Author: limpbizkit
Date: Thu Jan 1 14:50:02 2009
New Revision: 772
Added:
wiki/ModulesShouldBeFastAndSideEffectFree.wiki
Log:
Created wiki page through web user interface.
Added: wiki/ModulesShouldBeFastAndSideEffectFree.wiki
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+=Modules should be fast and side-effect free=
+Rather than using an external XML file for configuration, Guice modules
are written using regular Java code. Java is familiar, works with your IDE,
and survives automated refactoring.
+
+But the full power of the Java language comes at a cost: it's easy to do
_too much_ in a module. It's tempting to connect to a database connection
or to start an HTTP server in your Guice module. Don't do this! Doing
heavy-lifting in a module poses problems:
+ * *Modules start up, but they don't shut down.* Should you open a
database connection in your module, you won't have any hook to close it.
+ * *Modules should be tested.* If a module opens a database as a course
of execution, it becomes difficult to write unit tests for it.
+ * *Modules can be overridden.* Guice modules support
[http://google-guice.googlecode.com/svn/trunk/latest-javadoc/com/google/inject/util/Modules.html#override(com.google.inject.Module...)
overrides], allowing a production service to be substituted with a
lightweight or test one. When the production service is created as a part
of module execution, such overrides are ineffective.
+
+Rather than doing work in the module itself, define an interface that can
do the work at the proper level of abstraction. In our applications we use
a simple interface:
+{{{
+interface Service {
+ /**
+ * Starts the service. This method blocks until the service has
completely started.
+ */
+ void start() throws Exception;
+
+ /**
+ * Stops the service. This method blocks until the service has
completely shut down.
+ */
+ void stop();
+}
+}}}
+After creating the Injector, we finish bootstrapping our application by
starting the services.
+{{{
+ public static void main(String args) throws Exception {
+ Injector injector = Guice.createInjector(
+ new DatabaseModule(),
+ new WebserverModule(),
+ ...
+ );
+
+ Service databaseConnectionPool = injector.getInstance(
+ Key.get(Service.class, DatabaseService.class));
+ databaseConnectionPool.start();
+ addShutdownHook(databaseConnectionPool);
+
+ Service webserver = injector.getInstance(
+ Key.get(Service.class, WebserverService.class));
+ webserver.start();
+ addShutdownHook(webserver);
+ }
+}}}
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