Page History

...

...

In Spring term, CloudStack uses both singleton components and prototype components, Spring component is nothing else more than a Java POJO object, you can either declare it with @Component annotation or in <bean> declaration in applicationContext.xml or componentContext.xml. CloudStack uses a lot of singleton components, for example, managers, DAOs, following is an example of such components

@Local(value=ClusterDao.class)

public class ClusterDaoImpl extends GenericDaoBase<ClusterVO, Long> implements ClusterDao {

    protected final SearchBuilder<ClusterVO> PodSearch;

    protected final SearchBuilder<ClusterVO> HyTypeWithoutGuidSearch;

    protected final SearchBuilder<ClusterVO> AvailHyperSearch;

    protected final SearchBuilder<ClusterVO> ZoneSearch;

    protected final SearchBuilder<ClusterVO> ZoneHyTypeSearch;

    private static final String GET_POD_CLUSTER_MAP_PREFIX = "SELECT pod_id, id FROM cloud.cluster WHERE cluster.id IN( ";

    private static final String GET_POD_CLUSTER_MAP_SUFFIX = " )";

    @Inject
    protected HostPodDao 

_hostPodDao;

    public ClusterDaoImpl()

 {

    }

    // more 

code ...

}

 @Component

For most of low-level built-in components like DAOs, in Javelin practice, we use @Component annotation, since it saves us a lot of typing and can take advantage of Eclipse IDE's powerful refactoring and type-infer feature to help us coding. However, if you are developing high level components and you want to give flexibility for customer to choose to compose a particular CloudStack deployment, declare in componentContext.xml would be a better choice.

...

Following is an example of such hacking way, inside BaseCmd class, developer tried to resolve all the references for every used components into static variables at  BaseCmd. It also means that, when you add a new Command class to the system, you will have to remember to do something about your new service at BaseCmd classat BaseCmd class

 public abstract class BaseCmd {
    static public ConfigurationService _configService;
    static public AccountService _accountService;
    static public UserVmService _userVmService;
    static public ManagementService _mgr;
    // more code ...
}

With Spring, developer can now always use @Inject annotation to declare such reference. One of the top two reasons for us to try Spring experiments in Javelin, since with more complete dependency injection, it gives us a cleaner component coding practice. (the other top reason is that we have broader integration supports from many other third-party vendors, i.e., JUnit)

...

It is a good practice to separate component construction and initialization, always try to make your component be self-dependable in initialization, self-dependable initialization means the component is able to initialize itself with the very minimal dependency to other component's initialization status. Use @PostConstruct to mark such initialization method for Spring to automatically call for you. Following is an example.

 @Component

public class ConfigurationDaoImpl extends GenericDaoBase<ConfigurationVO, String> implements ConfigurationDao {

	@PostConstruct 	void initComponent() 

{		// more code...	} 
}

4.4 Be aware of Spring proxy mode

...

For regular business logic component developers, there are very few chances that you should be aware of, however, if you do you encounter some problems and suspect that you have fallen into such traps, checkout ApiServer/ApiDispatcher for a reference solution.

...

As a business logic component developer, you should avoid using any of the functions provided by ComponentContext, this can make the business component neutral to component container, since we now switched to standard javax @Inject auto-wiring annotation, it is always even possible for your component to run in containers that are not other than CloudStack

5. Component lifecyle

...