多线程断点下载

多线程下载 ```java public class MultiThreadDownloader {

private URL url;        // 目标地址
private File file;        // 本地文件
private long threadLen;    // 每个线程下载多少

private static final int THREAD_AMOUNT = 3;                // 线程数
private static final String DIR_PATH = "F:/Download";    // 下载目录

public MultiThreadDownloader(String address) throws IOException {    
    // 记住下载地址
    url = new URL(address);                                                            
    // 截取地址中的文件名, 创建本地文件
    file = new File(DIR_PATH, address.substring(address.lastIndexOf("/") + 1));        
}

public void download() throws IOException {
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setConnectTimeout(3000);

    // 获取文件总长度
    long totalLen = conn.getContentLength();                    
    // 计算每个线程要下载的长度
    // 总长度 如果能整除 线程数, 每条线程下载的长度就是 总长度 / 线程数
    // 总长度 如果不能整除 线程数, 那么每条线程下载长度就是 总长度 / 线程数 + 1
    threadLen = (totalLen + THREAD_AMOUNT - 1) / THREAD_AMOUNT;

    // 在本地创建一个和服务端大小相同的文件
    RandomAccessFile raf = new RandomAccessFile(file, "rw");    
    // 设置文件的大小, 写入了若干个0
    raf.setLength(totalLen);                                    
    raf.close();

    // 按照线程数循环
    for (int i = 0; i < THREAD_AMOUNT; i++) {
        // 开启线程, 每个线程将会下载一部分数据到本地文件中 
        new DownloadThread(i).start();       
    }     
}

private class DownloadThread extends Thread {
    // 用来标记当前线程是下载任务中的第几个线程
    private int id;

    public DownloadThread(int id) {
        this.id = id;
    }

    public void run() {
        // 从临时文件读取当前线程已完成的进度

        long start = id * threadLen;     
        long end = id * threadLen + threadLen - 1;
        System.out.println("线程" + id + ": " + start + "-" + end);

        try {
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setConnectTimeout(3000);
            // 设置当前线程下载的范围(start和end都包含)
            conn.setRequestProperty("Range", "bytes=" + start + "-" + end);

            InputStream in = conn.getInputStream();
            // 随机读写文件, 用来向本地文件写出
            RandomAccessFile raf = new RandomAccessFile(file, "rw");        
            // 设置保存数据的位置
            raf.seek(start);

            // 每次拷贝100KB
            byte[] buffer = new byte[1024 * 100];    
            int len;
            while ((len = in.read(buffer)) != -1) {
                // 从服务端读取数据, 写到本地文件
                raf.write(buffer, 0, len);            
            }
            raf.close();

            System.out.println("线程" + id + "下载完毕");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public static void main(String[] args) throws IOException {
    new MultiThreadDownloader("http://192.168.1.240:8080/14.Web/android-sdk_r17-windows.zip").download();
}

} ```

断点下载 ```java public class BreakpointDownloader { // 下载目录 private static final String DIR_PATH = "F:/Download";
// 总线程数 private static final int THREAD_AMOUNT = 3;

// 目标下载地址
private URL url;            
// 本地文件
private File dataFile;        
// 用来存储每个线程下载的进度的临时文件
private File tempFile;        
// 每个线程要下载的长度
private long threadLen;      
// 总共完成了多少  
private long totalFinish;    
// 服务端文件总长度
private long totalLen;        
// 用来记录开始下载时的时间
private long begin;

public BreakpointDownloader(String address) throws IOException {    
    url = new URL(address);                                                            
    dataFile = new File(DIR_PATH, address.substring(address.lastIndexOf("/") + 1));    
    tempFile = new File(dataFile.getAbsolutePath() + ".temp");                        
}

public void download() throws IOException {
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();
    conn.setConnectTimeout(3000);

    totalLen = conn.getContentLength();                                    
    threadLen = (totalLen + THREAD_AMOUNT - 1) / THREAD_AMOUNT;

    if (!dataFile.exists()) {                                            
        RandomAccessFile raf = new RandomAccessFile(dataFile, "rws");    
        raf.setLength(totalLen);                                        
        raf.close();
    }

    if (!tempFile.exists()) {                                            
        RandomAccessFile raf = new RandomAccessFile(tempFile, "rws");    
        for (int i = 0; i < THREAD_AMOUNT; i++)                           
            raf.writeLong(0);                                            
        raf.close();
    }

    for (int i = 0; i < THREAD_AMOUNT; i++) { 
        new DownloadThread(i).start();       
    }

    // 记录开始时间
    begin = System.currentTimeMillis();        
}

private class DownloadThread extends Thread {
    // 用来标记当前线程是下载任务中的第几个线程
    private int id;

    public DownloadThread(int id) {
        this.id = id;
    }

    public void run() {
        try {
            RandomAccessFile tempRaf = new RandomAccessFile(tempFile, "rws");        
            tempRaf.seek(id * 8);                        
            long threadFinish = tempRaf.readLong();        
            synchronized(BreakpointDownloader.this) {    
                totalFinish += threadFinish;            
            }

            long start = id * threadLen + threadFinish;      
            long end = id * threadLen + threadLen - 1;       
            System.out.println("线程" + id + ": " + start + "-" + end);

            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setConnectTimeout(3000);
            conn.setRequestProperty("Range", "bytes=" + start + "-" + end);

            InputStream in = conn.getInputStream();                              
            RandomAccessFile dataRaf = new RandomAccessFile(dataFile, "rws");    
            dataRaf.seek(start);

            byte[] buffer = new byte[1024 * 100];            
            int len;
            while ((len = in.read(buffer)) != -1) {
                dataRaf.write(buffer, 0, len);               
                threadFinish += len;                        
                tempRaf.seek(id * 8);                        
                tempRaf.writeLong(threadFinish);           
                synchronized(BreakpointDownloader.this) {   
                    totalFinish += len;                        
                }
            }
            dataRaf.close();
            tempRaf.close();

            System.out.println("线程" + id + "下载完毕");
            // 如果已完成长度等于服务端文件长度(代表下载完成)
            if (totalFinish == totalLen) {                    
                System.out.println("下载完成, 耗时: " + (System.currentTimeMillis() - begin));
                // 删除临时文件
                tempFile.delete();                            
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

public static void main(String[] args) throws IOException {
    new BreakpointDownloader("http://192.168.1.240:8080/14.Web/android-sdk_r17-windows.zip").download();
}

} ```

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Good Luck!