Context与SQLiteDatabase的方法openOrCreateDatabase的区别 (context、SQLiteOpenHelper)

    Context.openOrCreateDatabase 与 SQLiteDatabase.openOrCreateDatabase本质上完成的功能都一样,Context.openOrCreateDatabase最终是需要调用 SQLiteDatabase.openOrCreateDatabase来完成数据库的创建的。

也就是说, SQLiteDatabase类是android上对sqlite的最底层的封装,几乎所有的对数据库的操作最终都通过这个类来实现。

而Context里面提供的方法,是用于上下文的时候创建数据库,例如你在某个逻辑里面创建的数据库只是在特定的context里面,对于数据库的权限,交由context来管理,而这个逻辑可能是会提供给不止一个context


可以在程序上下文中用openOrCreateDatabase方法来创建和打开一个数据库,方法如下:

private static final String DATABASE_NAME = "betty_db”;

private static final String DATABASE_TABLE = “betty_table”;

private static final String DATABASE_CREATE =

“create table “ + DATABASE_TABLE +

“ ( _id integer primary key autoincrement,” +

“column_one text not null);”;

SQLiteDatabase myDatabase;

private void createDatabase() {

myDatabase = openOrCreateDatabase(DATABASE_NAME, Context.MODE_PRIVATE, null);

myDatabase.execSQL(DATABASE_CREATE);

}


转--------------------------------------------------------------------------------

 梳理SQLiteDatabase、openOrCreateDatabase、SQLiteOpenHelper

 

                    据我所知,android创建数据库可以通过以下方法:

                                                  

一、 SQLiteDatabase.openOrCreateDatabase(file, factory):(以下都在这个SQLiteDatabase类中)

                         1. 一个类名+方法就是个static方法:

                            public static SQLiteDatabase openOrCreateDatabase(File file, CursorFactory factory) {
                                      return openOrCreateDatabase(file.getPath(), factory);
                             }

                         2. 换了绝对路径。

                             public static SQLiteDatabase openOrCreateDatabase(String path, CursorFactory factory) {
                                    return openDatabase(path, factory, CREATE_IF_NECESSARY);
                             }

                         3.去调用了openDatabase方法。

                                      public static SQLiteDatabase openDatabase(String path, CursorFactory factory, int flags) {
                                              SQLiteDatabase db = null;
                                              try {
                                                  // Open the database.
                                                  return new SQLiteDatabase(path, factory, flags);
                                              } catch (SQLiteDatabaseCorruptException e) {
                                                  // Try to recover from this, if we can.
                                                  // TODO: should we do this for other open failures?
                                                  Log.e(TAG, "Deleting and re-creating corrupt database " + path, e);
                                                  EventLog.writeEvent(EVENT_DB_CORRUPT, path);
                                                  new File(path).delete();
                                                  return new SQLiteDatabase(path, factory, flags);
                                              }
                                          }

                        

                         4. 关键的就这行代码:return new SQLiteDatabase(path, factory, flags);其他的是异常处理。

                            也就是新建了这个类,我们创建数据库也可以这样直接new出来。

                         5.现在关键的工作就落在SQLiteDatabase这个类的构造函数上了。  

    private SQLiteDatabase(String path, CursorFactory factory, int flags) {
        if (path == null) {
            throw new IllegalArgumentException("path should not be null");
        }
        mFlags = flags;
        mPath = path;
        mLogStats = "1".equals(android.os.SystemProperties.get("db.logstats"));
        mSlowQueryThreshold = SystemProperties.getInt(LOG_SLOW_QUERIES_PROPERTY, -1);

        mLeakedException = new IllegalStateException(path +
            " SQLiteDatabase created and never closed");
        mFactory = factory;
        dbopen(mPath, mFlags);   <-----------------------------------------------------------
        mPrograms = new WeakHashMap<SQLiteClosable,Object>();
        try {
            setLocale(Locale.getDefault());
        } catch (RuntimeException e) {
            Log.e(TAG, "Failed to setLocale() when constructing, closing the database", e);
            dbclose();
            throw e;
        }
    }

                   6.起关键的就是 dbopen(mPath, mFlags);

                           private native void dbopen(String path, int flags);

                   7.看到这个native,就是jni咯!往下就是c++了。其中细体什么实现open或create就不用管了。感兴趣可以往下跟。

 

 

 二、context.openOrCreateDatabase(name, mode, factory);

                                     1、看android帮助是这样描述的:

                                      Open a new private SQLiteDatabase associated with this Context's application package.

                                      Create the database file if it doesn't exist.

                                      2 、看源码没发现细体的实现部分的代码,发现的话分享一下给我。搜了很多源码,只有这样:

                                           context.java中: 

                                           public abstract SQLiteDatabase openOrCreateDatabase(String name,int mode, CursorFactory factory);

                                           是个抽象方法,其子类ContextWrapper.java:

                                           public SQLiteDatabase openOrCreateDatabase(String name, int mode, CursorFactory factory) {
                                                         return mBase.openOrCreateDatabase(name, mode, factory);
                                          }

                                          又没见哪个子类去覆盖这个方法;

 

         小结:查网上一些资料也是说第二种通过context的方法,其实最终也是得通过SQLiteDatabase这个类中的方法。

                      我认为也是,因为1.context中也用到SQLiteDatabase 这个类啊,可以直接调用这个SQLiteDatabase 的方法嘛。

                                                      2.我们知道在data/data+加上这个应用的包名,就是这个应用程序存放私有数据的目录。那么这个应用程序

                                                       只要通过数据库名称就能找到其路径。而这个SQLiteDatabase的 openOrCreateDatabase(File file, CursorFactory factory)

                                                       再到openOrCreateDatabase(String path, CursorFactory factory);由此可猜这个openOrCreateDatabase(file...)应该是提供

                                                       给context的。

 

 

       (三) 剩下就是这个SQLiteOpenHelper

                        1. 都说这个SQLiteOpenHelper.java类是方便操作数据库的类。

                        2. 那分析一下源码

                                        public abstract class SQLiteOpenHelper

                         3.所以通常我们要使用个类就得去继承它。并去实现父类的抽象方法如:

                                        public class DatabaseHelper extends SQLiteOpenHelper
                                        {
                                         public DatabaseHelper(Context context, String name, CursorFactory cursorFactory, int version)
                                         {
                                          super(context, name, cursorFactory, version);
                                         }

                                         @Override
                                         public void onCreate(SQLiteDatabase db)
                                         {  
                                         }

                                         @Override
                                         public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion)
                                         {  
                                         }

                                         @Override
                                         public void onOpen(SQLiteDatabase db)
                                         {
                                          super.onOpen(db);
                                         }
                                        }

                         4. 好像就这么简单,其实什么也没做。那什么创建或打开数据库?

                         5.重要的是SQLiteOpenHelper中的getWritableDatabase和getReadableDatabase方法,你会发现getReadableDatabase中调用了getWritableDatabase

                         6.所以只要getWritableDatabase清楚getWritableDatabase是如何实现的,就很明朗了。

                                                                                                                                       看似复杂,  慢慢往下看,还有注释(同时要清楚这方法目的就是要创建或打开数据库)

 public synchronized SQLiteDatabase getWritableDatabase() {
        if (mDatabase != null && mDatabase.isOpen() && !mDatabase.isReadOnly()) {
            return mDatabase;  // The database is already open for business                  //如果已经打开了,直接返回。
        }

        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);                                                                        //以上没多大意义,可以不管
            } else {
                db = mContext.openOrCreateDatabase(mName, 0, mFactory);                    //关键是这行代码,是不是很熟悉
            }

            int version = db.getVersion();
            if (version != mNewVersion) {
                db.beginTransaction();
                try {
                    if (version == 0) {
                           onCreate(db);  //子类我们实现的方法(其实什么也没做,要想这第一次创建时做一些操作,

                                                     //就自己在子类的方法实现,如创建表)                                                                            
                    } else {
                        onUpgrade(db, version, mNewVersion);  //子类我们实现的方法(版本发生变化)
                    }
                    db.setVersion(mNewVersion);
                    db.setTransactionSuccessful();
                } finally {
                    db.endTransaction();
                }
            }

            onOpen(db);                                    //子类我们实现的方法                                            
            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();
            }
        }
    }




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