我想识别通过POST请求的请求正文发送的JSON中插入的不带引号(作为字符串)的数值：

例如,这将是错误的JSON格式,因为age字段不包含引号：

{
  "Student":{
    "Name": "John",
    "Age":  12
  }
}

正确的JSON格式为：

{
  "Student":{ 
    "Name": "John",
    "Age":  "12"
  }
}

在我的代码中,我已将age字段的数据类型定义为String,因此“ 12”应该是正确的输入.但是,即使使用12,也不会引发任何错误消息.

杰克逊似乎自动将数值转换为字符串.如何识别数值并返回消息？

到目前为止,我一直在尝试识别这些数值：

public List<Student> getMultiple(StudentDTO Student) {
    if(Student.getAge().getClass()==String.class) {
        System.out.println("Age entered correctly as String");
    } else{
        System.out.println("Please insert age value inside inverted commas");
    }
}

但是,如果插入不带引号的年龄,则不会在控制台上打印“请在反逗号内插入年龄值”.

解决方法:

默认情况下,当目标字段为String类型时,Jackson将标量值转换为String.这个想法是为String类型创建一个自定义反序列化器,并注释掉转换部分：

package jackson.deserializer;

import java.io.IOException;

import com.fasterxml.jackson.core.*;
import com.fasterxml.jackson.databind.*;
import com.fasterxml.jackson.databind.deser.std.StringDeserializer;


public class CustomStringDeserializer extends StringDeserializer 
{

    public final static CustomStringDeserializer instance = new CustomStringDeserializer();

    @Override
    public String deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        if (p.hasToken(JsonToken.VALUE_STRING)) {
            return p.getText();
        }
        JsonToken t = p.getCurrentToken();
        // [databind#381]
        if (t == JsonToken.START_ARRAY) {
            return _deserializeFromArray(p, ctxt);
        }
        // need to gracefully handle byte[] data, as base64
        if (t == JsonToken.VALUE_EMBEDDED_OBJECT) {
            Object ob = p.getEmbeddedObject();
            if (ob == null) {
                return null;
            }
            if (ob instanceof byte[]) {
                return ctxt.getBase64Variant().encode((byte[]) ob, false);
            }
            // otherwise, try conversion using toString()...
            return ob.toString();
        }
        // allow coercions for other scalar types
        // 17-Jan-2018, tatu: Related to [databind#1853] avoid FIELD_NAME by ensuring it's
        //   "real" scalar
        /*if (t.isScalarValue()) {
            String text = p.getValueAsString();
            if (text != null) {
                return text;
            }
        }*/
        return (String) ctxt.handleUnexpectedToken(_valueClass, p);
    }

}

现在注册此反序列化器：

@Bean
public Module customStringDeserializer() {
    SimpleModule module = new SimpleModule();
    module.addDeserializer(String.class, CustomStringDeserializer.instance);
    return module;
}

当发送一个整数并且期望使用String时,这是错误：

{“timestamp”:”2019-04-24T15:15:58.968+0000″,”status”:400,”error”:”Bad
Request”,”message”:”JSON parse error: Cannot deserialize instance of
java.lang.String out of VALUE_NUMBER_INT token; nested exception is
com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot
deserialize instance of java.lang.String out of VALUE_NUMBER_INT
token\n at [Source: (PushbackInputStream); line: 3, column: 13]
(through reference chain:
org.hello.model.Student[\”age\”])”,”path”:”/hello/echo”}