Riena/Getting Started with injecting services and extensions
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Getting Started with Injecting Services and Extensions
The Idea of Injecting Services and Extensions in Riena
Services and extensions are the main building blocks of Eclipse RCP applications. There is a long list of discussions and debates about when to use services and when to use extensions. So, we will not add another discussion here. We think that both are useful and have their permission.
One problem with both of them is that using them requires a lot of code that respects their life cycle and is quite error prone. Most of this code is embedded within the code that needs other services or extensions. This approach does not clearly separate concerns and it makes unit testing more difficult.
A common solution for this problem is the use of dependency injection frameworks such as Spring (http://springframework.org/) or HiveMind (http://hivemind.apache.org/).
Within Riena we are using a simple to use approach to inject services and extensions into objects (pojos). Our approach is code based, i.e. no xml configuration files. Furthermore we are using fluent interfaces (http://en.wikipedia.org/wiki/Fluent_interface) and conventions to simplify the injection definitions.
Service injecting examples:
Inject.service("ServiceClazz1").into(target).andStart(context); Inject.service("ServiceClazz2").useFilter(filter).into(target).bind("register").unbind("unregister").andStart(context); Inject.service("ServiceClazz3").useRanking().into(target).bind("register").unbind("unregister").andStart(context);
where ´target´ is the pojo that gets a service injected. When services come and go they will be injected into or removed from the pojos. The method names for injecting/removing can be specified with the bind() and unbind() methods. If they are not defined default method names are assumed.
Extensions get injected into pojos as instances of an interface. The interfaces need only to declare the getters. Dynamic proxies will be created for the interfaces which map the data from the extensions. A few simple rules are used for the mapping from extensions to the interfaces (lightweight xml/java mapping). Extension properties. i.e. attributes, sub-elements and element value can then accessed by getters in the interface definition. It is only necessary to define the interfaces for the mapping.
The extension injector does not evaluate the extension schema, so it can only trust that the extension and the interface match with regards to a few basic rules:
- one interface maps to one extension element type
- the interface has to be annotated with
- interface methods can contain getters prefixed with:
- Such prefixed methods enforce a default mapping to attribute or element names, i.e. the remainder of the methods name is interpreted as the attribute or element name. A simple name mangling is performed, e.g for the method getDatabaseURL the mapping looks for the attribute name databaseURL.
To enforce another name mapping a method can be annotated with @MapName("name"). The name specifies the name of the element or attribute. The extension element´s value can be retrieved by annotating the method with @MapValue(). The return type must be String. The method names of such annotated methods can be arbitrary
- The return type of a method indicates how the value of an attribute will be converted. If the return type is
- a primitive type or
java.lang.Stringthan the mapping converts the attribute's value to the corresponding type.
- an interface or an array of interfaces annotated with
@ExtensionInterfacethan the mapping tries to resolve to a nested element or to nested elements.
org.osgi.framework.Bundlethan the method returns the contributing bundle.
java.lang.Classthan the attribute is interpreted as a class name and a class instance will be returned.
- and finally if none of the above matches the mapping tries to create an new instance of the attribute´s value (interpreted as class name) each time it is called. If the extension attribute is not defined
nullwill be returned.
- a primitive type or
Extension injecting examples:
Inject.extension("extpoint1").into(target).andStart(context); Inject.extension("extpoint2").useType(interface).into(target).bind("configure").doNotReplaceSymbols().andStart(context); Inject.extension("extpoint3").expectExactly(1).into(target).andStart(context);