Are java.lang.Class methods thread safe?

Question

Under IBM JVM we have faced an issue when multiple threads are trying to call Class.getAnnotation at the same time on different objects (but with the same annotation). Threads are starting to deadlock waiting on a monitor inside a Hashtable, which is used as a cache for annotations in IBM JVM. The weirdest thing is that the thread that is holding this monitor is put into 'waiting on condition' state right inside Hashtable.get, making all other threads to wait indefinitely.

The support from IBM stated, that implementation of Class.getAnnotation is not thread safe.

Comparing to other JVM implementations (for example, OpenJDK) we see that they implement Class methods in thread safe manner. IBM JVM is a closed source JVM, they do publish some source code together with their JVM, but it's not enough to make a clear judgment whenever their implementation of Class is thread safe or not.

The Class documentation doesn't clearly state whenever its methods are thread safe or not. So is it a safe assumption to treat Class methods (getAnnotation in particular) as a thread safe or we must use sync blocks in multi threaded environment?

How do popular frameworks (ex. Hibernate) are mitigating this problem? We haven't found any usage of synchronization in Hibernate code that was using getAnnotation method.

Answer 1

Your problem might be related to bug fixed in version 8 of Oracle Java.

One thread calls isAnnotationPresent on an annotated class where the annotation is not yet initialised for its defining classloader. This will result in a call on AnnotationType.getInstance, locking the class object for sun.reflect.annotation.AnnotationType. getInstance will result in a Class.initAnnotationsIfNecessary for that annotation, trying to acquire a lock on the class object of that annotation.

In the meanwhile, another thread has requested Class.getAnnotations for that annotation(!). Since getAnnotations locks the class object it was requested on, the first thread can't lock it when it runs into Class.initAnnotationsIfNecessary for that annotation. But the thread holding the lock will try to acquire the lock for the class object of sun.reflect.annotation.AnnotationType in AnnotationType.getInstance which is hold by the first thread, thus resulting in the deadlock.

JDK-7122142 : (ann) Race condition between isAnnotationPresent and getAnnotations