Chaos Engineering: Metaspace OutOfMemoryError

JVM memory has the following regions: 

JVM memory regions

1. Young Generation
2. Old Generation
3. Metaspace
4. Others region

When you encounter ‘java.lang.OutOfMemoryError: Metaspace,’ it indicates that the Metaspace region in the JVM memory is getting saturated. Metaspace is the region where metadata details that are required to execute your application are stored. In a nutshell, they contain class definitions and method definitions of your application. To learn more about what gets stored in each of the JVM memory regions, you may refer to the "JVM Memory - Learn Easily" video. In this post, let’s discuss how one can simulate java.lang.OutOfMemoryError: Metaspace.

Simulating java.lang.OutOfMemoryError: Metaspace

To simulate ‘java.lang.OutOfMemoryError: Metaspace’, we wrote this program:

public class MetaspaceLeakProgram {      

     public static void main(String[] args) throws Exception {    
          
         ClassPool classPool = ClassPool.getDefault();       

         while (true) {                     

           // Keep creating classes dynamically!         
           String className = "com.buggyapp.MetaspaceObject" + UUID.randomUUID();         
           classPool.makeClass(className).toClass();     
          }   
     }    
}

This program leverages the ClassPool object from the open-source Javassist library. This ClassPool object is capable of creating new classes at runtime. Please take a close look at the above program. If you notice, this program keeps on creating new classes. Below is the sample class names generated by this program:

com.buggyapp.MetaspaceObject76a9a309-c9c6-4e5f-a302-8340eb3acdef 
com.buggyapp.MetaspaceObjectb9bd6832-bacd-4c7c-a6e6-3bfa19a85e80 
com.buggyapp.MetaspaceObject81d9d086-7245-4304-818f-0bfcbf319fd3 
com.buggyapp.MetaspaceObjecte27068b6-f4cb-498a-80d5-0e5b61c2ada0 
com.buggyapp.MetaspaceObject06f9d773-d365-48c8-a5cc-9c69b3178f4c 
:
: 
:

Whenever a new class is created, its corresponding class metadata definitions are created in the JVM’s Metaspace region. Since metadata definitions are created in Metaspace, its size starts to grow. When the maximum metaspace size is reached, the application will experience ‘java.lang.OutOfMemoryError: Metaspace.’

java.lang.OutOfMemoryError: Metaspace Causes

 ‘java.lang.OutOfMemoryError: Metaspace’ error happens because of two reasons:

1.  Metaspace region size is under-allocated 
2.  Memory leak in the Metaspace region

You can address reason #1 by increasing the Metaspace region size. You can do this by passing the JVM argument -XX:MaxMetaspaceSize. 

In order to address reason #2, you have to do proper troubleshooting. Read further about how to troubleshoot memory leaks in the Metaspace region.

Video