Play Framework Cookbook
In order to get to know more modules, you should not hesitate to take a closer look at the steadily increasing amount of modules available at the Play framework modules page at http://www.playframework.org/modules.
When beginning to understand modules, you should not start with modules implementing its persistence layer, as they are often the more complex ones.
In order to clear up some confusion, you should be aware of the definition of two terms throughout the article, as these two words with an almost identical meaning are used most of the time. The first is word is module and the second is plugin. Module means the little application which serves your main application, where as plugin represents a piece of Java code, which connects to the mechanism of plugins inside Play.
Creating and using your own module
Before you can implement your own functionality in a module, you should know how to create and build a module. This recipe takes a look at the module's structure and should give you a good start.
The source code of the example is available at examples/chapter5/module-intro.
How to do it...
It is pretty easy to create a new module. Go into any directory and enter the following:
play new-module firstmodule
This creates a directory called firstmodule and copies a set of predefined files into it. By copying these files, you can create a package and create this module ready to use for other Play applications. Now, you can run play build-module and your module is built. The build step implies compiling your Java code, creating a JAR file from it, and packing a complete ZIP archive of all data in the module, which includes Java libraries, documentation, and all configuration files. This archive can be found in the dist/ directory of the module after building it. You can just press Return on the command line when you are asked for the required Play framework version for this module. Now it is simple to include the created module in any Play framework application. Just put this in the in the conf/dependencies.yml file of your application. Do not put this in your module!
require:
- play
- customModules -> firstmodule
repositories:
- playCustomModules:
type: local
artifact: "/absolute/path/to/firstmodule/"
contains:
- customModules -> *
The next step is to run play deps. This should show you the inclusion of your module. You can check whether the modules/ directory of your application now includes a file modules/firstmodule, whose content is the absolute path of your module directory. In this example it would be /path/to/firstmodule. To check whether you are able to use your module now, you can enter the following:
play firstmodule:hello
This should return Hello in the last line. In case you are wondering where this is coming from, it is part of the commands.py file in your module, which was automatically created when you created the module via play new-module. Alternatively, you just start your Play application and check for an output such as the following during application startup:
INFO ~ Module firstmodule is available (/path/to/firstmodule)
The next step is to fill the currently non-functional module with a real Java plugin, so create src/play/modules/firstmodule/MyPlugin.java:
public class MyPlugin extends PlayPlugin {
public void onApplicationStart() {
Logger.info("Yeeha, firstmodule started");
}
}
You also need to create the file src/play.plugins:
1000:play.modules.firstmodule.MyPlugin
Now you need to compile the module and create a JAR from it. Build the module as shown in the preceding code by entering play build-module. After this step, there will be a lib/play- firstmodule.jar file available, which will be loaded automatically when you include the module in your real application configuration file. Furthermore, when starting your application now, you will see the following entry in the application log file. If you are running in development mode, do not forget to issue a first request to make sure all parts of the application are loaded:
INFO ~ Yeeha, firstmodule started
How it works...
After getting the most basic module to work, it is time go get to know the structure of a module. The filesystem layout looks like this, after the module has been created:
app/controllers/firstmodule
app/models/firstmodule
app/views/firstmodule
app/views/tags/firstmodule
build.xml
commands.py
conf/messages
conf/routes
lib
src/play/modules/firstmodule/MyPlugin.java
src/play.plugins
As you can see a module basically resembles a normal Play application. There are directories for models, views, tags, and controllers, as well as a configuration directory, which can include translations or routes. Note that there should never be an application.conf file in a module.
There are two more files in the root directory of the module. The build.xml file is an ant file. This helps to compile the module source and creates a JAR file out of the compiled classes, which is put into the lib/ directory and named after the module. The commands.py file is a Python file, which allows you to add special command line directives, such as the play firstmodule:hello command that we just saw when executing the Play command line tool.
The lib/ directory should also be used for additional JARs, as all JAR files in this directory are automatically added to classpath when the module is loaded.
Now the only missing piece is the src/ directory. It includes the source of your module, most likely the logic and the plugin source. Furthermore, it features a very important file called play.plugins. After creating the module, the file is empty. When writing Java code in the src/ directory, it should have one line consisting of two entries. One entry features the class to load as a plugin; where as the other entry resembles a priority. This priority defines the order in which to load all modules of an application. The lower the priority, the earlier the module gets loaded.
If you take a closer look at the PlayPlugin class, which MyPlugin inherits from, you will see a lot of methods that you can override. Here is a list of some of them accompanying a short description:
onLoad(): This gets executed directly after the plugin has been loaded. However, this does not mean that the whole application is ready!
bind(): There are two bind() methods with different parameters. These methods allow a plugin to create a real object out of arbitrary HTTP request parameters or even the body of a request. If you return anything different other than null in this method, the returned value is used as a parameter for controller whenever any controller is executed.
getStatus(), getJsonStatus(): Allows you to return an arbitrary string representing a status of the plugin or statistics about its usage. You should always implement this for production ready plugins in order to simplify monitoring.
enhance(): Performs bytecode enhancement.
rawInvocation(): This can be used to intercept any incoming request and change the logic of it. This is already used in the CorePlugin to intercept the @kill and @status URLs. This is also used in the DocViewerPlugin to provide all the existing documentation, when being in test mode.
serveStatic(): Allows for programmatically intercepting the serving of static resources. A common example can be found in the SASS module, where the access to the .sass file is intercepted and it is precomplied.
loadTemplate(): This method can be used to inject arbitrary templates into the template loader. For example, it could be used to load templates from a database instead of the filesystem.
detectChange(): This is only active in development mode. If you throw an exception in this method, the application will be reloaded.
onApplicationStart(): This is executed on application start and if in development mode, on every reload of your application. You should initiate stateful things here, such as connections to databases or expensive object creations. Be aware, that you have to care of thread safe objects and method invocations for yourself. For an example you could check the DBPlugin, which initializes the database connection and its connection pool. Another example is the JPAPlugin, which initializes the persistence manager or the JobPlugin, which uses this to start jobs on application start.
onApplicationReady(): This method is executed after all plugins are loaded, all classes are precompiled, and every initialization is finished. The application is now ready to serve requests.
afterApplicationStart(): This is currently almost similar to onApplicationReady().
onApplicationStop(): This method is executed during a graceful shutdown. This should be used to free resources, which were opened during the starting of the plugin. A standard example is to close network connections to database, remove stale file system entries, or clear up caches.
onInvocationException(): This method is executed when an exception, which is not caught is thrown during controller invocation. The ValidationPlugin uses this method to inject an error cookie into the current request.
invocationFinally(): This method is executed after a controller invocation, regardless of whether an exception was thrown or not. This should be used to close request specific data, such as a connection, which is only active during request processing.
beforeActionInvocation(): This code is executed before controller invocation. Useful for validation, where it is used by Play as well. You could also possibly put additional objects into the render arguments here. Several plugins also set up some variables inside thread locals to make sure they are thread safe.
onActionInvocationResult(): This method is executed when the controller action throws a result. It allows inspecting or changing the result afterwards. You can also change headers of a response at this point, as no data has been sent to the client yet.
onInvocationSuccess(): This method is executed upon successful execution of a complete controller method.
onRoutesLoaded(): This is executed when routes are loaded from the routes files. If you want to add some routes programmatically, do it in this method.
onEvent(): This is a poor man's listener for events, which can be sent using the postEvent() method.
onClassesChange(): This is only relevant in testing or development mode. The argument of this method is a list of freshly changed classes, after a recompilation. This allows the plugin to detect whether certain resources need to be refreshed or restarted. If your application is a complete shared-nothing architecture, you should not have any problems. Test first, before implementing this method.
addTemplateExtensions(): This method allows you to add further TemplateExtension classes, which do not inherit from JavaExtensions, as these are added automatically. At the time of this writing, neither a plugin nor anything in the core Play framework made use of this, with the exception of the Scala module.
compileAll(): If the standard compiler inside Play is not sufficient to compile application classes, you can override this method. This is currently only done inside the Scala plugin and should not be necessary in regular applications.
routeRequest(): This method can be used to redirect requests programmatically. You could possibly redirect any URL which has a certain prefix or treat POST requests differently. You have to render some result if you decide to override this method.
modelFactory(): This method allows for returning a factory object to create different model classes. This is needed primarily inside of the different persistence layers. It was introduced in play 1.1 and is currently only used by the JPA plugin and by the Morphia plugin. The model factory returned here implements a basic and generic interface for getting data, which is meant to be independent from the persistence layer. It is also used to provide a more generic fixtures support.
afterFixtureLoad(): This method is executed after a Fixtures.load() method has been executed. It could possibly be used to free or check some resources after adding batch data via fixtures.
Cleaning up after creating your module
When creating a module via Play new-module, you should remove any unnecessary cruft from your new module, as most often, not all of this is needed. Remove all unneeded directories or files, to make understanding the module as easy as possible.
Supporting Eclipse IDE
As play eclipsify does not work currently for modules, you need to set it up manually. A trick to get around this is to create and eclipsify a normal Play application, and then configure the build path and use "Link source" to add the src/ directory of the plugin.
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