Android數據庫ContentProvider封裝原理

大家都知道android數據庫使用的是SQLite，但是Google為了給我們簡化操作，可以不用經常編寫容易出錯的SQL語句，直接通過ContentProvider來封裝數據的query查詢、添加insert、刪除delete和更新update，我們就以android系統的SDK中的例子來給大家簡單的說明下吧。

 public class NotePadProvider extends ContentProvider {

    private static final String TAG = "NotePadProvider";

    private static final String DATABASE_NAME = "note_pad.db"; //數據庫存儲文件名，包含了.db後綴
    private static final int DATABASE_VERSION = 2; //數據庫版本號，這個是自己定義的，未來擴展數據庫時自己可以方便的定義升級規則
    private static final String NOTES_TABLE_NAME = "notes"; //表名

    private static HashMap<String, String> sNotesProjectionMap;  //常規的Notes
    private static HashMap<String, String> sLiveFolderProjectionMap; //LiveFoder內容

    private static final int NOTES = 1;
    private static final int NOTE_ID = 2;
    private static final int LIVE_FOLDER_NOTES = 3;

    private static final UriMatcher sUriMatcher; //這裡android123提示大家，通常我們操作數據庫的Uri比如content://android123/cwj/1103這樣的Uri均通過UriMatcher注冊並識別的。

    private static class DatabaseHelper extends SQLiteOpenHelper {  //數據庫輔助子類

        DatabaseHelper(Context context) {
            super(context, DATABASE_NAME, null, DATABASE_VERSION);
        }

        @Override
        public void onCreate(SQLiteDatabase db) {  //首次生成數據庫，執行sql命令創建一個表
            db.execSQL("CREATE TABLE " + NOTES_TABLE_NAME + " ("
                    + Notes._ID + " INTEGER PRIMARY KEY,"
                    + Notes.TITLE + " TEXT,"
                    + Notes.NOTE + " TEXT,"
                    + Notes.CREATED_DATE + " INTEGER,"
                    + Notes.MODIFIED_DATE + " INTEGER"
                    + ");");
        }

        @Override
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {  //剛來數據的版本，就是為了定義我們如果未來數據庫需要擴展，幫助用戶識別並根據規則自動升級數據庫文件
            Log.w(TAG, "Upgrading database from version " + oldVersion + " to "
                    + newVersion + ", which will destroy all old data");
            db.execSQL("DROP TABLE IF EXISTS notes"); //由於這裡沒有做細節處理，如果有新版本，刪除老的表，我們未來不能這樣處理，這僅僅是Google的例子而已所以刪除老版本數據
            onCreate(db);
        }
    }

    private DatabaseHelper mOpenHelper;

    @Override
    public boolean onCreate() {  //這裡重寫ContentProvider的onCreate方法做一些初始化操作
        mOpenHelper = new DatabaseHelper(getContext());
        return true;
    }

  //有關數據庫的查詢操作，android的SQLite提供了一個SQLiteQueryBuilder方法再次將SQL命令封裝了下，單獨分離出表名，排序方法等

    @Override
    public Cursor query(Uri uri, String[] projection, String selection, String[] selectionArgs,
            String sortOrder) {  
        SQLiteQueryBuilder qb = new SQLiteQueryBuilder();
        qb.setTables(NOTES_TABLE_NAME);

        switch (sUriMatcher.match(uri)) {
        case NOTES:
            qb.setProjectionMap(sNotesProjectionMap);
            break;

        case NOTE_ID:
            qb.setProjectionMap(sNotesProjectionMap);
            qb.appendWhere(Notes._ID + "=" + uri.getPathSegments().get(1));
            break;

        case LIVE_FOLDER_NOTES:
            qb.setProjectionMap(sLiveFolderProjectionMap);
            break;

        default:
            throw new IllegalArgumentException("Unknown URI " + uri);
        }

        String orderBy;
        if (TextUtils.isEmpty(sortOrder)) {
            orderBy = NotePad.Notes.DEFAULT_SORT_ORDER;
        } else {
            orderBy = sortOrder;
        }

        SQLiteDatabase db = mOpenHelper.getReadableDatabase(); 
        Cursor c = qb.query(db, projection, selection, selectionArgs, null, null, orderBy);

        c.setNotificationUri(getContext().getContentResolver(), uri);
        return c;
    }

    @Override
    public String getType(Uri uri) {
        switch (sUriMatcher.match(uri)) {
        case NOTES:
        case LIVE_FOLDER_NOTES:
            return Notes.CONTENT_TYPE;

        case NOTE_ID:
            return Notes.CONTENT_ITEM_TYPE;

        default:
            throw new IllegalArgumentException("Unknown URI " + uri);
        }
    }

  有關數據的插入操作，重寫ContentProvider的insert方法即可

    @Override
    public Uri insert(Uri uri, ContentValues initialValues) {
        if (sUriMatcher.match(uri) != NOTES) {
            throw new IllegalArgumentException("Unknown URI " + uri);
        }

        ContentValues values;
        if (initialValues != null) {
            values = new ContentValues(initialValues);
        } else {
            values = new ContentValues();
        }

        Long now = Long.valueOf(System.currentTimeMillis());

        if (values.containsKey(NotePad.Notes.CREATED_DATE) == false) {
            values.put(NotePad.Notes.CREATED_DATE, now);
        }

        if (values.containsKey(NotePad.Notes.MODIFIED_DATE) == false) {
            values.put(NotePad.Notes.MODIFIED_DATE, now);
        }

        if (values.containsKey(NotePad.Notes.TITLE) == false) {
            Resources r = Resources.getSystem();
            values.put(NotePad.Notes.TITLE, r.getString(android.R.string.untitled));
        }

        if (values.containsKey(NotePad.Notes.NOTE) == false) {
            values.put(NotePad.Notes.NOTE, "");
        }

        SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        long rowId = db.insert(NOTES_TABLE_NAME, Notes.NOTE, values);
        if (rowId > 0) {
            Uri noteUri = ContentUris.withAppendedId(NotePad.Notes.CONTENT_URI, rowId);
            getContext().getContentResolver().notifyChange(noteUri, null); //通知數據庫內容有改變
            return noteUri;
        }

        throw new SQLException("Failed to insert row into " + uri);
    }

    @Override
    public int delete(Uri uri, String where, String[] whereArgs) {
        SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        int count;
        switch (sUriMatcher.match(uri)) {
        case NOTES:
            count = db.delete(NOTES_TABLE_NAME, where, whereArgs);
            break;

        case NOTE_ID:
            String noteId = uri.getPathSegments().get(1);
            count = db.delete(NOTES_TABLE_NAME, Notes._ID + "=" + noteId
                    + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
            break;

        default:
            throw new IllegalArgumentException("Unknown URI " + uri);
        }

        getContext().getContentResolver().notifyChange(uri, null);
        return count;
    }

    @Override
    public int update(Uri uri, ContentValues values, String where, String[] whereArgs) {
        SQLiteDatabase db = mOpenHelper.getWritableDatabase();
        int count;
        switch (sUriMatcher.match(uri)) {
        case NOTES:
            count = db.update(NOTES_TABLE_NAME, values, where, whereArgs);
            break;

        case NOTE_ID:
            String noteId = uri.getPathSegments().get(1);
            count = db.update(NOTES_TABLE_NAME, values, Notes._ID + "=" + noteId
                    + (!TextUtils.isEmpty(where) ? " AND (" + where + ')' : ""), whereArgs);
            break;

        default:
            throw new IllegalArgumentException("Unknown URI " + uri);
        }

        getContext().getContentResolver().notifyChange(uri, null);
        return count;
    }

  最後我們需要在構造奔雷時就監聽Uri，如果處理的Uri需要其他程序獲知，需要在androidmanifest.XML文件中顯式的導出provider的Uri定義

    static {
        sUriMatcher = new UriMatcher(UriMatcher.NO_MATCH);
        sUriMatcher.addURI(NotePad.AUTHORITY, "notes", NOTES);
        sUriMatcher.addURI(NotePad.AUTHORITY, "notes/#", NOTE_ID);
        sUriMatcher.addURI(NotePad.AUTHORITY, "live_folders/notes", LIVE_FOLDER_NOTES);

        sNotesProjectionMap = new HashMap<String, String>();
        sNotesProjectionMap.put(Notes._ID, Notes._ID);
        sNotesProjectionMap.put(Notes.TITLE, Notes.TITLE);
        sNotesProjectionMap.put(Notes.NOTE, Notes.NOTE);
        sNotesProjectionMap.put(Notes.CREATED_DATE, Notes.CREATED_DATE);
        sNotesProjectionMap.put(Notes.MODIFIED_DATE, Notes.MODIFIED_DATE);

        // Support for Live Folders.
        sLiveFolderProjectionMap = new HashMap<String, String>();
        sLiveFolderProjectionMap.put(LiveFolders._ID, Notes._ID + " AS " +
                LiveFolders._ID);
        sLiveFolderProjectionMap.put(LiveFolders.NAME, Notes.TITLE + " AS " +
                LiveFolders.NAME);
        // Add more columns here for more robust Live Folders.
    }
}

  最後Android123幫助初學者簡單的概括下android的ContentProvider類的作用，就是盡可能的減少SQL語句的編寫在外部操作，封裝成方法，而有關SQL語言的執行在DatabaseHelper中也被簡化和分離出了，而SQL語句主要是體現在選擇表的字段，where這樣的條件限定語句大大減少了我們日常的開發。