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。