Android上的db4o 3.0+问题

问题描述：

我在Android 3.0+上遇到了db4o问题，因为事实证明，在创建db4o数据库时，默认情况下会使用某些网络apis。（我偶然发现了这个帖子：http://mavistechchannel.wordpress.com/2011/11/18/db4o-at-honeycomb-and-ice-cream-sandwich/关于它）Android上的db4o 3.0+问题

但是，我试图让数据库创建请求异步，但我想我遇到了一个问题，因为它在它完全创建之前调用数据库DB。（我现在得到一个锁定错误）有什么办法可以做到这一点同步？或者，至少等到它完成了？这里是我的db4o帮手：

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; 
    } 
} 
}

答

更新：This db4o bug has been fixed。如果你得到最新的8.1位，错误不应该发生，解决方法是绝对的：

当你试图获取数据库时，你会得到一个文件锁定的异常？对。

那么问题是，你没有等待异步任务完成，只是在Db4oHelperAsync.oc为空的情况下启动一个新任务。您基本上必须等到初始化完成后才使用Db4oHelperAsync.oc变量。所以你在Java同步领域。

例如，您可以这样做：同步Db4oHelperAsync.oc访问权限。当请求数据库等待，直到变量被设置。不幸的是，我不知道异步任务的确切行为。我的猜测是，它会在主要活动中运行.onPostExecute（）方法。这也意味着你不能等待它，因为这意味着你阻止了Activity-Thread并且.onPostExecute（）永远不会被执行。

这是我的草稿，我会尽力去做。我从来没有执行也没有编译它。它可能有同步问题。例如，当初始化失败时，它只会将应用程序挂在.db（）调用上，因为它会一直等待。所以要非常小心，并试图改善它：

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()); 
      } 
     } 
    } 
}

P.S.作为db4o的一个bug添加了这个问题：http://tracker.db4o.com/browse/COR-2269

感谢您的帮助 - 我没有完全遵循这一点，但足够接近。这有点令人沮丧，但是，哦！ – Hosemeyer 2011-12-22 18:56:57

答

感谢您发布这个问题，这是一个严重的Android乐趣扰流板。创建新的db4o数据库文件时，db4o通过调用java.net.InetAddress.getLocalHost（）。getHostName（）来生成它的唯一内部签名。这个电话没有发现异常情况。我们将为Android找到一个解决方法，并在这里修复时回到这里和我们的论坛。

2012年2月9日更新：该问题已得到修复，新版本在线。 http://community.versant.com/Blogs/db4o/tabid/197/entryid/1057/Default.aspx

Android上的db4o 3.0+问题

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