我使用这个类来管理连接到底层SQLiteDatabase

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();
    }
}

它包含了两个锁，一个用于读操作，另一个用于写。 但我还是偶尔会得到这样的异常：

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)

这意味着，数据库被莫名其妙地锁定试图在getWritableDatabase AQUIRE锁时。

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

我SQLiteOpenHelper是单模式，数据源仅使用的BasicDataSource作为父类。

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

什么是改善我能做些什么来避免SQLiteDatabaseLockedException在显示code？

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

推荐答案

在SQLite的，可以有任意多的读者，但是任何作家阻止所有其他读者和作者。

In SQLite, there can be arbitrarily many readers, but any writer blocks all other readers and writers.

您要为读者和作者使用一个锁。

You have to use a single lock for both readers and writers.

请注意，必须先把锁，只要你实际上是在访问数据库中保存。

Please note that locks must be held as long as you're actually accessing the database.

如果你想支持多个阅读器，使用实现ReadWriteLock,如ReentrantReadWriteLock.事情是这样的：

If you want to support multiple readers, use a lock that implements ReadWriteLock, such as ReentrantReadWriteLock. Something like this:

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();
        }
    }
}