czwartek, 25 listopada 2010

Don't repeat the Spring beans! (in Apache Camel and OSGi environment)

The 'Don't repeat the DAO!' article is a fundamental one for all Spring/ORM/Java world. The generic DAO pattern works well and is widely used in both commercial projects and open-source implementations.

But what if you only need one class for all of your DAOs? In one of my projects I just needed to put all records from a database view to a file. I decided not to use an ORM, but just Spring's JdbcTemplate, configured by view name and column to use in ORDER BY section. So I started to write Spring XML wiring and realized.... that I'm repeating the Spring beans! Every declaration looked something like:
<bean id="customerDAO" class="">
     <property name="table" value="V_CUSTOMERS"/>
     <property name="orderColumn" value="CUST_ID"/>

  <bean id="contractDAO" class="">
     <property name="table" value="V_CONTRACTS"/>
     <property name="orderColumn" value="CONTRACT_ID"/>

There were about 15 twin beans in my Spring XML files! That looked really ugly...

Googling a bit introduced to me concept of writing own Spring's BeanFactoryPostProcessor, in which I could instantiate DAOs and register them in CamelContext. My domain classes already had some annotations - I used camel-bindy component to specify output file format. So the idea was to write another annotation and pass DAOs configuration in it:

@GenericDao(type = "customer", table = "V_CUSTOMERS", orderColumn = "CUST_ID")
public class Customer { ... }

@GenericDao(type = "contract", table = "V_CONTRACTS", orderColumn = "CONTRACT_ID")
public class Contract { ... }

Then, the BeanFactoryPostProcessor could look like this:
public class GenericDaoBeanFactoryPostProcessor implements BeanFactoryPostProcessor {

  public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    Collection<Class<?>> classes = findClassesWithAnnotation(GenericDao.class); 
    for (Class<?> c : classes) {
      GeneratedFile gfa = c.getAnnotation(GenericDao.class);
      String type = gfa.type();
      GenericDaoImpl dao = new GenericDaoImpl();
      beanFactory.registerSingleton(type + "Dao", dao);

So it seems to be rather easy. But what with findClassesWithAnnotation method? My first attempt was to use Camel's DefaultPackageScanClassResolver implementation, and it worked fine in my unit tests. But when I deployed the bundle within an OSGi container (Servicemix 4.0), no annotated classes were found at all... Looking inside DefaultPackageScanClassResolver implementation showed the reason:
// osgi bundles should be skipped
 if (url.toString().startsWith("bundle:") || urlPath.startsWith("bundle:")) {
   log.trace("It's a virtual osgi bundle, skipping");
Loading classes from bundle is an OSGi container-specific matter, and Camel correctly doesn't implement it (more precisely: there is an OsgiPackageScanClassResolver implementation, but it uses OSGi APIs).

The solution is to use CamelContext's getPackageScanClassResolver() method to get correct resolver. In unit tests it returns default one, in OSGi environment - container-aware implementation.
public class GeneratedFileBeanFactoryPostProcessor implements BeanFactoryPostProcessor {
  protected CamelContext camelContext;

  public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    camelContext.getPackageScanClassResolver().findAnnotated(GenericDao.class, defaultPackage);

And now it works fine and Spring XML wirings look much better :)
Finally my route also uses PackageScanClassResolver to configure all endpoints:

   .recipientList(constant(makeEndpointsFromTypes())  // makes 'direct:type' endpoint for all object types

  for (String type: types) {

PS. Sure, all of my classes could just subclass GenericDaoImpl, but I didn't want to mix database logic with the file structure logic. Using annotations feels a bit more like configuration, not implementation itself ;)

1 komentarz:

  1. is "GenericDao" a class or a custom annotation? Can you please share your GenericDao code.