用Java實現斷點續傳 (HTTP)
斷點續傳的原理
其實斷點續傳的原理很簡單,就是在 Http 的請求上和一般的下載有所不同而已。
打個比方,瀏覽器請求伺服器上的一個文時,所發出的請求如下:
假設伺服器域名為 www.sjtu.edu.cn,檔名為 down.zip。
GET /down.zip HTTP/1.1
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-
excel, application/msword, application/vnd.ms-powerpoint, */*
Accept-Language: zh-cn
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)
Connection: Keep-Alive
伺服器收到請求後,按要求尋找請求的檔案,提取檔案的資訊,然後返回給瀏覽器,返回資訊如下:
200
Content-Length=106786028
Accept-Ranges=bytes
Date=Mon, 30 Apr 2001 12:56:11 GMT
ETag=W/"02ca57e173c11:95b"
Content-Type=application/octet-stream
Server=Microsoft-IIS/5.0
Last-Modified=Mon, 30 Apr 2001 12:56:11 GMT
所謂斷點續傳,也就是要從檔案已經下載的地方開始繼續下載。所以在客戶端瀏覽器傳給 Web 伺服器的時候要多加一條資訊 – 從哪裡開始。
下面是用自己編的一個”瀏覽器”來傳遞請求資訊給 Web 伺服器,要求從 2000070 位元組開始。
GET /down.zip HTTP/1.0
User-Agent: NetFox
RANGE: bytes=2000070-
Accept: text/html, image/gif, image/jpeg, *; q=.2, */*; q=.2
仔細看一下就會發現多了一行 RANGE: bytes=2000070-
這一行的意思就是告訴伺服器 down.zip 這個檔案從 2000070 位元組開始傳,前面的位元組不用傳了。
伺服器收到這個請求以後,返回的資訊如下:
206
Content-Length=106786028
Content-Range=bytes 2000070-106786027/106786028
Date =Mon, 30 Apr 2001 12:55:20 GMT
ETag=W/"02ca57e173c11:95b"
Content-Type=application/octet-stream
Server=Microsoft-IIS/5.0
Last-Modified=Mon, 30 Apr 2001 12:55:20 GMT
和前面伺服器返回的資訊比較一下,就會發現增加了一行: Content-Range=bytes 2000070-106786027/106786028
,返回的程式碼也改為 206 了,而不再是 200 了。
知道了以上原理,就可以進行斷點續傳的程式設計了。
Java 實現斷點續傳的關鍵幾點
1、 用什麼方法實現提交RANGE: bytes=2000070-
當然用最原始的 Socket 是肯定能完成的,不過那樣太費事了,其實 Java 的 net 包中提供了這種功能。程式碼如下:
URL url = new URL("http://www.sjtu.edu.cn/down.zip");
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection();
// 設定 User-Agent
httpConnection.setRequestProperty("User-Agent","NetFox");
// 設定斷點續傳的開始位置
httpConnection.setRequestProperty("RANGE","bytes=2000070");
// 獲得輸入流
InputStream input = httpConnection.getInputStream();
從輸入流中取出的位元組流就是 down.zip 檔案從 2000070 開始的位元組流。 大家看,其實斷點續傳用 Java 實現起來還是很簡單的吧。 接下來要做的事就是怎麼儲存獲得的流到檔案中去了。
2、 儲存檔案採用的方法。
我採用的是 IO 包中的 RandAccessFile 類。
操作相當簡單,假設從 2000070 處開始儲存檔案,程式碼如下:
RandomAccess oSavedFile = new RandomAccessFile("down.zip","rw");
long nPos = 2000070;
// 定位檔案指標到 nPos 位置
oSavedFile.seek(nPos);
byte[] b = new byte[1024];
int nRead;
// 從輸入流中讀入位元組流,然後寫到檔案中
while((nRead=input.read(b,0,1024)) > 0) {
oSavedFile.write(b,0,nRead);
}
怎麼樣,也很簡單吧。 接下來要做的就是整合成一個完整的程式了。包括一系列的執行緒控制等等。
斷點續傳核心的實現
主要用了 6 個類,包括一個測試類。
- SiteFileFetch.java 負責整個檔案的抓取,控制內部執行緒 (FileSplitterFetch 類 )。
- FileSplitterFetch.java 負責部分檔案的抓取。
- FileAccess.java 負責檔案的儲存。
- SiteInfoBean.java 要抓取的檔案的資訊,如檔案儲存的目錄,名字,抓取檔案的 URL 等。
- Utility.java 工具類,放一些簡單的方法。
- TestMethod.java 測試類。
我們來分別看看程式碼:
1、 SiteFileFetch.java
/**
* SiteFileFetch.java
*/
package NetFox;
import java.io.*;
import java.net.*;
public class SiteFileFetch extends Thread {
SiteInfoBean siteInfoBean = null; // 檔案資訊 Bean
long[] nStartPos; // 開始位置
long[] nEndPos; // 結束位置
FileSplitterFetch[] fileSplitterFetch; // 子執行緒物件
long nFileLength; // 檔案長度
boolean bFirst = true; // 是否第一次取檔案
boolean bStop = false; // 停止標誌
File tmpFile; // 檔案下載的臨時資訊
DataOutputStream output; // 輸出到檔案的輸出流
public SiteFileFetch(SiteInfoBean bean) throws IOException {
siteInfoBean = bean;
//tmpFile = File.createTempFile ("zhong","1111",new File(bean.getSFilePath()));
tmpFile = new File(bean.getSFilePath()+File.separator + bean.getSFileName()+".info");
if(tmpFile.exists ())
{
bFirst = false;
read_nPos();
}
else
{
nStartPos = new long[bean.getNSplitter()];
nEndPos = new long[bean.getNSplitter()];
}
}
public void run() {
// 獲得檔案長度
// 分割檔案
// 例項 FileSplitterFetch
// 啟動 FileSplitterFetch 執行緒
// 等待子執行緒返回
try{
if(bFirst){
nFileLength = getFileSize();
if(nFileLength == -1){
System.err.println("File Length is not known!");
}else if(nFileLength == -2){
System.err.println("File is not access!");
}else{
for(int i=0;i<nStartPos.length;i++){
nStartPos[i] = (long)(i*(nFileLength/nStartPos.length));
}
for(int i=0;i<nEndPos.length-1;i++){
nEndPos[i] = nStartPos[i+1];
}
nEndPos[nEndPos.length-1] = nFileLength;
}
}
// 啟動子執行緒
fileSplitterFetch = new FileSplitterFetch[nStartPos.length];
for(int i=0;i<nStartPos.length;i++){
fileSplitterFetch[i] = new FileSplitterFetch(siteInfoBean.getSSiteURL(),
siteInfoBean.getSFilePath() + File.separator + siteInfoBean.getSFileName(),
nStartPos[i],nEndPos[i],i);
Utility.log("Thread " + i + " , nStartPos = " + nStartPos[i] + ", nEndPos = "
+ nEndPos[i]);
fileSplitterFetch[i].start();
}
// fileSplitterFetch[nPos.length-1] = new FileSplitterFetch(siteInfoBean.getSSiteURL(),
siteInfoBean.getSFilePath() + File.separator
+ siteInfoBean.getSFileName(),nPos[nPos.length-1],nFileLength,nPos.length-1);
// Utility.log("Thread " +(nPos.length-1) + ",nStartPos = "+nPos[nPos.length-1]+", nEndPos = " + nFileLength);
// fileSplitterFetch[nPos.length-1].start();
// 等待子執行緒結束
//int count = 0;
// 是否結束 while 迴圈
boolean breakWhile = false;
while(!bStop){
write_nPos();
Utility.sleep(500);
breakWhile = true;
for(int i=0;i<nStartPos.length;i++){
if(!fileSplitterFetch[i].bDownOver){
breakWhile = false;
break;
}
}
if(breakWhile)
break;
//count++;
//if(count>4)
// siteStop();
}
System.err.println("檔案下載結束!");
}catch(Exception e){
e.printStackTrace ();
}
}
// 獲得檔案長度
public long getFileSize(){
int nFileLength = -1;
try{
URL url = new URL(siteInfoBean.getSSiteURL());
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection ();
httpConnection.setRequestProperty("User-Agent","NetFox");
int responseCode=httpConnection.getResponseCode();
if(responseCode>=400){
processErrorCode(responseCode);
return -2; //-2 represent access is error
}
String sHeader;
for(int i=1;;i++){
//DataInputStream in = new DataInputStream(httpConnection.getInputStream ());
//Utility.log(in.readLine());
sHeader=httpConnection.getHeaderFieldKey(i);
if(sHeader!=null){
if(sHeader.equals("Content-Length")){
nFileLength = Integer.parseInt(httpConnection.getHeaderField(sHeader));
break;
}
}
else
break;
}
}
catch(IOException e){e.printStackTrace ();}
catch(Exception e){e.printStackTrace ();}
Utility.log(nFileLength);
return nFileLength;
}
// 儲存下載資訊(檔案指標位置)
private void write_nPos(){
try{
output = new DataOutputStream(new FileOutputStream(tmpFile));
output.writeInt(nStartPos.length);
for(int i=0;i<nStartPos.length;i++){
// output.writeLong(nPos[i]);
output.writeLong(fileSplitterFetch[i].nStartPos);
output.writeLong(fileSplitterFetch[i].nEndPos);
}
output.close();
}
catch(IOException e){e.printStackTrace ();}
catch(Exception e){e.printStackTrace ();}
}
// 讀取儲存的下載資訊(檔案指標位置)
private void read_nPos(){
try{
DataInputStream input = new DataInputStream(new FileInputStream(tmpFile));
int nCount = input.readInt();
nStartPos = new long[nCount];
nEndPos = new long[nCount];
for(int i=0;i<nStartPos.length;i++){
nStartPos[i] = input.readLong();
nEndPos[i] = input.readLong();
}
input.close();
}
catch(IOException e){e.printStackTrace ();}
catch(Exception e){e.printStackTrace ();}
}
private void processErrorCode(int nErrorCode){
System.err.println("Error Code : " + nErrorCode);
}
// 停止檔案下載
public void siteStop(){
bStop = true;
for(int i=0;i<nStartPos.length;i++)
fileSplitterFetch[i].splitterStop();
}
}
2、 FileSplitterFetch.java
/**
*FileSplitterFetch.java
*/
package NetFox;
import java.io.*;
import java.net.*;
public class FileSplitterFetch extends Thread {
String sURL; //File URL
long nStartPos; //File Snippet Start Position
long nEndPos; //File Snippet End Position
int nThreadID; //Thread's ID
boolean bDownOver = false; //Downing is over
boolean bStop = false; //Stop identical
FileAccessI fileAccessI = null; //File Access interface
public FileSplitterFetch(String sURL,String sName,long nStart,long nEnd,int id)
throws IOException {
this.sURL = sURL;
this.nStartPos = nStart;
this.nEndPos = nEnd;
nThreadID = id;
fileAccessI = new FileAccessI(sName,nStartPos);
}
public void run(){
while(nStartPos < nEndPos && !bStop){
try{
URL url = new URL(sURL);
HttpURLConnection httpConnection = (HttpURLConnection)url.openConnection ();
httpConnection.setRequestProperty("User-Agent","NetFox");
String sProperty = "bytes="+nStartPos+"-";
httpConnection.setRequestProperty("RANGE",sProperty);
Utility.log(sProperty);
InputStream input = httpConnection.getInputStream();
//logResponseHead(httpConnection);
byte[] b = new byte[1024];
int nRead;
while((nRead=input.read(b,0,1024)) > 0 && nStartPos < nEndPos
&& !bStop) {
nStartPos += fileAccessI.write(b,0,nRead);
//if(nThreadID == 1)
// Utility.log("nStartPos = " + nStartPos + ", nEndPos = " + nEndPos);
}
Utility.log("Thread " + nThreadID + " is over!");
bDownOver = true;
//nPos = fileAccessI.write (b,0,nRead);
}
catch(Exception e){e.printStackTrace ();}
}
}
// 列印迴應的頭資訊
public void logResponseHead(HttpURLConnection con)
{
for(int i=1;;i++){
String header=con.getHeaderFieldKey(i);
if(header!=null)
//responseHeaders.put(header,httpConnection.getHeaderField(header));
Utility.log(header+" : "+con.getHeaderField(header));
else
break;
}
}
public void splitterStop(){
bStop = true;
}
}
3、 FileAccess.java
/**
*FileAccess.java
*/
package NetFox;
import java.io.*;
public class FileAccessI implements Serializable{
RandomAccessFile oSavedFile;
long nPos;
public FileAccessI() throws IOException{
this("",0);
}
public FileAccessI(String sName,long nPos) throws IOException{
oSavedFile = new RandomAccessFile(sName,"rw");
this.nPos = nPos;
oSavedFile.seek(nPos);
}
public synchronized int write(byte[] b,int nStart,int nLen){
int n = -1;
try{
oSavedFile.write(b,nStart,nLen);
n = nLen;
}catch(IOException e){
e.printStackTrace ();
}
return n;
}
}
4、 SiteInfoBean.java
/**
*SiteInfoBean.java
*/
package NetFox;
public class SiteInfoBean {
private String sSiteURL; //Site's URL
private String sFilePath; //Saved File's Path
private String sFileName; //Saved File's Name
private int nSplitter; //Count of Splited Downloading File
public SiteInfoBean() {
//default value of nSplitter is 5
this("","","",5);
}
public SiteInfoBean(String sURL,String sPath,String sName,int nSpiltter) {
sSiteURL= sURL;
sFilePath = sPath;
sFileName = sName;
this.nSplitter = nSpiltter;
}
public String getSSiteURL() {
return sSiteURL;
}
public void setSSiteURL(String value) {
sSiteURL = value;
}
public String getSFilePath() {
return sFilePath;
}
public void setSFilePath(String value) {
sFilePath = value;
}
public String getSFileName() {
return sFileName;
}
public void setSFileName(String value) {
sFileName = value;
}
public int getNSplitter() {
return nSplitter;
}
public void setNSplitter(int nCount) {
nSplitter = nCount;
}
}
5、 Utility.java
/**
*Utility.java
*/
package NetFox;
public class Utility {
public Utility() {}
public static void sleep(int nSecond) {
try{
Thread.sleep(nSecond);
}
catch(Exception e) {
e.printStackTrace ();
}
}
public static void log(String sMsg) {
System.err.println(sMsg);
}
public static void log(int sMsg) {
System.err.println(sMsg);
}
}
6、 TestMethod.java
/**
*TestMethod.java
*/
package NetFox;
public class TestMethod {
public TestMethod() { ///xx/weblogic60b2_win.exe
try{
SiteInfoBean bean = new SiteInfoBean("http://localhost/xx/weblogic60b2_win.exe",
"L:\\temp","weblogic60b2_win.exe",5);
//SiteInfoBean bean = new SiteInfoBean("http://localhost:8080/down.zip","L:\\temp",
"weblogic60b2_win.exe",5);
SiteFileFetch fileFetch = new SiteFileFetch(bean);
fileFetch.start();
} catch(Exception e) {
e.printStackTrace ();
}
}
public static void main(String[] args) {
new TestMethod();
}
}