MongoDB Java Driver 源码分析(9)：com.mongodb.DBport

　　DBPort 是表示数据库端口的类，分别用 call 和 say 方法实现读取和写入操作。
　　这两个方法都调用了 go 方法。

    // 读取操作
    Response call( OutMessage msg , DBCollection coll )
        throws IOException {
        return go( msg , coll );
    }

    // 写入操作
    void say( OutMessage msg )
        throws IOException {
        go( msg , null );
    }

    // 执行操作
    private synchronized Response go( OutMessage msg , DBCollection coll )
        throws IOException {
        return go( msg , coll , false );
    }

    // 执行操作
    private synchronized Response go( OutMessage msg , DBCollection coll , boolean forceReponse )
        throws IOException {

        // 正在处理请求
        if ( _processingResponse ){
            if ( coll == null ){
                // this could be a pipeline and should be safe
            }
            else {
                // this could cause issues since we're reading data off the wire
                throw new IllegalStateException( "DBPort.go called and expecting a response while processing another response" );
            }
        }

        // 增加调用次数计数
        _calls++;
    
        // _sorket 为空，打开连接
        if ( _socket == null )
            _open();
        
        if ( _out == null )
            throw new IllegalStateException( "_out shouldn't be null" );

        try {
            // 准备消息
            msg.prepare();

            // 输出
            msg.pipe( _out );
            
            if ( _pool != null )
                _pool._everWorked = true;
            
            if ( coll == null && ! forceReponse )
                return null;
            
            _processingResponse = true;

            // 返回结果
            return new Response( _sa , coll , _in , _decoder);
        }
        catch ( IOException ioe ){
            close();
            throw ioe;
        }
        finally {
            _processingResponse = false;
        }
    }

　　DBProt 的 go 方法调用了 OutMessage 的 prepare、pipe 等方法，实际上这些方法又是间接地通过 PoolOutputBuffer 实现的，这将在后面的文章中提到。

　　另外 DBPort 的 open 方法用于打开数据连接：

    // 打开连接
    boolean _open()
        throws IOException {
        
        long sleepTime = 100;

        final long start = System.currentTimeMillis();
        while ( true ){
            
            IOException lastError = null;

            try {
                // 创建 socket 并连接
                _socket = new Socket();
                _socket.connect( _addr , _options.connectTimeout );
                
                // 设置 socket 参数
                _socket.setTcpNoDelay( ! USE_NAGLE );
                _socket.setKeepAlive( _options.socketKeepAlive );
                _socket.setSoTimeout( _options.socketTimeout );

                // 获取输入输出流
                _in = new BufferedInputStream( _socket.getInputStream() );
                _out = _socket.getOutputStream();
                return true;
            }
            catch ( IOException ioe ){
               // ...
            }
            
            if ( ! _options.autoConnectRetry || ( _pool != null && ! _pool._everWorked ) )
                throw lastError;


            // 超时处理
            
            long sleptSoFar = System.currentTimeMillis() - start;

            if ( sleptSoFar >= CONN_RETRY_TIME_MS )
                throw lastError;
            
            if ( sleepTime + sleptSoFar > CONN_RETRY_TIME_MS )
                sleepTime = CONN_RETRY_TIME_MS - sleptSoFar;

            //  等待重试
            _logger.severe( "going to sleep and retry.  total sleep time after = " + ( sleptSoFar + sleptSoFar ) + "ms  this time:" + sleepTime + "ms" );
            ThreadUtil.sleep( sleepTime );
            sleepTime *= 2;
        }
    }