Question

我尝试从资源文件夹中读取现有的sqlite数据库！但是我收到了这个错误：

java.lang.runtimeexception unable to instantiate activity componentinfo

请帮帮我！

这是我的DataBaseHelper类：

public class DataBaseHelper extends SQLiteOpenHelper {

// The Android's default system path of your application database.
private static String DB_PATH = "/data/data/com.example.sqltest/databases/";

private static String DB_NAME = "test.sqlite";

private SQLiteDatabase myDataBase;

private final Context myContext;

/**`enter code here`
 * 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;
}

/**
 * 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();

    this.close();

    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);
}

// 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) {

}

// 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.

}

这是MainActivity类：

public class MainActivity extends Activity {
DataBaseHelper myDbHelper;

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

public MainActivity(Context context) {

    myDbHelper = new DataBaseHelper(this);

    try {

        myDbHelper.createDataBase();

    } catch (IOException ioe) {

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

    }

    try {

        myDbHelper.openDataBase();

    } catch (SQLException sqle) {

        throw sqle;

    }
}
}

任何人都可以提供最简单的代码来显示我现有的sqlite数据库的数据吗？

Answer 1

我想不出有任何理由在constructor为此活动子类（在你的情况下）做任何事情。通常你永远不会直接构建一个活动< / strong>，我们经常使用Intent，我猜这是你的代码问题。使用onCreate()而不是它的构造函数，并执行所有正常的静态设置 - 创建视图，绑定数据到onCreate（）中的列表，数据库等。

<强> EDIT1 ：
您可以轻松解决java.lang.runtimeexception unable to instantiate activity componentinfo，只更改MainActivity：

public class MainActivity extends Activity { DataBaseHelper myDbHelper; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); myDbHelper = new DataBaseHelper(this); try { myDbHelper.createDataBase(); } catch (IOException ioe) { throw new Error("Unable to create database"); } try { myDbHelper.openDataBase(); } catch (SQLException sqle) { throw sqle; } } }

<强> EDIT2 ：
确保将test.sqlite放在项目的assets文件夹中，项目包的名称为com.example.sqltest。
如果您的代码没有其他问题，它将工作。这意味着您的数据库已成功复制并可以使用。现在您必须访问并从数据库中获取内容。请注意课程结束时的注释：

添加您的公共帮助程序方法以访问和获取内容数据库。你可以通过“返回”来返回游标 myDataBase.query（....）“所以你很容易为它创建适配器你的意见。

如果您看到黑页，可能是因为您的布局是空的。您可以使用数据库为您的视图创建适配器，因此您的活动不会为空。

您可以在SQLiteOpenHelper课程中查看这些页面以获取更多详细信息：
Android SQLite Database and ContentProvider - Tutorial
Android SQLite Database Tutorial

无法为sqlite数据库实例化活动componentinfo

1 个答案: