Jackson库中objectMapper的用法

Jackson库中objectMapper用法

ObjectMapper类是Jackson库的主要类。它提供一些功能将转换成Java对象与SON结构互相转换，在项目中遇到过，故记录一下。

在 pom.xml 加入依赖

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.8.3</version>
</dependency>

创建一个实体类RiemannUser：

package com.test.objectMapper;
import java.io.Serializable;
import java.util.Date;
import java.util.List;
/**
* @author riemann
* @date 2019/05/27 22:48
*/
public class RiemannUser implements Serializable {
   private static final long serialVersionUID = 1L;
   private int id;
   private String message;
   private Date sendDate;
   private String nodeName;
   private List<Integer> intList;
   public RiemannUser() {
       super();
   }
   public RiemannUser(int id, String message, Date sendDate) {
       super();
       this.id = id;
       this.message = message;
       this.sendDate = sendDate;
   }
   public static long getSerialVersionUID() {
       return serialVersionUID;
   }
   public int getId() {
       return id;
   }
   public void setId(int id) {
       this.id = id;
   }
   public String getMessage() {
       return message;
   }
   public void setMessage(String message) {
       this.message = message;
   }
   public Date getSendDate() {
       return sendDate;
   }
   public void setSendDate(Date sendDate) {
       this.sendDate = sendDate;
   }
   public String getNodeName() {
       return nodeName;
   }
   public void setNodeName(String nodeName) {
       this.nodeName = nodeName;
   }
   public List<Integer> getIntList() {
       return intList;
   }
   public void setIntList(List<Integer> intList) {
       this.intList = intList;
   }
   @Override
   public String toString() {
       return "RiemannUser{" + "id=" + id + ", message='" + message + '\'' + ", sendDate=" + sendDate + ", nodeName='" + nodeName + '\'' + ", intList=" + intList + '}';
   }
}

先创建一个ObjectMapper，然后赋值一些属性：

public static ObjectMapper mapper = new ObjectMapper();
static {
   // 转换为格式化的json
   mapper.enable(SerializationFeature.INDENT_OUTPUT);
   // 如果json中有新增的字段并且是实体类类中不存在的，不报错
   mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
}

1、对象与json字符串、byte数组

@Test
public void testObject() throws JsonGenerationException, JsonMappingException, IOException {
   RiemannUser riemann = new RiemannUser(1,"Hello World", new Date());
   mapper.writeValue(new File("D:/test.txt"), riemann);//写到文件中
   //mapper.writeValue(System.out, riemann); //写到控制台
   String jsonStr = mapper.writeValueAsString(riemann);
   System.out.println("对象转json字符串: " + jsonStr);
   byte[] byteArr = mapper.writeValueAsBytes(riemann);
   System.out.println("对象转为byte数组：" + byteArr);
   RiemannUser riemannUser = mapper.readValue(jsonStr, RiemannUser.class);
   System.out.println("json字符串转为对象：" + riemannUser);
   RiemannUser riemannUser2 = mapper.readValue(byteArr, RiemannUser.class);
   System.out.println("byte数组转为对象：" + riemannUser2);
}

运行结果：

对象转json字符串: {
"id" : 1,
"message" : "Hello World",
"sendDate" : 1558971056693,
"nodeName" : null,
"intList" : null
}
对象转为byte数组：[B@31610302
json字符串转为对象：RiemannUser{id=1, message='Hello World', sendDate=Mon May 27 23:30:56 CST 2019, nodeName='null', intList=null}
byte数组转为对象：RiemannUser{id=1, message='Hello World', sendDate=Mon May 27 23:30:56 CST 2019, nodeName='null', intList=null}

2、list集合与json字符串

@Test
public void testList() throws JsonGenerationException, JsonMappingException, IOException {
   List<RiemannUser> riemannList = new ArrayList<>();
   riemannList.add(new RiemannUser(1,"a",new Date()));
   riemannList.add(new RiemannUser(2,"b",new Date()));
   riemannList.add(new RiemannUser(3,"c",new Date()));
   String jsonStr = mapper.writeValueAsString(riemannList);
   System.out.println("集合转为字符串：" + jsonStr);
   List<RiemannUser> riemannLists = mapper.readValue(jsonStr, List.class);
   System.out.println("字符串转集合：" + riemannLists);
}

运行结果：

集合转为字符串：[ {
"id" : 1,
"message" : "a",
"sendDate" : 1558971833351,
"nodeName" : null,
"intList" : null
}, {
"id" : 2,
"message" : "b",
"sendDate" : 1558971833351,
"nodeName" : null,
"intList" : null
}, {
"id" : 3,
"message" : "c",
"sendDate" : 1558971833351,
"nodeName" : null,
"intList" : null
} ]
字符串转集合：[{id=1, message=a, sendDate=1558971833351, nodeName=null, intList=null}, {id=2, message=b, sendDate=1558971833351, nodeName=null, intList=null}, {id=3, message=c, sendDate=1558971833351, nodeName=null, intList=null}]

3、map与json字符串

@Test
public void testMap() {
   Map<String, Object> testMap = new HashMap<>();
   testMap.put("name", "riemann");
   testMap.put("age", 27);
   testMap.put("date", new Date());
   testMap.put("user", new RiemannUser(1, "Hello World", new Date()));
   String jsonStr = null;
   try {
       jsonStr = mapper.writeValueAsString(testMap);
       System.out.println("Map转为字符串：" + jsonStr);
       Map<String, Object> testMapDes = null;
       try {
           testMapDes = mapper.readValue(jsonStr, Map.class);
           System.out.println("字符串转Map：" + testMapDes);
       } catch (IOException e) {
           e.printStackTrace();
       }
   } catch (JsonProcessingException e) {
       e.printStackTrace();
   }
}

Map转为字符串：{
"date" : 1558972169132,
"name" : "riemann",
"user" : {
"id" : 1,
"message" : "Hello World",
"sendDate" : 1558972169134,
"nodeName" : null,
"intList" : null
},
"age" : 27
}
字符串转Map：{date=1558972169132, name=riemann, user={id=1, message=Hello World, sendDate=1558972169134, nodeName=null, intList=null}, age=27}

4、修改转换时的日期格式：

@Test
public void testOther() throws IOException {
   // 修改时间格式
   mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
   RiemannUser riemannUser = new RiemannUser(1,"Hello World",new Date());
   riemannUser.setIntList(Arrays.asList(1,2,3));
   String jsonStr = mapper.writeValueAsString(riemannUser);
   System.out.println("对象转为字符串：" + jsonStr);
}

运行结果：

对象转为字符串：{
"id" : 1,
"message" : "Hello World",
"sendDate" : "2019-05-27 23:53:55",
"nodeName" : null,
"intList" : [ 1, 2, 3 ]
}

objectMapper的一些坑

相信做过Java 开发对这个类应该不陌生,没错,这个类是jackson提供的，主要是用来把对象转换成为一个json字符串返回到前端,

现在大部分数据交换都是以json来传输的,所以这个很重要,那你到底又对这个类有着有多少了解呢，下面我说一下我遇到的一些坑

首先，先把我要说的几个坑需要设置的属性贴出来先

ObjectMapper objectMapper = new ObjectMapper();
//序列化的时候序列对象的所有属性
objectMapper.setSerializationInclusion(Include.ALWAYS);

//反序列化的时候如果多了其他属性,不抛出异常
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

//如果是空对象的时候,不抛异常
objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);

//取消时间的转化格式,默认是时间戳,可以取消,同时需要设置要表现的时间格式
objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"))

简单说一下这个类的基本用法，以下采用代码块加截图的形式来说明和部分文字件数

package com.shiro.test;
import java.text.SimpleDateFormat;
import java.util.Date;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
public class Main2 {
public static void main(String[] args) throws Exception{
ObjectMapper objectMapper = new ObjectMapper();
//序列化的时候序列对象的所有属性
objectMapper.setSerializationInclusion(Include.ALWAYS);
//取消时间的转化格式,默认是时间戳,可以取消,同时需要设置要表现的时间格式
objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));

Person person = new Person(1, "zxc", new Date());
//这是最简单的一个例子,把一个对象转换为json字符串
String personJson = objectMapper.writeValueAsString(person);
System.out.println(personJson);

//默认为true,会显示时间戳
objectMapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, true);
personJson = objectMapper.writeValueAsString(person);
System.out.println(personJson);
}
}

输出的信息如下

objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false)的作用

package com.shiro.test;
import java.text.SimpleDateFormat;
import java.util.Date;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
public class Main2 {
public static void main(String[] args) throws Exception{
ObjectMapper objectMapper = new ObjectMapper();
//序列化的时候序列对象的所有属性
objectMapper.setSerializationInclusion(Include.ALWAYS);
//如果是空对象的时候,不抛异常,也就是对应的属性没有get方法
objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);

Person person = new Person(1, "zxc", new Date());

String personJson = objectMapper.writeValueAsString(person);
System.out.println(personJson);

//默认是true,即会抛异常
objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, true);
personJson = objectMapper.writeValueAsString(person);
System.out.println(personJson);
}
}

对应的person类此时为

package com.shiro.test;
import java.util.Date;
public class Person {
private Integer id;
private String name;
private Date birthDate;
//public Integer getId() {
//return id;
//}
//public void setId(Integer id) {
//this.id = id;
//}
//public String getName() {
//return name;
//}
//public void setName(String name) {
//this.name = name;
//}
//public Date getBirthDate() {
//return birthDate;
//}
//public void setBirthDate(Date birthDate) {
//this.birthDate = birthDate;
//}
@Override
public String toString() {
return "Person [id=" + id + ", name=" + name + ", birthDate=" + birthDate + "]";
}
public Person(Integer id, String name, Date birthDate) {
super();
this.id = id;
this.name = name;
this.birthDate = birthDate;
}

public Person() {
// TODO Auto-generated constructor stub
}
}

结果如下

package com.shiro.test;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
public class Main2 {
public static void main(String[] args) throws Exception{
ObjectMapper objectMapper = new ObjectMapper();
//序列化的时候序列对象的所有属性
objectMapper.setSerializationInclusion(Include.ALWAYS);
//反序列化的时候如果多了其他属性,不抛出异常
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);

//Person person = new Person(1, "zxc", new Date());

//String personJson = objectMapper.writeValueAsString(person);
//System.out.println(personJson);

//注意,age属性是不存在在person对象中的
String personStr = "{\"id\":1,\"name\":\"zxc\",\"age\":\"zxc\"}";

Person person = objectMapper.readValue(personStr, Person.class);
System.out.println(person);

//默认为true
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, true);
person = objectMapper.readValue(personStr, Person.class);
System.out.println(person);
}
}

执行后的结果如下

这些便是这几个属性的作用所以，由于第一个比较简单我就这样说一下吧

Include.ALWAYS 是序列化对像所有属性

Include.NON_NULL 只有不为null的字段才被序列化

Include.NON_EMPTY 如果为null或者 空字符串和空集合都不会被序列化

然后再说一下如何把一个对象集合转换为一个 Java里面的数组

package com.shiro.test;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import com.fasterxml.jackson.annotation.JsonInclude.Include;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.JavaType;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Main2 {
public static void main(String[] args) throws Exception{
ObjectMapper objectMapper = new ObjectMapper();
//序列化的时候序列对象的所有属性
objectMapper.setSerializationInclusion(Include.NON_DEFAULT);

Person person1 = new Person(1, "zxc", new Date());
Person person2 = new Person(2, "ldh", new Date());

List<Person> persons = new ArrayList<>();
persons.add(person1);
persons.add(person2);

//先转换为json字符串
String personStr = objectMapper.writeValueAsString(persons);

//反序列化为List<user> 集合,1需要通过 TypeReference 来具体传递值
List<Person> persons2 = objectMapper.readValue(personStr, new TypeReference<List<Person>>() {});

for(Person person : persons2) {
System.out.println(person);
}

//2,通过 JavaType 来进行处理返回
JavaType javaType = objectMapper.getTypeFactory().constructParametricType(List.class, Person.class);
List<Person> persons3 = objectMapper.readValue(personStr, javaType);

for(Person person : persons3) {
System.out.println(person);
}
}
}

来源：https://riemann.blog.csdn.net/article/details/90614414