思路

1. 使用http請求遠端檔案地址
2. 從響應體中獲取檔案頭資訊
3. 讀取響應體中的輸入流，並寫入本地檔案輸出流中

程式碼實現

package org.hanmeis;

import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import
 
 java.util.Scanner;
import java.util.Timer;
import java.util.TimerTask;
import java.util.logging.Logger;

/**
 * Created by zhao.wu on 2016/12/7.
 * 單執行緒下載
 * 使用方法：SingleDownloader [url]
 */
public class SingleDownloader {
    private static Logger logger = Logger.getLogger("SingleDownloader");

    public
 
 static void main(String[] args) throws IOException {
        String url;
        if(args.length<1){
            System.out.print("輸入下載地址：");
            url = new Scanner(System.in).nextLine();
        }else {
            url = args[0];
        }

        //校驗引數合法性 略

        SingleDownloader singleDownloader = new
 
 SingleDownloader(url);
        singleDownloader.connectServer();
        singleDownloader.startDownload();
    }

    private URL url;
    private String fileName;
    private long fileSize;
    private long preSize;
    private long currentSize;
    private InputStream is;
    private OutputStream os;
    private Timer timer = new Timer();

    private SingleDownloader(){
        logger.info("新任務加入下載佇列......");
    }

    private SingleDownloader(String url) throws MalformedURLException {
        this();
        this.url = new URL(url);
    }

    /**
     * 連線遠端伺服器，並獲取檔案資訊：大小，檔名
     * @throws IOException
     */
    private void connectServer() throws IOException {
        logger.info(String.format("連線伺服器：%s://%s",url.getProtocol(),url.getHost()));

        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setConnectTimeout(10*1000);

        this.fileSize = Long.parseLong(connection.getHeaderField("Content-Length"));

        String[] temp = url.getFile().split("/");
        if(temp.length!=0) {
            this.fileName = temp[temp.length - 1];
        }

        this.is =  connection.getInputStream();
        this.os = new FileOutputStream(this.fileName);

        logger.info(String.format("檔名：%s  大小：%.2fM", this.fileName, this.fileSize*1.0/1024/1024));
    }

    /**
     * 下載過程
     * @throws IOException
     */
    private void startDownload() throws IOException {

        logger.info(String.format("開始下載:%s",fileName));
        info();

        byte[] bs = new byte[5120];
        int len;
        while((len = is.read(bs))>0){
            os.write(bs);
            this.currentSize += len;
        }
        is.close();
        os.close();
        timer.cancel();
        logger.info(String.format("完成下載:%s",fileName));
    }

    /**
     * 定時計算下載狀態
     */
    private void info(){
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                long tempC = currentSize;
                long tempSize = tempC-preSize;
                preSize = tempC;
                double percent = (tempC*100.0)/fileSize;
                double speed = (tempSize*1.0)/1024/3;
                logger.info(String.format("檔案：%s 已完成：%.2f%%  速率：%.2fkb/s",fileName, percent, speed));
            }
        },0, 3000);
    }
}