You can make use of JSTL (just drop jstl-1.2.jar in /WEB-INF/lib) c:import tag to import an external resource, which will throw FileNotFoundException if the URL is invalid, which in turn can be catched using JSTL c:catch tag. You can finally use JSTL c:choose to check whether to display the iframe or the eventual error.
Here's an SSCCE, copy'n'paste'n'run it (with JSTL installed):
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!doctype html>
<html lang="en">
<head>
<title>SO question 2291085</title>
</head>
<body>
<c:set var="url" value="http://google.com" />
<c:catch var="e">
<c:import url="${url}" varReader="ignore" />
</c:catch>
<c:choose>
<c:when test="${empty e}">
<iframe src="${url}"></iframe>
</c:when>
<c:otherwise>
<p>Error! ${e}</p>
</c:otherwise>
</c:choose>
</body>
</html>
Change http://google.com to http://google.com/foo or something invalid, you'll see that the error shows instead.
Note that I used varReader="ignore" to have it buffered but unread, so that it won't entirely hauled in which may be expensive because after all you're requesting the same URL twice.
Update: Alternatively, you can use a Servlet for this which preprocesses the request inside doGet() method with help of java.net.URLConnection. Here's a kickoff example.
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
URL url = new URL("http://google.com");
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
int status = connection.getResponseCode(); // 200 = OK, 404 = Not Found, etc.
if (status == 200) {
request.setAttribute("url", url);
} else {
// Do your thing to set custom message or request another page.
}
request.getRequestDispatcher("/WEB-INF/page.jsp").forward(request, response);
}
...and in page.jsp then just have something like
<c:if test="${not empty url}">
<iframe src="${url}"></iframe>
</c:if>
Map the servlet on an url-pattern of something like /foo and call it on http:/example.com/contexty/foo instead of the JSP.