Java非法状态异常

Question

I am following a tutorial online, but i am getting an illegal state exception. Link to tutorial: http://www.developerfeed.com/android/tutorial/building-todo-list-app-android-using-sqlite

Here is the Database class:

public class TaskerDbHelper extends SQLiteOpenHelper {

private static final int DATABASE_VERSION = 1;

// Database Name
private static final String DATABASE_NAME = "taskerManager";

// tasks table name
private static final String TABLE_TASKS = "tasks";

// tasks Table Columns names
private static final String KEY_ID = "id";
private static final String KEY_TASKNAME = "taskName";
private static final String KEY_STATUS = "status";

public TaskerDbHelper(Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

@Override
public void onCreate(SQLiteDatabase db) {

    String sql = "CREATE TABLE IF NOT EXISTS " + TABLE_TASKS + " ( "
            + KEY_ID + " INTEGER PRIMARY KEY AUTOINCREMENT, " + KEY_TASKNAME
            + " TEXT, " + KEY_STATUS + " INTEGER)";
    db.execSQL(sql);

    db.close();
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldV, int newV) {
    // Drop older table if existed
    db.execSQL("DROP TABLE IF EXISTS " + TABLE_TASKS);
    // Create tables again
    onCreate(db);
}

// Adding new task
public void addTask(Task task) {
    SQLiteDatabase db = this.getWritableDatabase();

    ContentValues values = new ContentValues();
    values.put(KEY_TASKNAME, task.getTaskName()); // task name
    // status of task- can be 0 for not done and 1 for done
    values.put(KEY_STATUS, task.getStatus());

    // Inserting Row
    db.insert(TABLE_TASKS, null, values);
    db.close(); // Closing database connection
}

public List<Task> getAllTasks() {
    List<Task> taskList = new ArrayList<Task>();
    // Select All Query
    String selectQuery = "SELECT  * FROM " + TABLE_TASKS;

    SQLiteDatabase db = this.getWritableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);

    // looping through all rows and adding to list
    if (cursor.moveToFirst()) {
        do {
            Task task = new Task();
            task.setId(cursor.getInt(0));
            task.setTaskName(cursor.getString(1));
            task.setStatus(cursor.getInt(2));
            // Adding contact to list
            taskList.add(task);
        } while (cursor.moveToNext());
    }

    // return task list
    return taskList;
}

public void updateTask(Task task) {
    // updating row
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues values = new ContentValues();
    values.put(KEY_TASKNAME, task.getTaskName());
    values.put(KEY_STATUS, task.getStatus());
    db.update(TABLE_TASKS, values, KEY_ID + " = ?",new String[] {String.valueOf(task.getId())});
    db.close();
}

}

And here is what the log file says:

08-14 14:21:42.133: E/AndroidRuntime(10366): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.tasker/com.example.tasker.ViewTask}: java.lang.IllegalStateException: attempt to re-open an already-closed object: SQLiteDatabase: /data/data/com.example.tasker/databases/taskerManager

Answer 1

There are two problems in your code:

A) you must not close the db in onCreate . That method is part of opening the db so the db should be open afterwards. source-code

public void onCreate(SQLiteDatabase db) {
    ...
    db.close();
}

B) You close the database at the end of every task.

public void updateTask(Task task) {
    db = get..
    ...
    db.close();
}

But while doing that you are still using the same database via a Cursor

db = db.get..
Cursor cursor = db.rawQuery
while (cursor.moveToNext() {
     updateTask();
}
db.close();

SQLiteOpenHelper will always give you the same db object since there is only one database. Closing the connection in one method while using it in another one can lead to your problem as well.

You could probably solve the task problem by using the existing db object from the outer loop or simply not closing the db in updateTask but it is in practice much better not to close the database at all.

Especially if you hand out references to Cursor . You must not close the database before the cursor is closed because Cursor can re-query it's data using the database connection it knows about. That will happen in two cases:

If your code uses cursor.requery() probably indirectly through a bad CursorAdapter .

The second case is not as common. Cursor can only see a limited windows of the current query. If you move the cursor outside of that window it will re-query the required part using the database connection as well. Typical small databases are usually much smaller than the window so there is no need to move the window and no hidden requery will happen.

Closing resources is usually a good thing. Closing the database is in most cases not. It is also no problem not doing it. SQLite makes sure nothing bad happens since every modification to the database is guaranteed by the transaction safety of SQLite.

Answer 2

you are closing the databse-connection at the end of each method with db.close(). So if you start another method you will always have to re-enable the connection. with

getWritableDatabase();

you should get it done.

Or you don't close the connection at the end of every method. That totally depends on what your app is meant for.

Answer 3

you have not called db.close(); in

      public List<Task> getAllTasks() { 
    .....

}

It may lead that problem because when you try to call getWritableDatabase(); in next function at that time database is already being open. so call db.close(); and close it in getAllTasks() method also.