android - Android 3.0+ 问题上的 db4o

Question

我在 Android 3.0+ 上遇到 db4o 的问题，因为事实证明，在创建 db4o 数据库时，它默认使用一些网络 api。 (我偶然发现了这篇文章:http://mavistechchannel.wordpress.com/2011/11/18/db4o-at-honeycomb-and-ice-cream-sandwich/ 关于它)

但是，我试图使数据库创建请求异步，但我认为我遇到了在数据库完全创建之前调用数据库的问题，因为它锁定了数据库。 (现在我得到一个锁定错误)有什么办法可以同步吗？或者，至少要等到它完成？这是我的 db4o 助手:

public class Db4oHelperAsync implements Constants{

private static final String USE_INTERNAL_MEMORY_FOR_DATABASE = "USE_INTERNAL_MEMORY_FOR_DATABASE";

private static ObjectContainer oc = null;
private Context context; 

/**       
 * @param ctx
 */

public Db4oHelperAsync(Context ctx) {
    context = ctx;
}

/**
 * Create, open and close the database
 */
public ObjectContainer db() {


    if (oc == null || oc.ext().isClosed()) {

        if (Utilities.getPreferences(context).getBoolean(USE_INTERNAL_MEMORY_FOR_DATABASE, true)) {

            new GetDbFromInternalMemory().execute();

        } else {
            new GetDbFromSDCard().execute();
        }
        return oc;


    } else {

        return oc;

    }

}
/**
 * Configure the behavior of the database
 */

private EmbeddedConfiguration dbConfig() throws IOException {
    EmbeddedConfiguration configuration = Db4oEmbedded.newConfiguration();

    configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).objectField("name").indexed(true);
    configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).cascadeOnUpdate(true);
    configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).cascadeOnActivate(true);
    configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).cascadeOnDelete(true);

    configuration.common().objectClass(PersistentObjectWithoutCascadeOnDelete.class).objectField("name").indexed(true);
    configuration.common().objectClass(PersistentObjectWithoutCascadeOnDelete.class).cascadeOnUpdate(true);
    configuration.common().objectClass(PersistentObjectWithoutCascadeOnDelete.class).cascadeOnActivate(true);

    return configuration;
}

/**
 * Returns the path for the database location
 */

private String db4oDBFullPathInternal(Context ctx) {
    return ctx.getDir("data", 0) + "/" + "testapp.db4o";
}

private String db4oDBFullPathSdCard(Context ctx) {
    File path = new File(Environment.getExternalStorageDirectory(), ".testapp");
    if (!path.exists()) {
        path.mkdir();
    }
    return path + "/" + "testapp.db4o";
}

/**
 * Closes the database
 */

public void close() {
    if (oc != null)
        oc.close();
}

private class GetDbFromInternalMemory extends AsyncTask<Void, Void, ObjectContainer>{

    @Override
    protected ObjectContainer doInBackground(Void... params) {
        try {
            ObjectContainer obj = Db4oEmbedded.openFile(dbConfig(), db4oDBFullPathInternal(context));
            CLog.v("USING INTERNAL MEMORY FOR DATABASE");
            return obj;

        } catch (Exception ie) {
            ie.printStackTrace();
            CLog.e(Db4oHelper.class.getName(), ie.toString());
            return null;
        }
    }

    @Override
    protected void onPostExecute(ObjectContainer result)
    {
        oc = result;
    }
}

private class GetDbFromSDCard extends AsyncTask<Void, Void, ObjectContainer>{

    @Override
    protected ObjectContainer doInBackground(Void... params) {
        try {

            ObjectContainer obj = Db4oEmbedded.openFile(dbConfig(), db4oDBFullPathSdCard(context)); 
            CLog.v("USING SDCARD FOR DATABASE");
            SharedPreferences.Editor edit = Utilities.getPreferencesEditor(context);
            edit.putBoolean(USE_INTERNAL_MEMORY_FOR_DATABASE, true);
            edit.commit();

            return obj;

        } catch (Exception ie) {
            ie.printStackTrace();
            CLog.e(Db4oHelper.class.getName(), ie.toString());
            return null;
        }
    }

    @Override
    protected void onPostExecute(ObjectContainer result)
    {
        oc = result;
    }
}
}

Answer 1

更新:这db4o bug has been fixed .如果您获得最新的 8.1 位，则不应发生错误并且解决方法是绝对的:

尝试获取数据库时遇到文件锁定异常？对。

好吧，问题是您没有等待异步任务完成，而是开始一个新任务，以防 Db4oHelperAsync.oc 为空。您基本上必须等到初始化完成，然后才使用 Db4oHelperAsync.oc 变量。所以您在 Java 同步领域。

例如，您可以这样做:同步 Db4oHelperAsync.oc 访问。请求数据库时，等待变量设置。现在不幸的是我不知道异步任务的确切行为。我的猜测是它将在主要 Activity 上运行 .onPostExecute() 方法。这也意味着你不能只是等待它，因为这意味着你阻止了 Activity-Thread 并且 .onPostExecute() 将永远不会被执行。

这是我将尝试做的事情的草稿。我从未执行或编译过它。它可能有同步问题。例如，当初始化失败时，它只会将您的应用程序卡在 .db() 调用上，因为它会永远等待。所以要非常小心并尝试改进它:

public class Db4oHelperAsync implements Constants{

    private static final String USE_INTERNAL_MEMORY_FOR_DATABASE = "USE_INTERNAL_MEMORY_FOR_DATABASE";

    private static ObjectContainer oc = null;
    private static final Object lock = new Object();
    private Context context; 

    /**       
     * @param ctx
     */

    public Db4oHelperAsync(Context ctx) {
        context = ctx;
    }

    /**
     * Create, open and close the database
     */
    public ObjectContainer db() {       
        synchronized(lock){
            if (oc == null || oc.ext().isClosed()) {
                if (Utilities.getPreferences(context).getBoolean(USE_INTERNAL_MEMORY_FOR_DATABASE, true)) {
                    new GetDbFromInternalMemory().start();
                } else {
                    new GetDbFromSDCard().start();
                }
                while(oc==null){
                    this.wait()
                }
                return oc;
            } else {
                return oc;
            }
        }
    }
    /**
     * Configure the behavior of the database
     */

    private EmbeddedConfiguration dbConfig() throws IOException {
        EmbeddedConfiguration configuration = Db4oEmbedded.newConfiguration();

        configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).objectField("name").indexed(true);
        configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).cascadeOnUpdate(true);
        configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).cascadeOnActivate(true);
        configuration.common().objectClass(PersistentObjectWithCascadeOnDelete.class).cascadeOnDelete(true);

        configuration.common().objectClass(PersistentObjectWithoutCascadeOnDelete.class).objectField("name").indexed(true);
        configuration.common().objectClass(PersistentObjectWithoutCascadeOnDelete.class).cascadeOnUpdate(true);
        configuration.common().objectClass(PersistentObjectWithoutCascadeOnDelete.class).cascadeOnActivate(true);

        return configuration;
    }

    /**
     * Returns the path for the database location
     */

    private String db4oDBFullPathInternal(Context ctx) {
        return ctx.getDir("data", 0) + "/" + "testapp.db4o";
    }

    private String db4oDBFullPathSdCard(Context ctx) {
        File path = new File(Environment.getExternalStorageDirectory(), ".testapp");
        if (!path.exists()) {
            path.mkdir();
        }
        return path + "/" + "testapp.db4o";
    }

    /**
     * Closes the database
     */

    public void close() {

        synchronized(lock){
            if (oc != null)
                oc.close();
        }
    }

    private class GetDbFromInternalMemory extends Thread{

        @Override
        protected void run() {
            try {
                ObjectContainer obj = Db4oEmbedded.openFile(dbConfig(), db4oDBFullPathInternal(context));
                CLog.v("USING INTERNAL MEMORY FOR DATABASE");

                synchronized(Db4oHelperAsync.lock){
                    Db4oHelperAsync.oc = obj;
                    Db4oHelperAsync.lock.notifyAll()
                }
            } catch (Exception ie) {
                ie.printStackTrace();
                CLog.e(Db4oHelper.class.getName(), ie.toString());
            }
        }
    }

    private class GetDbFromSDCard extends Thread{

        @Override
        protected void run() {
            try {
                ObjectContainer obj = Db4oEmbedded.openFile(dbConfig(), db4oDBFullPathSdCard(context)); 
                CLog.v("USING SDCARD FOR DATABASE");
                SharedPreferences.Editor edit = Utilities.getPreferencesEditor(context);
                edit.putBoolean(USE_INTERNAL_MEMORY_FOR_DATABASE, true);
                edit.commit();

                synchronized(Db4oHelperAsync.lock){
                    Db4oHelperAsync.oc = obj;
                    Db4oHelperAsync.lock.notifyAll()
                }
            } catch (Exception ie) {
                ie.printStackTrace();
                CLog.e(Db4oHelper.class.getName(), ie.toString());
            }
        }
    }
}

附言将此问题作为错误添加到 db4o:http://tracker.db4o.com/browse/COR-2269