Simple caldav4j example in java and clojure

I was trying to access a caldav calendar through java and clojure to build a small application. But it wasn’t too easy.

caldav4j seemed to be very confusing and not so well maintained. There is no quick start guide and you are directed to have a look at the unit tests. But they are a compilation of interconnected classes and their hierarchies including a lot of deprecated classes being used – pretty confusing for a newcomer.

I had also problems using the alternative ical4j and it’s caldav connector. After some debugging it seemed incompatible with zimbra because of some strange filter constraints.

This might all be due to the fact that caldav is a complex protocol and I accepted to dig deeper into caldav4j.

Enough of the yacking, I traced down the necessary wire to build a basic example and hope it’ll help others looking for a quick start.



I tested it with zimbra. For google calendar a little more work might be required. The key might be in the class.


    HttpClient httpClient = new HttpClient();
    // I tried it with zimbra - but I had no luck using google calendar
    httpClient.getHostConfiguration().setHost("CALDAVHOST", 443, "https");
    String username = "username";
    UsernamePasswordCredentials httpCredentials = new UsernamePasswordCredentials(username, "secret");
    httpClient.getState().setCredentials(AuthScope.ANY, httpCredentials);
    // Need a proxy?
    //httpClient.getHostConfiguration().setProxy("phost", 8080);

    CalDAVCollection collection = new CalDAVCollection(
        "/dav/"+ username +"/Calendar",
        (HostConfiguration) httpClient.getHostConfiguration().clone(),
        new CalDAV4JMethodFactory(),

    GenerateQuery gq=new GenerateQuery();
    // TODO you might want to adjust the date
    gq.setFilter("VEVENT [20131001T000000Z;20131010T000000Z] : STATUS!=CANCELLED");
    // Get the raw caldav query
    // System.out.println("Query: "+ gq.prettyPrint());
    CalendarQuery calendarQuery = gq.generate();
    List<Calendar>calendars = collection.queryCalendars(httpClient, calendarQuery);

    for (Calendar calendar : calendars) {
      ComponentList componentList = calendar.getComponents().getComponents(Component.VEVENT);
      Iterator<VEvent> eventIterator = componentList.iterator();
      while (eventIterator.hasNext()) {
        VEvent ve =;
        System.out.println("Event: "+ ve.toString());


defn- create-query []
  (let [gq (GenerateQuery.)]
    ; Date Format yyyyMMdd
    (.setFilter gq "VEVENT [20131001T000000Z;20131010T000000Z] : STATUS!=CANCELLED")
    (.generate gq)))

(defn -main []
  (let [http-client (initialize-http-client)
        collection-path (:collection-path configuration)
        host-configuration (.clone (.getHostConfiguration http-client))
        method-factory (CalDAV4JMethodFactory.)
        collection (CalDAVCollection. collection-path host-configuration method-factory CalDAVConstants/PROC_ID_DEFAULT)
        calendars (.queryCalendars collection http-client (create-query))
    (doseq [cal calendars
            event (-> cal (.getComponents) (.getComponents Component/VEVENT))]
      (println "Event: " event)
      (println "\n\n"))))

Clojure core.logic cheatsheets

I am currently having a look at clojure core.logic and was missing some kind of reference documentation. There are a lot of examples and tutorials but I never had an overview about what’s available.

In a google groups discussion the idea of a reference documentation like clojure cheat sheets came up.!topic/clojure/ScJa_o4lP2k

So I decided to create one, view it directly: cheatsheet on github

Have a look at the github project:

Thanks to the original author of the cheatsheet generator Michael Fogus and Andy Fingerhut.

Thanks goes to xavriley who created a cool version of clojuredocs for core.logic which is where the cheatsheet entries link to.

How to install oracle java 7 update 11 (jdk-7u11) on debian linux

Currently you can’t install the new java sdk 7 update 11 from oracle using java_package. Usually you can simply follow this instruction: Installing oracle jre / sdk on debian.

Unfortunately the error is: No matching plugin was found.

But you can modify the script /usr/share/java-package/ to accept the new package:

<       "jdk-7u"[0-9]"-linux-i586.tar.gz") # SUPPORTED
<           j2se_version=1.7.0+update${archive_name:6:1}${revision}
>       "jdk-7u"[0-9][0-9]"-linux-i586.tar.gz") # SUPPORTED
>           j2se_version=1.7.0+update${archive_name:6:2}${revision}
<       "jdk-7u"[0-9]"-linux-x64.tar.gz") # SUPPORTED
<           j2se_version=1.7.0+update${archive_name:6:1}${revision}
>       "jdk-7u"[0-9][0-9]"-linux-x64.tar.gz") # SUPPORTED
>           j2se_version=1.7.0+update${archive_name:6:2}${revision}

Worked for me:

The Debian package has been created in the current directory. You can
install the package as root (e.g. dpkg -i oracle-j2sdk1.7_1.7.0+update11_amd64.deb).

(#:/tmp)- dpkg -i oracle-j2sdk1.7_1.7.0+update11_amd64.deb
(Reading database … 225711 files and directories currently installed.)
Preparing to replace oracle-j2sdk1.7 1.7.0+update7 (using oracle-j2sdk1.7_1.7.0+update11_amd64.deb) …
Unpacking replacement oracle-j2sdk1.7 …
Setting up oracle-j2sdk1.7 (1.7.0+update11) …

(#:/tmp)- java -version
java version „1.7.0_11“
Java(TM) SE Runtime Environment (build 1.7.0_11-b21)
Java HotSpot(TM) 64-Bit Server VM (build 23.6-b04, mixed mode)

There is also a bug report: Bug#698108: jdk7u11 not supported

Edit: also works for java 7 update 12 (jdk-7u12), java 7 update 13 (jdk-7u13), java 7 update 14 (jdk-7u14) and java 7 update 15 (jdk-7u15)

Edit: Oh boy, too many updates recently.

Clojure store persistent data structures gotcha – load-file size limit

In clojure – the data structure and their default representation are suited as readable and portable format to persist data. But there is a small pitfall in using it.

Data Structure example:

  :request-time 2636,
  :status 200,
    "server" "Server",
    "content-encoding" "gzip",

Be aware that this function is intended to load code only. If your data structures or a string in them grow bigger than around 65,535 it crashes.

Exception similar to:

java.lang.ClassFormatError: Unknown constant tag 49 in class file parse$eval13

Please use read-string instead.

Example: (read-string (slurp "data.clj"))

Source: Rich Hickey on google groups Google Groups

GWT 2.2.0 and EXT GWT no success

I tried to use EXT GWT with GWT 2.2.0 and had no success … no one told me I couldn’t do that.

Afterwards I found out there really is no accurate version requirement noted for EXT GWT. „GWT 1.5 or above is required“ from the helpcenter was not very helpful here.

What I started with and wanted to use was:

<!-- My Wish: From the Sencha Roadmap, not yet released: 3.0 -->

(maven Dependency configuration)

Which resulted in compilation errors:

$ mvn -Dmaven.test.skip package
java.lang.IncompatibleClassChangeError: Found interface, but class was expected at com.extjs.gxt.ui.rebind.core. TemplatesGenerator$SourceGenerator.validateType( at com.extjs.gxt.ui.rebind.core. TemplatesGenerator$SourceGenerator.generate( at com.extjs.gxt.ui.rebind.core. TemplatesGenerator.generate(

I found some board entries about that. So I had to downgrade GWT and a couple of libraries like the maven gwt plugin, gin and gwt log. Thanks to maven this was less horrible than it could have been.


So, dear vendors, please make more accurate requirement definitions.

Java DDOS 2.2250738585072012e-308 with curl

There seems to be an easy exploit for the problem going around in the news.

curl -H „Accept-Language: en-us;q=2.2250738585072012e-308“ URL

Apparently works on any Tomcat-server calling getLocale() on the request or most spring applications.

Meanwhile there is java patch for the problem, update your boxes!

Release Notes:
Security Alert:

For my servers it did not have an effect but maybe you had success? Please leave a comment.


You should all have received a java security update by now. If not, you could simply use the following code to reproduce the bug locally.

$ cat 
class Runhang {
  public static void main(String[] args) {
    double d = Double.parseDouble("2.2250738585072012e-308");
    System.out.println("Value: " + d);

$ javac
$ java Runhang &
... wait and kill it :)

FizzBuzz Aufgabe in Clojure

Es gibt eine Code Kata die nennt sich FizzBuzz Ich habe einmal versucht die Aufgabe in Clojure zu realisieren. Die meiste Zeit habe ich damit verbracht, geeignete Funktionen für meine Zwecke zu finden, da dies so ziemlich meine ersten Zeilen in Clojure sind.

(defn fizzbuzz [n] (cond
    (and (zero? (mod n 3)) (zero? (mod n 5))) "fizzbuzz"
    (zero? (mod n 3)) "fizz"
    (zero? (mod n 5)) "buzz"
    1 n
(map fizzbuzz (range 1 20))                          
(1 2 "fizz" 4 "buzz" "fizz" 7 8 "fizz" "buzz" 11 "fizz" 13
14 "fizzbuzz" 16 17 "fizz" 19)

Naja, ein Blick in die Code Beispiele der Seite verrät bessere Versionen. Es scheint aber als wären die Katas auch eine gute Möglichkeit um einfach eine Sprache zu lernen, vor allem Dank der Benutzer Lösungen.

Mehr über Clojure gibt es hier und in den Screencasts, welche meiner Meinung nach eine sehr schöne Möglichkeit sind etwas über die Sprache zu lernen.