Spring boot app stops with UnknownContentTypeException when trying to return JSON response from endpoint

I am trying to make a Spring boot app that uses the API endpoints of SWAPI and refine the results to some of them. To explain better I want to take the name of a character the user provided from a GET request and return to him the Starships this character has piloted. I am trying to use RestTemplate to make the calls to the SWAPI API and the refine these results to provide the ones I want. From the debugging I did there seems to be a problem when calling the resttemplate in the for loop in the controller class resulting to this :

2021-05-05 17:13:54.099 ERROR 272 --- [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.web.client.UnknownContentTypeException: Could not extract response: no suitable HttpMessageConverter found for response type [class com.example.swapispring.entity.Starship] and content type [text/html]] with root cause

org.springframework.web.client.UnknownContentTypeException: Could not extract response: no suitable HttpMessageConverter found for response type [class com.example.swapispring.entity.Starship] and content type [text/html]
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:126) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:778) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:711) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.client.RestTemplate.getForObject(RestTemplate.java:334) ~[spring-web-5.3.6.jar:5.3.6]
    at com.example.swapispring.controller.Controller.getStarships(Controller.java:36) ~[classes/: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:197) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:141) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:106) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:894) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:808) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1060) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:962) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:626) ~[tomcat-embed-core-9.0.45.jar:4.0.FR]
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883) ~[spring-webmvc-5.3.6.jar:5.3.6]
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:733) ~[tomcat-embed-core-9.0.45.jar:4.0.FR]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:227) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) ~[tomcat-embed-websocket-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.6.jar:5.3.6]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.6.jar:5.3.6]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201) ~[spring-web-5.3.6.jar:5.3.6]
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119) ~[spring-web-5.3.6.jar:5.3.6]
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:189) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:162) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:97) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:542) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:143) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:78) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:357) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:374) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:893) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1707) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49) ~[tomcat-embed-core-9.0.45.jar:9.0.45]
    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-9.0.45.jar:9.0.45]
    at java.base/java.lang.Thread.run(Thread.java:834) ~[na:na]

Main method

package com.example.swapispring;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;

@SpringBootApplication
public class SwapiWithSpringApplication {

    @Bean
    public RestTemplate getRestTemplate() {
        return new RestTemplate();
    }

    public static void main(String[] args) {
        SpringApplication.run(SwapiWithSpringApplication.class, args);
    }

}

Controller

package com.example.swapispring.controller;

import com.example.swapispring.entity.Character;
import com.example.swapispring.entity.InitialResponse;
import com.example.swapispring.entity.Starship;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

import java.util.ArrayList;
import java.util.List;

@RestController
@RequestMapping("/starships")
public class Controller {

    @Autowired
    private RestTemplate restTemplate;

    @RequestMapping("/{characterName}")
    public List<Starship> getStarships(@PathVariable("characterName") String characterName) {


        InitialResponse initialResponse = restTemplate.getForObject("https://swapi.dev/api/people/?search=" + characterName, InitialResponse.class);

        ArrayList<Starship> results = new ArrayList<>();

        for (Character tempCharacter:initialResponse.getCharacters()) {
          for (String tempStarship: tempCharacter.getStarships()) {
               Starship resultingObject = restTemplate.getForObject(tempStarship,Starship.class);
               results.add(resultingObject);
          }
        };
        return results;
    }
}

All entities (InitialResponse, Character, Starship) were created using jsonschema2pojo. I cannot provide the whole classes due to the character limit of the question.

P.S. I am very new and trying to improve the quality of my questions on stack overflow. If I have made mistakes on the format of my question, feel free to provide feedback on this matter aswell.

Answer

After many hours of debugging the issue was at the second restTemplate call in the nested for loop. The url for the call was “http” (e.g. http://swapi.dev/api/starships/12/) and not “https” so it was failing to get the response even though the link works normally in a browser as it automatically redirects to the “https” version. Fixed with the following:

package com.example.swapispring.controller;

import com.example.swapispring.entity.responseOfInhabitantsFromPlanetName.InitialResponseOfPlanets;
import com.example.swapispring.entity.responseOfInhabitantsFromPlanetName.Planet;
import com.example.swapispring.entity.responseOfInhabitantsFromPlanetName.Residents;
import com.example.swapispring.entity.responseOfStarshipsFromCharacterName.Character;
import com.example.swapispring.entity.responseOfStarshipsFromCharacterName.InitialResponseOfCharacters;
import com.example.swapispring.entity.responseOfStarshipsFromCharacterName.Starship;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

import java.util.ArrayList;

@RestController
@RequestMapping("/api")
public class Controller {

    @Autowired
    private RestTemplate restTemplate;

    //Endpoint to return JSON of starships that the given character has piloted
    @RequestMapping("/starships/{characterName}")
    public ArrayList<Starship> getStarships(@PathVariable("characterName") String characterName) {

        //Rest Template get for initial response JSON (The character)
        InitialResponseOfCharacters initialResponseOfCharacters = restTemplate.getForObject("https://swapi.dev/api/people/?search=" + characterName, InitialResponseOfCharacters.class);

        //Array list of starships to be returned as the final response
        ArrayList<Starship> results = new ArrayList<>();
        assert initialResponseOfCharacters != null;
        if (initialResponseOfCharacters.getCharacters().size() == 1) {
            for (Character tempCharacter : initialResponseOfCharacters.getCharacters()) {
                for (String tempStarship : tempCharacter.getStarships()) {
                    tempStarship = tempStarship.replaceAll("http", "https");
                    Starship resultingObject = restTemplate.getForObject(tempStarship, Starship.class);
                    results.add(resultingObject);
                }
            }
            //error handling
        } else {
            Starship starshipError = new Starship();
            starshipError.setAdditionalProperty("Error", "More than one or no characters found please refine your search");
            results.add(starshipError);
        }
        return results;
    }