Google Guice presentation at Montreal JUG - October 2010

Dependency
Injection
with...

Guice

Copyright © 2010 – Mathieu Carbou – Mycila.com - mathieu.carbou@gmail.com

About me

 Mathieu Carbou
 Expert Java Developer
 + 6 years Open-Source projects
 Code design, Web, Testing, Maven, Spring, WS,
JMS, EDA, OSGI , Security, Hibernate, ....
… and Google Guice !
 http://code.mycila.com
 http://blog.mycila.com
 http://www.testatoo.org


About the next hour

 Pre-DI anti-patterns
 Evolutions in the Java World
 DI advantages
 A little word about Spring...
 … and a long long time about Guice !


Pre-DI Anti-Patterns

 Manual instanciation

public final class SignupService {
public void signup(User user) {
EmailService emailService = new EmailService();
emailService.setHost("smtp.company.com");
emailService.setPort(25);
emailService.send(
"killerapp@company.com",
user.getEmail(),
"Welcome to Killer App !");
}
}



 Factory pattern

public final class SignupService {
EmailService emailService =
EmailServiceFactory.getEmailService();
emailService.send(
user.getEmail(),
}
}



 Service locator

// retreived from JNDI for example
ServiceLocator serviceLocator = getServiceLocator();
serviceLocator.getEmailService("emailService");
emailService.send(
user.getEmail(),
}



 Bean factory

// retreived Spring's bean factory
BeanFactory beanFactory = getBeanFactory();
EmailService) beanFactory.getBean("emailService");
emailService.send(
user.getEmail(),
}



 Design issues
 Strong coupling (locator, factory, identifier)
 No isolation
 Difficult to test, refactor and maintain
SignupService depends MailService
depends
depends

ServiceLocator

depends

UserService UserRepository UserRepository



 Potential issues
 Static references never garbadged-collected
 Static shared access issues (concurrency)
 Static lazy initialization often badly coded
 Bad responsibility for client and user classes


Evolutions

 Metadata representations
 XML
 Annotations

 Misconception about annotations
Annotations are metadata within classes introducing no dependencies at
runtime. A class can be resolved even if its annotations are not in the
classpath !


Evolutions

 JSR250 (already in JDK6, commes from J2EE)
 @Resource
 @PostConstruct
 @PreDestroy
 JSR330 (next JDK, jar available)
 @Inject
 @Provider
 @Named
 @Singleton
 @Scope


Evolutions

 Generics (type literals)
 List<Employee> != List<User>

 Some DI frameworks till rely on Class (Spring)
 May crash at runtime (type erasure) !

 New DI frameworks use TypeLiteral (Guice)
 Errors caugth at compile time !


Evolutions

 Example:

public final class FireService {

final List<Employee> employees;

@Inject
FireService(List<Employee> employees) {
this.employees = employees;
}

public void fireAll() {
for (Employee employee : employees)
employee.fire();
}
}


Evolutions

 Guice: Compile error !
final class FireModule extends AbstractModule {
protected void configure() {
bind(new TypeLiteral<List<Employee>>(){})
.toInstance(Arrays.asList(new User()));
}
}

 Spring (xml): compiles, but fails at runtime !
<bean name="fireService"
class="com.mycompany.killerapp.FireService">
<constructor-arg>
<list>
<bean class="com.mycompany.killerapp.User"/>
</list>
</constructor-arg>
</bean>


DI Definition

 A dependant is contacted by its dependencies

 Hollywood principle: Don't call me, I'll call you


DI Definition

 Example:

public final class FireService {

final List<Employee> employees;

@Inject
FireService(List<Employee> employees) {
this.employees = employees;
}

public void fireAll() {
for (Employee employee : employees)
employee.fire();
}
}


DI Advantages

 Easier testing & refactoring
 Easier maintenance
 Loose coupling
 Manage object lifetime
 Better design and use (interfaces)
 Hide implementation details
 Write less code (and less bugs)


About Spring Framework

 Everyone knows it
 You can compare

 Guice != Spring
 Guice = Advanced DI framework
 Spring = Integration Framework (JMX, JMS, JDBC,
TX, Hibernate, ...) built upon a DI stack

 You can use Spring classes with Guice


Google Guice !

 Open-Sourced by Google
 First public release in 2007
 http://code.google.com/p/google-guice/

 Project's lead: Bob Lee
 Cofounder of AOP Alliance
 Creator of JAdvise and Dynaop
 Strong AOP background


Google Guice !

 Version 3 to be released soon

 Standardized DI annotations (JSR330)

 Focus on:
 Easy to use
 Best DI capabilities

 Not an integration framework


Google Guice Ecosystem

 Guice Servlets
(DSL, DI and lifetime management for servlets and filters)

<listener>
<listener-class>
com.company.web.guice.GuiceConfig
</listener-class>
</listener>
<filter>
<filter-name>Guice Filter</filter-name>
<filter-class>
com.google.inject.servlet.GuiceFilter
</filter-class>
</filter>
<filter-mapping>
<filter-name>Guice Filter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>



 Guice Servlets
(DSL, DI and lifetime management for servlets and filters)

protected void configureServlets() {
Map<String, String> params = new HashMap<String, String>();

/* Shirio Security */
bind(IniShiroFilter.class).in(Scopes.SINGLETON);
filter("/*").through(IniShiroFilter.class);

/* Wicket */
params.put(WicketFilter.FILTER_MAPPING_PARAM, "/*");
filter("/page/*").through(WicketGuiceFilter.class, params);
bind(WebApplication.class).to(MyApplication.class);

/* Jersey */
serve("/rest/*").with(GuiceContainer.class, params);
}



 Guice Persistence (Hibernate, JPA, DB4O, ...)

protected void configurePersistence() {
workAcross(UnitOfWork.TRANSACTION).usingJpa("c4sJpaUnit");
bind(DataSource.class)
.toProvider(fromJndi(DataSource.class, "jdbc/c4sDS"))
.in(Singleton.class);
bind(PlayerRepository.class)
.to(JpaPlayerRepository.class)
bind(TeamRepository.class)
.to(JpaTeamRepository.class)
}



 Guice Persistence (Dynamic finders)

public interface TeamFinder {
@Finder(query="from Team")
List<Team> allTeams();
}



 Guice Jndi

class SqlModule extends AbstractModule {

bind(UserRepository.class).to(SQLUserRepository.class);
.toProvider(fromJndi(DataSource.class, "jdbc/h2DB"))

}
}



 Spring Integration
 Assisted inject (contextual DI)
interface PaymentFactory {
Payment create(Date startDate, Money amount);
}

// DI configuration
bind(Payment.class).to(PaymentImpl.class);
bind(PaymentFactory.class)
.toProvider(newFactory(PaymentFactory.class,
RealPayment.class));

// user code
Payement pay = factory.create(today, amount);



 Projects extending Guice capabilities:
 Peaberry (OSGI)
 Mycila-testing
 Guiceyfruit (JSR250, JNDI, ...)
 Mycila-guice (JSR250, scopes, module discovery, …)
 GuiceBerry
 [...]


Google Guice Integration


Guice relative Books


Google Guice Advantages

 Easier to use, test, maintain, refactor, learn
 Better power to complexity ratio than Spring
 Advanced DI concepts
 Favor good design
 Immutability, valid objects
 Thread-safe object
 No more java beans constraints
 Injectable DSL is possible


Google Guice Advantages

 No string identifier
 No more conflicts
 No more responsability issue
 No more XML !
 Static and compile-time verification
 Plus many things we'll see in details now !


Dive into Guice

 DI configuration (Java-based, DSL & Fluent API)
public final class SqlModule extends AbstractModule {
@Override
bind(UserRepository.class).to(SQLUserRepository.class);
.toProvider(fromJndi(DataSource.class,"jdbc/myDB"))
}

@Provides
Connection connection(DataSource dataSource)
throws SQLException {
return dataSource.getConnection();
}
}


Dive into Guice

 Dependency identification: Key class
 Supports type literals and raw types
 Bindings annotations
 Static type checking
 Errors caught at compile time, not at runtime


Dive into Guice

 Dependency identification
bind(Properties.class)
.annotatedWith(named("app-settings"))
.toProvider(fromJndi(Properties.class, "settings/app"));

bind(new TypeLiteral<GenericRepository<Employee>>(){})
.to(EmployeeRepository.class);

--------------------------------------------------------------

Key key =
Key.get(new TypeLiteral<GenericRepository<Employee>>(){});
GenericRepository<Employee> repo = injector.getInstance(key)

Properties props = injector.getInstance(
Key.get(Properties, named("app-settings")))


Dive into Guice

 Bootstrapping
Injector injector = Guice.createInjector(
Stage.PRODUCTION,
new MyModule(), new JSR250Module(), new SqlModule());


Dive into Guice

 Injection points: Constructors, Fields, Methods
public class BasicTemplateService {

@Inject
UserRepository repository

private final Settings settings;

@Inject
public BasicTemplateService(Settings settings) {
this.settings = settings;
}

@Inject
@Named("/app/messages/user")
void withMessageBundle(ResourceBundle rb) {
[...]
}
}

Dive into Guice

 AOP support
 Easy
 All casual cases
 Intercept:
 Methods
 Injections
 Matchers
 Classes
 Methods


Dive into Guice

 AOP support - Interceptors
bindInterceptor(
Matchers.inPackage(getPackage("com.company.repository")),
Matchers.annotatedWith(Test.class),
new MyAopAllianceInterceptor());

package com.company.repository;
class MyRepository {
@Test
void intercepted() {}
}


Dive into Guice

 AOP support – Injection listeners
bindListener(Matchers.any(), new TypeListener() {

public <I> void hear(final TypeLiteral<I> type,
final TypeEncounter<I> encounter) {

System.out.println("Meeting type: " + type);

encounter.register(new InjectionListener<I>() {

public void afterInjection(I injectee) {
System.out.println("Type has been injected");
}
});
}
});


Dive into Guice

 Providers
 Scope widening
 Delay construction
class CurrentLocale {

@Inject
Provider<HttpServletRequest> request;

public Locale get() {
[...]
}
}


Dive into Guice

 Scopes
 Object lifetime management
 By default: no scope
 Allows objects to be garbadged-collected shortly

 Misconception about singletons
 @Singleton IS a scope !
 Should only considered when really needed
 Constructing new objects has no cost in Java


Dive into Guice

 Scopes
bind(JamesBondMessage.class)
.in(new ExpiringSingleton(10, SECONDS));

bindScope(Concurrent.class, new ConcurrentSingleton());
bind(EnglishDataLoader.class).in(Concurrent.class);
bind(FrenchDataLoader.class).in(Concurrent.class);

@Concurrent
class SpanishDataLoader {
}


Dive into Guice

 And also
 Module overriding
 Modules.override(...).by()...
 Introspection API
 Elements.getElements()
 binding.acceptVisitor()
 […]
 Parent / Child injectors
 Private / Public bindings


Guice questions ?


Google Guice presentation at Montreal JUG - October 2010

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