Thrown by a method invocation on a proxy instance if its invocation handler's invoke method throws a checked exception (a Throwable that is not assignable to RuntimeException or Error) that is not assignable to any of the exception types declared in the throws clause of the method that was invoked on the proxy instance and dispatched to the invocation handler.

An UndeclaredThrowableException instance contains the undeclared checked exception that was thrown by the invocation handler, and it can be retrieved with the getUndeclaredThrowable() method. UndeclaredThrowableException extends RuntimeException, so it is an unchecked exception that wraps a checked exception.

As of release 1.4, this exception has been retrofitted to conform to the general purpose exception-chaining mechanism. The "undeclared checked exception that was thrown by the invocation handler" that may be provided at construction time and accessed via the getUndeclaredThrowable() method is now known as the cause, and may be accessed via the Throwable.getCause() method, as well as the aforementioned "legacy method."

Returns the Throwable instance wrapped in this UndeclaredThrowableException, which may be null.

This method predates the general-purpose exception chaining facility. The Throwable.getCause() method is now the preferred means of obtaining this information.