斷點續傳

對于熟用QQ的程序員，QQ的斷點續傳功能應該是印象很深刻的。因為它很實用也很方面。因此，在我們的上傳下載過程中，很好實現了斷點續傳的功能。
其實斷點續傳的原理很簡單，就在上傳的過程中，先去服務上進行查找，是否存在此文件，如果存在些文件，則比較服務器上文件的大小與本地文件的大小，如果服務器上的文件比本地的要小，則認為此文件上傳過程中應該可以進行斷點續傳。
在實現的過程中，RandomAccessFile類變得很有用。此類的實例支持對隨機存取文件的讀取和寫入。隨機存取文件的行為類似存儲在文件系統中的一個大型字節數組。存在指向該隱含數組的光標或索引，稱為文件指針；輸入操作從文件指針開始讀取字節，并隨著對字節的讀取而前移此文件指針。如果隨機存取文件以讀取/寫入模式創建，則輸出操作也可用；輸出操作從文件指針開始寫入字節，并隨著對字節的寫入而前移此文件指針。寫入隱含數組的當前末尾之后的輸出操作導致該數組擴展。該文件指針可以通過 getFilePointer 方法讀取，并通過 seek 方法設置。
RandomAccessFile類的skipBytes方法嘗試跳過輸入的 n 個字節以丟棄跳過的字節。如果從服務器上查得待上傳文件的大小n，則采用skipBytes方法可以跳過這n個字節，從而開始從新的地方開始進行斷點續傳。具體的方法說明可以參見JDK5的API說明。
可以在net.sf.jftp.net. DataConnection類的run方法中，可以看出上傳下載中斷點續傳的實現，代碼如下：

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，下載----------------------
                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，上傳----------------------
                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);
}
}