Java之Jackson框架
在Jackson框架中，提供了三种方式用来处理JSON数据：
流式API
在该方式下，使用JsonParser读取JSON数据，使用JsonGenerator写JSON数据。这种方式性能最佳(最低开销、最快速度读/写，其他两种方式基于该方式实现)
public static void write2JsonByStreamApi() throws Exception {
JsonFactory jf = new JsonFactory();
JsonGenerator jg = jf.createGenerator(new File("D:/user.json"),
JsonEncoding.UTF8);
jg.writeStartObject();
jg.writeObjectFieldStart("users");
jg.writeStringField("name", "jackson");
jg.writeNumberField("age", 13);
jg.writeEndObject();
jg.writeEndObject();
jg.flush();
jg.close();
}
树模型
在该模式下，将JSON数据以树的方式存储在内存中，使用ObjectMapper读取JSON数据生成树，树是JsonNode节点的集合
数据绑定
在该模式下，可以方便的将JSON和POJO相互转化，数据的绑定有两种变体：
- 简单数据绑定
简单数据绑定是指Map、List、String、Numbers、Boolean、以及null之间的相互转化，其转化对应如下：
| JSON Type | Java Type |
| Object | LinkedHashMap<String,Object> |
| Array | ArrayList<Object> |
| String | String |
| Number(整数) |
Integer、Long、BigInteger |
| Number(小数) | Double |
| True|False | Boolean |
| Null | Null |
- 完整数据绑定
完整数据绑定是指任何Java Bean类型上述简单数据绑定进行转换
/*********************************************/
/**父母信息类*/
public class Parent {
private String fathername = "";
private String mothername = "";
public Parent() {
/**JSON串转为Java对象时调用无惨构造函数*/
}
public Parent(String fname,String mname) {
this.fathername = fname;
this.mothername = mname;
}
public String getFathername() {
return fathername;
}
public void setFathername(String fathername) {
this.fathername = fathername;
}
public String getMothername() {
return mothername;
}
public void setMothername(String mothername) {
this.mothername = mothername;
}
}
/**教师信息类*/
public class Teacher {
private String name = "";
private int age = 0;
public Teacher() {
/**JSON串转为Java对象时调用无惨构造函数*/
}
public Teacher(String name,int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
}
/**学生信息类*/
public class Student {
private String name = "";
private String sex = "";
private int age = 0;
private Parent parent = null;
private Teacher[] teachers = null;
public Student() {
/**JSON串转为Java对象时调用无惨构造函数*/
}
public Student(String name,String sex,int age){
this.name = name;
this.sex = sex;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public Parent getParent() {
return parent;
}
public void setParent(Parent parent) {
this.parent = parent;
}
public Teacher[] getTeachers() {
return teachers;
}
public void setTeachers(Teacher[] teachers) {
this.teachers = teachers;
}
}
/*********************************************/
public class JSonUtil {
private static ObjectMapper mapper = null;
static {
mapper = new ObjectMapper();
}
public static void write2Json(Object obj) throws IOException {
mapper.writeValue(System.out, obj);
}
public static Object json2Object(String json,Class<? extends Object> clazz) throws Exception {
return mapper.readValue(json, clazz);
}
public static void main(String []args) throws Exception {
/**Java对象转化为JSON串*/
Parent parent = new Parent("张大龙","张园园");
Teacher teacher_1 = new Teacher("李大成",46);
Teacher teacher_2 = new Teacher("王大虎",43);
Student student = new Student("张明","男",22);
student.setParent(parent);
student.setTeachers(new Teacher[]{teacher_1,teacher_2});
JSonUtil.write2Json(student);
/**JSON串转换为Java对象*/
String json = "{\"name\":\"张明\",\"sex\":\"男\",\"age\":22," +"\"parent\":{\"fathername\":\"张大龙\",\"mothername\":\"张园园\"}," + "\"teachers\":[{\"name\":\"李大成\",\"age\":46}," + "{\"name\":\"王大虎\",\"age\":43}]}";
Student student = (Student) JSonUtil.json2Object(json, Student.class);
System.out.println(student.getParent().getFathername());
}
}
/*************************输出结果*****************************/
{"name":"张明","sex":"男","age":22,"parent":{"fathername":"张大龙","mothername":"张园园"},"teachers":[{"name":"李大成","age":46},{"name":"王大虎","age":43}]}
在上面的示例中，需要注意的是：
- 将JSON串转为Java对象时，需要Java对象提供无参的构造函数，并且要求Java对象具有Java Bean性质，也就是说要有setter/getter方法
- 将Java对象转为JSON字符串时，JSON串Object的key名称取自Java对象的属性名称，如果想改变JSON串Object的key名称需要重写自己的序列化器，在解析时需要重写反序列化器，并且在序列化器中指定JSON串顺序
/**序列化器*/
public class StudentSerializer extends JsonSerializer<Student> {
@Override
public void serialize(Student value, JsonGenerator js, SerializerProvider provider) throws IOException,
JsonProcessingException {
// TODO Auto-generated method stub
js.writeStartObject(); js.writeObjectField("学生姓名", value.getName());
js.writeObjectField("性别", value.getSex());
js.writeObjectField("年龄", value.getAge());
js.writeFieldName("父母信息"); js.writeStartObject();
js.writeObjectField("父亲姓名", value.getParent().getFathername());
js.writeObjectField("母亲姓名", value.getParent().getMothername());
js.writeEndObject(); js.writeArrayFieldStart("教师信息");
Teacher [] teachers = value.getTeachers();
for(Teacher teacher : teachers) {
js.writeStartObject();
js.writeObjectField("教师姓名", teacher.getName());
js.writeObjectField("教师年龄", teacher.getAge());
js.writeEndObject();
}
js.writeEndArray(); js.writeEndObject();
}
}
/**反序列化器*/
public class StudentDeserializer extends JsonDeserializer<Student> {
@Override
public Student deserialize(JsonParser parser, DeserializationContext context)
throws IOException, JsonProcessingException {
// TODO Auto-generated method stub
JsonNode node = parser.getCodec().readTree(parser);
String name = node.get("学生姓名").asText();
String sex = node.get("性别").asText();
int age = node.get("年龄").asInt();
JsonNode pNode = node.get("父母信息");
String fathername = pNode.get("父亲姓名").asText();
String mothername = pNode.get("母亲姓名").asText();
Parent parent = new Parent();
parent.setFathername(fathername);
parent.setMothername(mothername);
JsonNode tNodes = node.get("教师信息");
ArrayList<Teacher> list = new ArrayList<Teacher>();
for(JsonNode tNode : tNodes) {
String teachername = tNode.get("教师姓名").asText();
int teacherage = tNode.get("教师年龄").asInt();
Teacher teacher = new Teacher();
teacher.setName(teachername);
teacher.setAge(teacherage);
list.add(teacher);
}
Student student = new Student();
student.setName(name);
student.setAge(age);
student.setSex(sex);
student.setParent(parent);
student.setTeachers((Teacher[]) list.toArray(new Teacher[]{}));
return student;
}
}
/**************************************************************/
public class JSonUtil {
private static ObjectMapper mapper = null;
static {
mapper = new ObjectMapper();
/**注册序列化器与反序列化器*/
StudentSerializer ser = new StudentSerializer();
StudentDeserializer deser = new StudentDeserializer();
SimpleModule module = new SimpleModule();
module.addSerializer(Student.class, ser);
module.addDeserializer(Student.class, deser);
mapper.registerModule(module);
}
public static void write2Json(Object obj) throws IOException {
mapper.writeValue(System.out, obj);
}
public static Object json2Object(String json,Class<? extends Object> clazz) throws Exception {
return mapper.readValue(json, clazz);
}
public static void main(String []args) throws Exception {
Parent parent = new Parent("张大龙","张园园");
Teacher teacher_1 = new Teacher("李大成",46);
Teacher teacher_2 = new Teacher("王大虎",43);
Student student = new Student("张明","男",22);
student.setParent(parent);
student.setTeachers(new Teacher[]{teacher_1,teacher_2});
JSonUtil.write2Json(student);
String json = "{\"学生姓名\":\"张明\",\"性别\":\"男\",\"年龄\":22," + "\"父母信息\":{\"父亲姓名\":\"张大龙\",\"母亲姓名\":\"张园园\"}," + "\"教师信息\":[{\"教师姓名\":\"李大成\",\"教师年龄\":46}," + "{\"教师姓名\":\"王大虎\",\"教师年龄\":43}]}";
Student stu = (Student) JSonUtil.json2Object(json, Student.class);
System.out.println(stu.getParent().getFathername());
}
}
在上面的序列化示例中，可以看出需要将自定义的序列化器与反序列化器注册到ObjectMapper中，在Jackson2.x框架中，提供了注解方式
/*************序列化对象添加注解***************/
@JsonSerialize(using = StudentSerializer.class)
@JsonDeserialize(using = StudentDeserializer.class)
public class Student {
private String name = "";
private String sex = "";
private int age = 0;
private Parent parent = null;
private Teacher[] teachers = null;
public Student() { }
public Student(String name, String sex, int age) {
this.name = name;
this.sex = sex;
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public Parent getParent() {
return parent;
}
public void setParent(Parent parent) {
this.parent = parent;
}
public Teacher[] getTeachers() {
return teachers;
}
public void setTeachers(Teacher[] teachers) {
this.teachers = teachers;
}
}
/***********************************************/
public class JSonUtil {
private static ObjectMapper mapper = null;
static {
mapper = new ObjectMapper();
}
public static void write2Json(Object obj) throws IOException {
mapper.writeValue(System.out, obj);
}
public static Object json2Object(String json,Class<? extends Object> clazz) throws Exception {
return mapper.readValue(json, clazz);
}
public static void main(String []args) throws Exception {
Parent parent = new Parent("张大龙", "张园园");
Teacher teacher_1 = new Teacher("李大成", 46);
Teacher teacher_2 = new Teacher("王大虎", 43);
Student student = new Student("张明", "男", 22);
student.setParent(parent);
student.setTeachers(new Teacher[] { teacher_1, teacher_2 });
JSonUtil.write2Json(student);
String json = "{\"学生姓名\":\"张明\",\"性别\":\"男\",\"年龄\":22,"
+ "\"父母信息\":{\"父亲姓名\":\"张大龙\",\"母亲姓名\":\"张园园\"},"
+ "\"教师信息\":[{\"教师姓名\":\"李大成\",\"教师年龄\":46},"
+ "{\"教师姓名\":\"王大虎\",\"教师年龄\":43}]}";
Student stu = (Student) JSonUtil.json2Object(json, Student.class);
System.out.println(stu.getParent().getFathername()); }
}
- 泛型的数据绑定
除绑定到POJO和简单类型外，还有一个额外的变型：绑定到泛型容器，由于所谓的类型擦除【Java采用向后兼容的方式实现泛型】，需要进行特殊处理，这时需要借助TypeReference类
/*********************************************/
public class PersonMsg {
private String name;
private String sex;
public PersonMsg() { }
public PersonMsg(String name,String sex) {
this.name = name;
this.sex = sex;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
}
/*********************************************/
public class JSonUtil {
private static ObjectMapper mapper = null;
static {
mapper = new ObjectMapper();
}
public static void write2Json(Object obj) throws IOException {
mapper.writeValue(System.out, obj);
}
public static Object json2Object(String json,TypeReference<Map<String,PersonMsg>> typeReference) throws Exception {
return mapper.readValue(json, typeReference);
}
public static void main(String []args) throws Exception {
PersonMsg p0 = new PersonMsg("liming", "man");
PersonMsg p1 = new PersonMsg("lixiang", "woman");
Map<String, PersonMsg> map = new HashMap<String, PersonMsg>();
map.put("liming", p0);
map.put("lixiang", p1);
JSonUtil.write2Json(map);
String json = "{\"liming\":{\"name\":\"liming\",\"sex\":\"man\"}," +
"\"lixiang\":{\"name\":\"lixiang\",\"sex\":\"woman\"}}";
Map<String,PersonMsg> pmap = (Map<String, PersonMsg>) JSonUtil.json2Object(json, new TypeReference<Map<String,PersonMsg>>(){});
}
}
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