no suitable HttpMessageConverter found for request type [java.lang.Integer]

今天在使用Spring Template的时候遇到了这个异常：

no suitable HttpMessageConverter found for request type [java.lang.Integer]

Google了一下然后在stackoverflow上面找到了解决方案：

I have a method in Spring rest service.

  @RequestMapping(value = "test/process", method = RequestMethod.POST)
  public @ResponseBody MyResponse processRequest(String RequestId, int count)

I am using Spring RestTemplate to call this service like this.

  RestTemplate restTemplate = this.getRestTemplate();
  MultiValueMap<String, Object> map = new LinkedMultiValueMap<String, Object>();
  map.add("RequestId", RequestId);
  map.add("count", count); 
  restTemplate.postForObject(url, map,MyResponse.class);

When I try to invoke the client method I get the exception that no suitable HttpMessageConverter found for request type [java.lang.Integer]

org.springframework.http.converter.HttpMessageNotWritableException: Could not write request: no suitable HttpMessageConverter found for request type [java.lang.Integer]
at org.springframework.http.converter.FormHttpMessageConverter.writePart(FormHttpMessageConverter.java:310)
at org.springframework.http.converter.FormHttpMessageConverter.writeParts(FormHttpMessageConverter.java:270)
at org.springframework.http.converter.FormHttpMessageConverter.writeMultipart(FormHttpMessageConverter.java:260)
at org.springframework.http.converter.FormHttpMessageConverter.write(FormHttpMessageConverter.java:200)
at org.springframework.http.converter.FormHttpMessageConverter.write(FormHttpMessageConverter.java:1)
at org.springframework.web.client.RestTemplate$HttpEntityRequestCallback.doWithRequest(RestTemplate.java:596)

I know one of the ways is to pass all the parameters as String. But I might need to pass complex data types as parameters later. What is the ways to achieve this. I have googled and some option seem to be writing my own converters. How should I start about solving this problem.

The root cause of this error is that by specifying an Integer in the LinkedMultiValueMap, the RestTemplate will take that to mean that your request is a multipart request. There is no HttpMessageConverter registered by default that can handle writing values of type Integer to a request body.

As you said, you can handle this situation by changing the count to be a String. After all, there is no Integer type in HTTP request parameters. However, you were worried

But I might need to pass complex data types as parameters later.

Assume something like this

  public @ResponseBody MyResponse processRequest(String RequestId, int count, Complex complex) 

with

public class Complex {
    private String someValue;
    private int intValue;

    public String getSomeValue() {
        return someValue;
    }

    public void setSomeValue(String someValue) {
        this.someValue = someValue;
    }

    public int getIntValue() {
        return intValue;
    }

    public void setIntValue(int intValue) {
        this.intValue = intValue;
    }

    public String toString() {
        return someValue + " " + intValue;
    }
}

The the following will work just fine

MultiValueMap<String, Object> map = new LinkedMultiValueMap<String, Object>();
map.add("RequestId", "asd");
map.add("count", "42");
map.add("someValue", "complex");
map.add("intValue", "69");
restTemplate.postForObject(url, map,MyResponse.class);

Remember that the request parameters are used to populate the fields of model attributes by their names.

An even better solution would have you using a serialization standard like JSON or XML.I mean instead of sending request parameters, you should send JSON and XML. Spring supports those types with @RequestBody annotation on handler method parameters.