如何使用GSON进行自定义序列化？

Question

I have a case where I would like to use GSON custom serialization feature.

class Student{
   public String name;
   public int rollNumber;

   public Student(String name, int rollNumber){
        this.name = name;
        this.rollNumber = rollNumber;
   }

   public int getRollNumber(){
       return this.rollNumber;
   }

   public String getName(){
       return this.name;
   }


}
class School{

    public Student[] students;

    public School(Student[] students){
          this.students = students;
    }


   public Students[] getStudents(){
       return this.students;
   }

}

Now when I do

private static final Gson GSON = new GsonBuilder().disableHtmlEscaping().create();

Student[] students = new Student[2];

students[0] = new Student("sam", 1);

students[1] = new Student("tom", 2);

School school = new School(students);

GSON.toJson(school);

I get output like this:

[{"name":"sam","rollNumer":1},{"name":"tom","rollNumer":2}]

But I want it to be :

["student":{"name":"sam","rollNumer":1},"student":{"name":"tom","rollNumer":2}]

How do I achieve this using GSON custom serialization?

I have tried this and this . But did not help much.

Answer 1

This

["student":{"name":"sam","rollNumer":1},"student":{"name":"tom","rollNumer":2}]

is not valid JSON (you can verify it yourself using an online tool like jsonlint ). See details from the JSON specification :

Definition of an object :

An object is an unordered set of name/value pairs. An object begins with { (left brace) and ends with } (right brace). Each name is followed by : (colon) and the name/value pairs are separated by , (comma).

Definition of an array :

An array is an ordered collection of values. An array begins with [ (left bracket) and ends with ] (right bracket). Values are separated by , (comma).

Definition of a value :

A value can be a string in double quotes, or a number, or true or false or null, or an object or an array. These structures can be nested.

Your output defines a JSON array, but the objects in that array aren't properly surrounded with braces. A correct representation would look like this:

[{"student":{"name":"sam","rollNumer":1}}, {"student":{"name":"tom","rollNumer":2}}]

Which can be generated with this simple Gson TypeAdapter :

class StudentAdapter extends TypeAdapter<Student> {

    @Override
    public void write(final JsonWriter writer, final Student student)
            throws IOException {
        if (student == null) {
            writer.nullValue();
            return;
        }

        writer.beginObject();
        writer.name("student");
        writer.beginObject();
        writer.name("name");
        writer.value(student.getName());
        writer.name("rollNumber");
        writer.value(student.getRollNumber());
        writer.endObject();
        writer.endObject();
    }

    @Override
    public Student read(final JsonReader reader) throws IOException {
        if (reader.peek() == JsonToken.NULL) {
            reader.nextNull();
            return null;
        }

        final Student student = new Student();
        reader.beginObject();
        reader.nextName(); // discard the 'student' wrapper property
        reader.beginObject();
        while (reader.hasNext()) {
            final String attrName = reader.nextName();
            if ("name".equals(attrName)) {
                student.setName(reader.nextString());
            } else if ("rollNumber".equals(attrName)) {
                student.setRollNumber(reader.nextInt());
            }
        }
        reader.endObject();
        reader.endObject();

        return student;
    }
}

Test method:

@Test
public void testWriteSchoolsAsJSONWithGsonAndCustomOutput()
        throws Exception {
    final Gson gson = new GsonBuilder().registerTypeAdapter(Student.class,
            new StudentAdapter()).create();

    Student[] students = new Student[2];
    students[0] = new Student("sam", 1);
    students[1] = new Student("tom", 2);

    School school = new School(students);

    final String outputJson = gson.toJson(school);
    System.out.println(outputJson);

    school = gson.fromJson(outputJson, School.class);
    System.out.println(school);
}

And 'relevant' output:

{"students":[{"student":{"name":"sam","rollNumber":1}},{"student":{"name":"tom","rollNumber":2}}]}

Answer 2

First of all you can't get output as ["student":{"name":"sam","rollNumer":1},"student":{"name":"tom","rollNumer":2}] because you are trying to convert object into json not array of object.

I tried your code and I am getting following output

{"students":[{"name":"sam","rollNumber":1},{"name":"tom","rollNumber":2}]}

Here is code :

    Student[] students = new Student[2];

    students[0] = new Student("sam", 1);

    students[1] = new Student("tom", 2);

    School school = new School(students);

    Gson gson = new GsonBuilder().disableHtmlEscaping().create();

    System.out.println(gson.toJson(school));

Answer 3

You are getting the correct result according to your classes.

You have objects Student that look like this:

{"name":"sam", "rollNumer":1}

It's simple, an object in JSON is surrounded with {} and contains pairs of property/value...

Then, arrays in JSON are surrounded with [] and contains a number of objects/values separated by commas. As you have an array of Student you just have a number of objects like the one above separated by commas and all surrounded by [] .

And this is exactly what you have... everything makes sense!

OK, you want the JSON to contain "student" before each Student object, but what is this string "student" ? Where does it come from? In fact there's no such string anywhere in your classes, so how do you want Gson (or whatever library) to include it in the JSON?

If you really need that string, you need to include it somehow in your classes, and the simplest way I come up with is to have a Map instead of an array, like this:

public Map<String, Student> students;

And then add the students to the map like this:

Student student = new Student("sam", 1);
students.put("student", student);
//add other students...

Note that now you do have the string "student" , associated with a student as the key in the map...

Now you will have more or less the result you want, but not exactly! because the whole thing will be surrounded by {} instead of [] , because a Map is an object, not an array...

{"student":{"name":"sam","rollNumer":1}},{"student":{"name":"tom","rollNumer":2}}

I'm afraid the response exactly as you want it is not possible to get (as far as I understand)...