metaspace

Metaspace is a memory area in the Java Virtual Machine used to store metadata for classes loaded by the application. It was introduced in Java 8 to replace the older PermGen space and, unlike the Java heap, resides in native memory (off-heap). This design allows metaspace to grow automatically based on the needs of the running application, constrained only by the host system memory unless a maximum size is configured.

What it stores and how it behaves

Metaspace holds the metadata the JVM needs to define and work with loaded classes, such as information about class definitions, methods, fields, and related runtime structures. The data stored here is created as classes are loaded and can be reclaimed when their defining class loaders become eligible for garbage collection. This means that, in contrast to the fixed PermGen, metaspace size can expand and shrink over the lifetime of an application, subject to memory availability and any configured limits.

Configuration and limits

By default metaspace size is not fixed and can grow as needed. It can be bounded with -XX:MaxMetaspaceSize to cap native memory usage. The option -XX:MetaspaceSize sets the initial size at which the JVM may trigger a growth operation and GC. Other flags control how aggressively the JVM frees unused space. If metaspace runs out of memory, the JVM may throw java.lang.OutOfMemoryError: Metaspace.

Troubleshooting and monitoring

Common symptoms include OutOfMemoryError: Metaspace after extensive dynamic class loading or redeployments. Solutions include increasing MaxMetaspaceSize, ensuring proper unloading of classes by removing references to obsolete class loaders, reducing dynamic class generation, and analyzing classloading leaks. Metaspace usage can be monitored with tools like jcmd, jstat, VisualVM, or JVM native memory profiling.