Android資料庫ContentProvider封裝原理

大家都知道Android資料庫使用的是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來封裝資料的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"; //資料庫隱藏檔名，包含了.db尾碼
private static final int DATABASE_VERSION = 2; //資料庫版本號碼，這個是自己定義的，未來擴充資料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，通常我們操作資料庫的Uri比如content://android123/cwj/1103這樣的Uri均通過 UriMatcher註冊並識別的。

private static class DatabaseHelper extends SQLiteOpenHelper { //資料庫輔助子類

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

@Override
public void onCreate(SQLiteDatabase db) { //首次產生資料庫，執行sql命令建立一個表
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) { //剛來資料的版本，就是為了定義我們如果未來資料庫需要擴充，協助使用者識別並根據規則自動升級資料庫檔案
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資料
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;
}

//有關資料庫的查詢操作，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);
}
}

有關資料的插入操作，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需要其他程式獲知，需要在Androidmanifest.xml檔案中顯式的匯出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.
}
}

最後協助初學者簡單的概括下Android的ContentProvider類的作用，就是儘可能的減少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欄位，where這樣的條件限定語句大大減少了我們日常的開發。