ClassPrepareEvent

ClassPrepareEvent is an event type used in the Java Virtual Machine Tool Interface (JVMTI) and in debugging protocols such as JDWP/JDI. It signals that a class has been prepared by the JVM, meaning the class has been loaded and linked (verification and preparation have completed), but has not yet run its static initializers. The event marks the point at which the class’s metadata becomes available for inspection and instrumentation.

The event is typically delivered after a ClassLoadEvent and before ClassInitialize. Its payload provides a reference to the prepared class (for example, a ReferenceType or jclass), along with associated metadata such as the class name, binary name, and the class loader that defined it. Depending on the toolchain, additional information such as the class signature and generic signature may also be available.

In practice, ClassPrepareEvent is used by debuggers and instrumentation agents to observe when new classes enter the VM so that they can perform analysis or modify bytecodes before the class is initialized. Common use cases include selective instrumentation, setting up breakpoints or traces in newly loaded classes, and building class hierarchies or dependency graphs. Since the class has not yet executed its static initialization, tools must handle this window carefully, particularly with respect to any required initialization side effects or ordering constraints.

Notes:

- ClassPrepareEvent is distinct from ClassLoadEvent (loading) and ClassInitialize (execution of static initializers).

- It is most useful for agents that need to instrument or inspect classes at the moment they become available to the runtime.