ObjectMapper类
前言
ObjectMapper类是Jackson库的主要类。它提供一些功能将转换成Java对象匹配JSON结构，反之亦然
使用方式
使用Jackson，首先需要相关的jar包。对于使用maven的，需要添加以下依赖：
<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.5</version>
</dependency>
<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core -->
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-core</artifactId>
<version>2.9.5</version>
</dependency>
<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-annotations -->
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-annotations</artifactId>
<version>2.9.5</version>
</dependency>
实例演示
Java对象转换为JSON对象 json对象转化为java 对象
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json中有新增的字段并且是实体类类中不存在的，不报错
//objectMapper.configure( DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
//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"))
PS：还需要注意，如果ObjectMapper的configure设置FAIL_ON_EMPTY_BEANS为false，那么对应的实体类的属性没有get方法也不会抛出异常，但是这个属性默认的是true，即必须要有get方法，这个需要注意。
public class ObjectMapperTest {
public static void main(String[] args) throws IOException {
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.setSerializationInclusion(JsonInclude.Include.ALWAYS);
objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, true);
Person person = new Person(1, "tom", "123");
String jsonString = objectMapper.writeValueAsString(person);
System.out.println("JsonString: " + jsonString);
Person person1 = objectMapper.readValue(jsonString, Person.class);
System.out.println(person1.toString());
}
}
Java数组对象和JSON数组对象之间的转换
public static void getListData() {
ObjectMapper objectMapper = new ObjectMapper();
//Java数组转换为JSON数组
Person person = new Person(1, "tom", "123");
Person person1 = new Person(2, "jack", "123445");
List<Person> personList = new ArrayList<>();
personList.add(person);
personList.add(person1);
String jsonString = null;
try {
//Json数组转换为Java数组
jsonString = objectMapper.writeValueAsString(personList);
System.out.println("JsonString List: " + jsonString);
//JavaType
JavaType javaType = objectMapper.getTypeFactory().constructParametricType(List.class, Person.class);
List<Person> list = objectMapper.readValue(jsonString, javaType);
//打印出list中的值
for (Person person2 : list) {
System.out.println(person2.toString());
}
} catch (Exception e) {
e.printStackTrace();
}
}
参考:
- 封装对象User
package com.entity;
import java.io.Serializable;
public class User implements Serializable {
private static final long serialVersionUID = 1L;
private String userName;
private String age;
private String sex;
private String email;
private String phone;
public User() {
super();
}
public User(String userName, String age, String sex, String email, String phone) {
super();
this.userName = userName;
this.age = age;
this.sex = sex;
this.email = email;
this.phone = phone;
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getAge() {
return age;
}
public void setAge(String age) {
this.age = age;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public String getPhone() {
return phone;
}
public void setPhone(String phone) {
this.phone = phone;
}
@Override
public String toString() {
return "User{" +
"userName='" + userName + '\'' +
", age='" + age + '\'' +
", sex='" + sex + '\'' +
", email='" + email + '\'' +
", phone='" + phone + '\'' +
'}';
}
}
- ObjectMapper 对象预处理
public static ObjectMapper mapper = new ObjectMapper();
//涉及到一下问题需要预处理，否则直接在引用时new new ObjectMapper();即可
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);
}
ObjectMapper -> write (写）
//将对象信息写入指定文件
mapper.writeValue(new File("C:\\Users\\Chyb\\Desktop\\新建文本文档.txt"), user);
//将对象信息写到控制台
mapper.writeValue(System.out, user);
//将Map对象转换为JSON字符串（对象为Object对象，均可以转换为JOSN串）
String str = mapper.writeValueAsString( map )
String str = objectMapper.writeValueAsString( list );
//将对象转换为JSON串，依赖于对象中封装的get、set方法（没有get、set方法会报JSON转换异常）
String userJSON = mapper.writeValueAsString( user );
//将对象转换为byte数组
byte[] bytes = objectMapper.writeValueAsBytes( u );
//如果转换的对象中有日期元素，在mapper对象中填装时间格式化属性，那么接下来用mapper对下对象转换过的对象，转换后时间元素会被格式化，如果在填装时间格式化属性之前则不会格式化生效
mapper.setDateFormat( new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
ObjectMapper -> read (读）
//读取JSON数据转换实体对象，依赖于实体对象的无参构造方法，无参构造方法不存在则会报JSON序列化异常
User u = mapper.readValue( userJSON, User.class );
//读取文件内容，转换为实体对象
User fileUser = objectMapper.readValue( new File( "C:\\Users\\Chyb\\Desktop\\新建文本文档.txt" ), User.class );
//读取文件内容，转换为Map对象
Map map = objectMapper.readValue( new File( "C:\\Users\\Chyb\\Desktop\\新建文本文档.txt" ), Map.class );
//读取byte数组，填装至实体对象
User user = objectMapper.readValue( bytes, User.class );
