android系統內嵌了SQLite資料庫，優點是處理速度快，佔用記憶體少

在android應用中，圍繞著資料庫的操作主要是通過SQLiteDatabase來完成的，該類封裝了以下方法：

SQLiteDatabase是通過SQLiteOpenHelper來獲取的，該類是一個抽象類，開發基於資料庫儲存的android應用時，需要繼承該類，構建出自己的SQLiteOpenHelper例項並覆蓋其超類所宣告的抽象方法：
onCreate(SQLiteDatabase db)：用於建立資料庫表格
onUpgrade(SQLiteDatabase db,int oldVersion,int newVersion)：用於更新資料庫表結構
有了SQLiteOpenHelper之後，便可通過其getWritableDatabase()方法獲取SQLiteDatabase物件，用以完成對資料庫的CURD操作。

public class WordSQLiteHelper extends SQLiteOpenHelper{
	public static final String TABLE_WORDS = "words";
	public static final String COLUMN_ID = "_id";
	public static final String COLUMN_ENGLISH = "english";
	public static final String COLUMN_CHINESE = "chinese";
	public static final String COLUMN_PHRASE = "phrase";
	public static final String COLUMN_TRANSLATION = "translation";
	public static final String COLUMN_LEVEL = "level";
	private static final String DATABASE_NAME = "words.db";
	private static final int DATABASE_VERSION = 1;

	public WordSQLiteHelper(Context context){
		super(context, DATABASE_NAME, null, DATABASE_VERSION);
	}
	@Override
	public void onCreate(SQLiteDatabase db) {
		StringBuilder sql=new StringBuilder("create table ").append(TABLE_WORDS).append("(")
				.append(COLUMN_ID).append(" integer primary key autoincrement,")
				.append(COLUMN_ENGLISH).append(" text not null,")
				.append(COLUMN_CHINESE).append(" text not null,")
				.append(COLUMN_PHRASE).append(" text not null,")
				.append(COLUMN_LEVEL).append(" real not null,")
				.append(COLUMN_TRANSLATION).append(" text not null);");
		db.execSQL(sql.toString());
	}

	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
		Log.w(WordSQLiteHelper.class.getName(),"Upgrading database from version " + oldVersion + " to "
	            + newVersion + ", which will destroy all old data");
		db.execSQL("DROP TABLE IF EXISTS "+TABLE_WORDS);
		onCreate(db);
	}
}
 

在做web應用時，為了程式碼層次能夠更加清晰，通常將其劃分為3層結構(entity、dao和service)
dao的建立使得Service的編寫變得更加容易，android應用可基於SQLiteDatabase物件構建出自己的Dao例項

public class WordDao implements IBaseDao<Word>{
	private SQLiteDatabase database;
	private WordSQLiteHelper dbHelper;
	private String[] allColumns={WordSQLiteHelper.COLUMN_ID,WordSQLiteHelper.COLUMN_ENGLISH,
			WordSQLiteHelper.COLUMN_CHINESE,WordSQLiteHelper.COLUMN_PHRASE,WordSQLiteHelper.COLUMN_TRANSLATION,WordSQLiteHelper.COLUMN_LEVEL};
	public WordDao(Context context){
		dbHelper=new WordSQLiteHelper(context);
	}
	public void open(){
		database=dbHelper.getWritableDatabase();
	}
	public void close(){
		dbHelper.close();
	}
	@Override
	public void create(Word word){
		ContentValues values = new ContentValues();
		values.put(WordSQLiteHelper.COLUMN_ENGLISH,word.getEnglish());
		values.put(WordSQLiteHelper.COLUMN_CHINESE,word.getChinese());
		values.put(WordSQLiteHelper.COLUMN_PHRASE,word.getPhrase());
		values.put(WordSQLiteHelper.COLUMN_TRANSLATION,word.getTranslation());
		values.put(WordSQLiteHelper.COLUMN_LEVEL,word.getLevel());
		long insertId=database.insert(WordSQLiteHelper.TABLE_WORDS,null,values);
		word.setId(insertId);
	}
	@Override
	public void update(Word word){
		ContentValues values = new ContentValues();
		values.put(WordSQLiteHelper.COLUMN_ENGLISH,word.getEnglish());
		values.put(WordSQLiteHelper.COLUMN_CHINESE,word.getChinese());
		values.put(WordSQLiteHelper.COLUMN_PHRASE,word.getPhrase());
		values.put(WordSQLiteHelper.COLUMN_TRANSLATION,word.getTranslation());
		values.put(WordSQLiteHelper.COLUMN_LEVEL,word.getLevel());
		database.update(WordSQLiteHelper.TABLE_WORDS,values,WordSQLiteHelper.COLUMN_ID+"="+word.getId(),null);
	}
	@Override
	public void delete(Word word){
		long id=word.getId();
		database.delete(WordSQLiteHelper.TABLE_WORDS,WordSQLiteHelper.COLUMN_ID+"="+id,null);
	}
	@Override
	public List<Word> getAll(){
		List<Word> words=new ArrayList<Word>();
		Cursor cursor = database.query(WordSQLiteHelper.TABLE_WORDS,
		        allColumns, null, null, null, null, null);
		//Cursor cursor=database.rawQuery("select * from "+WordSQLiteHelper.TABLE_WORDS,new String[]{});
		cursor.moveToFirst();
		while(!cursor.isAfterLast()){
			words.add(encapsulate(cursor));
			cursor.moveToNext();
		}
		cursor.close();
		return words;
	}
	@Override
	public List<Word> findOnePage(int offset,int num){
		List<Word> words=new ArrayList<Word>();
		Cursor cursor = database.query(WordSQLiteHelper.TABLE_WORDS,
		        allColumns, null, null, null, null, null);
		cursor.moveToFirst();
		cursor.move(offset);
		int i=0;
		while(!cursor.isAfterLast()){
			if(i==num){
				break;
			}
			words.add(encapsulate(cursor));
			cursor.moveToNext();
			i++;
		}
		cursor.close();
		return words;
	}
	@Override
	public int getCount(){
		Cursor cursor = database.query(WordSQLiteHelper.TABLE_WORDS,
		        allColumns, null, null, null, null, null);
		return cursor.getCount();
	}
	private Word encapsulate(Cursor cursor){
		Word word=new Word();
		word.setId(cursor.getLong(0));
		word.setEnglish(cursor.getString(1));
		word.setChinese(cursor.getString(2));
		word.setPhrase(cursor.getString(3));
		word.setTranslation(cursor.getString(4));
		word.setLevel(cursor.getFloat(5));
		return word;
	}
}

在web應用中，dao例項一般通過spring容器來管理的，引入容器確保了dao在系統中只存在一個例項，由於每個dao都封裝了一個session，單例項的dao也避免了session資源的浪費。
我們可效仿spring的管理方式，編寫自己的dao容器，同時，在容器中宣告clear方法，當系統APP退出時，執行該方法對容器進行銷燬，銷燬容器的同時，關閉dao對應的session。

public class DaoContext {
	private Map<String,IBaseDao<?>> daoMap;
	private static DaoContext instance=new DaoContext();
	private DaoContext(){
		daoMap=new HashMap<String,IBaseDao<?>>();
	}
	public static DaoContext getInstance(){
		return instance;
	}
	public void addDao(IBaseDao<?> dao){
		String daoName=dao.getClass().getSimpleName();
		removeDao(daoName);
		daoMap.put(dao.getClass().getSimpleName(),dao);
		dao.open();
	}
	public void removeDao(String daoName){
		if(daoMap.containsKey(daoName)){
			daoMap.remove(daoName);
			IBaseDao<?> dao=getDao(daoName);
			dao.close();
		}
	}
	public IBaseDao<?> getDao(String daoName){
		return daoMap.get(daoName);
	}
	public void clear(){
		for(Entry<String, IBaseDao<?>> entry:daoMap.entrySet()){
			IBaseDao<?> dao=entry.getValue();
			dao.close();
		}
		daoMap.clear();
	}
}