How to identify the URL of an Java web application from within?

Question

My Java web application contains a startup servlet. Its init() method is invoked, when the web application server (Tomcat) is started. Within this method I need the URL of my web application. Since there is no HttpServletRequest, how to get this information?

Answer 1

You can't. Because there is no "URL of an Java web application" as seen "from within". A servlet is not tied to an URL, that is done from the outside. (Perhaps you have a Apache server that connects to a Tomcat - Tomcat can't know about it) It makes sense to ask a HttpServletRequest for its url, because we are speaking of the information of a event (the URL that was actually used to generate this request), it does not make sense to ask for a configuration URL.

Answer 2

A workaround could be to perform the initialization lazy when the first request arrives. You can implement a filter that do that once, e.g. by storing a boolean flag in a static variable and synchronizing access to the flag correctly. But it implies a little overhead because each subsequent request will go through the filter which then bypass the initialization. It was just a thought.

Answer 3

There is nothing in the servlet API that provides this information, plus any given resource may be bound to multiple URL's.

What you CAN do, is to inspect the servlet context when you receive an actual request and see what URL was used.

Answer 4

Here is how it works for me and probably for most configurations:

public static String getWebappUrl(ServletConfig servletConfig, boolean ssl) {
    String protocol = ssl ? "https" : "http";
    String host = getHostName();
    String context = servletConfig.getServletContext().getServletContextName();
    return protocol + "://" + host + "/" + context;
}

public static String getHostName() {
    String[] hostnames = getHostNames();
    if (hostnames.length == 0) return "localhost";
    if (hostnames.length == 1) return hostnames[0];
    for (int i = 0; i < hostnames.length; i++) {
        if (!"localhost".equals(hostnames[i])) return hostnames[i];
    }
    return hostnames[0];
}

public static String[] getHostNames() {
    String localhostName;
    try {
        localhostName = InetAddress.getLocalHost().getHostName();
    } catch (UnknownHostException ex) {
        return new String[] {"localhost"};
    }
    InetAddress ia[];
    try {
        ia = InetAddress.getAllByName(localhostName);
    } catch (UnknownHostException ex) {
        return new String[] {localhostName};
    }
    String[] sa = new String[ia.length];
    for (int i = 0; i < ia.length; i++) {
        sa[i] = ia[i].getHostName();
    }
    return sa;
}