Resolvers behind a JDK dynamic proxy build fine but fail on every field at runtime #822

A resolver wrapped in a JDK dynamic proxy (for example Spring AOP without proxyTargetClass) passes build(), but every field it resolves fails at query time:

IllegalArgumentException: object of type jdk.proxy2.$Proxy11 is not an instance of com.example.ProxiedQuery

Repro (master, 14.0.4-SNAPSHOT)

class ProxiedQuery : GraphQLQueryResolver {
    fun test(): String = "ok"
}

val resolver = ProxyFactory(ProxiedQuery()).proxy as GraphQLQueryResolver // JDK dynamic proxy
val schema = SchemaParser.newParser()
    .schemaString("type Query { test: String }")
    .resolvers(resolver)
    .build()
    .makeExecutableSchema()

GraphQL.newGraphQL(schema).build().execute("{ test }")
// data={test=null}
// errors=[ExceptionWhileDataFetching{path=[test], exception=java.lang.IllegalArgumentException: object of type jdk.proxy2.$Proxy11 is not an instance of ...ProxiedQuery}]

The same resolver behind a CGLIB proxy works.

Cause

Spring4AopProxyHandler.getTargetClass returns the target class, so the resolver methods are found on ProxiedQuery. MethodFieldResolver.invoke (MethodFieldResolver.kt:286) then calls them on the proxy instance, which isn't a ProxiedQuery.

The existing test SchemaParserTest."parser handles spring AOP proxied resolvers by default" uses this kind of proxy but only builds the schema, so it doesn't catch this.

Fix

Invoking the method on the unwrapped target would skip the proxy's interceptors (e.g. @PreAuthorize), so that's not an option. Either:

• fail at build() with a clear message, e.g. "resolver X is a JDK dynamic proxy, use class-based proxies", or
• call through Proxy.getInvocationHandler(proxy) so interceptors still run (needs checking against non-Spring handlers).

Found while triaging #410.