How to check if a URL exists or returns 404 with Java?

问题

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\");
}

This should print

exists
does not exists

TEST

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\"));           

Output

200
200
200
404

SOLUTION

Add the next line before .connect() and the output would be 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)\");

回答1:

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);

回答2:

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.

回答3:

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)

回答4:

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");

回答5:

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.

来源：https://stackoverflow.com/questions/1378199/how-to-check-if-a-url-exists-or-returns-404-with-java