClass();
		}
	}
}

In this example the source code iterates over a loop and generates classes at the runtime. All those generated class definitions end up consuming Metaspace. Class generation complexity is taken care of by the javassist library.

The code will keep generating new classes and loading their definitions to Metaspace until the space is fully utilized and the java.lang.OutOfMemoryError: Metaspace is thrown. When launched with -XX:MaxMetaspaceSize=64m then on Mac OS X my Java 1.8.0_05 dies at around 70,000 classes loaded.