如何使用Java检查URL是否存在或返回404？

Question 1

String urlString = "http://www.nbc.com/Heroes/novels/downloads/Heroes_novel_001.pdf";
URL url = new URL(urlString);
if(/* Url does not return 404 */) {
    System.out.println("exists");
} else {
    System.out.println("does not exists");
}
urlString = "http://www.nbc.com/Heroes/novels/downloads/Heroes_novel_190.pdf";
url = new URL(urlString);
if(/* Url does not return 404 */) {
    System.out.println("exists");
} else {
    System.out.println("does not exists");
}

这应该打印

exists
does not exists

测试

public static String URL = "http://www.nbc.com/Heroes/novels/downloads/";

public static int getResponseCode(String urlString) throws MalformedURLException, IOException {
    URL u = new URL(urlString); 
    HttpURLConnection huc =  (HttpURLConnection)  u.openConnection(); 
    huc.setRequestMethod("GET"); 
    huc.connect(); 
    return huc.getResponseCode();
}

System.out.println(getResponseCode(URL + "Heroes_novel_001.pdf")); 
System.out.println(getResponseCode(URL + "Heroes_novel_190.pdf"));   
System.out.println(getResponseCode("http://www.example.com")); 
System.out.println(getResponseCode("http://www.example.com/junk"));

输出量

200
200
200
404

解

在.connect（）之前添加下一行，输出将为200、404、200、404

huc.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.1.2) Gecko/20090729 Firefox/3.5.2 (.NET CLR 3.5.30729)");

Question 2

您可能要添加

HttpURLConnection.setFollowRedirects(false);
// note : or
//        huc.setInstanceFollowRedirects(false)

如果您不想遵循重定向（3XX）

您只需要一个“ HEAD”，而不是执行“ GET”。

huc.setRequestMethod("HEAD");
return (huc.getResponseCode() == HttpURLConnection.HTTP_OK);

Question 3

这为我工作：

URL u = new URL ( "http://www.example.com/");
HttpURLConnection huc =  ( HttpURLConnection )  u.openConnection (); 
huc.setRequestMethod ("GET");  //OR  huc.setRequestMethod ("HEAD"); 
huc.connect () ; 
int code = huc.getResponseCode() ;
System.out.println(code);

感谢您的建议。

Question 4

通过调用使用HttpUrlConnectionopenConnection()URL对象来。

从连接读取后，getResponseCode（）将为您提供HTTP响应。

例如

   URL u = new URL("http://www.example.com/"); 
   HttpURLConnection huc = (HttpURLConnection)u.openConnection(); 
   huc.setRequestMethod("GET"); 
   huc.connect() ; 
   OutputStream os = huc.getOutputStream(); 
   int code = huc.getResponseCode();

（未测试）

Question 5

您的代码没有错。是NBC.com对您进行欺骗。当NBC.com决定您的浏览器无法显示PDF时，无论您请求什么，即使它不存在，它也只会发送回一个网页。

您需要通过告诉它您的浏览器可以欺骗它，例如

conn.setRequestProperty("User-Agent",
    "Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.0.13) Gecko/2009073021 Firefox/3.0.13");

Question 6

根据给出的答案和问题中的信息，这是您应该使用的代码：

public static boolean doesURLExist(URL url) throws IOException
{
    // We want to check the current URL
    HttpURLConnection.setFollowRedirects(false);

    HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();

    // We don't need to get data
    httpURLConnection.setRequestMethod("HEAD");

    // Some websites don't like programmatic access so pretend to be a browser
    httpURLConnection.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.1.2) Gecko/20090729 Firefox/3.5.2 (.NET CLR 3.5.30729)");
    int responseCode = httpURLConnection.getResponseCode();

    // We only accept response code 200
    return responseCode == HttpURLConnection.HTTP_OK;
}

当然可以测试和工作了。