各个类型和Json类型的相互转换

ObjectMapper类(com.fasterxml.jackson.databind.ObjectMapper)是Jackson的主要类，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。

对应maven：

<dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-core</artifactId><version>2.12.2</version><scope>runtime</scope></dependency><dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-annotations</artifactId><version>2.12.2</version><scope>runtime</scope></dependency><dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-databind</artifactId><version>2.12.2</version><scope>runtime</scope></dependency>

1、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.regi sterModule(new JavaTimeModule());}

2、ObjectMapper的常用方法

1）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
}

2）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@3327bd23Student [ name: Hyl, age: 20 ]

3）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}]

4）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}

5）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 ]
}

6）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());}}