How do I parse an array using RestTemplate in Spring Framework?

Question

I've this piece of JSON data which comes through Fixer API for currency exchange.

http://data.fixer.io/api/latest?access_key=API_KEY&symbols=USD,INR&format=1

{
  "success":true,
  "timestamp":1593947225,
  "base":"EUR",
  "date":"2020-07-05",
  "rates":{
    "USD":1.1245,
    "INR":83.977026
  }
}

I need to read the rates nested array using RestTemplate and I tried to make this classes to make it works with no luck!

LatestRatesController.java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Bean;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;


@RestController
public class LatestRatesController {
    
    @Value("${access_key}")
    private String access_key;
    
    @Autowired
    private RestTemplate restTemplate;
    
      @Bean
        public RestTemplate restTemplate() {
            return new RestTemplate();
        }
      
    @GetMapping("/latest-rates/from/EUR/to/{to}")
    public ExchangeValue retrieveExchangeValue(@PathVariable("to") String to)
    {
        ExchangeValue exchangeValue = restTemplate.getForObject("http://data.fixer.io/api/latest?access_key=" + access_key
                +"&symbols="+ to+"&format=1", ExchangeValue.class);
        
        return exchangeValue;
        
    }
}

ExchangeValue.java

public class ExchangeValue {

    private boolean success;
    private BigDecimal timestamp;
    private String base;
    private String date;
    private List<String> rates;
    
    protected ExchangeValue() {
        
    }
    
    public ExchangeValue(boolean success, BigDecimal timestamp, String base, String date) {
        super();
        this.success = success;
        this.timestamp = timestamp;
        this.base = base;
        this.date = date;
    }
    
    public List<String> getRates() {
        return rates;
    }

    public void setRates(List<String> rates) {
        this.rates = rates;
    }

    public boolean isSuccess() {
        return success;
    }
    public void setSuccess(boolean success) {
        this.success = success;
    }
    public BigDecimal getTimestamp() {
        return timestamp;
    }
    public void setTimestamp(BigDecimal timestamp) {
        this.timestamp = timestamp;
    }
    public String getBase() {
        return base;
    }
    public void setBase(String base) {
        this.base = base;
    }
    public String getDate() {
        return date;
    }
    public void setDate(String date) {
        this.date = date;
    }
    
}

Rates.java

public class Rates {

    private String USD;

    public Rates(String uSD) {
        super();
        USD = uSD;
    }

    public String getUSD() {
        return USD;
    }

    public void setUSD(String uSD) {
        USD = uSD;
    }
    
}

and actually I need to read any other currencies might pass through the API URI so the Rates class I made it to test only with USD one, and when try to running this I got this error

Whitelabel Error Page
This application has no explicit mapping for /error, so you are seeing this as a fallback.

Sun Jul 05 14:44:12 EEST 2020
There was an unexpected error (type=Internal Server Error, status=500).
Error while extracting response for type [class com.fixer.microservices.latestratesservice.ExchangeValue] and content type [application/json;Charset=UTF-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize instance of `java.util.ArrayList<java.lang.Object>` out of START_OBJECT token; nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.ArrayList<java.lang.Object>` out of START_OBJECT token at [Source: (PushbackInputStream); line: 6, column: 11] (through reference chain: com.fixer.microservices.latestratesservice.ExchangeValue["rates"])
org.springframework.web.client.RestClientException: Error while extracting response for type [class com.fixer.microservices.latestratesservice.ExchangeValue] and content type [application/json;Charset=UTF-8]; nested exception is org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize instance of `java.util.ArrayList<java.lang.Object>` out of START_OBJECT token; nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.ArrayList<java.lang.Object>` out of START_OBJECT token
 at [Source: (PushbackInputStream); line: 6, column: 11] (through reference chain: com.fixer.microservices.latestratesservice.ExchangeValue["rates"])
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:120)
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:741)
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:674)
    at org.springframework.web.client.RestTemplate.getForObject(RestTemplate.java:315)
    at com.fixer.microservices.latestratesservice.LatestRatesController.retrieveExchangeValue(LatestRatesController.java:30)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:190)
    at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:138)
    at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:105)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:879)
    at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:793)
    at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:87)
    at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:1040)
    at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:943)
    at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:1006)
    at org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:898)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
    at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:883)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:231)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.RequestContextFilter.doFilterInternal(RequestContextFilter.java:100)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.FormContentFilter.doFilterInternal(FormContentFilter.java:93)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.boot.actuate.metrics.web.servlet.WebMvcMetricsFilter.doFilterInternal(WebMvcMetricsFilter.java:93)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:201)
    at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:119)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:193)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:166)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:202)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:541)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:139)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:92)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:74)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:343)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:373)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:65)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:868)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1590)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)
Caused by: org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: Cannot deserialize instance of `java.util.ArrayList<java.lang.Object>` out of START_OBJECT token; nested exception is com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.ArrayList<java.lang.Object>` out of START_OBJECT token
 at [Source: (PushbackInputStream); line: 6, column: 11] (through reference chain: com.fixer.microservices.latestratesservice.ExchangeValue["rates"])
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:275)
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:257)
    at org.springframework.web.client.HttpMessageConverterExtractor.extractData(HttpMessageConverterExtractor.java:105)
    ... 58 more
Caused by: com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot deserialize instance of `java.util.ArrayList<java.lang.Object>` out of START_OBJECT token
 at [Source: (PushbackInputStream); line: 6, column: 11] (through reference chain: com.fixer.microservices.latestratesservice.ExchangeValue["rates"])
    at com.fasterxml.jackson.databind.exc.MismatchedInputException.from(MismatchedInputException.java:59)
    at com.fasterxml.jackson.databind.DeserializationContext.reportInputMismatch(DeserializationContext.java:1464)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1238)
    at com.fasterxml.jackson.databind.DeserializationContext.handleUnexpectedToken(DeserializationContext.java:1148)
    at com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer.handleNonArray(StringCollectionDeserializer.java:274)
    at com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer.deserialize(StringCollectionDeserializer.java:183)
    at com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer.deserialize(StringCollectionDeserializer.java:173)
    at com.fasterxml.jackson.databind.deser.std.StringCollectionDeserializer.deserialize(StringCollectionDeserializer.java:21)
    at com.fasterxml.jackson.databind.deser.impl.MethodProperty.deserializeAndSet(MethodProperty.java:129)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:371)
    at com.fasterxml.jackson.databind.deser.BeanDeserializer.deserialize(BeanDeserializer.java:164)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4482)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3487)
    at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:269)
    ... 60 more

Answer 1

You need to use Map<String, Double> rates instead of List<String> rates as json structure in ExchangeValue class.

public class ExchangeValue {

    private boolean success;
    private BigDecimal timestamp;
    private String base;
    private String date;
    private Map<String, Double> rates;
    ...
}

Answer 2

Your rates object is not a JSONArray instead it's another JSONObject. So to cast it to Java Object, the better data structure would be a hash map.

Thus you dont need Rates class.

And you can define your ExchangeValue class like this:

public class ExchangeValue {

  private boolean success;
  private BigDecimal timestamp;
  private String base;
  private String date;
  private Map<String,Double> rates;

  protected ExchangeValue() {

  }

  public ExchangeValue(boolean success, BigDecimal timestamp, String base, String date, Map<String, Double> rates) {
    this.success = success;
    this.timestamp = timestamp;
    this.base = base;
    this.date = date;
    this.rates = rates;
  }

  public Map<String, Double> getRates() {
    return rates;
  }

  public void setRates(Map<String, Double> rates) {
    this.rates = rates;
  }

  public boolean isSuccess() {
    return success;
  }
  public void setSuccess(boolean success) {
    this.success = success;
  }
  public BigDecimal getTimestamp() {
    return timestamp;
  }
  public void setTimestamp(BigDecimal timestamp) {
    this.timestamp = timestamp;
  }
  public String getBase() {
    return base;
  }
  public void setBase(String base) {
    this.base = base;
  }
  public String getDate() {
    return date;
  }
  public void setDate(String date) {
    this.date = date;
  }

}

Answer 3

In your case rates are not List<String> but rather Map<String, Double>.

There are no square brackets [] (list) but curly ones {} (in this case a map).