Can't serialize Json to Java object using Jackson ObjectMapper - json

Question

I'm writing a Java spring boot mvc application with possibility of data export / import. I wrote a wrapper class, that shoud serialize / deserialize data for Student class. It works good for export, but during import an error appears
com.fasterxml.jackson.databind.exc.MismatchedInputException: Root name
'student' does not match expected ('students') for type [simple type,
class org.bajiepka.courseApp.wrappers.Response] at [Source:
(FileInputStream); line: 2, column: 3]
here is my maven jackson dependency:
<dependency>
<groupId>com.fasterxml.jackson.core</groupId>
<artifactId>jackson-databind</artifactId>
<version>2.9.8</version>
</dependency>
I use wrapper class Response for list of students,
package org.bajiepka.courseApp.wrappers;
import com.fasterxml.jackson.annotation.JsonProperty;
import com.fasterxml.jackson.annotation.JsonRootName;
import org.bajiepka.courseApp.domain.Student;
#JsonRootName("import")
public class Response {
private Iterable<Student> students;
#JsonProperty("students")
public Iterable<Student> getStudents(){
return students;
}
public void setStudents(Iterable<Student> students) {
this.students = students;
}
}
And this is my Student class:
#Data
#Entity
public class Student {
private #Id #GeneratedValue Long id;
private #Version int version;
#NotNull
private String name;
private String address;
private String phone;
private Integer gradeBook;
private float averageProgress;
public Student() {}
}
}
This is the method that created an export file:
public String write(boolean toFile){
ObjectMapper mapper = new ObjectMapper();
mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);
mapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
ObjectWriter writer = mapper.writer().withDefaultPrettyPrinter();
try {
String result = writer.writeValueAsString(response);
if (toFile){
result = fileService.writeToFile(result);
}
return result;
} catch (JsonProcessingException e) {
e.printStackTrace();
}
return "";
}
after export i get the file with json, that i can't convert to java objects:
{
"student" : [ {
"id" : 1,
"version" : 0,
"name" : "Rachel Jessica Parker",
"address" : "Pentaho",
"phone" : "111-22-33",
"gradeBook" : 1000121,
"averageProgress" : 0.0
}, {
"id" : 2,
"version" : 0,
"name" : "Bobby Jackson Junior",
"address" : "Illinois",
"phone" : "222-33-44",
"gradeBook" : 1000122,
"averageProgress" : 0.0
}, {
"id" : 3,
"version" : 0,
"name" : "Sammy Smith Carlson",
"address" : "Pennsylvania",
"phone" : "333-44-55",
"gradeBook" : 1000123,
"averageProgress" : 0.0
}, {
"id" : 4,
"version" : 0,
"name" : "Harry Dale Harrison",
"address" : "Detroit",
"phone" : "444-55-66",
"gradeBook" : 1000124,
"averageProgress" : 0.0
}, {
"id" : 5,
"version" : 0,
"name" : "Lindsey jefferson Conly",
"address" : "Washington",
"phone" : "555-66-77",
"gradeBook" : 1000125,
"averageProgress" : 0.0
}, {
"id" : 6,
"version" : 0,
"name" : "Mo Williams Jr.",
"address" : "New York",
"phone" : "666-77-88",
"gradeBook" : 1000126,
"averageProgress" : 0.0
} ]
}
And finally this is the method for conversion:
#GetMapping(value = "/import")
public String importFile(#RequestParam Long id){
File importFile = new File(exchangeFileService.findById(id).getName());
if (importFile.exists()) {
try (FileInputStream stream = new FileInputStream(importFile)) {
ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
TypeReference<Response> typeReference = new TypeReference<>(){};
Response response = mapper.readValue(stream, typeReference);
} catch (IOException e) {
e.printStackTrace();
}
}
return "redirect:/exchange/upload";
}
i get mentioned above MismatchInputException at mapper.readValue(stream, typeReference)
ObjectMapper should return me a Response with a list of students but it doesn't...
#UPDATE
I've managed to find the cause of the error.
During the Object -> Json serialization jackson root name is missed...
I've added manually
{
**"import" : {**
"student" : [ {
"id" : 1,
"version" : 0,
"name" : "Rachel Jessica Parker",
"address" : "Pentaho",
"phone" : "111-22-33",
"gradeBook" : 1000121,
"averageProgress" : 0.0
}, ... ]
,
"course" : [ {
"id" : 7,
"version" : 0,
"name" : "Physics for 9-th grade",
"number" : 100292910,
"cost" : 25000.0,
"modules" : 0,
"max_COURSES_PER_STUDENT" : 3,
"modules_PER_COURSE" : 10
}, ... ]
**}**
}
Also i've managed to extend Response class, as it was required... Now i'm trying to find the reason in ObjectMapper during serialization...

Answer 1

The problem was caused due to Java object -> Json serialization without root level.
To add it to Json i've just added it at mapper by withRootName() method and everything works well from now on!
mapper.writer().withDefaultPrettyPrinter().withRootName("import")