`DummyInvocationUtils.methodOn(…)` fails for methods returning `Object` on JDK 17

[nealeu]

See this commit for the issue and the workaround.

In testing to upgrade to JDK 17 (note Lombok upgrade as per #1637 is required first), we found some test failures where we had Object as the return type (because String is final so cound not be used for enhancement).

The result is the following exception:

org.springframework.aop.framework.AopConfigException: Unexpected AOP exception; nested exception is java.lang.NullPointerException: Cannot invoke "Object.getClass()" because "cause" is null

	at org.springframework.aop.framework.CglibAopProxy.getProxy(CglibAopProxy.java:215)
	at org.springframework.aop.framework.ProxyFactory.getProxy(ProxyFactory.java:110)
	at org.springframework.hateoas.server.core.DummyInvocationUtils.getProxyWithInterceptor(DummyInvocationUtils.java:193)
	at org.springframework.hateoas.server.core.DummyInvocationUtils.access$000(DummyInvocationUtils.java:38)
	at org.springframework.hateoas.server.core.DummyInvocationUtils$InvocationRecordingMethodInterceptor.invoke(DummyInvocationUtils.java:90)
	at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
	at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:750)
	at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:692)
	at org.springframework.hateoas.server.mvc.DummyInvocationUtilsUnitTest$SampleController$$EnhancerBySpringCGLIB$$920d7ce9.returnsStringAsObject(<generated>)

In our case we were able to change the method return type to CharSequence. I think for JDK 16+ Object is perhaps now not supportable - or some of the JDK9+ code in the AOP code is not handling Object due to some JDK changes, so this may need escalating into Spring core.

[odrotbohm]

Thanks for bringing this up. Fixed in main already. Backporting currently.

[nealeu]

Thanks Oliver