Android数据库ContentProvider封装原理

大家都知道Android数据库使用的是SQLite，但是Google为了给我们简化操作，可以不用经常编写容易出错的SQL语句，直接通过ContentProvider来封装数据的query查询、添加insert、删除delete和更新update，我们就以Android系统的SDK中的例子来给大家简单的说明下吧。

public class NotePadProvider extends ContentProvider {

private static final String TAG = "NotePadProvider";

private static final String DATABASE_NAME = "note_pad.db"; //数据库存储文件名，包含了.db后缀

private static final int DATABASE_VERSION = 2; //数据库版本号，这个是自己定义的，未来扩展数据库时自己可以方便的定义升级规则

private static final String NOTES_TABLE_NAME = "notes"; //表名

private static HashMap sNotesProjectionMap; //常规的Notes

private static HashMap sLiveFolderProjectionMap; //LiveFoder内容

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; //这里Android123提示大家，通常我们操作数据库的Uri比如content://android123/cwj/1103这样的Uri均通过UriMatcher注册并识别的。

private static class DatabaseHelper extends SQLiteOpenHelper { //数据库辅助子类

DatabaseHelper(Context context) {

super(context, DATABASE_NAME, null, DATABASE_VERSION);

}

@Override

public void onCreate(SQLiteDatabase db) { //首次生成数据库，执行sql命令创建一个表

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) { //刚来数据的版本，就是为了定义我们如果未来数据库需要扩展，帮助用户识别并根据规则自动升级数据库文件

Log.w(TAG, "Upgrading database from version " + oldVersion + " to "

+ newVersion + ", which will destroy all old data");

db.execSQL("DROP TABLE IF EXISTS notes"); //由于这里没有做细节处理，如果有新版本，删除老的表，我们未来不能这样处理，这仅仅是Google的例子而已所以删除老版本数据

onCreate(db);

}

private DatabaseHelper mOpenHelper;

@Override

public boolean onCreate() { //这里重写ContentProvider的onCreate方法做一些初始化操作

mOpenHelper = new DatabaseHelper(getContext());

return true;

}

//有关数据库的查询操作，Android的SQLite提供了一个SQLiteQueryBuilder方法再次将SQL命令封装了下，单独分离出表名，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);

}

有关数据的插入操作，重写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); //通知数据库内容有改变

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;

}

最后我们需要在构造奔雷时就监听Uri，如果处理的Uri需要其他程序获知，需要在Androidmanifest.xml文件中显式的导出provider的Uri定义

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();

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();

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.

}

最后安博中程网站帮助初学者简单的概括下Android的ContentProvider类的作用，就是尽可能的减少SQL语句的编写在外部操作，封装成方法，而有关SQL语言的执行在DatabaseHelper中也被简化和分离出了，而SQL语句主要是体现在选择表的字段，where这样的条件限定语句大大减少了我们日常的开发。

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