基於HTTP協議的下載功能實現

  超文本傳輸協議 (HTTP-HyperText Transfer Protocol)是一種使用極為廣泛的協議，它由請求和響應構成，是一種無狀態的應用層協議。設計HTTP協議的初衷是為了提供一種傳輸HTML（HyperText Markup Language，超文本標記語言）的協議和方法。通過HTTP協議請求的資源由URI（Uniform Resource Identifiers，統一資源標識符）來標識。通常，我們通過URI就能訪問到萬維網服務器提供的數據，如HTML文檔、音視頻等各種格式的數據。因此，基於HTTP協議，我們可以實現簡單的下載器功能。

  HTTP下載器的功能很簡單，下載器使用指定的URI發送GET請求到服務器，服務器返回請求響應，下載器根據響應狀態碼判斷請求成功與否。一般情況下，請求一個資源成功后的狀態碼應為200（HTTP_OK）或者206(Partial-Content)。其他狀態碼視為失敗（具體的HTTP狀態碼請參見：http://zh.wikipedia.org/wiki/HTTP%E7%8A%B6%E6%80%81%E7%A0%81）。若請求成功，HTTP響應頭中將會帶有資源的相關信息，其中，我們最關心的是資源大小，一般以Content-Length字段返回（HTTP頭字段請參考：http://msdn.microsoft.com/zh-cn/library/aa287673(v=vs.71).aspx），若有Range字段的情況下，也可以通過Range字段的返回值來計算得到資源的大小，這里的資源大小單位為byte。

  下面就以Java語言來實現一個基於HTTP協議的下載器。因測試工程是為項目所編寫，實現了一套完整的下載管控機制（如下載記錄管理、多任務管理、斷點續下、意外中斷自動恢復下載、狀態通知等功能），為保密起見，以下只公開下載部分核心代碼：

  下載狀態類：

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;
}

  下載類：

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;
    }
}

 更多代碼詳情請參考：TransferLibrary——一個Android文件傳輸庫，主要實現基於Http的文件上傳和下載，簡單方便，支持多任務下載，斷點續傳等等，歡迎小伙伴們使用交流：D