基于HTTP协议的下载功能实现

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