将 SQLite 表数据导出为 XML

Question

我在尝试将我的 SQLite 数据库表导出到 XML 文件时遇到空指针异常。

StackTrace： PasteBin Link

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更新 2： 我只是不明白传入的内容（在主活动中初始化和调用 exportData 方法时）：DatabaseAssistant DA = new DatabaseAssistant(myContext, **WHAT_IS_PASSED_HERE?**); DA.exportData();
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更新： 提到我在哪里获得 NPE 的行是：com.astix.reachout.DatabaseAssistant.exportData(DatabaseAssistant.java:72 & com.astix.reachout.ReachOutMain$9.onClick(ReachOutMain.java:347)

_exporter.startDbExport( _db.getPath() ); 在DA.exportData() 中可用
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我提到了this answer。 我的数据库助手代码：

public class DatabaseAssistant
{
    public String filNameFullPath;
    public long filNameTS;
    public String EXPORT_FILE_NAME;
    //private static final String EXPORT_FILE_NAME = "/sdcard/datanaexport.xml";

    private Context _ctx;
    private SQLiteDatabase _db;
    private Exporter _exporter;

    public String newfilename(){
        System.out.println("inside newfilename()");

        filNameTS = System.currentTimeMillis();
        filNameFullPath = Environment.getExternalStorageDirectory().getPath();
        EXPORT_FILE_NAME = filNameFullPath + "/" + filNameTS +".xml";
        System.out.println("new file name: " + EXPORT_FILE_NAME);
        return EXPORT_FILE_NAME;
    }

    public DatabaseAssistant( Context ctx, SQLiteDatabase db )
    {
        _ctx = ctx;
        _db = db;

        newfilename();
        try
        {
            System.out.println("inside try databaseAssitant() -- file name: " + EXPORT_FILE_NAME);
            // create a file on the sdcard to export the
            // database contents to
            File myFile = new File( EXPORT_FILE_NAME );
                        myFile.createNewFile();

                        FileOutputStream fOut =  new FileOutputStream(myFile);
                        BufferedOutputStream bos = new BufferedOutputStream( fOut );

            _exporter = new Exporter( bos );
        }
        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
    }

    public void exportData( )
    {
        log( "Exporting Data" );

        try
        {
            System.out.println("inside try exportData()");
            _exporter.startDbExport( _db.getPath() );

            // get the tables out of the given sqlite database
                    String sql = "SELECT * FROM sqlite_master";

                    Cursor cur = _db.rawQuery( sql, new String[0] );
                    Log.d("db", "show tables, cur size " + cur.getCount() );
                    cur.moveToFirst();

                    String tableName;
                    while ( cur.getPosition() < cur.getCount() )
                    {
                        tableName = cur.getString( cur.getColumnIndex( "name" ) );
                        log( "table name " + tableName );

                        // don't process these two tables since they are used
                        // for metadata
                        if ( ! tableName.equals( "android_metadata" ) &&
                        ! tableName.equals( "sqlite_sequence" ) )
                        {
                            exportTable( tableName );
                        }

                        cur.moveToNext();
                    }
                _exporter.endDbExport();
            _exporter.close();
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
    }

    private void exportTable( String tableName ) throws IOException
    {
        _exporter.startTable(tableName);

        // get everything from the table
        String sql = "select * from " + tableName;
        Cursor cur = _db.rawQuery( sql, new String[0] );
        int numcols = cur.getColumnCount();

        log( "Start exporting table " + tableName );

//      // logging
//      for( int idx = 0; idx < numcols; idx++ )
//      {
//          log( "column " + cur.getColumnName(idx) );
//      }

        cur.moveToFirst();

        // move through the table, creating rows
        // and adding each column with name and value
        // to the row
        while( cur.getPosition() < cur.getCount() )
        {
            _exporter.startRow();
            String name;
            String val;
            for( int idx = 0; idx < numcols; idx++ )
            {
                name = cur.getColumnName(idx);
                val = cur.getString( idx );
                log( "col '" + name + "' -- val '" + val + "'" );

                _exporter.addColumn( name, val );
            }

            _exporter.endRow();
            cur.moveToNext();
        }

        cur.close();

        _exporter.endTable();
    }

    private void log( String msg )
    {
        Log.d( "DatabaseAssistant", msg );
    }

    class Exporter
    {
        private static final String CLOSING_WITH_TICK = "'>";
        private static final String START_DB = "<export-database name='";
        private static final String END_DB = "</export-database>";
        private static final String START_TABLE = "<table name='";
        private static final String END_TABLE = "</table>";
        private static final String START_ROW = "<row>";
        private static final String END_ROW = "</row>";
        private static final String START_COL = "<col name='";
        private static final String END_COL = "</col>";

        private BufferedOutputStream _bos;

        public Exporter() throws FileNotFoundException
        {
            this( new BufferedOutputStream(
                    _ctx.openFileOutput( EXPORT_FILE_NAME,
                    Context.MODE_WORLD_READABLE ) ) );
        }

        public Exporter( BufferedOutputStream bos )
        {
            _bos = bos;
        }

        public void close() throws IOException
        {
            if ( _bos != null )
            {
                _bos.close();
            }
        }

        public void startDbExport( String dbName ) throws IOException
        {
            String stg = START_DB + dbName + CLOSING_WITH_TICK;
            _bos.write( stg.getBytes() );
        }

        public void endDbExport() throws IOException
        {
            _bos.write( END_DB.getBytes() );
        }

        public void startTable( String tableName ) throws IOException
        {
            String stg = START_TABLE + tableName + CLOSING_WITH_TICK;
            _bos.write( stg.getBytes() );
        }

        public void endTable() throws IOException
        {
            _bos.write( END_TABLE.getBytes() );
        }

        public void startRow() throws IOException
        {
            _bos.write( START_ROW.getBytes() );
        }

        public void endRow() throws IOException
        {
            _bos.write( END_ROW.getBytes() );
        }

        public void addColumn( String name, String val ) throws IOException
        {
            String stg = START_COL + name + CLOSING_WITH_TICK + val + END_COL;
            _bos.write( stg.getBytes() );
        }
    }

    class Importer
    {

    }

}

在我的主要活动中（在按钮的 onClick 下调用）：

DatabaseAssistant DA = new DatabaseAssistant(myContext, mySQLiteDatabase);
DA.exportData();

这里^（上面声明为）：

private Context myContext;
private SQLiteDatabase mySQLiteDatabase;

任何建议都是可以理解的.. 谢谢

Answer 1

_exporter 或 _db 是 null。

您的构造函数初始化了这两个变量，所以很可能给构造函数的mySQLiteDatabase 已经是null。 （尽管如果构造函数中发生任何错误，您将得到一个无效的_exporter；最好抑制异常并继续进行，就好像什么都没发生一样。这里不是这种情况，因为没有堆栈跟踪。）