Sunday, 3 August 2014

Teaching ClassLoaders, Reflection and NIO2

As a leader of the London Java Community and as a developer in several firms I have noticed one trait across Java developers. We are spoilt! We have nice APIs, abstractions, IDEs and build systems which means that often we don't appreciate some of the lower levels of what is actually happening within the Java ecosystem. Most people can get by without this knowledge, however in the situation that there is an outage or a problem and we simply have no idea why something has started to fail, it's not the best time to start reading up on advanced Java topics. 

Sometimes we think we have enough information, because in a topic like class loading it might be enough to say "It's how Java loads classes". In my opinion this lack of knowledge is fairly dangerous, so in the Advanced Java course I recently taught with Mallon Associates I wanted to build an example that would capture the attention of the delegates. It covers four topics that we may be forgiven for forgetting Java is capable of.

The topics that I was aiming to teach amongst others were:

  • NIO2 file watch service API
  • Classloaders
  • Compilation of code at runtime
  • Reflection
The example I came up with was the WatchingClassLoader. The WatchingClassLoader is a toy example which watches a folder for the arrival of a new file. When a file lands in the inbox it checks to see whether it is a .class file or .java file. If it is a .class file the class loader will load it, if it is a .java file it first compiles it and then loads the compiled version. The main thread also holds a loop waiting for input into the terminal, this allows the user to specify a class and see via reflection the methods of the now loaded class within system. As the class loader is hierarchical if it is asked for a class it doesn't know about it defers to the parent. It works with simple classes that are in the default package (i.e. no package) - though it could be extended to be smarter. 


The WatchService is a neat API that allows you to register a Path to monitor for a particular file based event to occur. You can specify the type of event that you are interested in using the StandardWatchEventKinds class. Using the .take() method on the watcher blocks until that event occurs. In the final example this runs on a separate thread.

Class Loader

It is possible to write your own class loader in Java. By default the class loader in Java looks on the classpath specified when running the java command - but more often this is provided from your build system specifying the path to all the library jars your running application will require. Class loading is lazy, so a class is only loaded into the system when it is first required - this is why your code can sometimes run for years and then when it goes down a spurious code path you suddenly find yourself with a ClassNotFoundException.

Writing a custom class loader gives you the opportunity to change this default behaviour. Examples of class loaders might be that when a class is requested it is loaded from a remote network location or your class files are stored in an encrypted format and the class loader is responsible for decrypting the file. In this case I just want to load the class in from where I am watching for new files to be added. To create your own class loader you need to extend the ClassLoader abstract class.

At minimum you need to be looking to override the loadClass method and basically get a byte array of the contents of class file to load. From here you need to call resolveClass and defineClass to correctly load the class into the virtual machine. My system works by checking if the class aiming to be loaded is visible in the directory I am watching - otherwise it defers to the parent class loader to load the class. Class loaders are hierarchical in nature so this is an acceptable way of delegating upwards the loading of the class. The situation that this occurs in my mini example is when the user types something like java.lang.String into the console. A cutdown example of how to do this is below.

Compiling on the Fly

One feature that surprised many of the people who took my course was that it is possible to invoke the Java Compiler from within an executing Java process. ToolProvider allows us to get hold of the JavaCompiler and from there we can pass in a CompilationTask which contains the .java file we are looking to compile. This is best shown by example: 


The final stage is reflection, which allows the user to look at the .java file or .class file they have added to the running system. In a demo this is the closest we get to the wow moment :). Reflection is the Java mechanism that allows inspection of a class at runtime. Most people are using reflection, even if they don't know they are doing so, as it is heavily used in frameworks such as Spring. In the example I just print out the methods as this is enough to prove the point, though on the actual course we use reflection for many other purposes including to invoke methods and generally prove what is possible. The simple snippet for allowing reflection on our classes via a custom class loader looks like this:

The Full Project

You can find the full toy example that you can run and play around with on GitHub. As this is a teaching example, there are plenty of places that this could be taken further - so I welcome any pull requests or tidy up to the code. It was actually something originally produced via live coding in front of the class, which has been a fairly tricky learning curve to get used to. That said now I wouldn't run a course without live coding, as it's where you get the most questions and although it's massively stressful it makes for a great session.

Saturday, 12 July 2014

Update and New Career

I thought it was about time I dropped an update onto the blog about why I've been quiet recently and what has been going on.

Early this year I decided to change career paths slightly and become a technical trainer. The highlights of my career as a developer have been to help others improve and learn from others in an informal supportive environment. I am now lucky enough to be able to do this full time designing new and fun ways of helping others learn Java, Python, C++ and .NET technologies. I work for my own company James Gough Ltd, with one primary client Mallon Associates. So far I've run ten training courses and had people booking onto my future courses, it's been challenging but one of the best things I've done. I have a fairly packed end to 2014 as I gear up for our 15 week training program.

As well as working on private training I have also teamed up with Richard Warburton and Raoul-Gabriel Urma. I am excited to announce will be offering public Java 8 courses with a focus on Lambdas and preparing Java developers for the changes that are right on our doorstep. Java as we know it is going to change, for years some developers have been dragging their feet with Java 1.4 style syntax and missing some key changes. An appreciation for the Object Oriented/Functional relationship is now required to both be able to design and build code that is succinct and solves the business problem in the best way.

This year is very busy with it being my first year in business and getting married so sadly I've had to put conferences on hold for a year, but I'm hoping that early next year I can attend jfokus.

Friday, 24 January 2014

Live Coding on Java 8 Date Time API

Yesterday I recorded a live coding session on java.time. Below is all the information so you can code along if you like:

The Video

The Source Code

All the source code is freely available on github you can get this running the following command:

The (brief) details for setting up the IDE can be found in the readme.

The Slides

Monday, 20 January 2014

Java 8 Date Time: Happy Martin Luther King Day

Today is Martin Luther King Day in the US. I had totally forgotten about this holiday until I saw a few tweets this morning about it. The interesting thing about this holiday is that rather than being on a specific day, it is on the third Monday of January.

As I'm currently preparing some JSR-310 material, I thought it would be interesting to share (and discuss) TemporalQuery - a new interface in Java 8 that makes it possible to write custom queries against the new DateTime classes. There is often a use case in business that an event happens that is unique to a particular domain, an example would be Futures market rolls of contracts or time periods of validity.

TemporalQuery allows us to capture the logic to perform an operation against a temporal class and return an object representing what we were querying. The interface is generic so we can choose what we return. This is a really neat design feature in Java 8 allowing us to externalise logic outside of the core API, but encapsulate the functionality as a data operation.

TemporalQuery is also a FunctionalInterface so it can be used as a lambda on a stream of temporals. In the example below we just consider one date. To find the next Martin Luther King day we can take the current date and perform the following: NextMartinLutherKingDayQuery());

The NextMartinLutherKingDayQuery is our implementation of a TemporalQuery:

Here are some things to note about the implementation:

  • In the private method I take a date at the beginning of January of the year passed. Using a TemporalAdjuster I manipulate that date to cycle it forward to where I want to be. TemporalAdjusters add more power than just plusDays or minusMonths to allow us to work on more expressive situations with date and time. 
    • The TemporalAdjusters class has some really nice methods as used here, or you can write your own TemporalAdjuster if it doesn't exist. 
    • NextOrSame is useful for this type of operation where we don't know what day we are on, but if it's the day we are querying we don't want to advance the day. We use it to iterate to the third Monday of the month of January in the year in question.
  • I then use a the Period class to find out from the date passed whether Martin Luther King Day has passed, or it is today. If it has passed I return next year's date, otherwise returning this year's date. Period is a nice abstraction so I don't have to start subtracting millisecond values etc - yes we've all seen it. 
If you're interested in playing around with this I suggest downloading the latest Java 8 beta and having a go. 

Happy Martin Luther King Day!

Saturday, 18 January 2014

User Group Visit to San Francisco

Why am I in San Francisco? 

Is it:
  • A micro brewery pub crawl?
  • The fact it's almost 20°C tomorrow?
  • To attend a Brazilian BBQ in January? 
These things are certainly going to be a nice bonus of being here, however that's not the main reason. I'm here representing the London Java Community at Oracle's big annual user group leaders convention. The purpose of the conference is to bring leaders of a variety of different Oracle user groups together to discuss best practices and drive improvement of communities and involvement for all those concerned. Particularly this year the topics I am interested in and will be participating in are:
  • Using social media - I'm particularly interested in discussing handling negative discussion and feedback via this extremely public forum.
  • Turning members into leaders and growing speaker talent. This has been an active stream in the LJC and Trisha Gee and team have been doing a fantastic job of running workshop and helping people gain the courage to make that first talk. 
  • Gamifiying your user group experience and improving user group content. I'm mainly interested in this because I get motivated by badges and awards for certain things. I think it could put a fun spin on how we run certain events in the LJC.
I plan to blog about interesting conversations and what comes out of these themes over the course of the week. The summit will only be the beginning of the discussions and I look forward to seeing more about what our members think to some of the ideas and which may work for us. 

When discussing any kind of conference and benefits of conferences I always say that it doesn't have to be a specific talk that motivates you but a catalyst to change. All the major turning points in my career have been down to ideas gained at conferences. It will be interesting to be involved in a non technical conference across a range of different products, experiences and backgrounds. 

I have very kindly had partial sponsorship from the LJC and the JCP Program Office to be here this week and I can't thank them both enough for the opportunity. 

I am now a JCP Executive Committee Representative

Why are the JCP helping to sponsor my visit here? After 3 years of working on JSRs with the London Java Community with the LJC JCP committee I have now stepped up my involvement and represent the LJC directly with other JCP executive committee members. I am attending my first face-to-face meeting at the end of this week. I'm really thrilled to be working with so many fantastic organisations and individuals to help maintain and form standards within Java. As the meeting has official minutes it's unlikely I will post anything further.