方法一：
　　　　　　　URL serverUrl = new URL("http://localhost:8090/Demo/clean.sql");
            HttpURLConnection urlcon = (HttpURLConnection) serverUrl.openConnection();

            String message = urlcon.getHeaderField(0);
            if (StringUtils.hasText(message) && message.startsWith("HTTP/1.1 404")) {
                System.out.println("不存在");
            }else{
                System.out.println("存在"+message);
            }
            
            方法二：
            URL url = new URL("http://localhost:8090/Demo/clean.sql");
            HttpURLConnection urlcon2 = (HttpURLConnection) url.openConnection();
            Long TotalSize=Long.parseLong(urlcon2.getHeaderField("Content-Length"));  
            if (TotalSize>0){
                System.out.println("存在");
                
            }else{
                System.out.println("不存在");
            }

String filepath ="http://22.58.123.146:8002/bidpu/upload/E3502000072/20180426/1984fa0b9f6/招告/20180426招標公告.pdf"
try {
    URL pathUrl = new URL(filepath);
    HttpURLConnection urlcon = (HttpURLConnection) pathUrl.openConnection();
    if(urlcon.getResponseCode()>=400){ 
       System.out.println("檔案不存在");
    }else{
       System.out.println("檔案存在");
   }
}catch (Exception e) {
    System.out.print("請求失敗");
}