Android WebView 数据库存储返回 null

这是我的问题。我正在尝试将我的 Web 应用程序中的数据本地存储在 HTML5 数据库中。问题是，我尝试用我的javascript打开的数据库返回null。

这是安卓代码：

public class NotepadActivity extends Activity {
WebView main_view = null;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //Get rid of the android title bar
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.main);
        main_view = (WebView) findViewById(R.id.main_webview);
        WebSettings settings = main_view.getSettings();
        settings.setJavaScriptEnabled(true);
        settings.setDatabasePath("/data/data/"+this.getPackageName()+"/databases/");
        settings.setDomStorageEnabled(true);
        main_view.loadUrl("file:///android_asset/index.html");
        main_view.setWebViewClient(new WebViewClient() {
            @Override
            public boolean shouldOverrideUrlLoading(WebView view, String url) {
                return false;
            }
            @Override
            public void onReceivedError(WebView view, int errorCod,String description,    String failingUrl) {
                Toast.makeText(NotepadActivity.this, "Error: " + description , Toast.LENGTH_LONG).show();
            }   
        });
        main_view.setWebChromeClient(new WebChromeClient() {
               public void onConsoleMessage(String message, int lineNumber, String sourceID) {
                 Log.d("Notepad", message + " -- From line "
                                 + lineNumber + " of "
                                 + sourceID);
              }
        });
     }
     public void onExceededDatabaseQuota(String url, String
            databaseIdentifier, long currentQuota, long estimatedSize, long
            totalUsedQuota, WebStorage.QuotaUpdater quotaUpdater) {
                        quotaUpdater.updateQuota(estimatedSize * 2);
            } 
}

以及相关的 JavaScript：

     /* DB and result values */
    DB_name: "NotepadDB",
    DB_version: "1",
    DB_displayName: "Notepad Database",
    DB_sizeEstimate: 5 * 1024 * 1024,
    DB: null,
 ....

    /* Open a connection to the DB (or create if it doesn't exist) */
    openConnection: function() {
        App.log("App > DB > openConnection > Attempting to access database");
        App.val.DB = window.openDatabase(App.val.DB_name, App.val.DB_version, 
                        App.val.DB_displayName, App.val.DB_sizeEstimate, App.DB.initializeDatabase);
    },
    /* Only called when the database is initially created */
    initializeDatabase: function(database) {
        App.val.DB = database;
        //Create the required table
        App.val.DB.transaction(
            function (tx) {tx.executeSql('CREATE TABLE ' + App.schema.tableName + ' (' + App.schema.ID+ ' int unique, ' + App.schema.title + ' text, ' + App.schema.content + ' text, ' + App.schema.date + ' datetime)',
                [],
                function (tx, res) {
                    App.log("App > DB > intializeDatabase > Table Created Successfully");
                    App.DB.loadAllNotes();
                },
                function (tx, err) {
                    App.log("App > DB > intializeDatabase > ERROR - Table creation failed - code: " + err.code + ", message: " + err.message);
                    App.error("Unable to create database. Please try again later.");
                });
            }
        );
    },
    loadAllNotes: function() {
        if(App.val.DB != null) {
            //Do some stuff
        } else {
            App.log("App > DB > loadAllNotes > Database was NULL");
        }
    }

在我的日志猫日志中，我看到 App.log（"App> DB> loadAllNotes> Database is NULL"）;正在执行。

我在这里缺少什么吗？为什么数据库为空？