用Java实现FTP批量大文件上传下载(三)

五、断点续传

对于熟用QQ的程序员，QQ的断点续传功能应该是印象很深刻的。因为它很实用也很方面。因此，在我们的上传下载过程中，很实现了断点续传的功能。

其实断点续传的原理很简单，就在上传的过程中，先去服务上进行查找，是否存在此文件，如果存在些文件，则比较服务器上文件的大小与本地文件的大小，如果服务器上的文件比本地的要小，则认为此文件上传过程中应该可以进行断点续传。

在实现的过程中，RandomAccessFile类变得很有用。此类的实例支持对随机存取文件的读取和写入。随机存取文件的行为类似存储在文件系统中的一个大型字节数组。存在指向该隐含数组的光标或索引，称为文件指针；输入操作从文件指针开始读取字节，并随着对字节的读取而前移此文件指针。如果随机存取文件以读取/写入模式创建，则输出操作也可用；输出操作从文件指针开始写入字节，并随着对字节的写入而前移此文件指针。写入隐含数组的当前末尾之后的输出操作导致该数组扩展。该文件指针可以通过 getFilePointer 方法读取，并通过 seek 方法进行设置。

RandomAccessFile类的skipBytes方法尝试跳过输入的 n 个字节以丢弃跳过的字节。如果从服务器上查得待上传文件的大小n，则采用skipBytes方法可以跳过这n个字节，从而开始从新的地方开始进行断点续传。具体的方法说明可以参见JDK5的API说明。

可以在net.sf.jftp.net. DataConnection类的run方法中，可以看出上传下载中断点续传的实现，代码如下：

public void run()
    {
        try
        {
        newLine = con.getCRLF();

            if(Settings.getFtpPasvMode())
            {
                try
                {
                    sock = new Socket(host, port);
                    sock.setSoTimeout(Settings.getSocketTimeout());
                }
                catch(Exception ex)
                {
                    ok = false;
                    debug("Can't open Socket on port " + port);
                }
            }
            else
            {
                //Log.debug("trying new server socket: "+port);
                try
                {
                    ssock = new ServerSocket(port);
                }
                catch(Exception ex)
                {
                    ok = false;
                    Log.debug("Can't open ServerSocket on port " + port);
                }
            }
        }
        catch(Exception ex)
        {
            debug(ex.toString());
        }

        isThere = true;

        boolean ok = true;

        RandomAccessFile fOut = null;
        BufferedOutputStream bOut = null;
        RandomAccessFile fIn = null;

        try
        {
            if(!Settings.getFtpPasvMode())
            {
                int retry = 0;

                while((retry++ < 5) && (sock == null))
                {
                    try
                    {
                        ssock.setSoTimeout(Settings.connectionTimeout);
                        sock = ssock.accept();
                    }
                    catch(IOException e)
                    {
                        sock = null;
                        debug("Got IOException while trying to open a socket!");

                        if(retry == 5)
                        {
                            debug("Connection failed, tried 5 times - maybe try a higher timeout in Settings.java");
                        }

                finished = true;

                        throw e;
                    }
                    finally
                    {
                        ssock.close();
                    }

                    debug("Attempt timed out, retrying");
                }
            }

            if(ok)
            {
                byte[] buf = new byte[Settings.bufferSize];
                start = System.currentTimeMillis();

                int buflen = 0;

                //---------------download，下载----------------------
                if(type.equals(GET) || type.equals(GETDIR))
                {
                    if(!justStream)
                    {
                        try
                        {
                            if(resume)
                            {
                                File f = new File(file);
                                fOut = new RandomAccessFile(file, "rw");
                                fOut.skipBytes((int) f.length());
                                buflen = (int) f.length();
                            }
                            else
                            {
                                if(localfile == null)
                                {
                                    localfile = file;
                                }

                                File f2 = new File(Settings.appHomeDir);
                                f2.mkdirs();

                                File f = new File(localfile);

                                if(f.exists())
                                {
                                    f.delete();
                                }

                                bOut = new BufferedOutputStream(new FileOutputStream(localfile),
                                                                Settings.bufferSize);
                            }
                        }
                        catch(Exception ex)
                        {
                            debug("Can't create outputfile: " + file);
                            ok = false;
                            ex.printStackTrace();
                        }
                    }

                
                //---------------upload，上传----------------------
                if(type.equals(PUT) || type.equals(PUTDIR))
                {
                    if(in == null)
                    {
                        try
                        {
                            fIn = new RandomAccessFile(file, "r");
                            
                            if(resume)
                            {
                                fIn.skipBytes(skiplen);
                            }
                            
                            //fIn = new BufferedInputStream(new FileInputStream(file));
                        }
                        catch(Exception ex)
                        {
                            debug("Can't open inputfile: " + " (" + ex + ")");
                            ok = false;
                        }
                    }
                    
                    if(ok)
                    {
                        try
                        {
                            out = new BufferedOutputStream(sock.getOutputStream());
                        }
                        catch(Exception ex)
                        {
                            ok = false;
                            debug("Can't get OutputStream");
                        }
                        
                        if(ok)
                        {
                            try
                            {
                                int len = skiplen;
                                char b;
                                
                                while(true)
                                {
                                    int read;
                                    
                                    if(in != null)
                                    {
                                        read = in.read(buf);
                                    }
                                    else
                                    {
                                        read = fIn.read(buf);
                                    }
                                    
                                    len += read;
                                    
                                    //System.out.println(file + " " + type+ " " + len + " " + read);
                                    if(read == -1)
                                    {
                                        break;
                                    }
                                    
                                    if(newLine != null) 
                                    {
                                        byte[] buf2 = modifyPut(buf, read);
                                        out.write(buf2, 0, buf2.length);
                                    }
                                    else 
                                    {
                                        out.write(buf, 0, read);
                                    }
                                    
                                    con.fireProgressUpdate(file, type, len);
                                    
                                    if(time())
                                    {
                                        //   Log.debugSize(len, false, false, file);
                                    }
                                    
                                    if(read == StreamTokenizer.TT_EOF)
                                    {
                                        break;
                                    }
                                }
                                
                                out.flush();
                                
                                //Log.debugSize(len, false, true, file);
                            }
                            catch(IOException ex)
                            {
                                ok = false;
                                debug("Error: Data connection closed.");
                                con.fireProgressUpdate(file, FAILED, -1);
                                ex.printStackTrace();
                            }
                        }
                    }
                }
            }
        }
        catch(IOException ex)
        {
            Log.debug("Can't connect socket to ServerSocket");
            ex.printStackTrace();
        }
        finally
        {
            try
            {
                if(out != null)
                {
                    out.flush();
                    out.close();
                }
            }
            catch(Exception ex)
            {
                ex.printStackTrace();
            }
            
            try
            {
                if(bOut != null)
                {
                    bOut.flush();
                    bOut.close();
                }
            }
            catch(Exception ex)
            {
                ex.printStackTrace();
            }
            
            try
            {
                if(fOut != null)
                {
                    fOut.close();
                }
            }
            catch(Exception ex)
            {
                ex.printStackTrace();
            }
            
            try
            {
                if(in != null && !justStream)
                {
                    in.close();
                }
                
                if(fIn != null)
                {
                    fIn.close();
                }
            }
            catch(Exception ex)
            {
                ex.printStackTrace();
            }
        }
        
        try
        {
            sock.close();
        }
        catch(Exception ex)
        {
            debug(ex.toString());
        }
        
        if(!Settings.getFtpPasvMode())
        {
            try
            {
                ssock.close();
            }
            catch(Exception ex)
            {
                debug(ex.toString());
            }
        }
        
        finished = true;
        
        if(ok)
        {
            con.fireProgressUpdate(file, FINISHED, -1);
        }
        else
        {
            con.fireProgressUpdate(file, FAILED, -1);
        }
    }

待续
......