Rest API: Json参数格式错误时显示Json原文

后台提供API给前端调用，一般使用HttpMessageConverter把Json字符串转换成对象，如果存在格式问题则会抛出异常HttpMessageNotReadableException。但是后台拿不到原始的Json字符串，不方便定位到确切的格式错误。 本文介绍一个简易方法，可以在出错时，展示Json原文信息。

一开始想寻找其他方式，当抛出异常的时候，就从HttpServletRequest读取body内容。但是做不到，因为在做Json读取的时候，InputStream已经被读取了，无法再次获取到body内容。所以只能通过自定义MessageConverter的方式。

其实也可以，把request中inpustream读取出来保存，再提供一个getInputStream，参见 How to read request.getInputStream() multiple times

自定义MessageConverter

• Spring4

import com.google.common.io.CharStreams;

public class HttpMessageConverter extends GsonHttpMessageConverter {

    @Override
    public Object read(Type type, Class contextClass, HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
        TypeToken token = getTypeToken(type);
        return readTypeToken(token, inputMessage);
    }

    private Object readTypeToken(TypeToken token, HttpInputMessage inputMessage) throws IOException {
        Reader json = new InputStreamReader(inputMessage.getBody(), getCharset(inputMessage.getHeaders()));
        //先获取Json原始字符串
        final String jsonBody = CharStreams.toString(json);
        try {
            return this.getGson().fromJson(jsonBody, token.getType());
        } catch (JsonParseException ex) {
            //解析失败，则在异常中输出Json原始字符串
            throw new HttpMessageNotReadableException(String.format("JSON parse error: %s%n%s", ex.getMessage(), jsonBody), ex);
        }
    }

    private Charset getCharset(HttpHeaders headers) {
        if (headers == null || headers.getContentType() == null || headers.getContentType().getCharset() == null) {
            return DEFAULT_CHARSET;
        }
        return headers.getContentType().getCharset();
    }
}

• Spring 5
  在Spring 5中实现起来更加简单

public class HttpMessageConverter extends GsonHttpMessageConverter {
    @Override
    protected Object readInternal(Type resolvedType, Reader reader) throws Exception {
        String jsonBody = CharStreams.toString(reader);
        try {
            return this.getGson().fromJson(jsonBody, resolvedType);
        } catch (JsonParseException ex) {
            //解析失败，则在异常中输出Json原始字符串
            throw new JsonParseException(String.format("JSON parse error: %s%n%s", ex.getMessage(), jsonBody), ex);
        }
    }
}

配置自定义MessageConverter

spring-mvc.xml

结果

假设参数对象如下：

@Data
public class Person {
    private String name;
    private Integer age;
    private Boolean gender;
    private List hobbies;
}

如果收到错误格式的Json消息，则会抛出异常（hobbies应该是一个数组，参数中是字符串basketball）

org.springframework.http.converter.HttpMessageNotReadableException: JSON parse error: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 5 column 14 path $.hobbies
{
    "name": "tenmao",
    "gender": true,
    "age": 28,
    "hobbies": "basketball"
}; nested exception is com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_ARRAY but was STRING at line 5 column 14 path $.hobbies

参考

• How to read request.getInputStream() multiple times