ObjectMapper使用详细介绍
ObjectMapper使用详细介绍
1.简介
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。它使用JsonParser和JsonGenerator的实例实现JSON实际的读/写。它可以帮助我们快速的进行各个类型和Json类型的相互转换。可以使用ObjectMapper进行json和对象间的转换
Jackson ObjectMapper(com.fasterxml.jackson.databind.ObjectMapper)是使用Jackson解析JSON最简单的方法。
Jackson ObjectMapper可以从字符串、流或文件解析JSON，并创建Java对象或对象图来表示已解析的JSON
将JSON解析为Java对象也称为从JSON反序列化Java对象
Jackson ObjectMapper也可以从Java对象创建JSON. 从Java对象生成JSON的过程也被称为序列化Java对象到JSON
2.依赖:
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.9</version>
</dependency>
3.ObjectMapper的常用配置
private static final ObjectMapper mapper;
public static ObjectMapper getObjectMapper(){
return this.mapper;
}
static{
//创建ObjectMapper对象
mapper = new ObjectMapper()
//configure方法 配置一些需要的参数
// 转换为格式化的json 显示出来的格式美化
mapper.enable(SerializationFeature.INDENT_OUTPUT);
//序列化的时候序列对象的那些属性
//JsonInclude.Include.NON_DEFAULT 属性为默认值不序列化
//JsonInclude.Include.ALWAYS 所有属性
//JsonInclude.Include.NON_EMPTY 属性为 空（“”） 或者为 NULL 都不序列化
//JsonInclude.Include.NON_NULL 属性为NULL 不序列化
mapper.setSerializationInclusion(JsonInclude.Include.ALWAYS);
//反序列化时,遇到未知属性会不会报错
//true - 遇到没有的属性就报错 false - 没有的属性不会管，不会报错
mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
//如果是空对象的时候,不抛异常
mapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
// 忽略 transient 修饰的属性
mapper.configure(MapperFeature.PROPAGATE_TRANSIENT_MARKER, true);
//修改序列化后日期格式
mapper.configure(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS, false);
mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
//处理不同的时区偏移格式
mapper.disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS);
mapper.registerModule(new JavaTimeModule());
}
4.ObjectMapper的常用方法
1、字符串转实体类
/**
* 字符串转实体类
*
* @param mapper
* @param jsonString
*/
public static void stringToStudent(ObjectMapper mapper, String jsonString) {
jsonString = "{\"id\":1001,\"name\":\"Tom\",\"info\":\"一个小男孩\"}";
try {
Student student = mapper.readValue(jsonString, Student.class);
System.out.println("student = " + student);
//student = JacksonTest.Student(id=1001, name=Tom, info=一个小男孩)
} catch (Exception e) {
e.printStackTrace();
}
}
2、实体类转字符串
/**
* 实体类转字符串
*
* @param mapper
* @param jsonString
*/
public static void studentToString(ObjectMapper mapper, String jsonString) {
Student student = new Student();
student.setId(1002);
student.setName("李四");
student.setInfo("一个小女孩");
try {
jsonString = mapper.writeValueAsString(student);
System.out.println("jsonString = " + jsonString);
//jsonString = {"id":1002,"name":"李四","info":"一个小女孩"}
} catch (Exception e) {
e.printStackTrace();
}
}
3、字符串转 List<实体类>
/**
* 字符串转 List<实体类>
*
* @param mapper
* @param jsonString
*/
public static void stringToStudentList(ObjectMapper mapper, String jsonString) {
jsonString = "[{\"id\":1001,\"name\":\"Tom\",\"info\":\"一个小男孩\"},{\"id\":1002,\"name\":\"Jane\",\"info\":\"一个小女孩\"}]";
try {
Student[] stu = mapper.readValue(jsonString, Student[].class);
for (Student student : stu) {
System.out.println("student = " + student);
//student = JacksonTest.Student(id=1001, name=Tom, info=一个小男孩)
//student = JacksonTest.Student(id=1002, name=Jane, info=一个小女孩)
}
String ss = mapper.writeValueAsString(stu);
System.out.println("ss = " + ss);
//ss = [{"id":1001,"name":"Tom","info":"一个小男孩"},{"id":1002,"name":"Jane","info":"一个小女孩"}]
} catch (Exception e) {
e.printStackTrace();
}
}
4.json字符串转对象
ObjectMapper mapper = new ObjectMapper();
String jsonString = "{\"name\":\"Hyl\", \"age\":20}";
//将字符串转换为对象
Student student = mapper.readValue(jsonString, Student.class);
System.out.println(student);
//将对象转换为json字符串
jsonString = mapper.writeValueAsString(student);
System.out.println(jsonString);
结果：
Student [ name: Hyl, age: 20 ]
{
"name" : "Hyl",
"age" : 20
}
5.数组和对象之间转换
//对象转为byte数组
byte[] byteArr = mapper.writeValueAsBytes(student);
System.out.println(byteArr);
//byte数组转为对象
Student student= mapper.readValue(byteArr, Student.class);
System.out.println(student);
结果：
[B@3327bd23
Student [ name: Hyl, age: 20 ]
6.集合和json字符串之间转换
List<Student> studentList= new ArrayList<>();
studentList.add(new Student("hyl1" ,20 , new Date()));
studentList.add(new Student("hyl2" ,21 , new Date()));
studentList.add(new Student("hyl3" ,22 , new Date()));
studentList.add(new Student("hyl4" ,23 , new Date()));
String jsonStr = mapper.writeValueAsString(studentList);
System.out.println(jsonStr);
List<Student> studentList2 = mapper.readValue(jsonStr, List.class);
System.out.println("字符串转集合：" + studentList2 );
结果：
[ {
"name" : "hyl1",
"age" : 20,
"sendTime" : 1525164212803
}, {
"name" : "hyl2",
"age" : 21,
"sendTime" : 1525164212803
}, {
"name" : "hyl3",
"age" : 22,
"sendTime" : 1525164212803
}, {
"name" : "hyl4",
"age" : 23,
"sendTime" : 1525164212803
} ]
[{name=hyl1, age=20, sendTime=1525164212803}, {name=hyl2, age=21, sendTime=1525164212803}, {name=hyl3, age=22, sendTime=1525164212803}, {name=hyl4, age=23, sendTime=1525164212803}]
7.map和json字符串之间转换
Map<String, Object> testMap = new HashMap<>();
testMap.put("name", "22");
testMap.put("age", 20);
testMap.put("date", new Date());
testMap.put("student", new Student("hyl", 20, new Date()));
String jsonStr = mapper.writeValueAsString(testMap);
System.out.println(jsonStr);
Map<String, Object> testMapDes = mapper.readValue(jsonStr, Map.class);
System.out.println(testMapDes);
结果：
{
"date" : 1525164212803,
"name" : "22",
"student" : {
"name" : "hyl",
"age" : 20,
"sendTime" : 1525164212803,
"intList" : null
},
"age" : 20
}
{date=1525164212803, name=22, student={name=hyl, age=20, sendTime=1525164212803, intList=null}, age=20}
8.日期转json字符串
// 修改时间格式
mapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
Student student = new Student ("hyl",21, new Date());
student.setIntList(Arrays.asList(1, 2, 3));
String jsonStr = mapper.writeValueAsString(student);
System.out.println(jsonStr);
结果：
{
"name" : "hyl",
"age" : 21,
"sendTime" : "2020-07-23 13:14:36",
"intList" : [ 1, 2, 3 ]
}
9.readTree()方法
此方法更灵活，可以只将用户感兴趣的Json串信息值提取出来。主要利用ObjectMapper提供的readTree和Jackson提供的JsonNode类来实现
String test="{"results":[{"objectID":357,"geoPoints":[{"x":504604.59802246094,"y":305569.9150390625}]},{"objectID":358,"geoPoints":[{"x":504602.2680053711,"y":305554.43603515625}]}]}";
//此Json串比较复杂，包含了嵌套数组的形式，具有通用性。
//2.2.2.2实现反序列化
JsonNode node= objectMapper.readTree(test); //将Json串以树状结构读入内存
JsonNode contents=node.get("results");//得到results这个节点下的信息
for(int i=0;i<contents.size();i++) //遍历results下的信息，size()函数可以得节点所包含的的信息的个数，类似于数组的长度
{
System.out.println(contents.get(i).get("objectID").getIntValue()); //读取节点下的某个子节点的值
JsonNode geoNumber=contents.get(i).get("geoPoints");
for(int j=0;j<geoNumber.size();j++) //循环遍历子节点下的信息
{
System.out.println(geoNumber.get(j).get("x").getDoubleValue()+" "+geoNumber.get(j).get("y").getDoubleValue());
}
}
参考:
https://blog.csdn.net/u011213044/article/details/120329436
https://blog.csdn.net/weixin_43811057/article/details/124584279
