如何检查 URL 是否存在或使用 Java 返回404？

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");
}

这个应该能打印出来

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

200
200
200
404

解决方案

在. connect ()之前添加下一行，输出将是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)");
141490 次浏览
小开

Use HttpUrlConnection by calling openConnection() on your URL object.

getResponseCode() will give you the HTTP response once you've read from the connection.

e.g.

   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();

(not tested)

小开

There is nothing wrong with your code. It's the NBC.com doing tricks on you. When NBC.com decides that your browser is not capable of displaying PDF, it simply sends back a webpage regardless what you are requesting, even if it doesn't exist.

You need to trick it back by telling it your browser is capable, something like,

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");
小开

You may want to add 

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

if you don't want to follow redirection (3XX)

Instead of doing a "GET", a "HEAD" is all you need.

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

this worked for me: 

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);

thanks for the suggestions above.

小开

Based on the given answers and information in the question, this is the code you should use:

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;
}

Of course tested and working.


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