java - 在没有默认构造函数的内部类时使用 Jackson 解码 JSON

Question

我使用 Jackson 库编码了一个具有内部类(非静态)的类对象，它工作得非常好。但是，我在取消编码对象时遇到问题。原因是我的内部类没有任何默认构造函数。我在这里复制了示例代码:

public class JsonMarshallUnMarshall {
public static void main(String[] args) {
Person person = new Person();
person.setId(2222);
person.setName("My Name");
Person.Student student = person.new Student(44887);
person.setStudent(student);
String personJsonValue = null;

// Marshalling
System.out.println("====Marshalling====");
ObjectMapper mapper = new ObjectMapper();
mapper.getSerializationConfig().addMixInAnnotations(Person.Student.class, MixIn.class);
mapper.getDeserializationConfig().addMixInAnnotations(Person.Student.class, MixIn.class);

try {
personJsonValue = mapper.writeValueAsString(person);
System.out.println(personJsonValue);
} catch (JsonGenerationException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}

// UnMarshalling
System.out.println("====Unmarshalling====");
Person unMarshalledPerson = null;
try {
unMarshalledPerson = mapper
.readValue(personJsonValue, Person.class);
System.out.println("id: " + unMarshalledPerson.getId());
System.out.println("name: " + unMarshalledPerson.getName());
System.out.println("student id: " + unMarshalledPerson.getStudent().getStudentId());
} catch (JsonParseException e) {
e.printStackTrace();
} catch (JsonMappingException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}

class Person {
private int id;
private String name;
private Student student;
public Person() {
}
public Student getStudent() {
return student;
}
public void setStudent(Student student) {
this.student = student;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public class Student {
private int studentId;
public int getStudentId() {
return studentId;
}
public void setStudentId(int studentId) {
this.studentId = studentId;
}
public Student(int studentId) {
this.studentId = studentId;
}
}
}

abstract class MixIn {
public MixIn(@JsonProperty("studentId") int newStudentId) {
}
@JsonProperty("studentId")
abstract int getNewStudentId();
}

在执行代码时，我得到一个异常:

org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class com.tnsi.cnam.batch.utilities.Person$Student]: can not instantiate from JSON object (need to add/enable type information?)
at [Source: java.io.StringReader@1abab88; line: 1, column: 41] (through reference chain: com.tnsi.cnam.batch.utilities.Person["student"])

有什么办法可以解决这个问题...我不能放置默认构造函数，因为实际的类来自第三方 jar。

Answer 1

当然。但你可能不会喜欢它。您需要编写自己的自定义反序列化器。可以找到一个关于如何执行此操作的非常详尽的示例 here .