How to fix 'class java.util.LinkedHashMap cannot be cast to class' when using Java 8 and Java 11

Question

I'm working with a micro-service built using Java 11, this service has a dependency built in Java 8. Dependency has rest-clients in it and there is a method that does this:

 public <T> ResponseEntity<my.company.Wrapper<T>> getForList(String url, HttpHeaders headers) {
 try {
 return template.exchange(url, GET, new HttpEntity<>(headers), new ParameterizedTypeReference<Wrapper<T>>() { });
 } catch (RestClientException restEx) {
 logger.error("RestClientException!, restEx: {}", restEx);
 throw restEx;
 }
 }

Rest call is just successful, response -> <200,Wrapper [ elements = [{id=dbfc2557-1738-45e4-8ecd-235d35158957, .....

then, at micro-service (with Java 11), there is a point when it uses a java Stream:

 var found = enttsResponse.stream()
 .map(MyCompanyResponse::getId)
 .collect( toList() );

Exception message is:

java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class com.mycompany.commons.v2.entitlement.MyCompanyResponse (java.util.LinkedHashMap is in module java.base of loader 'bootstrap'; com.mycompany.commons.v2.entitlement.MyCompanyResponse is in unnamed module of loader 'app')

I'm familiar to Java8, but not Java 11, I think is something related to Class Loaders (from Java 9)

Workaround are accepted, thanks in advance !!!

Full stack trace:

java.lang.ClassCastException: class java.util.LinkedHashMap cannot be cast to class com.mycompane.commons.v2.entitlement.MyCompanyResponse (java.util.LinkedHashMap is in module java.base of loader 'bootstrap'; com.mycompane.commons.v2.entitlement.MyCompanyResponse is in unnamed module of loader 'app')
 at java.base/java.util.stream.ReferencePipeline3ドル1ドル.accept(ReferencePipeline.java:195) ~[na:na]
 at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1654) ~[na:na]
 at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484) ~[na:na]
 at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474) ~[na:na]
 at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913) ~[na:na]
 at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234) ~[na:na]
 at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578) ~[na:na]
 at com.mycompane.service.organization.core.SomeAmazingService.validateEntitlements(OrganizationService.java:105) ~[main/:na]
 at com.mycompane.service.organization.core.SomeAmazingService.createOrganization(OrganizationService.java:66) ~[main/:na]
 at com.mycompane.service.organization.core.SomeAmazingService$$FastClassBySpringCGLIB$490ドルb1633.invoke(<generated>) ~[main/:na]
 at org.springframework.cglib.proxy.MethodProxy.invoke(MethodProxy.java:218) ~[spring-core-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.invokeJoinpoint(CglibAopProxy.java:749) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:163) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.transaction.interceptor.TransactionAspectSupport.invokeWithinTransaction(TransactionAspectSupport.java:295) ~[spring-tx-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:98) ~[spring-tx-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:688) ~[spring-aop-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at com.mycompane.service.organization.core.SomeAmazingService$$EnhancerBySpringCGLIB$7ドルe4fe77b.createOrganization(<generated>) ~[main/:na]
 at com.mycompane.service.organization.web.OrganizationController.createOrganization(OrganizationController.java:53) ~[main/:na]
 at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:na]
 at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:na]
 at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:na]
 at java.base/java.lang.reflect.Method.invoke(Method.java:566) ~[na:na]
 at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:892) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:797) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1039) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:942) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1005) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:908) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:660) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:882) ~[spring-webmvc-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:741) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.boot.actuate.web.trace.servlet.HttpTraceFilter.doFilterInternal(HttpTraceFilter.java:88) ~[spring-boot-actuator-2.1.6.RELEASE.jar:2.1.6.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:320) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.invoke(FilterSecurityInterceptor.java:127) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.access.intercept.FilterSecurityInterceptor.doFilter(FilterSecurityInterceptor.java:91) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.access.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:119) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.session.SessionManagementFilter.doFilter(SessionManagementFilter.java:137) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.authentication.AnonymousAuthenticationFilter.doFilter(AnonymousAuthenticationFilter.java:111) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.servletapi.SecurityContextHolderAwareRequestFilter.doFilter(SecurityContextHolderAwareRequestFilter.java:170) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.savedrequest.RequestCacheAwareFilter.doFilter(RequestCacheAwareFilter.java:63) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at com.mycompane.commons.v2.security.JwtAuthenticationTokenFilter.successfulAuthentication(JwtAuthenticationTokenFilter.java:68) ~[spring-commons-2.63.jar:na]
 at org.springframework.security.web.authentication.AbstractAuthenticationProcessingFilter.doFilter(AbstractAuthenticationProcessingFilter.java:240) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.authentication.logout.LogoutFilter.doFilter(LogoutFilter.java:116) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.header.HeaderWriterFilter.doFilterInternal(HeaderWriterFilter.java:74) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.context.SecurityContextPersistenceFilter.doFilter(SecurityContextPersistenceFilter.java:105) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.context.request.async.WebAsyncManagerIntegrationFilter.doFilterInternal(WebAsyncManagerIntegrationFilter.java:56) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.security.web.FilterChainProxy$VirtualFilterChain.doFilter(FilterChainProxy.java:334) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy.doFilterInternal(FilterChainProxy.java:215) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.security.web.FilterChainProxy.doFilter(FilterChainProxy.java:178) ~[spring-security-web-5.1.5.RELEASE.jar:5.1.5.RELEASE]
 at org.springframework.web.filter.DelegatingFilterProxy.invokeDelegate(DelegatingFilterProxy.java:357) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.filter.DelegatingFilterProxy.doFilter(DelegatingFilterProxy.java:270) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:99) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:92) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.web.filter.HiddenHttpMethodFilter.doFilterInternal(HiddenHttpMethodFilter.java:93) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.cloud.sleuth.instrument.web.ExceptionLoggingFilter.doFilter(ExceptionLoggingFilter.java:50) ~[spring-cloud-sleuth-core-2.1.2.RELEASE.jar:2.1.2.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at brave.servlet.TracingFilter.doFilter(TracingFilter.java:99) ~[brave-instrumentation-servlet-5.6.5.jar:na]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.filterAndRecordMetrics(WebMvcMetricsFilter.java:114) ~[spring-boot-actuator-2.1.6.RELEASE.jar:2.1.6.RELEASE]
 at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:104) ~[spring-boot-actuator-2.1.6.RELEASE.jar:2.1.6.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:200) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:109) ~[spring-web-5.1.8.RELEASE.jar:5.1.8.RELEASE]
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:490) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:408) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:853) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1587) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128) ~[na:na]
 at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628) ~[na:na]
 at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) ~[tomcat-embed-core-9.0.21.jar:9.0.21]
 at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]

• Naman: what is the type of enttsResponse?
• Abiral Khanal: If workaround works, and you only need the ids, you can check if the map has id and return the id from map instead.
• Holger: This has nothing to do with Java 11 vs Java 8. Type tokens do not work that way. When you instantiate new ParameterizedTypeReference<Wrapper<T>>() { }, it will only know that it has to produce a Wrapper, but still doesn’t know what T is. You have to remove the getForList method and replace the caller with the template.exchange(url, GET, new HttpEntity<>(headers), new ParameterizedTypeReference<Wrapper<MyCompanyResponse>>() { }); expression.
• Alex Andrade: @Holger call is ResponseEntity<Wrapper<EntitlementResponse>> response = rest.getForList(uriBuilder.toUriString(), headers);
• Holger: Well, then you have to use template.exchange(uriBuilder.toUriString(), GET, new HttpEntity<>(headers), new ParameterizedTypeReference<Wrapper<EntitlementResponse>>() { });, but the point remains. The type token must capture an actual type ("reifiable type") rather than a type variable. Otherwise, the framework doesn’t know what to produce and will fall back to collections (i.e. LinkedHashMap).

Answer 1

You have a type token of the form new ParameterizedTypeReference<Wrapper<T>>() { }, referring to a type variable. This doesn’t work due to type erasue (the reason why such type tokens exist in the first place).

The framework receiving the type token will only know that it has to produce a Wrapper, but still doesn’t know what T is. So it doesn’t know what to produce and will fall back to collections (i.e. LinkedHashMap).

You have to replace the caller of the generic method with an expression referring to the known reifiable type, i.e.

template.exchange(uriBuilder.toUriString(), GET, new HttpEntity<>(headers),
 new ParameterizedTypeReference<Wrapper<EntitlementResponse>>() { })

• Alex Andrade: solution was easy, just changed to: public <T> ResponseEntity<T> get(String url, HttpHeaders headers, ParameterizedTypeReference<T> type ) { ... }
• Holger: @AlexAndrade that’s also a viable solution. The crucial aspect is the creation of the type token at the caller.

Answer 2

It's incredible that it doesn't work is the base of Generics principle

I have a generics class <S,T> where S is a request body and T is response body of a webclient in a reactive mode get new Bearer Token or return latest token, if it doens't expried, and calling server by that token without using .block*() methods in no blocking mode.

In my class I need to use my class which is Client<S, T> and a method return T type responsebody and pass requestbody parameter as S type but using T in below and it doesn't work:

public Mono<T> getResponse( @NotBlank String logHash,
 @NotBlank String url,
 @NotNull @NotBlank @NotEmpty S requestBody,
 @NotNull @NotBlank @NotEmpty Class<T> responseBodyClass) {
 return getToken()
 .flatMap(token -> 
 webClient
 .post()
 .uri(url)
 .header("Authorization", "Bearer " + token)
 .body(BodyInserters.fromValue(requestBody))
 .retrieve()
 .onStatus(HttpStatus.INTERNAL_SERVER_ERROR::equals, response -> response.bodyToMono(String.class).map(Exception::new))
 .onStatus(HttpStatus.BAD_REQUEST::equals, response -> response.bodyToMono(String.class).map(Exception::new))
 .bodyToMono(new ParameterizedTypeReference<T>() {})
 .doOnSuccess(response -> log.debug("{} full url {} response {}", logHash, this.serverUrl + url, response))
 .doOnError(ex -> {
 log.error("{} {}", logHash, ex.getMessage());
 ex.printStackTrace();
 })
 )
 .doOnSuccess(token -> log.debug("{} url {} response {}", logHash, this.serverUrl + url, token))
 .doOnError(ex -> {
 log.error("{} {}", logHash, ex.getMessage());
 ex.printStackTrace();
 });

I need to substitute in previous code this line (It's incredible because type must be part of this bean customized dinamically by Spring Framework AOP in the code, I think that a solution for AOP Spring Framework is add a final object Class<T> and in runtime substitute T with correct class passed in constructor by @Autowired annotation as private final value not only verify type class on return):

.bodyToMono(new ParameterizedTypeReference<T>() {})

with responseBody Class Type which is a .class of my responseBody to avoid exception because JVM can't find T type for return body object:

.bodyToMono(responseBody)

What type of Generics Type is this implementatio?
I autowire class I pass Type in autowiring how is possibile that this can't find Type?

new ParameterizedTypeReference<T>() {}

caller:

 private Client<CheckCustomerBody, ApimResponse> client = null;

And in method that use client it I need to passawhen call it

 client = new Client<CheckCustomerBody, ApimResponse>();
 client.configure( WebClient.builder(),
 ClientType.XXXX_XX,
 channelEnum,
 logHash,
 Optional.empty(),
 Optional.empty(),
 Optional.empty()
 );
 return client
 .getResponse( logHash,
 WebOrderConstants.API_EXT_CHECK,
 request,
 ApimResponse.class)
 .map(apimResponse -> validationProductChange(
 apimResponse.getResponse(),
 customer.getClientAccountCode(),
 logHash
 )
 ? "OK"
 : "KO"
 )
 .doOnError(ex -> {
 log.error("{} END {} : Error using d365 service", logHash, prefix);
 ex.printStackTrace();
 });