As mentioned in the previous post, the SimpleFramework API and server is a minimal alternative in the world of Java application servers like Glassfish, Tomcat. Hosted on GitHub under an Apache v2.0 license, they offer an embeddable HTTP, Servlet and WebSocket capable stack and API. They claim their performance is better than other Java based web servers.
The Dependency Injection Framework Trap
Whether they live up to their name, and is truly easy and simple to use is somewhat debatable, though. They favour Spring for dependency injection, which means part of the code is specified in XML, and is “magically” coupled during runtime. There might be benefits to that, but the downside is that it becomes difficult to read and debug. In fact, their demo application will not start for whatever reason, and all that is offered in terms of output is an InvalidArgumentException without further details deep down from the Spring code.
Don’t get me wrong, the Google Guice framework is not much better. It might specify its module coupling in Java code instead of XML, however, when it comes to following the code and debugging, it is just as obscure. Implementations tend to be hidden behind layers of interfaces. As for the promise of easily re-wireable code, they both fall short. When it comes down to it, a refactoring will typically involve changes in both class content and class relationships, so a dependency injection framework is more likely to get in the way than help.
Finally, when it comes to testability, dependency injection is of course crucial. Being able to inject mocks or other testing objects keeps unit tests small. However, a wiring framework is not needed to describe the class relationships. In fact, my personal experience is that Guice based code is more cumbersome to use in test related code. It takes considerable effort to track down which classes to include in a unit test, and which to leave out, and then to create a test module of it all. At least Spring leaves the class constructors intact and simple.
Making it work
So, with that rant out of the way, what does it take to make the Simple Framework WebSocket Chat Demo work? My solution was to simply shortcut the 114 line XML Spring configuration, and write the same class relations in Java. The following 20 lines achieves the same as the XML.
Furthermore, it’s worth noting that the SimpleFramework is structured in multiple Maven projects, with dependencies between each other, as seen below the code snippet. It highlights another potential problem with the project. A lot of the server code required to make the Chat demo run is in another demo package. It suggest quite a lot of custom code has to be written to use the server API.
The GitHub Maven packages and dependencies at the time of writing:
It is not entirely clear whether the last demo example, “simple-demo-rest”, depends on the “simple-chat” project as the XML suggests, or whether the duplicate ChatRoom class was intended to be used. It might be that the XML was copied, but not updated - highlighting the previous point about hard to maintain and read dependency injection code.
Focusing only on the “simple-demo-chat” example then, this would be what it could look like in the Eclipse Package Explorer, with each directory created as a separate Java project.
Finally, here’s the full main-file hack to make the chat example work. Notice the required helper classes from demo packages.
Following the post about the small HTTP server using the the com.sun.net.httpserver API, I thought I’d try to make it work with WebSockets. I’ll save the suspense; it wont work. And I’m not the first to have failed at it, but it’s always more fun to search for the solution afterwards..
The problem is that the httpserver package is not designed for the persistent bi-directional connection used by the WebSocket protocol. The only part which will work is the handshake, which transmits a HTTP like request and response. However, as soon as that response is sent, the connection is closed, and thus the WebSocket object in the browser client will close.
There is a WebSocket API in Java EE, however it will require an application server like Glassfish or Tomcat to run. Another option is the SimpleFramework server, which also includes a WebSocket implementation. More about that in a later post.
The only part of the code worth mentioning here is the handshake response from the server. It sets the correct headers and encodes the server accept key according to the specification. Connecting to this server will therefore result in a valid websocket object on the client, as long as handle() method on the server does not return.
JavaFx is a comprehensive platform and set of APIs to develop desktop, web and even mobile applications. Although intended to replace the Swing GUI API, it can get significantly more complex, with FXML for XML based object specification; CSS styling; and special IDE tool chains.
Since JDK 8, it is supposedly included in the main install, however, on Debian I had to install it separately:
There’s a few Java libraries for creating terminal and ncurses like applications. They abstract away the ANSI escape codes and low level control like tput. I’ve briefly looked at three of them: Lanterna, Charva and JCurses. They all come with basic terminal control, and some widgets and window managing to create terminal based GUI applications.
JCurses seems to be the oldest, and possibly unmaintained. It requires ncurses to be installed, and also a native library. It is available through the Maven Central repository, however, make sure to set up the path for access to the libjcurses.so (in the tar-ball from SourceForge, or within the .jar from Maven). Documentation is sparse, but it seems it’s possible to interface with the terminal with little code.
Charva is also ncurses based and requires a native lib. One of it’s selling points is that it can act as a drop-in replacement for java.awt UIs. It seems unlikely that that will work well for all but the simplest UIs, however, at least it offers an easy transition path.
Finally, Lanterna seems to be the best maintained, and clearly documented project of the three. It is unique in that is handles all terminal details itself, and does not require external libraries. It is compatible with Linux, Windows, Cygwin, OS X terminals. Furthermore, if launched from an X or similar GUI based environment instead of a terminal, it will create its own GUI window into which it will render the terminal. Neat.
The drawback with Lanterna is its rather verbose and involved API. For “serious” apps, maybe that’s OK, but for simple positioning of the cursor, it can get a bit too much. The follow renders a text label with a border:
Lanterna is available through Maven Central, so the following gradle.build lines will do.
Here’s the full test example, including a main-method to easily run it stand-alone from the terminal.
The unofficial com.sun packages which is still part of the main JDK, include a few convenience classes for running a HTTP server. However, since these are not part of the official API, there’s typically a warning in the IDEs not to use them. The annotation @SuppressWarnings(“restriction”) can also be used to ignore this warning. Besides that, they will work fine.
Here’s what it takes to start the server on a given port, and serve static files from a specific directory tree:
The second argument to the create() method, which is currently 0, is the number of queued incoming queries. Setting a higher number here will allow the server to handle more parallel incoming requests, however potentially at the expense of overall throughput.
The start() is non-blocking, so a typically server application would have to go into an internal loop.
As for the handler, it listens to requests where the path of the URI starts with the specified string, in this case “/static”. It can then use the request URI to map this to hard-coded files, or files with the same name as in this example:
Finally, to test the running server, here’s two unit tests. The first downloads a file which exists, and prints its lines. The second requests a file which does not exist. Notice that the 404 return code passed from sendResponseHeaders() in the handler, is enough to make the openStream() call throw a FileNotFoundException on the client side.
Here’s the main class:
And here’s the Handler for static files:
The bitcoinj library is easy to use for Bitcoin wallet and transaction functions for both native Java and Android applications. Although there are certain features missing, it seems mature enough to be included in a Bitcoin walled app or service.
Sometimes the source code leaves a bit to be desired in structure and readability: anonymous inner classes and other deep nesting blocks sometimes makes it difficult to follow; inheritance is often used where composition would have been be better; the Collections classes could have been used over arrays in many places. All of this might come back to haunt the developers later, for now they seem to be plowing on.
At least the basics are straight forward. The following code will read a test walled from disk, or create a new one if it does not already exist. The TestNet3 block chain and network is used. Since the bitcoinj library relies heavily on the Google Guava (com.google.common) classes, there are frequent artifacts of the threading and callback handling showing up. In this example, we want the code to block and wait, therefore the extra await-functions are required.
The nice thing about the test block chain is that it is a real public live block chain, with miners and a block chain explorer, but with no value in the coins. In fact, you can get free coins to test with from faucet.xeno-genesis.com or tpfaucet.appspot.com. (The latter has been timing out over the last days).
To get some free test coins, run the following code, wait for the prompt which shows the next receiving address, and head over to faucet.xeno-genesis.com to ask them to send some money there. It should show up as received within a few seconds. Your wallet now contains some coins.
Since the test network is a real network with real miners, it’s good etiquette to return your test coins to the pool for others to use once you’re done with them. The following code takes care of that, returning them to the TP Faucet default return address “n2eMqTT929pb1RDNuqEnxdaLau1rxy3efi”. You can return all your coins, or just a fraction if you want to experiment more. This will also wait a few seconds for the callback confirmation.
Finally, it’s worth noting that bitcoinj is a “live” library, in development and with the latest update available through Gradle. To make this work, there’s a few settings and dependencies to take care of. The logging framework used by bitcoinj is SL4J, and an actual implementation library (e.g. “sl4j-simple”) is also need. It can be downloaded, or included as a Gradle build dependency as seen below.
Then, your source code directory structure might not match the default Gradle “main”, “test” structure. My current structure keeps all source code under the directory “src”, so I have specified that.
Gradle integrates OK with Eclipse, but be careful with the “refresh” option. It tends to insist on changing the classpath setting of the project, so the packages disappear. It’s a good idea to keep the .classpath setting file under version control.
The following listing shows all the tests. It demonstrates similar functionality as seen in the ForwardingService class in the main bitcoinj getting started guide. Hopefully, the code is a bit easier to read and run this way.
Working with MIDI in Java is easy. The standard API have classes covering MIDI file I/O, device I/O, sequencing and sound synthesis. This comprhensive tutorial covers most aspects. It also helps to know something about the MIDI format. Juan Bello’s slide deck is an excellent introduction. midi.org is also a good source, including their full message reference table.
To play a single C note through the default included “Gervill” soft synthezier, the following snippet will do. As commented in the code, there’s come discrepancy on pitch or octave labelling, but the 60th note is still on the save octave as A at 440 Hz.
This gives a brief overview of the MIDI devices on the system, both software based and hardware devices. Depending on drivers and OS, the various devices might show up under different names and types.
Finally, the following methods demonstream MIDI file I/O.
The full test case can be seen below. There’s a few more helper classes and details in the same package here. Then there’s some special implementation and details for the Roland TB-03 and TR-8 devices here.
When writing reusable code, we often want it to be as general and flexible as possible. So layers of abstractions are added; generic types; abstract class hierarchies; and let’s not forget the Factory pattern. Joel Spolsky had a famous rant about the factory factory factory pattern, and it can get ugly in the real world as well.
One of the reasons for the often clunky factory class is that it has not been possible to pass methods and constructors. Method references in Java 8 changes that, even for constructors. They can be passed for any class or array, e.g. String::new, String::new. Combined with generics, the type of the newly created object can also be specified.
In the example class below, the constructor happen to take two arguments, first a String and then an int. Therefore, the BiFunction function method fits, however, it would probably be more appropriate to define a more specific functional interface, which would also make the code more readable. The return value is of the type T, which should then be the same type as the generated object. The use is demonstrated in the test method below.
The restriction with this setup is of course that the number of arguments to the constructor is fixed. We could write fixes around that as well, but that would require the general class to know something about the classes it is instantiating, which defetes the purpose. There’s always the old Factory class, though.
Now, it can be argued that this still constitutes a factory pattern, even if an external factory class is not used. The methods of Collectors highlights this, e.g.:
The full code listing of the example: