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I use this class to manage connection to underlying SQLiteDatabase

public class BasicDataSource {

    protected DatabaseHandler dbHelper;
    protected volatile SQLiteDatabase readable_database;
    protected volatile SQLiteDatabase writable_database;
    protected Object read_lock = new Object();
    protected Object write_lock = new Object();
    protected Context context;

    protected BasicDataSource(Context ctx) {
        dbHelper = DatabaseHandler.getInstance(ctx);
        getReadableDatabase();
        dbHelper.onCreate(getWritableDatabase());
        this.context = ctx;
    }

    public synchronized void close() {
        dbHelper.close();
    }

    protected void closeInsertHelpers(InsertHelper... helpers) {
        for (InsertHelper ih : helpers) {
            if (ih != null)
                ih.close();
        }
    }

    protected SQLiteDatabase getReadableDatabase() {
        synchronized (read_lock) {
            if (readable_database == null || !readable_database.isOpen()) {
                readable_database = dbHelper.getReadableDatabase();
            }
            return readable_database;
        }
    }

    protected SQLiteDatabase getWritableDatabase() {
        synchronized (write_lock) {
            if (writable_database == null || !writable_database.isOpen()) {
                writable_database = dbHelper.getWritableDatabase();
            }
            return writable_database;
        }
    }

    protected synchronized void open() throws SQLException {
        getReadableDatabase();
        getWritableDatabase();
    }
}

It contains two locks, one for read, second for write. But I'm still occasionally getting this kind of exception:

java.lang.RuntimeException: An error occured while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:299)
        at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
        at java.util.concurrent.FutureTask.run(FutureTask.java:137)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
        at java.lang.Thread.run(Thread.java:856)
Caused by: android.database.sqlite.SQLiteDatabaseLockedException: database is locked (code 5): , while compiling: PRAGMA journal_mode
        at android.database.sqlite.SQLiteConnection.nativePrepareStatement(Native Method)
        at android.database.sqlite.SQLiteConnection.acquirePreparedStatement(SQLiteConnection.java:882)
        at android.database.sqlite.SQLiteConnection.executeForString(SQLiteConnection.java:627)
        at android.database.sqlite.SQLiteConnection.setJournalMode(SQLiteConnection.java:313)
        at android.database.sqlite.SQLiteConnection.setWalModeFromConfiguration(SQLiteConnection.java:287)
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:215)
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:193)
        at android.database.sqlite.SQLiteConnectionPool.openConnectionLocked(SQLiteConnectionPool.java:463)
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:185)
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:177)
        at android.database.sqlite.SQLiteDatabase.openInner(SQLiteDatabase.java:804)
        at android.database.sqlite.SQLiteDatabase.open(SQLiteDatabase.java:789)
        at android.database.sqlite.SQLiteDatabase.openDatabase(SQLiteDatabase.java:694)
        at android.app.ContextImpl.openOrCreateDatabase(ContextImpl.java:804)
        at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:221)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
        at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
        at com.mycompany.myapplication.sql.BasicDataSource.getWritableDatabase(BasicDataSource.java:57)
        at com.mycompany.myapplication.sql.datasources.SomeDataSource.fillUpDatabaseMethod(SomeDataSource.java:264)
        at com.mycompany.myapplication.sql.datasources.SomeDataSource.renewCacheMethod(SomeDataSource.java:560)
        at com.mycompany.myapplication.activities.lists.ListsActivity$Worker.doInBackground(ListsActivity.java:315)
        at com.mycompany.myapplication.activities.lists.ListsActivity$Worker.doInBackground(ListsActivity.java:1)
        at android.os.AsyncTask$2.call(AsyncTask.java:287)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
        ... 4 more
android.database.sqlite.SQLiteDatabaseLockedException: database is locked (code 5): , while compiling: PRAGMA journal_mode
        at android.database.sqlite.SQLiteConnection.nativePrepareStatement(Native Method)
        at android.database.sqlite.SQLiteConnection.acquirePreparedStatement(SQLiteConnection.java:882)
        at android.database.sqlite.SQLiteConnection.executeForString(SQLiteConnection.java:627)
        at android.database.sqlite.SQLiteConnection.setJournalMode(SQLiteConnection.java:313)
        at android.database.sqlite.SQLiteConnection.setWalModeFromConfiguration(SQLiteConnection.java:287)
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:215)
        at android.database.sqlite.SQLiteConnection.open(SQLiteConnection.java:193)
        at android.database.sqlite.SQLiteConnectionPool.openConnectionLocked(SQLiteConnectionPool.java:463)
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:185)
        at android.database.sqlite.SQLiteConnectionPool.open(SQLiteConnectionPool.java:177)
        at android.database.sqlite.SQLiteDatabase.openInner(SQLiteDatabase.java:804)
        at android.database.sqlite.SQLiteDatabase.open(SQLiteDatabase.java:789)
        at android.database.sqlite.SQLiteDatabase.openDatabase(SQLiteDatabase.java:694)
        at android.app.ContextImpl.openOrCreateDatabase(ContextImpl.java:804)
        at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:221)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
        at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
        at com.mycompany.myapplication.sql.BasicDataSource.getWritableDatabase(BasicDataSource.java:57)
        at com.mycompany.myapplication.sql.datasources.SomeDataSource.fillUpDatabaseMethod(SomeDataSource.java:264)
        at com.mycompany.myapplication.sql.datasources.SomeDataSource.renewCacheMethod(SomeDataSource.java:560)
        at com.mycompany.myapplication.activities.lists.ListsActivity$Worker.doInBackground(ListsActivity.java:315)
        at com.mycompany.myapplication.activities.lists.ListsActivity$Worker.doInBackground(ListsActivity.java:1)
        at android.os.AsyncTask$2.call(AsyncTask.java:287)
        at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
        at java.util.concurrent.FutureTask.run(FutureTask.java:137)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
        at java.lang.Thread.run(Thread.java:856)

Which means, the database is somehow locked when trying to aquire lock in getWritableDatabase.

My SQLiteOpenHelper is singleton pattern, and DataSources are only using BasicDataSource as parent class.

What is the improvement I can do to avoid SQLiteDatabaseLockedException in showed code?

4

1 回答 1

16

在 SQLite 中，可以有任意多个读取器，但是任何写入器都会阻止所有其他读取器和写入器。

您必须为读者和作者使用一个锁。

请注意，只要您实际访问数据库，就必须持有锁。

如果要支持多个阅读器，请使用实现 的锁ReadWriteLock，例如ReentrantReadWriteLock. 像这样的东西： 

class MyData {
    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
    private final Lock r = rwl.readLock();
    private final Lock w = rwl.writeLock();

    public Data ReadSomething(int id) {
        r.lock();
        try {
            Cursor c = readableDatabase.query(...);
            return c.getString(0);
        } finally {
            r.unlock();
        }
    }

    public void ChangeSomething(int id, int value) {
        w.lock();
        try {
            writeableDatabase.update(...);
        } finally {
            w.unlock();
        }
    }
}
于 2012-10-06T09:57:20.627 回答