Best practices for exposing multiple tables using content providers in Android

Question

I'm building an app where I have a table for events and a table for venues. I want to be able to grant other applications access to this data. I have a few questions related to best practices for this kind of problem.

1. How should I structure the database classes? I currently have classes for EventsDbAdapter and VenuesDbAdapter, which provide the logic for querying each table, while having a separate DbManager (extends SQLiteOpenHelper) for managing database versions, creating/upgrading databases, giving access to database (getWriteable/ReadeableDatabase). Is this the recommended solution, or would I be better off either consolidating everything to one class (ie. the DbManager) or separation everything and letting each Adapter extends SQLiteOpenHelper?

2. How should I design content providers for multiple tables? Extending the previous question, should I use one Content Provider for the whole app, or should I create separate providers for Events and Venues?

Most examples I find only deal with single table apps, so I would appreciate any pointers here.

Tuba

Answer 1

It's probably a bit late for you, but others may find this useful.

First you need to create multiple CONTENT_URIs

public static final Uri CONTENT_URI1 = 
    Uri.parse("content://"+ PROVIDER_NAME + "/sampleuri1");
public static final Uri CONTENT_URI2 = 
    Uri.parse("content://"+ PROVIDER_NAME + "/sampleuri2");

Then you expand your URI Matcher

private static final UriMatcher uriMatcher;
static {
    uriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
    uriMatcher.addURI(PROVIDER_NAME, "sampleuri1", SAMPLE1);
    uriMatcher.addURI(PROVIDER_NAME, "sampleuri1/#", SAMPLE1_ID);      
    uriMatcher.addURI(PROVIDER_NAME, "sampleuri2", SAMPLE2);
    uriMatcher.addURI(PROVIDER_NAME, "sampleuri2/#", SAMPLE2_ID);      
}

Then create your tables

private static final String DATABASE_NAME = "sample.db";
private static final String DATABASE_TABLE1 = "sample1";
private static final String DATABASE_TABLE2 = "sample2";
private static final int DATABASE_VERSION = 1;
private static final String DATABASE_CREATE1 =
    "CREATE TABLE IF NOT EXISTS " + DATABASE_TABLE1 + 
    " (" + _ID1 + " INTEGER PRIMARY KEY AUTOINCREMENT," + 
    "data text, stuff text);";
private static final String DATABASE_CREATE2 =
    "CREATE TABLE IF NOT EXISTS " + DATABASE_TABLE2 + 
    " (" + _ID2 + " INTEGER PRIMARY KEY AUTOINCREMENT," + 
    "data text, stuff text);";

Don't forget to add the second DATABASE_CREATE to onCreate()

You are going to use a switch-case block to determine what table is used. This is my insert code

@Override
public Uri insert(Uri uri, ContentValues values) {
    Uri _uri = null;
    switch (uriMatcher.match(uri)){
    case SAMPLE1:
        long _ID1 = db.insert(DATABASE_TABLE1, "", values);
        //---if added successfully---
        if (_ID1 > 0) {
            _uri = ContentUris.withAppendedId(CONTENT_URI1, _ID1);
            getContext().getContentResolver().notifyChange(_uri, null);    
        }
        break;
    case SAMPLE2:
        long _ID2 = db.insert(DATABASE_TABLE2, "", values);
        //---if added successfully---
        if (_ID2 > 0) {
            _uri = ContentUris.withAppendedId(CONTENT_URI2, _ID2);
            getContext().getContentResolver().notifyChange(_uri, null);    
        }
        break;
    default: throw new SQLException("Failed to insert row into " + uri);
    }
    return _uri;                
}

You will need to devide up the delete, update, getType, etc. Wherever your provider calls for DATABASE_TABLE or CONTENT_URI you will add a case and have DATABASE_TABLE1 or CONTENT_URI1 in one and #2 in the next and so on for as many as you want.

Goodnatured

Thanks for your answer, this was pretty close to the solution I ended up using. I find that complex providers working with several tables get a lot of switch-statements, which doesn't seem all that elegant. But I understand that's the way most people do it. – Tuba

Is the notifyChange really supposed to use the _uri and not the original uri? – Darren

Is this the accepted standard with Android? It works, obviously, but seems a little "clunky". – Dane

Can always just use the switch statements as a router of sorts. Then provide separate methods to serve each resource. query, queryUsers, queryUser, queryGroups, queryGroup This is how the built-in contacts provider does it. com.android.providers.contacts.ContactsProvider2.java github.com/android/platform_packages_providers_contactsprovider/… – Marjie

Given that the question requests recommendation for the best practice database class design, I would add that tables should be defined in their own class, with state class members exposing attributes such as table and column name. – Science

@Science Could you give an example please using that technique? – Varnish

@Dane Yes, there's an online course on Android data storage at Udacity (taught by Google themselves) that deals with this. udacity.com/course/android-basics-data-storage--ud845 – Uther

Answer 2

I recommend checking out the source code for the Android 2.x ContactProvider. (Which can be found online). They handle cross table queries by providing specialized views that you then run queries against on the back end. On the front end they are accessible to the caller via various different URIs through a single content provider. You'll probably also want to provide a class or two for holding constants for your table field names and URI strings. These classes could be provided either as an API include or as a drop in class, and will make it much easier for the consuming application to use.

Its a bit complex, so you might also want to check out how the calendar as well to get an idea of what you do and do not need.

You should only need a single DB adapter and a single Content provider per database (not per table) to do most of the work, but you can use multiple adapters/providers if you really want to. It just makes things a bit more complicated.

Valance

com.android.providers.contacts.ContactsProvider2.java github.com/android/platform_packages_providers_contactsprovider/… – Marjie

@Marloke Thanks. Ok, I understand that even the Android team use the switch solution, but this part you mentioned : They handle cross table queries by providing specialized views that you then run queries against on the back end. On the front end they are accessible to the caller via various different URIs through a single content provider . Do you think you could explain that a little more in detail? – Cantata

Answer 3

One ContentProvider can serve multiple tables, but they should be somewhat related. It will make a difference if you intend to sync your providers. If you want seperate syncs for, let's say Contacts, Mail or Calendar, you will need different providers for each of them, even if they end up being in the same database or are synced with the same service, because Sync Adapters are tied directly to a particular provider.

As far as I can tell, you can only use a single SQLiteOpenHelper per database, though, since it stores its meta information in a table within the database. So if your ContentProviders access the same database, you'll have to share the Helper somewhow.

Fictionalize

Answer 4

Note: This is a clarification/modification to the answer provide by Opy.

This approach subdivides each of the insert, delete, update, and getType methods with switch statements in order to handle each of your individual tables. You'll use a CASE to identify each table (or uri) to be referenced. Each CASE then maps to one of your tables or URI's. E.g., TABLE1 or URI1 is selected in CASE #1, etc. for all the tables your app employs.

Here's an example of the approach. This is for the insert method. It's implemented a bit differently from Opy's but performs the same function. You can select the style you prefer. I also wanted to be sure insert returns a value even if the table insertion fails. In that case it returns a -1.

  @Override
  public Uri insert(Uri uri, ContentValues values) {
    int uriType = sURIMatcher.match(uri);
    SQLiteDatabase sqlDB; 

    long id = 0;
    switch (uriType){ 
        case TABLE1: 
            sqlDB = Table1Database.getWritableDatabase();
            id = sqlDB.insert(Table1.TABLE_NAME, null, values); 
            getContext().getContentResolver().notifyChange(uri, null);
            return Uri.parse(BASE_PATH1 + "/" + id);
        case TABLE2: 
            sqlDB = Table2Database.getWritableDatabase();
            id = sqlDB.insert(Table2.TABLE_NAME, null, values); 
            getContext().getContentResolver().notifyChange(uri, null);
            return Uri.parse(BASE_PATH2 + "/" + id);
        default: 
            throw new SQLException("Failed to insert row into " + uri); 
            return -1;
    }       
  }  // [END insert]

Denicedenie

Answer 5

I found best demo and explanation for ContentProvider and I think it has followed Android Standards.

Contract Classes

 /**
   * The Content Authority is a name for the entire content provider, similar to the relationship
   * between a domain name and its website. A convenient string to use for content authority is
   * the package name for the app, since it is guaranteed to be unique on the device.
   */
  public static final String CONTENT_AUTHORITY = "com.androidessence.moviedatabase";

  /**
   * The content authority is used to create the base of all URIs which apps will use to
   * contact this content provider.
   */
  private static final Uri BASE_CONTENT_URI = Uri.parse("content://" + CONTENT_AUTHORITY);

  /**
   * A list of possible paths that will be appended to the base URI for each of the different
   * tables.
   */
  public static final String PATH_MOVIE = "movie";
  public static final String PATH_GENRE = "genre";

and Inner Classes:

 /**
   * Create one class for each table that handles all information regarding the table schema and
   * the URIs related to it.
   */
  public static final class MovieEntry implements BaseColumns {
      // Content URI represents the base location for the table
      public static final Uri CONTENT_URI =
              BASE_CONTENT_URI.buildUpon().appendPath(PATH_MOVIE).build();

      // These are special type prefixes that specify if a URI returns a list or a specific item
      public static final String CONTENT_TYPE =
              "vnd.android.cursor.dir/" + CONTENT_URI  + "/" + PATH_MOVIE;
      public static final String CONTENT_ITEM_TYPE =
              "vnd.android.cursor.item/" + CONTENT_URI + "/" + PATH_MOVIE;

      // Define the table schema
      public static final String TABLE_NAME = "movieTable";
      public static final String COLUMN_NAME = "movieName";
      public static final String COLUMN_RELEASE_DATE = "movieReleaseDate";
      public static final String COLUMN_GENRE = "movieGenre";

      // Define a function to build a URI to find a specific movie by it's identifier
      public static Uri buildMovieUri(long id){
          return ContentUris.withAppendedId(CONTENT_URI, id);
      }
  }

  public static final class GenreEntry implements BaseColumns{
      public static final Uri CONTENT_URI =
              BASE_CONTENT_URI.buildUpon().appendPath(PATH_GENRE).build();

      public static final String CONTENT_TYPE =
              "vnd.android.cursor.dir/" + CONTENT_URI + "/" + PATH_GENRE;
      public static final String CONTENT_ITEM_TYPE =
              "vnd.android.cursor.item/" + CONTENT_URI + "/" + PATH_GENRE;

      public static final String TABLE_NAME = "genreTable";
      public static final String COLUMN_NAME = "genreName";

      public static Uri buildGenreUri(long id){
          return ContentUris.withAppendedId(CONTENT_URI, id);
      }
  }

Now creating Database using SQLiteOpenHelper:

public class MovieDBHelper extends SQLiteOpenHelper{
    /**
     * Defines the database version. This variable must be incremented in order for onUpdate to
     * be called when necessary.
     */
    private static final int DATABASE_VERSION = 1;
    /**
     * The name of the database on the device.
     */
    private static final String DATABASE_NAME = "movieList.db";

    /**
     * Default constructor.
     * @param context The application context using this database.
     */
    public MovieDBHelper(Context context){
        super(context, DATABASE_NAME, null, DATABASE_VERSION);
    }

    /**
     * Called when the database is first created.
     * @param db The database being created, which all SQL statements will be executed on.
     */
    @Override
    public void onCreate(SQLiteDatabase db) {
        addGenreTable(db);
        addMovieTable(db);
    }

    /**
     * Called whenever DATABASE_VERSION is incremented. This is used whenever schema changes need
     * to be made or new tables are added.
     * @param db The database being updated.
     * @param oldVersion The previous version of the database. Used to determine whether or not
     *                   certain updates should be run.
     * @param newVersion The new version of the database.
     */
    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {

    }

    /**
     * Inserts the genre table into the database.
     * @param db The SQLiteDatabase the table is being inserted into.
     */
    private void addGenreTable(SQLiteDatabase db){
        db.execSQL(
                "CREATE TABLE " + MovieContract.GenreEntry.TABLE_NAME + " (" +
                        MovieContract.GenreEntry._ID + " INTEGER PRIMARY KEY, " +
                        MovieContract.GenreEntry.COLUMN_NAME + " TEXT UNIQUE NOT NULL);"
        );
    }

    /**
     * Inserts the movie table into the database.
     * @param db The SQLiteDatabase the table is being inserted into.
     */
    private void addMovieTable(SQLiteDatabase db){
        db.execSQL(
                "CREATE TABLE " + MovieContract.MovieEntry.TABLE_NAME + " (" +
                        MovieContract.MovieEntry._ID + " INTEGER PRIMARY KEY, " +
                        MovieContract.MovieEntry.COLUMN_NAME + " TEXT NOT NULL, " +
                        MovieContract.MovieEntry.COLUMN_RELEASE_DATE + " TEXT NOT NULL, " +
                        MovieContract.MovieEntry.COLUMN_GENRE + " INTEGER NOT NULL, " +
                        "FOREIGN KEY (" + MovieContract.MovieEntry.COLUMN_GENRE + ") " +
                        "REFERENCES " + MovieContract.GenreEntry.TABLE_NAME + " (" + MovieContract.GenreEntry._ID + "));"
        );
    }
}

Content Provider:

public class MovieProvider extends ContentProvider {
    // Use an int for each URI we will run, this represents the different queries
    private static final int GENRE = 100;
    private static final int GENRE_ID = 101;
    private static final int MOVIE = 200;
    private static final int MOVIE_ID = 201;

    private static final UriMatcher sUriMatcher = buildUriMatcher();
    private MovieDBHelper mOpenHelper;

    @Override
    public boolean onCreate() {
        mOpenHelper = new MovieDBHelper(getContext());
        return true;
    }

    /**
     * Builds a UriMatcher that is used to determine witch database request is being made.
     */
    public static UriMatcher buildUriMatcher(){
        String content = MovieContract.CONTENT_AUTHORITY;

        // All paths to the UriMatcher have a corresponding code to return
        // when a match is found (the ints above).
        UriMatcher matcher = new UriMatcher(UriMatcher.NO_MATCH);
        matcher.addURI(content, MovieContract.PATH_GENRE, GENRE);
        matcher.addURI(content, MovieContract.PATH_GENRE + "/#", GENRE_ID);
        matcher.addURI(content, MovieContract.PATH_MOVIE, MOVIE);
        matcher.addURI(content, MovieContract.PATH_MOVIE + "/#", MOVIE_ID);

        return matcher;
    }

    @Override
    public String getType(Uri uri) {
        switch(sUriMatcher.match(uri)){
            case GENRE:
                return MovieContract.GenreEntry.CONTENT_TYPE;
            case GENRE_ID:
                return MovieContract.GenreEntry.CONTENT_ITEM_TYPE;
            case MOVIE:
                return MovieContract.MovieEntry.CONTENT_TYPE;
            case MOVIE_ID:
                return MovieContract.MovieEntry.CONTENT_ITEM_TYPE;
            default:
                throw new UnsupportedOperationException("Unknown uri: " + uri);
        }
    }

    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs, String sortOrder) {
        final SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        Cursor retCursor;
        switch(sUriMatcher.match(uri)){
            case GENRE:
                retCursor = db.query(
                        MovieContract.GenreEntry.TABLE_NAME,
                        projection,
                        selection,
                        selectionArgs,
                        null,
                        null,
                        sortOrder
                );
                break;
            case GENRE_ID:
                long _id = ContentUris.parseId(uri);
                retCursor = db.query(
                        MovieContract.GenreEntry.TABLE_NAME,
                        projection,
                        MovieContract.GenreEntry._ID + " = ?",
                        new String[]{String.valueOf(_id)},
                        null,
                        null,
                        sortOrder
                );
                break;
            case MOVIE:
                retCursor = db.query(
                        MovieContract.MovieEntry.TABLE_NAME,
                        projection,
                        selection,
                        selectionArgs,
                        null,
                        null,
                        sortOrder
                );
                break;
            case MOVIE_ID:
                _id = ContentUris.parseId(uri);
                retCursor = db.query(
                        MovieContract.MovieEntry.TABLE_NAME,
                        projection,
                        MovieContract.MovieEntry._ID + " = ?",
                        new String[]{String.valueOf(_id)},
                        null,
                        null,
                        sortOrder
                );
                break;
            default:
                throw new UnsupportedOperationException("Unknown uri: " + uri);
        }

        // Set the notification URI for the cursor to the one passed into the function. This
        // causes the cursor to register a content observer to watch for changes that happen to
        // this URI and any of it's descendants. By descendants, we mean any URI that begins
        // with this path.
        retCursor.setNotificationUri(getContext().getContentResolver(), uri);
        return retCursor;
    }

    @Override
    public Uri insert(Uri uri, ContentValues values) {
        final SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        long _id;
        Uri returnUri;

        switch(sUriMatcher.match(uri)){
            case GENRE:
                _id = db.insert(MovieContract.GenreEntry.TABLE_NAME, null, values);
                if(_id > 0){
                    returnUri =  MovieContract.GenreEntry.buildGenreUri(_id);
                } else{
                    throw new UnsupportedOperationException("Unable to insert rows into: " + uri);
                }
                break;
            case MOVIE:
                _id = db.insert(MovieContract.MovieEntry.TABLE_NAME, null, values);
                if(_id > 0){
                    returnUri = MovieContract.MovieEntry.buildMovieUri(_id);
                } else{
                    throw new UnsupportedOperationException("Unable to insert rows into: " + uri);
                }
                break;
            default:
                throw new UnsupportedOperationException("Unknown uri: " + uri);
        }

        // Use this on the URI passed into the function to notify any observers that the uri has
        // changed.
        getContext().getContentResolver().notifyChange(uri, null);
        return returnUri;
    }

    @Override
    public int delete(Uri uri, String selection, String[] selectionArgs) {
        final SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        int rows; // Number of rows effected

        switch(sUriMatcher.match(uri)){
            case GENRE:
                rows = db.delete(MovieContract.GenreEntry.TABLE_NAME, selection, selectionArgs);
                break;
            case MOVIE:
                rows = db.delete(MovieContract.MovieEntry.TABLE_NAME, selection, selectionArgs);
                break;
            default:
                throw new UnsupportedOperationException("Unknown uri: " + uri);
        }

        // Because null could delete all rows:
        if(selection == null || rows != 0){
            getContext().getContentResolver().notifyChange(uri, null);
        }

        return rows;
    }

    @Override
    public int update(Uri uri, ContentValues values, String selection, String[] selectionArgs) {
        final SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        int rows;

        switch(sUriMatcher.match(uri)){
            case GENRE:
                rows = db.update(MovieContract.GenreEntry.TABLE_NAME, values, selection, selectionArgs);
                break;
            case MOVIE:
                rows = db.update(MovieContract.MovieEntry.TABLE_NAME, values, selection, selectionArgs);
                break;
            default:
                throw new UnsupportedOperationException("Unknown uri: " + uri);
        }

        if(rows != 0){
            getContext().getContentResolver().notifyChange(uri, null);
        }

        return rows;
    }
}

I hope it will helps you.

Demo on GitHub: https://github.com/androidessence/MovieDatabase

Full Article : https://guides.codepath.com/android/creating-content-providers

References:

• http://code.tutsplus.com/tutorials/android-sdk_content-providers--mobile-5549

• http://www.grokkingandroid.com/android-tutorial-writing-your-own-content-provider/

• http://developer.android.com/guide/topics/providers/content-providers.html

• https://thenewcircle.com/s/post/1375/android_content_provider_tutorial

• http://www.grokkingandroid.com/android-tutorial-content-provider-basics/

• http://androidessence.com/

Note : I copied code just because if link of demo or article may be remove in future.

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