如何在android中使用自己的数据文件

Most all of the Android examples and tutorials out there assume you want to create and populate your database at runtime and not to use and access an independent, preloaded database with your Android application.

The method I'm going to show you takes your own SQLite database file from the "assets" folder and copies into the system database path of your application so the SQLiteDatabase API can open and access it normally.

 

1. Preparing the SQLite database file.

Assuming you already have your sqlite database created, we need to do some modifications to it.
If you don't have a sqlite manager I recommend you to download the opensource available for Win/Linux/Mac.

Open your database and add a new table called "android_metadata", you can execute the following SQL statement to do it:

CREATE TABLE "android_metadata" ("locale" TEXT DEFAULT 'en_US')

Now insert a single row with the text 'en_US' in the "android_metadata" table:

INSERT INTO "android_metadata" VALUES ('en_US')

Then, it is necessary to rename the primary id field of your tables to "_id" so Android will know where to bind the id field of your tables.
You can easily do this with by pressing the edit table button , then selecting the table you want to edit and finally selecting the field you want to rename.

After renaming the id field of all your data tables to "_id" and adding the "android_metadata" table, your database it's ready to be used in your Android application.

Modified database

Note: in this image we see the tables "Categories" and "Content" with the id field renamed to "_id" and the just added table "android_metadata".

2. Copying, opening and accessing your database in your Android application.

Now just put your database file in the "assets" folder of your project and create a Database Helper class by extending the SQLiteOpenHelper class from the "android.database.sqlite" package.

Make your DataBaseHelper class look like this:

public class DataBaseHelper extends SQLiteOpenHelper{
 
    //The Android's default system path of your application database.
    private static  DB_PATH = "/data/data/YOUR_PACKAGE/databases/";
 
    private static  DB_NAME = "myDBName";
 
    private SQLiteDatabase myDataBase; 
 
    private final  myContext;
 
    /**
     * Constructor
     * Takes and keeps a reference of the passed context in order to access to the application assets and resources.
     * @param context
     */
    public DataBaseHelper( context) {
 
    	super(context, DB_NAME, null, 1);
        this.myContext = context;
    }	
 
  /**
     * Creates a empty database on the system and rewrites it with your own database.
     * */
    public void createDataBase() throws {
 
    	boolean dbExist = checkDataBase();
 
    	if(dbExist){
    		//do nothing - database already exist
    	}else{
 
    		//By calling this method and empty database will be created into the default system path
               //of your application so we are gonna be able to overwrite that database with our database.
        	this.getReadableDatabase();
 
        	try {
 
    			copyDataBase();
 
    		} catch ( e) {
 
        		throw new ("Error copying database");
 
        	}
    	}
 
    }
 
    /**
     * Check if the database already exist to avoid re-copying the file each time you open the application.
     * @return true if it exists, false if it doesn't
     */
    private boolean checkDataBase(){
 
    	SQLiteDatabase checkDB = null;
 
    	try{
    		 myPath = DB_PATH + DB_NAME;
    		checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
 
    	}catch(SQLiteException e){
 
    		//database does't exist yet.
 
    	}
 
    	if(checkDB != null){
 
    		checkDB.close();
 
    	}
 
    	return checkDB != null ? true : false;
    }
 
    /**
     * Copies your database from your local assets-folder to the just created empty database in the
     * system folder, from where it can be accessed and handled.
     * This is done by transfering bytestream.
     * */
    private void copyDataBase() throws {
 
    	//Open your local db as the input stream
    	 myInput = myContext.getAssets().open(DB_NAME);
 
    	// Path to the just created empty db
    	 outFileName = DB_PATH + DB_NAME;
 
    	//Open the empty db as the output stream
    	 myOutput = new (outFileName);
 
    	//transfer bytes from the inputfile to the outputfile
    	byte[] buffer = new byte[1024];
    	int length;
    	while ((length = myInput.read(buffer))>0){
    		myOutput.write(buffer, 0, length);
    	}
 
    	//Close the streams
    	myOutput.flush();
    	myOutput.close();
    	myInput.close();
 
    }
 
    public void openDataBase() throws {
 
    	//Open the database
         myPath = DB_PATH + DB_NAME;
    	myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
 
    }
 
    @Override
	public synchronized void close() {
 
    	    if(myDataBase != null)
    		    myDataBase.close();
 
    	    super.close();
 
	}
 
	@Override
	public void onCreate(SQLiteDatabase db) {
 
	}
 
	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
 
	}
 
        // Add your public helper methods to access and get content from the database.
       // You could return cursors by doing "return myDataBase.query(....)" so it'd be easy
       // to you to create adapters for your views.
 
}

That's it.
Now you can create a new instance of this DataBaseHelper class and call the createDataBase() and openDataBase() methods. Remember to change the "YOUR_PACKAGE" to your application package namespace (i.e: com.examplename.myapp) in the DB_PATH string.

       ...
 
        DataBaseHelper myDbHelper = new DataBaseHelper();
        myDbHelper = new DataBaseHelper(this);
 
        try {
 
        	myDbHelper.createDataBase();
 
 	} catch ( ioe) {
 
 		throw new ("Unable to create database");
 
 	}
 
 	try {
 
 		myDbHelper.openDataBase();
 
 	}catch( sqle){
 
 		throw sqle;
 
 	}
 
        ...