基于Android如何实现将数据库保存到SD卡

概述有时候为了需要，会将数据库保存到外部存储或者SD卡中（对于这种情况可以通过加密数据来避免数据被破解），比如一个应用支持多个数据，每个数据都需要有一个对应的数据库，并且数据库中的信息量特别大时，这显然更应

有时候为了需要，会将数据库保存到外部存储或者SD卡中（对于这种情况可以通过加密数据来避免数据被破解），比如一个应用支持多个数据，每个数据都需要有一个对应的数据库，并且数据库中的信息量特别大时，这显然更应该将数据库保存在外部存储或者SD卡中，因为RAM的大小是有限的；其次在写某些测试程序时将数据库保存在SD卡更方便查看数据库中的内容。

AndroID通过sqliteOpenHelper创建数据库时默认是将数据库保存在'/data/data/应用程序名/databases'目录下的，只需要在继承sqliteOpenHelper类的构造函数中传入数据库名称就可以了，但如果将数据库保存到指定的路径下面，都需要通过重写继承sqliteOpenHelper类的构造函数中的context，因为：在阅读sqliteOpenHelper.java的源码时会发现：创建数据库都是通过Context的openorCreateDatabase方法实现的，如果我们需要在指定的路径下创建数据库，就需要写一个类继承Context，并复写其openorCreateDatabase方法，在openorCreateDatabase方法中指定数据库存储的路径即可，下面为类sqliteOpenHelper中getWritableDatabase和getReadableDatabase方法的源码，sqliteOpenHelper就是通过这两个方法来创建数据库的。

/**  * Create and/or open a database that will be used for reading and writing.  * The first time this is called,the database will be opened and  * {@link #onCreate},{@link #onUpgrade} and/or {@link #onopen} will be  * called.  *  * <p>Once opened successfully,the database is cached,so you can  * call this method every time you need to write to the database.  * (Make sure to call {@link #close} when you no longer need the database.)  * Errors such as bad permissions or a full disk may cause this method  * to fail,but future attempts may succeed if the problem is fixed.</p>  *  * <p >Database upgrade may take a long time,you  * should not call this method from the application main thread,including  * from {@link androID.content.ContentProvIDer#onCreate ContentProvIDer.onCreate()}.  *  * @throws sqliteException if the database cannot be opened for writing  * @return a read/write database object valID until {@link #close} is called  */ public synchronized sqliteDatabase getWritableDatabase() {  if (mDatabase != null) {   if (!mDatabase.isopen()) {    // darn! the user closed the database by calling mDatabase.close()    mDatabase = null;   } else if (!mDatabase.isReadonly()) {    return mDatabase; // The database is already open for business   }  }  if (mIsInitializing) {   throw new IllegalStateException("getWritableDatabase called recursively");  }  // If we have a read-only database open,someone Could be using it  // (though they shouldn't),which would cause a lock to be held on  // the file,and our attempts to open the database read-write would  // fail waiting for the file lock. To prevent that,we acquire the  // lock on the read-only database,which shuts out other users.  boolean success = false;  sqliteDatabase db = null;  if (mDatabase != null) mDatabase.lock();  try {   mIsInitializing = true;   if (mname == null) {    db = sqliteDatabase.create(null);   } else {    db = mContext.openorCreateDatabase(mname,mFactory,mErrorHandler);   }   int version = db.getVersion();   if (version != mNewVersion) {    db.beginTransaction();    try {     if (version == 0) {      onCreate(db);     } else {      if (version > mNewVersion) {       onDowngrade(db,version,mNewVersion);      } else {       onUpgrade(db,mNewVersion);      }     }     db.setVersion(mNewVersion);     db.setTransactionSuccessful();    } finally {     db.endTransaction();    }   }   onopen(db);   success = true;   return db;  } finally {   mIsInitializing = false;   if (success) {    if (mDatabase != null) {     try { mDatabase.close(); } catch (Exception e) { }     mDatabase.unlock();    }    mDatabase = db;   } else {    if (mDatabase != null) mDatabase.unlock();    if (db != null) db.close();   }  } } /**  * Create and/or open a database. This will be the same object returned by  * {@link #getWritableDatabase} unless some problem,such as a full disk,* requires the database to be opened read-only. In that case,a read-only  * database object will be returned. If the problem is fixed,a future call  * to {@link #getWritableDatabase} may succeed,in which case the read-only  * database object will be closed and the read/write object will be returned  * in the future.  *  * <p >like {@link #getWritableDatabase},this method may  * take a long time to return,so you should not call it from the  * application main thread,including from  * {@link androID.content.ContentProvIDer#onCreate ContentProvIDer.onCreate()}.  *  * @throws sqliteException if the database cannot be opened  * @return a database object valID until {@link #getWritableDatabase}  * or {@link #close} is called.  */ public synchronized sqliteDatabase getReadableDatabase() {  if (mDatabase != null) {   if (!mDatabase.isopen()) {    // darn! the user closed the database by calling mDatabase.close()    mDatabase = null;   } else {    return mDatabase; // The database is already open for business   }  }  if (mIsInitializing) {   throw new IllegalStateException("getReadableDatabase called recursively");  }  try {   return getWritableDatabase();  } catch (sqliteException e) {   if (mname == null) throw e; // Can't open a temp database read-only!   Log.e(TAG,"Couldn't open " + mname + " for writing (will try read-only):",e);  }  sqliteDatabase db = null;  try {   mIsInitializing = true;   String path = mContext.getDatabasePath(mname).getPath();   db = sqliteDatabase.openDatabase(path,sqliteDatabase.OPEN_Readonly,mErrorHandler);   if (db.getVersion() != mNewVersion) {    throw new sqliteException("Can't upgrade read-only database from version " +      db.getVersion() + " to " + mNewVersion + ": " + path);   }   onopen(db);   Log.w(TAG,"Opened " + mname + " in read-only mode");   mDatabase = db;   return mDatabase;  } finally {   mIsInitializing = false;   if (db != null && db != mDatabase) db.close();  } }

通过上面的分析可以写出一个自定义的Context类，该类继承Context即可，但由于Context中有除了openorCreateDatabase方法以外的其它抽象函数，所以建议使用非抽象类Contextwrapper，该类继承自Context，自定义的DatabaseContext类源码如下：

public class DatabaseContext extends Contextwrapper { public DatabaseContext(Context context){  super( context ); } /**  * 获得数据库路径，如果不存在，则创建对象对象  * @param name  * @param mode  * @param factory  */ @OverrIDe public file getDatabasePath(String name) {  //判断是否存在sd卡  boolean sdExist = androID.os.Environment.MEDIA_MOUNTED.equals(androID.os.Environment.getExternalStorageState());  if(!sdExist){//如果不存在,return null;  }else{//如果存在   //获取sd卡路径   String dbDir= fileUtils.getFlashBPath();   dbDir += "DB";//数据库所在目录   String dbPath = dbDir+"/"+name;//数据库路径   //判断目录是否存在，不存在则创建该目录   file dirfile = new file(dbDir);   if(!dirfile.exists()){    dirfile.mkdirs();   }   //数据库文件是否创建成功   boolean isfileCreateSuccess = false;    //判断文件是否存在，不存在则创建该文件   file dbfile = new file(dbPath);   if(!dbfile.exists()){    try {        isfileCreateSuccess = dbfile.createNewfile();//创建文件    } catch (IOException e) {     e.printstacktrace();    }   }else{    isfileCreateSuccess = true;   }   //返回数据库文件对象   if(isfileCreateSuccess){    return dbfile;   }else{    return null;   }  } } /**  * 重载这个方法，是用来打开SD卡上的数据库的，androID 2.3及以下会调用这个方法。  *   * @param name  * @param mode  * @param factory  */ @OverrIDe public sqliteDatabase openorCreateDatabase(String name,int mode,sqliteDatabase.CursorFactory factory) {  sqliteDatabase result = sqliteDatabase.openorCreateDatabase(getDatabasePath(name),null);  return result; } /**  * AndroID 4.0会调用此方法获取数据库。  *   * @see androID.content.Contextwrapper#openorCreateDatabase(java.lang.String,int,*   androID.database.sqlite.sqliteDatabase.CursorFactory,*   androID.database.DatabaseErrorHandler)  * @param name  * @param mode  * @param factory  * @param errorHandler  */ @OverrIDe public sqliteDatabase openorCreateDatabase(String name,CursorFactory factory,DatabaseErrorHandler errorHandler) {  sqliteDatabase result = sqliteDatabase.openorCreateDatabase(getDatabasePath(name),null);  return result; }}

在继承sqliteOpenHelper的子类的构造函数中，用DatabaseContext的实例替代context即可：

DatabaseContext dbContext = new DatabaseContext(context);super(dbContext,mDatabasename,null,VERSION);

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