DATAREST-1213 - Unproxy JDK proxy to get version information for ETag

[darioseidl]

This is a fix for https://jira.spring.io/browse/DATAREST-1213. In some version after org.springframework.data:spring-data-rest-webmvc:2.4.4.RELEASE, HTTP PATCH and PUT requests with a projection would fail (using PATCH/PUT with projections makes sense when using return-body-on-update/return-body-on-create).

When using a projection, the payload in the PersistentEntityResource in RepositoryEntityController ends up as a dynamic JDK proxy and ETag.getVersionInformation fails with

java.lang.IllegalArgumentException: Target bean of type org.springframework.data.rest.webmvc.$Proxy129 is not of type of the persistent entity (org.springframework.data.rest.webmvc.jpa.Category)!: org.springframework.data.rest.webmvc.$Proxy129	at org.springframework.util.Assert.instanceCheckFailed(Assert.java:655)	at org.springframework.util.Assert.isInstanceOf(Assert.java:574)	at org.springframework.data.mapping.model.BasicPersistentEntity.verifyBeanType(BasicPersistentEntity.java:551)	at org.springframework.data.mapping.model.BasicPersistentEntity.getPropertyAccessor(BasicPersistentEntity.java:453)	at org.springframework.data.rest.webmvc.support.ETag.getVersionInformation(ETag.java:169)	at org.springframework.data.rest.webmvc.support.ETag.from(ETag.java:87)	at org.springframework.data.rest.webmvc.support.ETag.verify(ETag.java:105)	at org.springframework.data.rest.webmvc.RepositoryEntityController.patchItemResource(RepositoryEntityController.java:412) 

I'm not quite sure what's the canonical way to unproxy a JDK proxy in Spring Data, the proxy implement TargetAware so that seems to be an easy solution.

I would be very happy if this could be reviewed and merged. This bug affects us in two projects and currently prevents us from upgrading to any recent version of Spring Boot / Spring Data in one of them.

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[odrotbohm]

That's already been merged with 26410a0.