original source : https://medium.com/@amit.bhandari/storing-java-objects-other-than-primitive-types-in-room-database-11e45f4f6d22
compile "android.arch.persistence.room:runtime:1.0.0-alpha3"annotationProcessor "android.arch.persistence.room:compiler:1.0.0-alpha3"compile 'com.google.code.gson:gson:2.6.2'
First 2 lines for including Room DB in our project and 3rd one is Gson which we will use to convert our object to String.
Our modified User.java looks something like this. Even though you can’t see any getter or setter method here, you need to include them or make the fields public for Room to access it.
User.java
@Entitypublic class User { @PrimaryKey private int uId; private String uName; private ArrayList<String> uPets = new ArrayList<>(); public User() { } public User(int id, String name, List<String> pets){ this.uId = id; this.uName = name; this.uPets.addAll(pets); }; //getters setters removed for brevity}
User DAO stays more or less the same. You can include methods to fetch particular user from user id or name, I will leave that as an exercise for you.
UserDAO.java
@Daopublic interface UserDAO { @Insert(onConflict = REPLACE) void insertUser(User user); @Query("SELECT * FROM User") List<User> getUsers(); //include any methods to fetch specific user // @Query("SELECT * FROM User")}
And our UserDB will also stay the same except small difference.
UserDB.java
@Database (entities = {User.class},version = 1)@TypeConverters({Converters.class})
public abstract class UserDB extends RoomDatabase { public abstract UserDAO userDAO();}
Noticed that @TypeConverters annotation we provided along with @Database annotation, this will be the class which will tell Room how to convert ArrayList object to one of SQLite data type. We will implement methods to convert ArrayList to String for storing it in DB and String back to ArrayList for getting back original User object.
Type Converter specifies additional type converters that Room can use. The TypeConverter is added to the scope of the element so if you put it on a class / interface, all methods / fields in that class will be able to use the converters.
Without further delay, we will see how our type converter looks in our case.
UserDAO.java
public class Converters { @TypeConverter public static ArrayList<String> fromString(String value) { Type listType = new TypeToken<ArrayList<String>>() {}.getType(); return new Gson().fromJson(value, listType); } @TypeConverter public static String fromArrayLisr(ArrayList<String> list) { Gson gson = new Gson(); String json = gson.toJson(list); return json; }}
As you can see, we have 2 methods in TypeConverter.
- public static String fromArrayLisr(ArrayList<String> list) : This method takes our arraylist object as parameter and returns string representation for it so that it can be stored in Room Database. Very simple and easy way to convert any object to string is converting it into its JSON equivalent. Just creating Gson object and calling toJson method with our object as parameter is enough.
- public static ArrayList<String> fromString(String value) : While reading data back from Room Database, we get JSON form of our arraylist which we need to convert back. We will use Gson method fromJson by providing JSON string as parameter. But while converting back, we also need to provide the class of original object (in our case, arraylist), but providing arraylist is not enough here as Gson will not be able know what kind of list it has to form. That’s why we used Type to provide the type of list we want Gson to form for us from the JSON string.
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참고사항) @Bindable 에 대하여
