Question

我正在尝试使用Contentproviders设置数据库并遇到麻烦。当我使用调试器逐步执行代码时，它表明代码首先从ContentProvider类而不是主类运行！这怎么可能呢？谁能帮我？提前致谢！

第一个代码是主代码，第二个代码是提供者类

public class MedF1 extends Activity {

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.drug_list); 

    ListView drugListView;
    ArrayAdapter<Drug> aa;
    ArrayList<Drug> drugs = new ArrayList<Drug>();

    drugListView = (ListView)this.findViewById(R.id.list1);
    DrugProvider.DatabaseHelper mDbHelper1 = new DrugProvider.DatabaseHelper(this);



    //Creation of the Database here
    try {

    mDbHelper1.createDataBase();

    } catch (IOException ioe) {

    throw new Error("Unable to create database");

    }

    try {

    mDbHelper1.openDataBase();

    }catch(SQLException sqle){

    throw sqle;

    }

    //Database created now to fill the view
    int layoutID = android.R.layout.simple_list_item_1;
    aa = new ArrayAdapter<Drug>(this, layoutID , drugs);
    drugListView.setAdapter(aa);

    //
    loadDrugsFromProvider();    
}

这是ContentProvider。调试器显示访问的第一步是“DatabaseHelper”构造函数！这是正常的吗？主类的onCreate方法不应该首先出现吗？

 public class DrugProvider extends ContentProvider {

//Variable declaration omitted for brevity.

private static SQLiteDatabase myDataBase;

// Creation of the database and its basic parameters
public static class DatabaseHelper extends SQLiteOpenHelper {

    public final Context myContext;

    public DatabaseHelper(Context context) {

        super(context, DB_NAME, null, 1);

        this.myContext = context;

    }

    public void createDataBase() throws IOException {

        boolean dbExist = checkDataBase();

        if (dbExist) {
            // do nothing
        } else {

            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 transferring byte stream.
     * */
    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);
        }

        // Close the streams
        myOutput.flush();
        myOutput.close();
        myInput.close();

    }

    public void openDataBase() throws SQLException {

        // Open the database
        String 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) {

    }

Answer 1

正如迈克所说，请移动以下行：

DrugProvider.DatabaseHelper mDbHelper1 = new DrugProvider.DatabaseHelper(this);

进入onCreate（）方法。大声笑我从来没有回答过一个问题，感觉如此......无益。不知道为什么有人不会简单地回答这个问题，但是你可以去。

Android代码不是从主类开始的！

1 个答案: