Martin Odersky, the author of this programming language, is giving an introduction into the Scala on coursera. It will be a seven week long, free online class with weekly home-work assignments and a certificate at the end.

You can signup here.

But let’s say we have a Spring MVC application with a login form which contains a user name and password field, but the application does not use any of the supported methods to store the user data. How can we authenticate the user anyway?

The easiest way in my opinion is to create a new authentication provider. So we simply need to implement the AuthenticationProvider interface.

The authenticate() function of the class must return an UsernamePasswordAuthenticationToken instance if the authentication is successful or null otherwise. You can choose another token, simple check the classes implementing AbstractAuthenticationToken. But for our scenario this should be enough.

It is important to populate the list of authorities we grant the user. I used the standard user role (“ROLE_USER”).

In the real-world you might want to add a member variable to the authentication provider pointing to a bean which contains the code for authenticating an user, here I just hard-coded it (name must be “admin”, password “system”).

@Component
public class CustomAuthenticationProvider implements AuthenticationProvider {

	@Override
	public Authentication authenticate(Authentication authentication) throws AuthenticationException {
		String name = authentication.getName();
		String password = authentication.getCredentials().toString();
		if (name.equals("admin") && password.equals("system")) {
			List<GrantedAuthority> grantedAuths = new ArrayList<>();
			grantedAuths.add(new SimpleGrantedAuthority("ROLE_USER"));
			Authentication auth = new UsernamePasswordAuthenticationToken(name, password, grantedAuths);
			return auth;
		} else {
			return null;
		}
	}

	@Override
	public boolean supports(Class<?> authentication) {
		return authentication.equals(UsernamePasswordAuthenticationToken.class);
	}
}

Now we need to declare the new authentication provider in our configuration:

<authentication-manager>
	<authentication-provider ref="customAuthenticationProvider"/>
</authentication-manager>

And that’s it!

One of the more well-known is MongoDB, a document-oriented, schemaless database, which I started to like very much. MongoDB is easy to set up and allows me to store objects into the database right away, without thinking about table layout and hacking SQL installation scripts. Of course it also has its downsides, but later more on it.

Currently I take an online class on MongoDB for developers, which is provided by the makers of the database at 10gen.com for free. The online class consists of a couple of video lectures and regular home work assignments. After seven weeks the class will end with a final programming project and of course a 10gen MongoDB developer certificate.

The homework is easy and the course is progressing with a slow pace which is ideal if you cannot or don’t want to spend to much time on it.

I don’t think you can join the current class, but at the end of February (25th) a new course starts:

“M101J MongoDB for Java Developers”

So if you are interested in taking a closer look, then join the course. So far I like it. It gives a good introduction, talks not just about the programming API, but also about best practices in document design and performance tips. You will not be a professional mongo developer after the course, but I think it provides everything you need to start using MongoDB and is also a good starting point to dive deeper into the issue.

I started this blog for making notes for myself, share code snippets and help fellow programmers who got stuck with similar problems. But the internet moved on during the past two years and there seem to be dozens of web sites now which are probably better suited for doing all these tasks. Nevertheless I like blogs, they are much more personal than tweets and Gist code dumps. So I decided to continue sharing code, ideas and opinions … just don’t expect me to be as frequent as two years ago ;-)

Before this post becomes completely useless, here is a small brain teaser for our Javascript ninjas out there:

What is the following expression evaluating to?

"3" -+-+-+ "1" + "1" / "3" * "6" + "2"

The answer and in depth explanation can be found on wtfjs.com which makes me regularly laugh and cry.

As a small test I want to write a programme which reads numbers from stdin, spawns a new process to calculate the Fibonacci number for each input on a different CPU and prints the result to stdout.

First we need a function which is called when spawning a new process, which should calculate the result and print it to the console. In my case this is a simple IO Monad, which gets a MVar as it’s first argument. The function will wait for input from the MVar and then call the Fibonacci function.

This MVar can be thought of as a synchronised box where a thread can store a value and another thread can read from it. So MVars are just the poor men’s message channel which can hold one value at the time only. Of course there are also real channels in Haskell which act like the message queues in Scala or Erlang, see Control.Concurrent.Chan for more informations.

printResult mvar =  do
	value <- takeMVar mvar
	print $ "fib(" ++ show value ++ ")=" ++ show (fib value)

fib 0 = 0
fib 1 = 1
fib n =  fib (n - 1) + fib (n - 2)

Now we need to read the input from stdin, create a MVar object and invoke the printResult function. The cool thing about Haskell is the forkIO function which is really convenient. It takes an arbitrary IO expr and invokes it in a new thread:

import Control.Monad -- needed for "forever"
import System.Exit     -- needed for exitSuccess

main = forever $ do
	line <- getLine
	when (line == "quit") exitSuccess
	mvar <- newEmptyMVar
	forkIO $ printResult mvar
	putMVar mvar (read line :: Integer)

Now we have to compile our programme with the “-threaded” flag:

ghc -threaded fib.hs -o fib

The RTS allows us now to define the number of processors the application runs on with the -Nnum flag. If I want to use three processors for parallelisation I can run the programme simply with:

./fib +RTS -N4

Cracking open five of the best open source easter eggs

reinventing desktop human-computer interaction: 10/GUI

So here are my results from a short trip into the adventurous realm of multicore programming in Java. To make things a little bit simpler in this post I will demonstrate to run several functions calculating a high Fibonacci number instead of using a more complicated real world example. The calculations will be distributed via a thread pool and all classes used in this example belong to the Java standard library, so there won’t be a need to install any extra packages.

The first thing we will need is a class implementing the java.util.concurrent.Callable interface in which fib(int n) will be a static function returning an integer:

class FibTask implements Callable<Integer> {
  private int n;
  public FibTask(int n) {
    this.n = n;
  }

  @Override
  public Integer call() throws Exception {
    return fib(this.n);
  }
}

Before we start to build our thread pool we can retrieve the amount of CPUs/cores of our system from the Java runtime:

int cpus = Runtime.getRuntime().availableProcessors();

OK, so the next step is building a thread pool. If you are looking for a pool with specific parameters and behaviour you can build one by using the ThreadPoolExecutor class. But luckily the concurrent package already offers a nice short cut to create a pool with a fixed size:

ExecutorService pool = Executors.newFixedThreadPool(cpus);

To execute functions in the pool you can either use it’s execute method or even more convenient: invokeAll. This method takes a list of tasks which have to implement the Callable interface executes them and returns a list of Future objects which hold the results of the Callable objects. The cool thing about invokeAll is it makes sure all tasks are executed before returning anything. So for all elements in the returned list Future.isDone() will be true:

List<FibTask> tasks = new ArrayList<FibTask>();			
for (int i = 0; i < cpus; i++) {
  tasks.add(new FibTask(45));
}			
List<Future<Integer>> results = pool.invokeAll(tasks);

The results list holds the Future values now. If the pool is no longer needed, it should be closed properly by calling it’s shutdown() method. Before we can retrieve and print out the results we need to check for any errors occurred during the calculation:

for (Future<Integer> result : results) {
  if (result.isCancelled()) {
    System.err.println("Life is a bitch");
  } else {
    try {
      System.out.println("result: " + result.get());
    } catch (ExecutionException e) {
      System.out.println(e.getCause());
    }
  }
}

And this is all you need to run your code on multiple cores. Couldn’t be easier!

Hopefully HTML5 will make everything a little bit simpler. Anyway … I just want to create a simple box containing text centred in the middle of it. This box will be used as a button on a website. So what I need is to specify vertical and horizontal alignments. The horizontal alignment attribute is called “text-align” which I guess is a relict from the good old days when people where used to browse hypertext documents. But to put the text in the centre of the box I had to shoot myself into the foot … twice!

First I needed to create a table with one row and one entry and then I was able to use misuse the table alignment options to centre the text vertically with an attribute called “vertical-align”. Why is there no “horizontal-align”? I don’t know. Why do I have to use a table? I don’t know. Don’t ask questions when writing CSS/HTML code. Never!

Here is the result:

<html><head></head><body>

<div style="background-color:grey;width:10%;height:10%;position:absolute;top:10%;left:40%">
  <table style="width:100%;height:100%">
    <tr><td style="vertical-align:middle;text-align:center">
      This is a test
    </td></tr>
  </table>
</div>

</body></html>

Hooray! Please don’t ask me how long I needed to figure this out :/
I wonder if there is a more elegant solution …

The style of the div element can be used to define the box, Mozilla’s Gecko engine supports some really cool CSS features like round edges, shadows and gradients (in the case of Firefox 3.6). But of course it won’t work with other browsers, it would be to easy if just everyone could write CSS/HTML code without studying computer science first. However, if you have Firefox just look and be amazed of my mighty text box:

This is a test

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