- Memory region (outside the Java heap, in native memory).
- Stores class metadata: method tables, field info, constant pools, vtables, and class loader structures.
- Introduced in Java 8, replacing PermGen.
📅 When is it used?
- At class loading time, when a new class is loaded by a class loader.
- Each class has its metadata allocated inside the loader’s Metaspace.
- Static fields are stored in heap, but metadata (e.g., methods, structure) → Metaspace.
- PermGen problem: fixed size → frequent
OutOfMemoryError: PermGen space.
- Metaspace advantage: grows dynamically, limited by system memory or
-XX:MaxMetaspaceSize.
- Memory can be reclaimed if the class loader is GC’d.
🔑 Key Point
- Classes loaded by bootstrap/system loaders → live for the life of JVM.
- Classes loaded by custom loaders (e.g., Tomcat, OSGi) → freed when loader is GC’d.