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why is ResponseEntity <List<Object>> returning null?

Time：03-25

I have the following code and it works with getMapping("/users/{userId}") but it does not return the list of users and displays "Internal server error "

I have attached the code for the classes I have used and I have even created another class which contains list of User to be used instead of List<User> but it did not work and the output was null , so I would be gratefull if anyone could help me about it .

//User

@Setter
@Getter
@ToString
@JsonIgnoreProperties(ignoreUnknown = true)
public class User {

    private  int id;
    private String title;
    private String description;
    private int price;
    private float discountPercentage ;
         private float rating ;
         private int stock;
         private String brand;
         private String category;
         private String thumbnail;
         private String[] images;


    public User() {
    }
}

//UserService

 public List<User> getUsers() {
    ResponseEntity<List<User>> responseEntity=restTemplate.exchange("https://dummyjson.com/
     produts/",HttpMethod.GET, null, new ParameterizedTypeReference<List<User>>(){
            });
  List<User> users= responseEntity.getBody();
   return users;
}

//UserController

@GetMapping("/users")
public ResponseEntity<List<User>> getUsers(){
    return new ResponseEntity<List<User>>( userService.getUsers(),HttpStatus.OK);
}

//JSON

{
    "User": [
        {
            "id": 1,
            "title": "iPhone 9",
            "description": "An apple mobile which is nothing like apple",
            "price": 549,
            "discountPercentage": 12.96,
            "rating": 4.69,
            "stock": 94,
            "brand": "Apple",
            "category": "smartphones",
            "thumbnail": "https://dummyjson.com/image/i/products/1/thumbnail.jpg",
            "images": [
                "https://dummyjson.com/image/i/products/1/1.jpg",
                "https://dummyjson.com/image/i/products/1/2.jpg",
                "https://dummyjson.com/image/i/products/1/3.jpg",
                "https://dummyjson.com/image/i/products/1/4.jpg",
                "https://dummyjson.com/image/i/products/1/thumbnail.jpg"
            ]
        },
        {
            "id": 2,
            "title": "iPhone X",
            "description": "SIM-Free, Model A19211 6.5-inch Super Retina HD display with OLED technology A12 Bionic chip with ...",
            "price": 899,
            "discountPercentage": 17.94,
            "rating": 4.44,
            "stock": 34,
            "brand": "Apple",
            "category": "smartphones",
            "thumbnail": "https://dummyjson.com/image/i/products/2/thumbnail.jpg",
            "images": [
                "https://dummyjson.com/image/i/products/2/1.jpg",
                "https://dummyjson.com/image/i/products/2/2.jpg",
                "https://dummyjson.com/image/i/products/2/3.jpg",
                "https://dummyjson.com/image/i/products/2/thumbnail.jpg"
            ]
        }
],
    "total": 100,
    "skip": 0,
    "limit": 30
}

//Error

threw exception [Request processing failed; nested exception is org.springframework.web.client.RestClientException: Error while extracting response for type [java.util.List<com.example.demo.entity.User>] and content type [application/json;charset=utf-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize value of type `java.util.ArrayList<com.example.demo.entity.User>` from Object value (token `JsonToken.START_OBJECT`); nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<com.example.demo.entity.User>` from Object value (token `JsonToken.START_OBJECT`)

at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 1, column: 1]] with root cause



com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize value of type `java.util.ArrayList<com.example.demo.entity.User>` from Object value (token `JsonToken.START_OBJECT`)

at [Source: (org.springframework.util.StreamUtils$NonClosingInputStream); line: 1, column: 1]

at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1741) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1515) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1462) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.handleNonArray(CollectionDeserializer.java:392) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:252) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.deser.std.CollectionDeserializer.deserialize(CollectionDeserializer.java:28) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.deser.DefaultDeserializationContext.readRootValue(DefaultDeserializationContext.java:322) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4674) ~[jackson-databind-2.13.1.jar:2.13.1]

at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3682) ~[jackson-databind-2.13.1.jar:2.13.1]

at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:380) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:343) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:105) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.client.RestTemplate$ResponseEntityResponseExtractor.extractData(RestTemplate.java:1037) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.client.RestTemplate$ResponseEntityResponseExtractor.extractData(RestTemplate.java:1020) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:778) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:711) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.client.RestTemplate.exchange(RestTemplate.java:631) ~[spring-web-5.3.16.jar:5.3.16]

at com.example.demo.service.UserService.getUsers(UserService.java:36) ~[classes/:na]

at com.example.demo.controller.UserController.getUsers(UserController.java:30) ~[classes/:na]

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_322]

at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_322]

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_322]

at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_322]

at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:150) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:117) ~[spring-webmvc-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:895) ~[spring-webmvc-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1067) ~[spring-webmvc-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:963) ~[spring-webmvc-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.16.jar:5.3.16]

at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.3.16.jar:5.3.16]

at javax.servlet.http.HttpServlet.service(HttpServlet.java:655) ~[tomcat-embed-core-9.0.58.jar:4.0.FR]

at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.3.16.jar:5.3.16]

at javax.servlet.http.HttpServlet.service(HttpServlet.java:764) ~[tomcat-embed-core-9.0.58.jar:4.0.FR]

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.58.jar:9.0.58]

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.16.jar:5.3.16]

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.16.jar:5.3.16]

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.16.jar:5.3.16]

at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:117) ~[spring-web-5.3.16.jar:5.3.16]

at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:197) ~[tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:540) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:135) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:359) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:399) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:889) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1735) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1191) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.tomcat.util.threads.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:659) [tomcat-embed-core-9.0.58.jar:9.0.58]

at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61) [tomcat-embed-core-9.0.58.jar:9.0.58]

at java.lang.Thread.run(Thread.java:750) [na:1.8.0_322]

CodePudding user response：

As exception suggest, response can not map to List<User>, beside it i suggest to use Optional to manage all null possibilities as well

yourServiceReturnTypeAsOptional
       .map(vs -> ResponseEntity.status(HttpStatus.OK).body(vs))
              .orElseGet(() ->ResponseEntity.status(HttpStatus.NO_CONTENT)
              .build())

CodePudding user response：

You can use custom wrapper class:

public class UserList {

private List<User> users;

public List<User> getUsers() {
    return this.users;
}

public void setUsers(List<User> users) {
    this.users = users;
}

Then:

UserList userList =restTemplate.exchange("https://dummyjson.com/
     produts/",HttpMethod.GET, null, new ParameterizedTypeReference<UserList.class>(){
            });
List<User> users= userList.getUsers();

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