1 Dependency Injection Sandro Pedrazzini Approfondimento Dependency Injection.
Context and Dependency Injection
32
Context & Dependency Injection A Cocktail of Guice and Seam, the missing ingredients for Java EE 6 Werner Keil 21/04/2011
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A Cocktail of Guice and Seam, the missing ingredients for Java EE 6
Transcript of Context and Dependency Injection
Context & Dependency Injection
A Cocktail of Guice and Seam, the missing ingredients for Java EE 6
Werner Keil
21/04/2011
Without Dependency Injection
2
Explicit Constructor
class Stopwatch { final TimeSource timeSource; Stopwatch () { timeSource = new AtomicClock(...); } void start() { ... } long stop() { ... }}
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Without Dependency Injection
3
Using a Factory
class Stopwatch {
final TimeSource timeSource; Stopwatch () {
timeSource = DefaultTimeSource.getInstance();
}
void start() { ... }
long stop() { ... }
}
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Somewhat like in java.util.Calendar
Without Dependency Injection
4
Mock Data
void testStopwatch() { TimeSource original = DTS.getInstance(); DefaultTimeSource.setInstance(new MockTimeSource());
try {Stopwatch sw = new Stopwatch();
... } finally { DTS.setInstance(original); } }
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With Dependency Injection
5
@Inject
class Stopwatch { final TimeSource timeSource; @Inject Stopwatch(TimeSource timeSource) {
this.timeSource = TimeSource; } void start() { ... } long stop() { ... } }
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JSR-330
@Inject Identifies injectable constructors, methods, and fields .
This works for both static and non-static elements. @Qualifier
Identifies qualifier annotations to declare binding types of your API
@Named A name (string) based qualifier Others may be declared application - or domain-specific
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Definition
JSR-330
@Provider Provides instances of an (injectable) type. Allows
Retrieving multiple instances. Lazy or optional retrieval of an instance. Breaking circular dependencies. Abstracting scope
@Singleton Identifies a type that the injector only instantiates once. See the well-known Design Pattern
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Implementation
DEMO
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JSR-330
1. Minimal but usable API: A small API surface seems more important than making it as convenient as possible for clients. There are only two methods in the proposed API that could be considered convenience, added because they seemed to pull their weight
InjectionConfigurer.inject and InjectionSpec.inject(Object)
2. Builder-style API: so-called "fluent APIs“• Open to other styles.
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Configuration
JSR-330
3. Abstract classes instead of interfaces: 2 main types (InjectionConfigurer and InjectionSpec) are abstract classes
instead of interfaces. This allows new methods to be added later in a binary-
compatible way.
4. Separate configuration API• The API does not specify how an instance of
InjectionConfigurer is obtained. • Otherwise we have to standardize the injector API itself,
something outside of the stated scope of JSR-330.www.catmedia.us 10
Configuration (2)
JSR-330
5. Names are different from Guice's configuration API: This is mostly
to keep this separate from existing configuration APIs, but also so
that new concepts (like "Binding") don't have to be defined.
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Configuration (3)
More about JSR-330, seehttp://code.google.com/p/atinject
orhttp://jcp.org/en/jsr/summary?id=330
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Formerly known as WebBeans…
13
JSR-299
The lifecycle and interactions of stateful components bound to well-defined lifecycle contexts, where the set of contexts is extensible
A sophisticated, type safe dependency injection mechanism, including a facility for choosing between various components that implement the same Java interface at deployment time
An event notification model
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Services
JSR-299
Integration with the Unified Expression Language (EL), allowing any component to be used directly within a JSF or JSP page
The ability to decorate injected components A web conversation context in addition to the three
standard web contexts defined by the Java Servlets specification
An SPI allowing portable extensions to integrate cleanly with the Java EE environment
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Services (2)
• Sophisticated,• Type Safe,• Dependency
Injection,…
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IN MEMORIAM
17
Pietro Ferrero Jr.
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11 September 1963 – 18 April 2011
JSR-299
A (nonempty) set of bean types A (nonempty) set of bindings A scope A deployment type Optionally, a bean EL name A set of interceptor bindings A bean implementation
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Bean Attributes
JSR-299
• A custom implementation of Context may be associated with a scope type by calling BeanManager.addContext().
public void addContext(Context context);
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Context
JSR-299
• BeanManager.getContext() retrieves an active context object associated with the a given scope:
public Context getContext(Class<? extends Annotation> scopeType);
→ Context used in a broader meaning than some other parts of Java (Enterprise)
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Context (2)
JSR-299
@SessionScoped
public class PaymentStrategyProducer {
private PaymentStrategyType paymentStrategyType;
public void setPaymentStrategyType(
PaymentStrategyType type) {
paymentStrategyType = type;
}
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Restricted Instantiation
JSR-299
@Produces PaymentStrategy getPaymentStrategy(@CreditCard PaymentStrategy creditCard,
@Cheque PaymentStrategy cheque,
@Online PaymentStrategy online) {
switch (paymentStrategyType) {
case CREDIT_CARD: return creditCard;
case CHEQUE: return cheque;
[…]www.catmedia.us 22
Restricted Instantiation (2)
JSR-299
[…]
case ONLINE: return online;
default: throw new IllegalStateException();
} }
}
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Restricted Instantiation (3)
JSR-299
• Then the following observer method will always be notified of the event:
• public void afterLogin(@Observes LoggedInEvent event) { ... }
• Whereas this observer method may or may not be notified, depending upon the value of user.getRole():
• public void afterAdminLogin(@Observes @Role("admin") LoggedInEvent event) { ... }
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Event Observer
JSR-299
• As elsewhere, binding types may have annotation members:
@BindingType@Target(PARAMETER)@Retention(RUNTIME)public @interface Role {String value();
}
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Event Binding
JSR-299
Actually @Role could extend @Named from JSR-330 JSRs here potentially not consequently streamlined…?
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Event Binding (2)
JSR-299
public interface Bean<T>extends Contextual<T> {public Set<Type> getTypes();public Set<Annotation> getBindings();public Class<? extends Annotation>
getScopeType();
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Portable Extensions
JSR-299
public Class<? extends Annotation>
getDeploymentType();public String getName();public Class<?> getBeanClass();public boolean isNullable();
public Set<InjectionPoint> getInjectionPoints();
}www.catmedia.us 28
Portable Extensions (2)
DEMO
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More about JSR-299, see
http://www.seamframework.org/JCP
http://jcp.org/en/jsr/summary?id=299or
• http://www.developermarch.com/developersummit/sessions.html#session66
• (cancelled ;-/)www.catmedia.us 32
Further Reading• Java.net
http://www.java.net/• Java™ EE Patterns and Best Practices
http://kenai.com/projects/javaee-patterns /
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