条件下序列化时Gson忽略字段

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.annotations.Expose;

class MyClass {
    @Expose
    private String field1;

    private String field2;

    // getters and setters
}

public class Main {
    public static void main(String[] args) {
        MyClass obj = new MyClass();
        obj.setField1("value1");
        obj.setField2("value2");

        Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create();
        String json = gson.toJson(obj);
        System.out.println(json);
    }
}