java - 每个表问题的 SQLite 助手类

Question

我在我的 android 应用程序中使用 SQLite db，并为 db 中的每个表设置了帮助程序类。有很多表，这些类只处理每个表的所有 crud 方法。我在这里按照示例 #6

当实例化助手类的多个实例时，我遇到了问题。该数据库仅在一次实例化一个助手类时可用。

public class ProjectsHelper {

    SQLiteDatabase db;
    DataBaseHelper dbHelper;
    private String TABLE_NAME = "Projects";

    public ProjectsHelper(Context context){
        if(dbHelper == null){
            dbHelper = new DataBaseHelper(context);
        }
    }

    public void open() throws SQLException {
        try {
            if(!isOpen()){
                db = dbHelper.getWritableDatabase();
            }
        } catch (Exception e){
            e.printStackTrace();
        }
    }

    private boolean isOpen(){
        return db != null && db.isOpen();
    }

    public void close() {
        if(isOpen()){
            dbHelper.close();
            db.close();
            dbHelper = null;
            db = null;
        }
    }

    public long insert(Project obj){
        ContentValues values = new ContentValues();
        values.put("ProjName", obj.getProjName().trim());
        if(!Utilities.empty(obj.getProjDesc())){
            values.put("ProjDesc", obj.getProjDesc().trim());
        }
        if(!Utilities.empty(obj.getProjNumber())){
            values.put("ProjNum", obj.getProjNumber().trim());
        }
        values.put("ServerID", obj.getServerID());
        values.put("GUID", obj.getGUID());
        values.put("ModDate", obj.getModDate());

        long id = db.insert(TABLE_NAME, null, values);
        return id;
    }
}


public class DataBaseHelper extends SQLiteOpenHelper {

    //The Android's default system path of your application database.
    public static String DB_PATH = "/data/data/com.siscoders.arscompnentinspector/databases/";

    public static String DB_NAME = "ARSInspection.db";

    private SQLiteDatabase myDataBase;

    private final Context 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 context) {

        super(context, DB_NAME, null, 1);
        this.myContext = context;

        try {
            createDataBase();
        } catch (IOException ioe) {
            throw new Error("Unable to create database");
        }
        try {
            //openDataBase();
        }catch(SQLException sqle){
            throw sqle;
        }
    }

    /**
     * Creates a empty database on the system and rewrites it with your own database.
     * */
    public void createDataBase() throws IOException {

        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 (IOException e) {

                throw new Error("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{
            String 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 IOException{

    //Open your local db as the input stream
    InputStream myInput = myContext.getAssets().open(DB_NAME);

    // Path to the just created empty db
    String outFileName = DB_PATH + DB_NAME;

    //Open the empty db as the output stream
    OutputStream myOutput = new FileOutputStream(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);
    }
    myOutput.flush();
    myOutput.close();
    myInput.close();

}

@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) {

}
}

标签： javaandroidsqliteandroid-sqlite