candi-binding-tutorial
- 1. Java Injection (CanDI) Pattern Tutorial June 15, 2009 Contents 1 Overview 1 1.1 Tutorial Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Java Injection API . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Service Pattern 4 2.1 Using Services from PHP and JSP . . . . . . . . . . . . . . . . . 5 3 Resource XML Configuration Pattern 6 4 Startup Pattern 9 5 Plugin/Extension Pattern 9 1 Overview The four main CanDI patterns share a common goal: improve code with a declarative injection style. When Java classes cleanly describe their dependen- cies, their lifecycles, and their exports, they are self-documenting. You can read the classes and understand how they will behave, i.e. you don’t need to read the code side-by-side with XML configuration to understand the system’s behavior. The custom, typesafe binding annotations are key to CanDI’s self- documentation, because injection points clearly describe the resources or ser- vices they expect with adjective-oriented annotations. Because the annotations are true Java classes, they are documented in JavaDoc and verified by the com- piler. The small number of meaningful adjectives means they don’t impose a significant coding burden, and are well worth the small extra development time. CanDI Application Patterns PATTERN DESCRIPTION Service Pattern Organize the application as a collec- tion of services. 1
- 2. 1.1 Tutorial Architecture 1 OVERVIEW Resource Configuration Pattern Bind and resources with declara- tive annotations and configure with XML. Startup Pattern Use @Startup beans to initialize ap- plication state. Plugin/Extension Pattern Discover plugin/extension classes for a service. This tutorial describes four main CanDI design patterns: services, resources, startup and extensions. Services center an application’s design by encapsulating management and data. Resources are the configurable interface between the user and the application. Startup initializes the application. And extensions allow sophisticated applications to tailor their behavior to the user’s needs. 1.1 Tutorial Architecture @Current @ApplicationScoped MyServiceBean @Blue BlueResourceBean SetServlet GetServlet @Current MyService @Current MyService @Blue MyResource @Any Instance<MyResource> @Red MyResource @Red RedResourceBean @Startup MyStartupBean Since the purpose of the service pattern is encapsulating state and manag- ment for multiple clients, the tutorial shows a single service used by multiple servlets and by PHP and JSP scripts. Services are typically singletons in the application and use @Current to mark the binding. The resource pattern configures a driver class and properties in XML for an application resource. The resource tutorial uses MyResource as a general resource API, like DataSource or EntityManager , and application specific bindings @Red and @Blue . Because resource APIs are general, they need an application-specific description to document their purpose in the code. Binding annotations are simple, clear adjectives, and typically only a small number are needed. The driver classes like BlueResourceBean are typically selected and configured in an XML, like selecting and configuring a database. Startup initialization is needed by most applications, and can use the CanDI startup pattern to document the startup classes and avoid unnecessary XML. 2
- 3. 1.2 Java Injection API 1 OVERVIEW Because CanDI discovers beans through classpath scanning, you can create startup beans with just a @Startup annotation and a @PostConstruct method. A plugin or extension capability can improve the flexibility of many appli- cations, even if designed for internal use. The plugin pattern uses CanDI’s discovery process for the plugin capability without requiring a new infrastruc- ture. The tutorial reuses the MyResource API as a plugin API and grab all implementations using the CanDI Instance interface and the @Any annotation. 1.2 Java Injection API The most important CanDI classes are just three annotations: @Current , @BindingType and @ApplicationScoped , because many applications will pri- marily use the service and resource patterns. By using these three annotations effectively, you can improve the readability and maintainability of your appli- cation’s services and resources. Service and Resource Pattern CanDI classes ANNOTATION/CLASS DESCRIPTION @ApplicationScoped scope annotation marking the service as a singleton @BindingType descriptive application bindings are marked with this meta-annotation @Current Default binding for unique beans (service pattern). Applications which provide scripting access to services or resources will use the @Named annotation to provide a scripting name for the beans. Scripting Support CanDI classes ANNOTATION/CLASS DESCRIPTION Named Scriping and JSP/JSF EL access to CanDI beans (service pattern) The startup pattern uses two additional annotations, @Startup to mark the bean as needing creation on container start, and @PostConstruct marking a method to be initialized. Startup Pattern CanDI classes ANNOTATION/CLASS DESCRIPTION @Startup Starts a bean when the container starts. @PostConstruct Calls an initialization method the bean is created. 3
- 4. 2 SERVICE PATTERN A plugin or extension architecture uses two additional CanDI classes to easily find plugins discovered during CanDI’s classpath scanning. Instance<T> provides an iterator over all the discovered and configured beans, and @Any selects all beans independent of their @BindingType . Plugin/Extension Pattern CanDI classes ANNOTATION/CLASS DESCRIPTION Instance<T> Programmatic access to all imple- mentations of an interface. @All Selects all matching beans for an in- terface. 2 Service Pattern Because services are often unique in an application, the service interface is generally enough to uniquely identify the service. In CanDI, the @Current annotation injects a unique service to a client class. A declarative style applies to both the service declaration and the service use, by annotating the service scope as @ApplicationScoped , and annotating the client injection as @Current . By describing the function on the class itself, CanDI’s annotations improve the readability and maintainability of service classes. package example; import javax.enterprise.inject.Current; ... public class GetServlet extends HttpServlet { private @Current MyService _service; ... } Example: GetServlet.java Users of the service will access it through an interface like MyService . The implementation will be a concrete class like MyServiceBean . The interface API in CanDI is a plain Java interface with no CanDI-specific annotations or references. 4
- 5. 2.1 Using Services from PHP and JSP 2 SERVICE PATTERN package example; public interface MyService { public void setMessage(String message); public String getMessage(); } Example: MyService.java All the information relevant to the class deployment is on the class itself, be- cause the service implementation is discovered through CanDI’s classpath scan- ning. In other words, The service’s deployment is self-documenting. Since ser- vices are generally singletons, they will typically have the @ApplicationScoped annotation. Other annotations are optional and describe the service registra- tion or behavior. For example, the tutorial uses the @Named tag, because the test.jsp and test.php need a named reference. Scripting beans use the @Named annotation on a CanDI bean for integration with the JSP EL expression language and with PHP. Nonscripting beans do not declare a @Named annotation because CanDI uses the service type and binding annotations for matching. package example; import javax.enterprise.context.ApplicationScoped import javax.enterprise.inject.Named; @ApplicationScoped @Named("myService") public class MyServiceBean implements MyService { private String _message = "default"; public void setMessage(String message) { _message = message; } public String getMessage() { return _message; } } Example: MyServiceBean.java 2.1 Using Services from PHP and JSP CanDI is designed to integrate closely with scripting languages like PHP and JSP. The scripting languages locate a CanDI service or resource using a string, 5
- 6. 3 RESOURCE XML CONFIGURATION PATTERN because scripting lacks the strong typing needed for full dependency injection. As mentioned above, the name of a CanDI service is declared by the @Named anntation on the bean itself. The PHP or JSP code will use the name to obtain a reference to the bean. For PHP, the function call is java bean as follows: <?php $myService = java_bean("myService"); echo $myService->getMessage(); ?> Example: test.php While PHP has a function access to the CanDI service or resource, JSP and JSF grab the CanDI bean with using the JSP expression language. Any CanDI bean with a @Named annotation automatically becomes available to EL expressions as follows: message: ${myService.message} Example: test.jsp 3 Resource XML Configuration Pattern Resources like databases, and queues fit multiple roles in an application and need configuration and description beyond their generic DataSource and BlockingQueue APIs. While services are generally unique and can use the @Current binding, resources will generally create custom @BindingType anno- tations to identify and document the resource. CanDI encourages a small number of binding annotations used as adjectives to describe resources. A typical medium application like a mail list manager might use half a dozen custom binding adjectives, and may need fewer or more depending on the number of unique resources. Each database, queue, mail, and JPA EntityManager will generally have a unique name. If users need customiza- tion and configuration of internal resources, you may need additional binding types. If the application has a single database, it might only have one binding annotation, or might even use @Current . The purpose of the binding annotation is to self-document the resource in the client code. If the application uses @ShoppingCart database and a @ProductCatalog database, the client code will bind by their description. The code declares its dependencies in a readable way, and lets CanDI and the con- figuration provide the resource it needs. 6
- 7. 3 RESOURCE XML CONFIGURATION PATTERN The tutorial has @Red resource, configured in XML because the user might need to customize the configuration. The resource client, SetServlet , uses the adjective annotation in the field declaration as follows: public class SetServlet extends HttpServlet { private @Red MyResource _red; private @Blue MyResource _blue; ... } Example: SetServlet.java The XML is short and meaningful, because it’s only required for customiza- tion, not for wiring and binding. Databases and JMS queues will need to config- ure the database driver and add the binding adjective. Applications resources can also be configured in XML if exposing the configuration is useful to your users, but unique internal classes like most services will stay out of the XML. In our example, we let the users configure the data field of the resource and let them choose the implementation class. The XML configuration for a bean needs three pieces of data: the driver class, the descriptive binding annotation, and any customization data. Be- cause the driver is the most important, CanDI uses the class as the XML tag and uses the package as the XML namespace. While scanning the XML, the driver class is top and prominent, reflecting its importance. In the example, <example:BlueResourceBean> is the driver class. <example:BlueResourceBean xmlns:example="urn:java:example"> ... </example:BlueResourceBean> Example: BlueResourceBean instance configuration In CanDI, the binding annotation is also an XML tag, represented by its classname and package. In CanDI, classes and annotations get XML tags with camel-case names matching the classname, and XML for properties are lower case. The case distinguishes annotations from properties in the XML, improving XML readability. <example:Blue xmlns:example="urn:java:example"/> Example: @Blue annotation configuration Properties of a resource use the standard beans-style names, so <example:data> sets the bean’s setData property. CanDI converts the XML string value to the property’s actual value. In this case, the conversion is trivial, 7
- 8. 3 RESOURCE XML CONFIGURATION PATTERN but CanDI can convert to integers, doubles, enumerations, classes, URLs, etc. Beans have all the configuration capabilities as Resin beans in the resin.xml and resin-web.xml, because Resin uses CanDI for its own internal configuration. <web-app xmlns="http://caucho.com/ns/resin" xmlns:example="urn:java:example"> <example:BlueResourceBean> <example:Blue/> <example:data>blue resource</example:data> </example:BlueResourceBean> <example:RedResourceBean> <example:Red/> <example:data>red resource</example:data> </example:RedResourceBean> </web-app> Example: resin-web.xml Binding types should generally be descriptive adjectives, so it can describe the injection clearly. Anyone reading code should understand immediately which resource it’s using. The tutorial’s @Blue binding annotation itself is a normal Java annotation marked by a CanDI @BindingType annotation. Because of its importance and because there are only a small number of custom annota- tions, it’s important to spend time choosing a good descriptive name for the annotation. package example; import static java.lang.annotation.ElementType.*; import static java.lang.annotation.RetentionPolicy.*; import java.lang.annotation.*; import javax.enterprise.inject.BindingType; @BindingType @Documented @Target({TYPE, METHOD, FIELD, PARAMETER}) @Retention(RUNTIME) public @interface Blue { } Example: Blue.java The resource implementation itself is straightforward. When the resource is a singleton, it will need a @ApplicationScoped annotation, just like a service. By default, CanDI will inject a new instance of the bean at every injection point. 8
- 9. 5 PLUGIN/EXTENSION PATTERN package example; public class BlueResourceBean { private String _data; public void setData(String data) { _data = data; } } Example: BlueResourceBean.java 4 Startup Pattern The @Startup annotation marks a bean as initializing on server startup. Be- cause the startup bean is discovered through classpath scanning like the other beans, the initialization is controlled by the startup class itself. In other words, looking at the startup class is sufficient, because it doesn’t rely on XML for startup. The startup bean uses the @PostConstruct annotation on an initial- ization method to start initialization code. package example; import javax.annotation.PostConstruct; import javax.ejb.Startup; import javax.enterprise.inject.Current; @Startup public class MyStartupBean { private @Current MyService _service; @PostConstruct public void init() { _service.setMessage(this + ": initial value"); } } Example: MyStartupBean.java 5 Plugin/Extension Pattern A plugin or extension architecture can make an application more flexible and configurable. For example, a filtering system, or blueprints or custom ac- tions can add significant power to an application. The plugin pattern uses CanDI’s discovery system to find all implementations of the plugin interface. 9
- 10. 5 PLUGIN/EXTENSION PATTERN The Instance iterator together with the special @Any binding annotation gives all implementations of a resource. The CanDI Instance interface has two uses: return a unique instance pro- grammatically with the get() method, and list all instances for a plugin capa- bility. Since Instance implements the JDK’s Iterable interface, you can use it in a for loop. Each returned instance obeys the standard CanDI scoping rules, either returning the single value for @ApplicationScoped singletons, or creating a new instance for the default. The @Any annotation works with Instance to select all values. Because bindings default to the @Current binding type, we need to override the default to get all instances. package example; import javax.enterprise.inject.Any; import javax.enterprise.inject.Instance; ... public class GetServlet extends HttpServlet { @Any Instance<MyResource> _resources; public void service(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { PrintWriter out = response.getWriter(); for (MyResource resource : _resources) { out.println("resource: " + resource); } } } Example: GetServlet.java 10