Preferred Java way to ping an HTTP URL for availab

问题:

I need a monitor class that regularly checks whether a given HTTP URL is available. I can take care of the \"regularly\" part using the Spring TaskExecutor abstraction, so that\'s not the topic here. The question is: What is the preferred way to ping a URL in java?

Here is my current code as a starting point:

try {
    final URLConnection connection = new URL(url).openConnection();
    connection.connect();
    LOG.info(\"Service \" + url + \" available, yeah!\");
    available = true;
} catch (final MalformedURLException e) {
    throw new IllegalStateException(\"Bad URL: \" + url, e);
} catch (final IOException e) {
    LOG.info(\"Service \" + url + \" unavailable, oh no!\", e);
    available = false;
}

1. Is this any good at all (will it do what I want)?
2. Do I have to somehow close the connection?
3. I suppose this is a GET request. Is there a way to send HEAD instead?

回答1:

Is this any good at all (will it do what I want?)

You can do so. Another feasible way is using java.net.Socket.

public static boolean pingHost(String host, int port, int timeout) {
    try (Socket socket = new Socket()) {
        socket.connect(new InetSocketAddress(host, port), timeout);
        return true;
    } catch (IOException e) {
        return false; // Either timeout or unreachable or failed DNS lookup.
    }
}

There\'s also the InetAddress#isReachable():

boolean reachable = InetAddress.getByName(hostname).isReachable();

This however doesn\'t explicitly test port 80. You risk to get false negatives due to a Firewall blocking other ports.

---

Do I have to somehow close the connection?

No, you don\'t explicitly need. It\'s handled and pooled under the hoods.

---

I suppose this is a GET request. Is there a way to send HEAD instead?

You can cast the obtained URLConnection to HttpURLConnection and then use setRequestMethod() to set the request method. However, you need to take into account that some poor webapps or homegrown servers may return HTTP 405 error for a HEAD (i.e. not available, not implemented, not allowed) while a GET works perfectly fine. Using GET is more reliable in case you intend to verify links/resources not domains/hosts.

---

Testing the server for availability is not enough in my case, I need to test the URL (the webapp may not be deployed)

Indeed, connecting a host only informs if the host is available, not if the content is available. It can as good happen that a webserver has started without problems, but the webapp failed to deploy during server\'s start. This will however usually not cause the entire server to go down. You can determine that by checking if the HTTP response code is 200.

HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
connection.setRequestMethod(\"HEAD\");
int responseCode = connection.getResponseCode();
if (responseCode != 200) {
    // Not OK.
}

// < 100 is undetermined.
// 1nn is informal (shouldn\'t happen on a GET/HEAD)
// 2nn is success
// 3nn is redirect
// 4nn is client error
// 5nn is server error

For more detail about response status codes see RFC 2616 section 10. Calling connect() is by the way not needed if you\'re determining the response data. It will implicitly connect.

For future reference, here\'s a complete example in flavor of an utility method, also taking account with timeouts:

/**
 * Pings a HTTP URL. This effectively sends a HEAD request and returns <code>true</code> if the response code is in 
 * the 200-399 range.
 * @param url The HTTP URL to be pinged.
 * @param timeout The timeout in millis for both the connection timeout and the response read timeout. Note that
 * the total timeout is effectively two times the given timeout.
 * @return <code>true</code> if the given HTTP URL has returned response code 200-399 on a HEAD request within the
 * given timeout, otherwise <code>false</code>.
 */
public static boolean pingURL(String url, int timeout) {
    url = url.replaceFirst(\"^https\", \"http\"); // Otherwise an exception may be thrown on invalid SSL certificates.

    try {
        HttpURLConnection connection = (HttpURLConnection) new URL(url).openConnection();
        connection.setConnectTimeout(timeout);
        connection.setReadTimeout(timeout);
        connection.setRequestMethod(\"HEAD\");
        int responseCode = connection.getResponseCode();
        return (200 <= responseCode && responseCode <= 399);
    } catch (IOException exception) {
        return false;
    }
}

回答2:

Instead of using URLConnection use HttpURLConnection by calling openConnection() on your URL object.

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

here is code:

    HttpURLConnection connection = null;
    try {
        URL u = new URL(\"http://www.google.com/\");
        connection = (HttpURLConnection) u.openConnection();
        connection.setRequestMethod(\"HEAD\");
        int code = connection.getResponseCode();
        System.out.println(\"\" + code);
        // You can determine on HTTP return code received. 200 is success.
    } catch (MalformedURLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } finally {
        if (connection != null) {
            connection.disconnect();
        }
    }

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

Hope this helps.

回答3:

You could also use HttpURLConnection, which allows you to set the request method (to HEAD for example). Here\'s an example that shows how to send a request, read the response, and disconnect.

回答4:

The following code performs a HEAD request to check whether the website is available or not.

public static boolean isReachable(String targetUrl) throws IOException
{
    HttpURLConnection httpUrlConnection = (HttpURLConnection) new URL(
            targetUrl).openConnection();
    httpUrlConnection.setRequestMethod(\"HEAD\");

    try
    {
        int responseCode = httpUrlConnection.getResponseCode();

        return responseCode == HttpURLConnection.HTTP_OK;
    } catch (UnknownHostException noInternetConnection)
    {
        return false;
    }
}

回答5:

here the writer suggests this:

public boolean isOnline() {
    Runtime runtime = Runtime.getRuntime();
    try {
        Process ipProcess = runtime.exec(\"/system/bin/ping -c 1 8.8.8.8\");
        int     exitValue = ipProcess.waitFor();
        return (exitValue == 0);
    } catch (IOException | InterruptedException e) { e.printStackTrace(); }
    return false;
}

Possible Questions

• Is this really fast enough?Yes, very fast!
• Couldn’t I just ping my own page, which I want to request anyways? Sure! You could even check both, if you want to differentiate between “internet connection available” and your own servers beeing reachable What if the DNS is down? Google DNS (e.g. 8.8.8.8) is the largest public DNS service in the world. As of 2013 it serves 130 billion requests a day. Let ‘s just say, your app not responding would probably not be the talk of the day.

read the link. its seems very good

EDIT: in my exp of using it, it\'s not as fast as this method:

public boolean isOnline() {
    NetworkInfo netInfo = connectivityManager.getActiveNetworkInfo();
    return netInfo != null && netInfo.isConnectedOrConnecting();
}

they are a bit different but in the functionality for just checking the connection to internet the first method may become slow due to the connection variables.

回答6:

Consider using the Restlet framework, which has great semantics for this sort of thing. It\'s powerful and flexible.

The code could be as simple as:

Client client = new Client(Protocol.HTTP);
Response response = client.get(url);
if (response.getStatus().isError()) {
    // uh oh!
}

回答7:

wrt 2.: you\'d better do close it. However, it may depend on the particular implementation of URLConnection which is used. I have just finished tracking down a resource leak in our system just because of that. An app generated lots of hanging connections (according to lsof; we\'re running it on JDK1.6) and the reason was we have used exactly the piece of code you have shown. TCP Connections were not closed neither e.g. returned to pool etc. - they were left in ESTABILISHED state. In this case, the proper scenario is the one shown by YoK - cast it to (HttpURLConnection) and invoke .disconnect().