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Difference between revisions of "Eclipse4/RCP/Dependency Injection"
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** JSR 330 annotations: @Inject, @PostConstruct, @PreDestroy, @Named | ** JSR 330 annotations: @Inject, @PostConstruct, @PreDestroy, @Named | ||
** E4AP-specific annotations: @Preference, @Event, @UIEvent | ** E4AP-specific annotations: @Preference, @Event, @UIEvent | ||
| + | * Order of injection classes: Constructor, @Injected variables, @Injected methods, @PostConstruct | ||
| + | ** no guarantees of ordering within injection classes; methods requiring values from injected variables should either be called from @PostConstruct or be injected with both variables | ||
* List of available services | * List of available services | ||
** E4AP services | ** E4AP services | ||
Revision as of 14:34, 7 April 2011
One often-heard complaint of using the Eclipse 3.x Platform was that finding and obtaining services required navigating a deep chain of dependencies (e.g., obtaining an IStatusLineManager). Accessing the services required using singletons, which is problematic for app servers like RAP/Riena. Dependency Injection (DI) is one approach to circumvent these problems: rather than require client code to know how to access a service, the client instead describes the service required, and the platform is responsible for configuring the object with an appropriate service.
- DI is not new. Pointers to other projects and books (e.g., Dhanji Prasanna's Dependency Injection)
- Different approaches: separate configuration, or annotating POJOs with injection points.
The Eclipse 4 Application Platform provides a [| JSR 330]-compatible dependency injection (DI) framework, similar to [Spring] or [Guice]. Instead of PlatformUI.getWorkbench().getHelpSystem(), E4AP plugins can have the HelpSystem injected into an object directly.
Using the E4AP DI Framework
- E4AP's injector uses annotations
- JSR 330 annotations: @Inject, @PostConstruct, @PreDestroy, @Named
- E4AP-specific annotations: @Preference, @Event, @UIEvent
- Order of injection classes: Constructor, @Injected variables, @Injected methods, @PostConstruct
- no guarantees of ordering within injection classes; methods requiring values from injected variables should either be called from @PostConstruct or be injected with both variables
- List of available services
- E4AP services
- OSGi services
- plugins: org.eclipse.e4.core.di, org.eclipse.e4.core.di.extensions, and org.eclipse.e4.ui.di
- Accessing the IEclipseContext
- Adding, setting, or modifying variables
- Why is modify different from set
- Adding, setting, or modifying variables
Advantages / Disadvantages
DI provides a number of advantages:
- Clients are able to write POJOs and list the services they need. The DI framework provides
- Useful for testing: the assumptions are placed in the DI container rather than in the client code
DI has some disadvantages too:
- Concerns about discoverability of services - cannot use code completion to find out what is available.