在开发中，经常需要测试Restful接口，
需要生成POJO简单对象的JSON字符串，
如果对象类的字段比较多，
手工生成的时候会很麻烦，
下面提供一个基于Jackson的工具类，
能够根据POJO简单对象类，
初始化对象类中字段的默认值，
然后生成JSON字符串。
简单对象类UserEntity.java如下，
省略了get/set/toString等方法:
package com.yuwen.spring.demo.entity;
import com.baomidou.mybatisplus.annotation.IdType;
import com.baomidou.mybatisplus.annotation.TableField;
import com.baomidou.mybatisplus.annotation.TableId;
import com.baomidou.mybatisplus.annotation.TableName;
import java.io.Serializable;
import java.time.LocalDate;
import java.time.LocalDateTime;
public class UserEntity implements Serializable {
private static final long serialVersionUID = 1L;
private Long id;
private String name;
private String email;
private LocalDate birthday;
private LocalDateTime createTime;
private LocalDateTime updateTime;
}
3.Jackson工具类
工具类JacksonJsonGenerator.java如下，
每次使用的时候只需要修改targetClass：
package com.yuwen.spring.jackson.generator;
import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.time.LocalDate;
import java.time.LocalDateTime;
import java.time.LocalTime;
import java.time.format.DateTimeFormatter;
import org.apache.commons.lang3.StringUtils;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.ReflectionUtils.FieldCallback;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.datatype.jsr310.JavaTimeModule;
import com.fasterxml.jackson.datatype.jsr310.ser.LocalDateSerializer;
import com.fasterxml.jackson.datatype.jsr310.ser.LocalDateTimeSerializer;
import com.fasterxml.jackson.datatype.jsr310.ser.LocalTimeSerializer;
import com.yuwen.spring.demo.entity.UserEntity;
public class JacksonJsonGenerator {
// 指定需要生成JSON的Java类
public static Class targetClass = UserEntity.class;
public static void main(String[] args) throws Exception {
Object object = initObjectDefaultValue(targetClass);
genetateJson(object);
}
private static Object initObjectDefaultValue(Class targetClass)
throws InstantiationException, IllegalAccessException {
Object object = targetClass.newInstance();
// 根据属性类型给属性设默认值
FieldCallback fc = new FieldCallback() {
@Override
public void doWith(Field field) throws IllegalArgumentException, IllegalAccessException {
Class fieldType = field.getType();
// final修饰的字段无法设置新的值
if (Modifier.isFinal(field.getModifiers())) {
return;
}
String type = fieldType.getSimpleName();
Object fieldValue = StringUtils.EMPTY;
switch (type) {
case "String":
fieldValue = StringUtils.EMPTY;
break;
case "int":
case "Integer":
fieldValue = 0;
break;
case "long":
case "Long":
fieldValue = 0L;
break;
case "LocalDate":
fieldValue = LocalDate.now();
break;
case "LocalDateTime":
fieldValue = LocalDateTime.now();
break;
default:
fieldValue = StringUtils.EMPTY;
break;
}
try {
field.setAccessible(true);
field.set(object, fieldValue);
} catch (Exception e) {
e.printStackTrace();
}
}
};
ReflectionUtils.doWithFields(targetClass, fc);
return object;
}
private static void genetateJson(Object object) throws JsonProcessingException {
System.out.println("POJO对象：");
System.out.println(object);
ObjectMapper mapper = new ObjectMapper();
// 用于序列化Java8新的时间类型
JavaTimeModule javaTimeModule = new JavaTimeModule();
javaTimeModule.addSerializer(LocalDateTime.class,
new LocalDateTimeSerializer(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
javaTimeModule.addSerializer(LocalDate.class, new LocalDateSerializer(DateTimeFormatter.ISO_LOCAL_DATE));
javaTimeModule.addSerializer(LocalTime.class, new LocalTimeSerializer(DateTimeFormatter.ISO_LOCAL_DATE));
mapper.registerModule(javaTimeModule);
String json = mapper.writerWithDefaultPrettyPrinter().writevalueAsString(object);
System.out.println("JSON字符串：");
System.out.println(json);
}
}
4.生成JSON字符串
运行main方法输出结果：
POJO对象：
UserEntity{id=0, name=, email=, birthday=2021-11-19,
createTime=2021-11-19T15:10:32.874, updateTime=2021-11-19T15:10:32.874}
JSON字符串：
{
"id" : 0,
"name" : "",
"email" : "",
"birthday" : "2021-11-19",
"createTime" : "2021-11-19T15:10:32.874",
"updateTime" : "2021-11-19T15:10:32.874"
}
5.更多的数据类型
在initObjectDefaultValue方法中,
修改swith-case增加更多的case，
可以支持更多的数据类型，
也可以修改类型对应的默认值。
Jackson无法序列化Java8新的时间类型，
异常如下：
Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Java 8 date/time type `java.time.LocalDate` not supported by default: add Module "com.fasterxml.jackson.datatype:jackson-datatype-jsr310" to enable handling (through reference chain: com.yuwen.spring.demo.entity.UserEntity["birthday"])
因为birthday字段为LocalDate类型，
按照提示在pom.xml新增
如下依赖：
com.fasterxml.jackson.datatype jackson-datatype-jsr3102.12.5
然后在ObjectMapper注册JavaTimeModule即可:
ObjectMapper mapper = new ObjectMapper(); mapper.registerModule(new JavaTimeModule());7.其他方案
使用Jsr310NullKeySerializer，
也可以设置字段值为NULL时的序列化动作，
但是只能设置为""空字符串，
同时由于无法获取值为NULL的字段的类型，
无法为数据类型设置对应的默认值。
Jsr310NullKeySerializer.java代码如下：
package com.fasterxml.jackson.datatype.jsr310.ser.key; import java.io.IOException; import java.util.Map; import com.fasterxml.jackson.core.JsonGenerator; import com.fasterxml.jackson.databind.JsonMappingException; import com.fasterxml.jackson.databind.JsonSerializer; import com.fasterxml.jackson.databind.SerializerProvider; @Deprecated // since 2.10 -- not sure why module should provide general purpose null serializer // (maybe add in databind) public class Jsr310NullKeySerializer extends JsonSerializer
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