jackson 常见问题

org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type

org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class test.jackson.Employee]: can not instantiate from JSON object (need to add/enable type information?)

 at [Source: c:\temp\employee.json; line: 1, column: 2]

    at org.codehaus.jackson.map.JsonMappingException.from(JsonMappingException.java:163)

    at org.codehaus.jackson.map.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:483)

    at org.codehaus.jackson.map.deser.BeanDeserializer.deserialize(BeanDeserializer.java:350)

    at org.codehaus.jackson.map.ObjectMapper._readMapAndClose(ObjectMapper.java:2395)

    at org.codehaus.jackson.map.ObjectMapper.readValue(ObjectMapper.java:1549)

    at test.jackson.JSONToJavaExample.main(JSONToJavaExample.java:19)

一般来说，解决上面问题从下面几个方面入手：

1、是否缺少默认构造函数

2、是否是类的访问修饰符问题，即jackson访问不到。

jackson 的UnrecognizedPropertyException错误

java代码如下：

1. public class Student implements Serializable{ 
2.     private static final long serialVersionUID = 685922460589405829L; 
3.  
4.     private String name; 
5.     private String age; 
6.  
7.    /*get set略*/ 
8. } 

json字符串如下：

"{\"address\":\"hunan\",\"name\":\"china\",\"age\":\"5000\"}"

这种情况下，使用jackson从string转换为object的时候，会抛出下面异常：

1. java.lang.IllegalArgumentException: org.codehaus.jackson.map.exc.UnrecognizedPropertyException: Unrecognized field "address" (Class util.Student), not marked as ignorable 

意思是说Student类里没有address这个属性，所以无法正常转化，同时还指明了not marked as ignorable，即没有标明可忽略的特性

解决方案：

1、在类上添加忽略声明

@JsonIgnoreProperties(ignoreUnknown=true)

2、更改str2obj方法

   ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
    projectVO = objectMapper.readValue(yourjsonstring, Test.class);

Jackson annotation - How to rename element names?

you can rename a property just adding

@JsonProperty("contractor")

And by default Jackson use the getter and setter to serialize and deserialize.

Using a custom JsonSerializer.

public class Response {
  private String status;
  private String error;

  @JsonProperty("p")
  @JsonSerialize(using = CustomSerializer.class)
  private Object data;

  // ...
}

public class CustomSerializer extends JsonSerializer<Object> {
  public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
    jgen.writeStartObject();
    jgen.writeObjectField(value.getClass().getName(), value);
    jgen.writeEndObject();
  }
}

Could not read JSON: Can not construct instance of java.util.Date from String value ‘2012-07-21 12:11:12‘: not a valid representation

1. "yyyy-MM-dd‘T‘HH:mm:ss.SSSZ"
2. "yyyy-MM-dd‘T‘HH:mm:ss.SSS‘Z‘"
3. "EEE, dd MMM yyyy HH:mm:ss zzz"
4. "yyyy-MM-dd"

当尸实体中存在Date类型，但是json字符串中是字符串类型

只支持以上几种格式否则报错

1.  
2. /** 
3. *  java日期对象经过Jackson库转换成JSON日期格式化自定义类 
4. * @author godfox 
5. * @date 2010-5-3 
6. */ 
7. public class CustomDateSerializer extends JsonSerializer<Date> { 
8.  
9.         @Override 
10.         public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException { 
11.                 SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd"); 
12.                 String formattedDate = formatter.format(value); 
13.                 jgen.writeString(formattedDate); 
14.         } 
15. } 

然后在你的POJO上找到日期的get方法

Java代码  

1. @JsonSerialize(using = CustomDateSerializer.class)  
2.        public Date getCreateAt() {  
3.                return createAt;  
4.        }  

1. @JsonSerialize(using = CustomDateSerializer.class) 
2.        public Date getCreateAt() { 
3.                return createAt; 
4.        } 

1. //null的JSON序列 
2.     private class NullSerializer extends JsonSerializer<Object> { 
3.         public void serialize(Object value, JsonGenerator jgen, 
4.                 SerializerProvider provider) throws IOException, 
5.                 JsonProcessingException { 
6.             jgen.writeString(""); 
7.         } 
8.     } 

在spring的配置文件中定义这个自定义的Mapper

在spring的配置文件中定义这个自定义的Mapper

Xml代码

1. <!-- 启动 MVC注解 -->
2. <mvc:annotation-driven>
3. <mvc:message-converters>
4. <bean class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
5. <property name="objectMapper" ref="customObjectMapper" />
6. </bean>
7. </mvc:message-converters>
8. </mvc:annotation-driven>
9. <!-- 自定义的JSON ObjectMapper -->
10. <bean id="customObjectMapper" class="com.xixi.web4j.util.CustomObjectMapper" />

就这么简单，不用再像每个POJO对像上去@JsonSerializer(using...)了。

以后对所有DATE类型字段序列化JSON，都会默认传为“yyyy-MM-dd HH:mm:ss”格式，对于字段为null的也会转为“”。如果你对null有别的处理。你可以在自定义的NullSerializer修改你想要的格式。