超文本传输协议 (HTTP-HyperText Transfer Protocol)是一种使用极为广泛的协议，它由请求和响应构成，是一种无状态的应用层协议。设计HTTP协议的初衷是为了提供一种传输HTML（HyperText Markup Language，超文本标记语言）的协议和方法。通过HTTP协议请求的资源由URI（Uniform Resource Identifiers，统一资源标识符）来标识。通常，我们通过URI就能访问到万维网服务器提供的数据，如HTML文档、音视频等各种格式的数据。因此，基于HTTP协议，我们可以实现简单的下载器功能。
HTTP下载器的功能很简单，下载器使用指定的URI发送GET请求到服务器，服务器返回请求响应，下载器根据响应状态码判断请求成功与否。一般情况下，请求一个资源成功后的状态码应为200（HTTP_OK）或者206(Partial-Content)。其他状态码视为失败（具体的HTTP状态码请参见：http://zh.wikipedia.org/wiki/HTTP%E7%8A%B6%E6%80%81%E7%A0%81）。若请求成功，HTTP响应头中将会带有资源的相关信息，其中，我们最关心的是资源大小，一般以Content-Length字段返回（HTTP头字段请参考：http://msdn.microsoft.com/zh-cn/library/aa287673(v=vs.71).aspx），若有Range字段的情况下，也可以通过Range字段的返回值来计算得到资源的大小，这里的资源大小单位为byte。
下面就以Java语言来实现一个基于HTTP协议的下载器。因测试工程是为项目所编写，实现了一套完整的下载管控机制（如下载记录管理、多任务管理、断点续下、意外中断自动恢复下载、状态通知等功能），为保密起见，以下只公开下载部分核心代码：
下载状态类：
public class Status {
public static final int ERROR = -1;
public static final int WAITING = 0;
/**
* Paused by system automatically
*/
public static final int PAUSED = 1;
/**
* Paused by user manually
*/
public static final int PAUSED_MANUALLY = 2;
public static final int DOWNLOADING = 3;
public static final int FINISHED = 4;
}下载类：
package com.irwin.downloader; import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL; import org.apache.http.HttpStatus; public class Downloader implements Runnable {
private static final int BUFFER_SIZE = 32 * 1024; private String mUrl = null; private String mPath = null; private long mStart = 0; private long mCurrentBytes = 0; private int mStatus = Status.WAITING; private long mSize = 0; private boolean mRunning = true; private boolean mTerminate = false; public void setUrl(String url) {
mUrl = url;
} public String getUrl() {
return mUrl;
} public void setPath(String path) {
mPath = path;
} public String getPath() {
return mPath;
} public void setRange(long start, long end) {
if (start >= 0) {
mStart = start;
}
mSize = end;
} public void setTerminate(boolean terminate) {
mTerminate = terminate;
} public boolean isRunning() {
return mRunning;
} public int getStatus() { return mStatus;
} public long getSize() {
return mSize;
} public long getCurrentBytes() {
return mCurrentBytes;
} public void download(String url, String path, long rangeStart)
throws Exception {
if (rangeStart > 0 && rangeStart >= mSize) {
mStatus = Status.FINISHED;
return;
}
InputStream inStream = null;
RandomAccessFile file = null;
try {
URL downUrl = new URL(url);
HttpURLConnection http = (HttpURLConnection) downUrl
.openConnection();
http.setConnectTimeout(5 * 1000);
//Use Get method
http.setRequestMethod("GET");
//Accept all format of data.
http.setRequestProperty("Accept", "*/*");
http.setRequestProperty("Charset", "UTF-8"); //Data block to download.
http.setRequestProperty("Range", "bytes=" + rangeStart + "-");
http.setRequestProperty("User-Agent", "Client/4.0");
http.setRequestProperty("Connection", "Keep-Alive");
http.connect();
if (http.getResponseCode() != HttpStatus.SC_OK
&& http.getResponseCode() != HttpStatus.SC_PARTIAL_CONTENT) {
throw new IllegalAccessException("Invalid request: "
+ http.getResponseCode());
}
if (mSize <= 0) {
long total = getContentLen(http); if (total <= 0) {
throw new IllegalAccessException("Invalid content-length: "
+ total);
}
mSize = total;
}
mStatus = Status.DOWNLOADING;
inStream = http.getInputStream();
byte[] buffer = new byte[BUFFER_SIZE];
File tmp=new File(path+".tmp");
file = new RandomAccessFile(tmp, "rwd");
file.setLength(mSize);
file.seek(rangeStart);
int offset = 0;
mCurrentBytes = rangeStart;
while (true) {
if (mTerminate) {
mStatus = Status.PAUSED_MANUALLY;
break;
}
if ((offset = inStream.read(buffer)) != -1) {
file.write(buffer, 0, offset);
mCurrentBytes += offset;
} else {
break;
}
} file.close();
file = null;
inStream.close();
inStream = null;
if (mSize == mCurrentBytes) {
// Rename
mStatus = Status.FINISHED;
File f = new File(path);
tmp.renameTo(f);
} } catch (Exception e) {
throw e;
} finally {
if (inStream != null) {
try {
inStream.close();
} catch (Exception e2) {
// TODO: handle exception
}
}
if (file != null) {
try {
file.close();
} catch (Exception e2) {
// TODO: handle exception
}
}
}
} @Override
public void run() {
if (getUrl() != null && getPath() != null) {
if (!mTerminate) {
try {
download(getUrl(), getPath(), mStart);
} catch (Exception e) {
e.printStackTrace();
mStatus = Status.ERROR;
}
}
} else {
mStatus = Status.ERROR;
}
mRunning = false;
} private long getContentLen(HttpURLConnection conn) throws Exception {
long len = conn.getContentLength();
if (len <= 0) {
try {
len = Long.parseLong(conn.getHeaderField("Content-Length"));
} catch (Exception e) {
// TODO: handle exception
} }
if (len <= 0) {
//Try to calculate size from 'Range' field.
String range = conn.getHeaderField("Range");
if (range != null) {
String[] array = range.replace("bytes=", "").split("-");
if (array.length == 2) {
len = Long.parseLong(array[1]) - Long.parseLong(array[0]);
}
}
}
return len;
}
}
更多代码详情请参考：TransferLibrary——一个Android文件传输库，主要实现基于Http的文件上传和下载，简单方便，支持多任务下载，断点续传等等，欢迎小伙伴们使用交流：D