java 多线程实现下载文件

---自己记录
多线程下载文件

1. 使用调取
   `public class methodTest {
   public static String filePath="\deme.zip"; //文件保存地址
   public static String fileUrl="https://download.java.net/java/GA/jdk19/877d6127e982470ba2a7faa31cc93d04/36/GPL/openjdk-19_windows-x64_bin.zip";//文件地址

   public static void main(String[] args) {
   System.out.println("开始");
   //查看文件在不在
   UseThread useThread = new UseThread();
   useThread.downFile(filePath,fileUrl); //filePath:本地保存地点+文件名 fileUrl：线上文件地址

   System.out.println("结束");
   }
   }`

2. 下载文件
   `@Slf4j
   public class UseThread {
   //调用线程
   //限制线程数量
   private int threadNum = 3;

   public void downFile(String localPath,String onlinePath){
   int fileLength = 0 ; //要下载文件的长度
   //首先链接Url
   try {
   //获取链接
   URL url = new URL(onlinePath);
   HttpURLConnection conn = (HttpURLConnection) url.openConnection();
   int responseCode = conn.getResponseCode();
   //链接成功？
   if( responseCode == 200 ){
   //获取文件大小，多线程 指针位置不同需要提前设置
   fileLength = conn.getContentLength();
   //判断 本地是否存在
   File file = new File(localPath);

   if( !file.exists() ){
   //本地文件不存在，创建
   file.createNewFile();
   }
   //随机访问文件，设置本地文件长度
   RandomAccessFile raf = new RandomAccessFile(localPath,"rwd");
   raf.setLength(fileLength);

   //每个线程需要下载的长度
   int blockSize = fileLength/threadNum;

   for( int i = 0 ; i < threadNum ; i++){
   //计算每个文件开始-结束下载放入位置
   int startSize = iblockSize;
   int endSize = ( i + 1 )blockSize - 1;
   //开启线程
   new DownThread_2(localPath,onlinePath,"线程:"+i,startSize,endSize).start();
   }

   //线程是否结束，显示进度条
   //DownLoadParam 类内存储着是否完成 下载数量
   while (DownLoadParam.finish) {
   Thread.sleep(500); //间隔0.5秒计算一下
   if (DownLoadParam.downSize == fileLength) {
   DownLoadParam.finish = false;
   System.out.println("下载完成：100%");
   } else {
   System.out.println("已经下载了：" + ((int) (float) DownLoadParam.downSize / (float) fileLength * 100) + "%");
   }
   }
   }
   }catch (Exception e ){
   log.error("userThread:error",e);
   }

   }
   }
   `

3. 多线程
   `@Slf4j
   public class DownThread_2 extends Thread{
   //下载文件地址
   private String localFilePath;
   //文件地址
   private String onlineFilePath;
   //线程名称-自己传过来
   private String threadName;
   //开始位置
   private int startPosition;
   //结束位置
   private int endPosition;

   public DownThread_2(String localFilePath,String onlineFilePath,String threadName,
   Integer startPosition,int endPosition){
   this.localFilePath = localFilePath ;
   this.onlineFilePath = onlineFilePath ;
   this.threadName = threadName ;
   this.startPosition = startPosition ;
   this.endPosition = endPosition ;
   }

   @Override
   public void run() {
   try {

   //url 链接onlinePath
   URL url = new URL(onlineFilePath);
   HttpURLConnection conn = (HttpURLConnection) url.openConnection();
   //设置 下载的 开始结束 range？
   conn.setRequestProperty("range","bytes="+startPosition+"-"+endPosition);
   //超时连接
   conn.setConnectTimeout(500);

   //链接响应代码 206 才是成功 因为设置了range
   int responseCode = conn.getResponseCode();

   if( responseCode == 206 ){

   //成功后 读数据
   BufferedInputStream bis = new BufferedInputStream(conn.getInputStream());
   //开始读写数据
   RandomAccessFile raf = new RandomAccessFile(localFilePath,"rwd");
   raf.seek(startPosition);
   byte[] bytes = new byte[1024];
   int len ;

   while( (len = bis.read(bytes)) > -1){

   raf.write(bytes,0,len);

   //加锁唯一
   synchronized (DownLoadParam.class){
   DownLoadParam.downSize = DownLoadParam.downSize+len;
   }

   }

   raf.close();
   bis.close();
   log.info("线程{}开始下载:{}---{}",threadName,startPosition,endPosition);
   }

   }catch ( Exception e){
   log.error("线程{}，异常",threadName,e);
   }
   }
   }
   `

4. 共享参数
   public class DownLoadParam { //已经下载多少 静态 在方法区 共享资源 public static int downSize = 0; //是否下载完 public static boolean finish = true; }