What Causes java.lang.OutOfMemoryError: Metaspace in Java 8 Applications?

What are the potential causes of java.lang.OutOfMemoryError: Metaspace in a Java 8 application running on Tomcat?

The 'java.lang.OutOfMemoryError: Metaspace' error occurs when the Java Virtual Machine (JVM) runs out of memory in the Metaspace area, which is used for class metadata. Unlike the PermGen space in previous versions of Java (Java 7 and earlier), the Metaspace is dynamically allocated and not limited by a fixed size, but it can still run out of memory due to various factors, particularly in applications using heavy reflection or custom class loaders.

// Example to increase `MaxMetaspaceSize` in Tomcat startup script
JAVA_OPTS="-server -Xms8g -Xmx8g -XX:MaxMetaspaceSize=4096m ..."

Causes

• **Heavy Use of Reflection:** Reflection can lead to many classes being loaded, especially if classes are being dynamically generated or modified during execution. This can cause Metaspace to fill up quickly.
• **Custom Class Loaders:** Using custom class loaders can lead to memory leaks or accumulation of class definitions if they are not managed properly, preventing unused classes from being garbage collected.
• **Frequent Class Loading/Unloading:** If your application is frequently loading and unloading classes (e.g., due to redeployments in a server environment), this can lead to increased Metaspace usage.
• **High Traffic Volume:** An increase in traffic might lead to more class instances being created and subsequently loaded, which consumes additional Metaspace.

Solutions

• **Increase MaxMetaspaceSize:** Consider increasing the `-XX:MaxMetaspaceSize` parameter to allocate more memory for Metaspace. Currently set to 3200m, you could try increasing it to 4096m or higher based on your application's needs.
• **Monitor Class Loading:** Utilize tools such as VisualVM or Java Mission Control to monitor class loading and Metaspace usage over time to identify potential leaks or unnecessary class loading.
• **Optimize Use of Reflection:** Reduce reliance on reflection wherever possible. If dynamic class generation is necessary, ensure that classes are unloaded properly when no longer needed.
• **Review Custom Class Loaders:** Investigate and review your custom class loader implementations to ensure that they do not retain references to classes that are not in use.

Mistake: Setting a very high MaxMetaspaceSize without monitoring.

Solution: Increased limits can mask underlying issues. Always monitor and profile your application.

Mistake: Not releasing class references after usage in custom class loaders.

Solution: Ensure that classes are cleaned up and references are cleared for better memory management.