java.util.Collections$UnmodifiableMap problem : code included

Question

I am building a facebook platform web app using GWT and hosting it on App Engine.

I am adding validation code that uses supplied query string parameters in the callback url. GWT allows me to get these parameters by calling Window.Location.getParameterMap() and the returned Map is immutable.

I may be wrong however I think this problem has nothing to do with FB, GWT or App Engine specifically and is more down to my misunderstanding something about Map objects.

I don't think that my code attempts to modify the supplied Map but the error I get seems to suggest that my code is trying to modify an immutable Map.

Can someone please take a look and let me know where I am modifying an unmodifiable Map?

I would supply a stack trace but I can't find a way to get a stack trace for this to display in App Engine logs.

Thanks in advance for any and all help :-)

/**
 * Validation Test
 * To generate the signature for these arguments:
 * 1. Remove the fb_sig key and value pair.
 * 2. Remove the "fb_sig_" prefix from all of the keys.
 * 3. Sort the array alphabetically by key.
 * 4. Concatenate all key/value pairs together in the format "k=v".
 * 5. Append your secret key.
 * 6. Take the md5 hash of the whole string.
 * @param fbQueryStringParams
 * @return String
 */
public String test(Map<String,List<java.lang.String>> fbQueryStringParams) {

    String appSecret = TinyFBClient.APP_SECRET;
    String fbSig = fbQueryStringParams.get("fb_sig").get(0);
    StringBuilder sb = new StringBuilder();     
    TreeMap<String,String> sortedMap = new TreeMap<String,String>();

    // Get a Set view of the Map of query string parameters.
    Set<Map.Entry<String,List<java.lang.String>>> mapEntries = fbQueryStringParams.entrySet();

    // Iterate through the Set view, inserting into a SortedMap all Map.Entry's
    // that do not have a Key value of "fb_sig".
    Iterator<Map.Entry<String,List<java.lang.String>>> i = mapEntries.iterator();
    while(i.hasNext()) {

        Map.Entry<String,List<java.lang.String>> mapEntry = i.next();

        if(!mapEntry.getKey().equals("fb_sig")) { // 1. Remove the fb_sig key and value pair.

            sortedMap.put(mapEntry.getKey(),mapEntry.getValue().get(0)); // 3. Sort the array alphabetically by key.

        }

    }

    // Get a Set view of the Map of alphabetically sorted Map.Entry objects.
    Set<Map.Entry<String,String>> sortedMapEntries = sortedMap.entrySet();

    // Iterate through the Set view, appending the concatenated key's and value's
    // to a StringBuilder object.
    Iterator<Map.Entry<String,String>> ii = sortedMapEntries.iterator();
    while(ii.hasNext()) {

        Map.Entry<String,String> mapEntry = ii.next();

        // 4. Concatenate all key/value pairs together in the format "k=v".
        sb.append(mapEntry.getKey().replaceAll("fb_sig_","")); // 2. Remove the "fb_sig_" prefix from all of the keys.
        sb.append("=");
        sb.append(mapEntry.getValue());

    }

    sb.append(appSecret); // 5. Append your secret key.

    String md5 = DigestUtils.md5Hex(sb.toString()); // 6. Take the md5 hash of the whole string.

    // Build and return an output String for display.
    StringBuilder output = new StringBuilder();
    output.append("fbSig = "+fbSig);
    output.append("<br/>");
    output.append("md5 = "+md5);
    return output.toString();

}

Answer 1

copy the Windows.Location.getParameterMap() in a HashMap and it will work:

So you send new HashMap>( Windows.Location.getParameterMap()) over RPC that works.

The problem is that unmodifiableMap is not Serializable for GWT. I know that it has a Serializable marker, but in GWT it works a little bit different. Most collection classes have a custom GWT implementation and some are not 100% compatible.

Answer 2

I don't see any unmodifiable collections.

Your code is pretty complicated. If I understood it right, then this should be equivalent. I wouldn't use Map.Entry objects and the TreeMap has a handy constructor for your needs. And finally, I'd prefer the 'forall' loop over the iterator.

public String test(Map<String, List<java.lang.String>> fbQueryStringParams) {
    String appSecret = TinyFBClient.APP_SECRET;
    String fbSig = fbQueryStringParams.get("fb_sig").get(0);
    StringBuilder sb = new StringBuilder();
    TreeMap<String, List<String>> sortedMap = new TreeMap<String, List<String>>(fbQueryStringParams);
    sortedMap.remove("fbSig"); // remove the unwanted entry

    for (String key, sortedMap.keySet()) {
        List<String> values = sortedMap.get(key);
        String printableKey = key.replaceAll("fb_sig_", ""));
        String value = "EMPTY LIST";

        if (!values.isEmpty()) {
            // This could have been your problem, you always
            // assume, all lists in the map are not empty
            value = values.get(0);
        }

        sb.append(String.format("%s=%s", printableKey, value);
    }

    sb.append(appSecret);
    String md5 = DigestUtils.md5Hex(sb.toString());

    // Build and return an output String for display.
    StringBuilder output = new StringBuilder();
    output.append("fbSig = " + fbSig);
    output.append("<br/>");
    output.append("md5 = " + md5);
    return output.toString();
}

While refactoring I found one possible bug: when you create the sorted map in your code, you assume, all lists in the map are not empty. So the first empty list will cause a NPE in the first loop.

Answer 3

Do a System.out.println(fbQueryStringParams.getClass()); at the start of the message (or log it or whatever you need to be able to see what it is).

If that argument is passed to you from the system it is very likely wrapped as an unmodifiable collection since they don't want you altering it.

Answer 4

Did I understand it correctly that you are doing a Window.Location.getParameterMap in your client code and sending it to the server in a RPC call ? In that case ... the question is: is that ParameterMap serializable ? Not all implementations are in fact supported in GWT. So it might just be that your server code is not even called but that it crashes before it can send the request. Did you see any warning during GWT compilation ?

The code, although the implementation can be cleaned up and indeed you can have a NPE, is NOT modifying the supplied parameter Map or the List in the Map values. So the problem is probably somewhere else.

Why don't you run your application in hosted mode (or development mode as they call it in GWT 2.0) ?

David