Google Gson 面接の質問と回答

Google Gson is a Java library that can be used to convert Java Objects into respective JSON format. In another way, it can used to convert the JSON into equivalent java objects. There are some other java libraries also capable of doing this conversion, but Gson stands among very few which do not require any pre-annotated java classes OR sourcecode of java classes in any way.

Gson also support the old java classes which had not support of generics in them for type information. It just work with these legacy classes smoothly.

Gson object can be created in two ways. First way gives you a quick Gson object ready for faster coding, while second way uses GsonBuilder to build a more sophisticated Gson object.

Gson gson = new Gson();

To convert the java objects to JSON format, use toJson() method.

Employee employee = new Employee();

employee.setId(1);

employee.setFirstName("Arindam");

employee.setLastName("Ghosh");

employee.setRoles(Arrays.asList("FINANCE", "MANAGER"));

Gson gson = new Gson();

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

To convert the JSON to java object, use fromJson() method.

Gson gson = new Gson();

System.out.println(

    gson.fromJson("{'id':1,'firstName':'Arindam','lastName':'Ghosh','roles':['FINANCE','MANAGER']}", 

    Employee.class));

     

Employee [id=1, firstName=Arindam, lastName=Ghosh, roles=[FINANCE, MANAGER]]

In most of the cases, Gson library is smart enough to create instances even if any class does not provide default no-args constructor. But, if you found any problem using a class having no no-args constructor, you can use InstanceCreator support. You need to register the InstanceCreator of a java class type with Gson first before using it.

For example, Department.java does not have any default constructor.

public class Department

{

   public Department(String deptName)

   {

      this.deptName = deptName;

   }

 

   private String deptName;

 

   public String getDeptName()

   {

      return deptName;

   }

 

   public void setDeptName(String deptName)

   {

      this.deptName = deptName;

   }

    

   @Override

   public String toString()

   {

      return "Department [deptName="+deptName+"]";

   }

}

And our Employee class has reference of Department as:

public class Employee

{

   private Integer id;

   private String firstName;

   private String lastName;

   private List<String> roles;

   private Department department; //Department reference

    

   //Other setters and getters

}

To use Department class correctly, you need to register an InstanceCreator for Department.java as below:

class DepartmentInstanceCreator implements InstanceCreator<Department> {

   public Department createInstance(Type type)

   {

      return new Department("None");

   }

}

 

//Now use the above InstanceCreator as below

 

GsonBuilder gsonBuilder = new GsonBuilder();

Gson gson = gsonBuilder.create();

 

System.out.println(

            gson.fromJson("{'id':1,'firstName':'Arindam','lastName':'Ghosh','roles':['FINANCE','MANAGER'],'department':{'deptName':'Finance'}}", 

            Employee.class));

             

Employee [id=1, firstName=Arindam, lastName=Ghosh, roles=[FINANCE, MANAGER], department=Department [deptName=Finance]]