Gson: How to exclude specific fields from Serialization without annotations

Question

I\'m trying to learn Gson and I\'m struggling with field exclusion. Here are my classes

public class Student {    
  private Long                id;
  privat         


        

Answer 1

So, you want to exclude firstName and country.name. Here is what your ExclusionStrategy should look like

    public class TestExclStrat implements ExclusionStrategy {

        public boolean shouldSkipClass(Class arg0) {
            return false;
        }

        public boolean shouldSkipField(FieldAttributes f) {

            return (f.getDeclaringClass() == Student.class && f.getName().equals("firstName"))||
            (f.getDeclaringClass() == Country.class && f.getName().equals("name"));
        }

    }

If you see closely it returns true for Student.firstName and Country.name, which is what you want to exclude.

You need to apply this ExclusionStrategy like this,

    Gson gson = new GsonBuilder()
        .setExclusionStrategies(new TestExclStrat())
        //.serializeNulls() <-- uncomment to serialize NULL fields as well
        .create();
    Student src = new Student();
    String json = gson.toJson(src);
    System.out.println(json);

This returns:

{ "middleName": "J.", "initials": "P.F", "lastName": "Fry", "country": { "id": 91}}

I assume the country object is initialized with id = 91L in student class.

---

You may get fancy. For example, you do not want to serialize any field that contains "name" string in its name. Do this:

public boolean shouldSkipField(FieldAttributes f) {
    return f.getName().toLowerCase().contains("name"); 
}

This will return:

{ "initials": "P.F", "country": { "id": 91 }}

---

EDIT: Added more info as requested.

This ExclusionStrategy will do the thing, but you need to pass "Fully Qualified Field Name". See below:

    public class TestExclStrat implements ExclusionStrategy {

        private Class c;
        private String fieldName;
        public TestExclStrat(String fqfn) throws SecurityException, NoSuchFieldException, ClassNotFoundException
        {
            this.c = Class.forName(fqfn.substring(0, fqfn.lastIndexOf(".")));
            this.fieldName = fqfn.substring(fqfn.lastIndexOf(".")+1);
        }
        public boolean shouldSkipClass(Class arg0) {
            return false;
        }

        public boolean shouldSkipField(FieldAttributes f) {

            return (f.getDeclaringClass() == c && f.getName().equals(fieldName));
        }

    }

Here is how we can use it generically.

    Gson gson = new GsonBuilder()
        .setExclusionStrategies(new TestExclStrat("in.naishe.test.Country.name"))
        //.serializeNulls()
        .create();
    Student src = new Student();
    String json = gson.toJson(src);
    System.out.println(json); 

It returns:

{ "firstName": "Philip" , "middleName": "J.", "initials": "P.F", "lastName": "Fry", "country": { "id": 91 }}