Post on 05-Dec-2014
description
transcript
Effective Dependency Injection
Nicholas Behrens and Jerome DochezGoogle Inc.
This talk
1. Quick DI refresher
2. Guidelines for effective DI
3. Composing applications using DI
4. Testing code using DI
5. Applying DI to existing code
1. Quick DI Refresher
Declare what you need, not how to get it
PaymentProcessor(
CreditCardService creditCardService) {
this.creditCardService = creditCardService;
this.riskAssessor =
new RiskAssessorImpl(...);
}
DI = Dependency Injection
DI = Dependency Injection
+ Loose coupling, facilitate reuse and testing- Complexity, runtime failures
Many frameworks with many tools
Important to establish good conventions
DI Framework
● Providers● Bindings● @Inject● Module [= collection of Bindings]● Injector [= collection of Modules]
○ injector.getInstance(type)
● Scopes
2. Guidelines for effective DI
Explicit > Implicit
● Ensure explicit bindings for all injected types○ bind(RequestFilter.class);
● Implicit bindings dangerous○ Removing an explicit binding○ Pulling in undesired explicit binding
Inject exactly what you need
Bad:PaymentProcessor(User user) {
this.account = user.getAccount();
}
Better:PaymentProcessor(Account account) {
this.account = account;
}
Prefer constructor injection
● private final fields
● Immutability, thread-safety
● Objects valid post-construction
● Match developer expectations
Avoid work in constructors
● Separation of concerns, facilitates testing,
"standard" constructor
● In Guice: runtime ProvisionException
easily propagates up to top level
● Initialize after constructing object graph
If you must do work in a constructor
interface DataStoreProvider<T> extends CheckedProvider<T> {
T get() throws DataStoreException;
}
Guice-specific
If you must do work in a constructor (cont'd)@CheckedProvides(DataStoreProvider.class)
Contacts provideContacts(UserId userId, DataStore dataStore)
throws DataStoreException {
return dataStore.readUserContacts(userId);
}
@Override protected void configure() {
install(ThrowingProviderBinder.forModule(this));
}
Guice-specific
@Inject ContactsFormatter(
DataStoreProvider<Contacts> contactsProvider) {
this.contactsProvider = contactsProvider;
}
String formatContacts() throws DataStoreException {
Contacts contacts = contactsProvider.get(); // throws
...
}
If you must do work in a constructor (cont'd)
Guice-specific
Problems
● More complex, verbose bindings
● More setup work for tests
● Business logic creep
● Viral
If you must do work in a constructor (cont'd)
Guice-specific
● No longer declaring specific deps
● Injector allows dep on anything
● Easy to start using Injector, hard to stop
Avoid direct deps on Injector
Avoid binding very general types
BadbindConstant().to(8080);
@Inject int port;
BetterbindConstant().annotatedWith(ForHttp.class)
.to(8080);
@Inject @ForHttp int port;
At first it seems convenient...@Named("http") int httpPort;
...but then you do@Named("HTTP") int httpPort;
No help from the IDE, more runtime errors.Harder to track down injection sites.
Avoid parameterized binding annotations
Consistently use constants (enums!) for parameters:
@ForPort(Ports.HTTP) int httpPort;
@FeatureEnabled(Features.X) boolean xEnabled;
Using parameterized binding annotations safely
Make it easy to compose modules
Avoid● bind().toInstance(...)● requestStaticInjection(...)● Serial eager initialization of objects
Keep modules fast and side-effect free
Guice-specific
Prefer Null Objects over @Nullable@Provides AbuseService provideAbuseService(
@ForAbuse String abuseServiceAddress) {
if (abuseServiceAddress.isEmpty()) { return null; }
return new AbuseServiceImpl(abuseServiceAddress);
}
class AbuseChecker {
@Inject AbuseChecker(
@Nullable AbuseService abuseService) { ... }
void doAbuseCheck(...) {
if (abuseService != null) { ... )
}
Bind StubAbuseService instead of null. Simplify AbuseChecker.
Use Provider to avoid unnecessary provisioning@Provides AbuseService provideAbuseService(
@UseNewAbuseService boolean useNewAbuseService,
Provider<OldAbuseServiceImpl> oldAbuseService,
Provider<NewAbuseServiceImpl> newAbuseService) {
return useNewAbuseService.get()
? newAbuseService.get()
: oldAbuseService.get();
}
Scoping as an alternative to contextpublic interface PipelineVisitor() {
void visit(TypeToVisit typeToVisit);
}
public class Pipeline() {
private final PipelineVisitor[] visitors = {
new Visitor1(), new Visitor2(), ...
}
public void visit(TypeToVisit typeToVisit) {
...
}
}
Requirements creep in, you add more parameters...public interface PipelineVisitor() {
void visit(TypeToVisit typeToVisit, User user);}public interface PipelineVisitor() {
void visit(TypeToVisit typeToVisit, User user, Logger logger);
}public interface PipelineVisitor() {
void visit(TypeToVisit typeToVisit, User user, Logger logger, RequestParams params);
}
and so on...
Ah yes, the Context to the rescuepublic interface PipelineVisitor() {
void visit(PipelineContext context);}public Class PipelineContext {
TypeToVisit typeToVisit;User user;Logging logging;... and many more ...
}
and adapts the Pipeline itself.public class Pipeline() {
private final PipelineVisitor[] visitors = { ... };OutputType visit(TypeToVisit in, User user, Logging
log){
OutputType out = new OutputType();PipelineContext context =
new PipelineContext(in, user, log);for (PipelineVisitor visitor : visitors) {
visitor.visit(context);}
}}
what about testing, isolation,...
First : remove the contextpublic interface Visitor {
void visit(TypeToVisit in);}
public class LoggerVisitor {private final Logger logger;@Inject public LoggerVisitor(Logger logger) {...}void visit(InputType in, Output out) {
logger.log(Level.INFO, "Visiting " + in);}public class UserDependentVisitor {
private final User user;...
}
Second : use a Multi-Binderclass VisitorsModule extend AbstractModule {
@Override protected void configure() {Multibinder<PipelineVisitor> visitorBinder =
Multibinder.newSetBinder(binder(), PipelineVisitor.class);
visitorBinder.addBinding().to(LoggerVisitor.class);visitorBinder.addBinding()
.to(UserDependentVisitor.class);}
Now we can inject a Set<Visitor>
Inject into Pipelinepublic class Pipeline() {
private final Set<Provider<PipelineVisitor.class>> visitors
@InjectPipeline(Set<Provider<PipelineVisitor.class>> visitors) {
this.visitors = visitors;}
public void visit(TypeToVisit in) {for (Provider<PipelineVisitor.class> visitor :
visitors){
visitor.get().visit(in);...
Testing Visitors and Pipelinepublic void testVisitor() {
Visitor v = new FirstVisitor(...);v.visit(in);assert(...)
}
public void testPipeline() {Visitor v = Mockito.mock(Visitor.class);Pipeline p = new Pipeline(Sets.of(Providers.of(v)));TypeToVisit in = new TypeToVisit(...);p.visit(in);Mockito.verify(v).visit(eq(in),
any(Output.class));}
3. Composing applications using DI
Modular Java!
Use a Service-Based Architecture ● All services defined as an interface● Implementations package-private● Package-level Module binds No need for OSGi unless you need: ● Runtime extensibility● Runtime versioning
API: (interfaces, enums, binding annotations)
public interface SpamChecker {
void checkIsSpam(...);
}
Implementation: (package private)
class SpamCheckerImpl implements SpamChecker {
void checkIsSpam(...) { ... }
}
Modular Java: Declare API and Impl
In the same package as SpamChecker*:
public class SpamCheckerModule extends AbstractModule {
@Override
public void configure() {
bind(SpamCheckerImpl.class).in(Singleton.class);
bind(SpamChecker.class).to(SpamCheckerImpl.class);
}
}
To use in your app, install SpamCheckerModule and then inject SpamChecker as needed.
Modular Java: Colocate bindings
Modular Java: Document deps
Document what your module requires:
class SpamCheckerModule ... {
@Override protected void configure() {
requireBinding(Clock.class);
requireBinding(CryptoService.class);
...
}
}
Modular Java: Package it all up
1. Mavena. All public interfaces in -api.jar moduleb. All implementations and modules in -impl.jarc. api module should only be used for compiling
dependent modules, impl module should be used for assembling the application
2. OSGia. Export interfaces and Module onlyb. Avoid versioning at runtime
4. Testing code using DI
Unit testing a class using DI
public class SubjectUnderTest {
@Inject
SubjectUnderTest(Service1 s1, Service2 s2) {
// Assign to fields
}
String doSomething() { ... }
}
Unit testing a class using DI (cont'd)
Service1 mock1 = Mockito.mock(Service1.class);
Service2 mock2 = Mockito.mock(Service2.class);
SubjectUnderTest sut = new SubjectUnderTest(
mock1, mock2);
// configure mock
Mockito.when(mock1).service(eq("param"))
.thenReturn("some return value);
// test your service
sut.doSomething();
// verify the results...
Module.override to overcome problematic bindings for testing:
Module testModule = Modules
.override(new AcmeModule())
.with(new AbstractModule() {
@Override protected void configure() {
bind(AuthChecker.class)
.toInstance(new StubAuthChecker());
}
});
Avoid overriding too many bindings
Testing bindings
System (end-to-end) tests a must with DI
Discover runtime failures in tests or your users will discover them for you
System tests
5. Applying DI to existing code
Rewiring an existing code base to use DI will take time
Don't need to use everywhere (but consistency is good)
Migrating to DI
Approaches● Bottom-up, separate injectors for subparts● Top-down, single injector pushed down
Often need to compromise on ideal patterns● Multiple Injectors● Directly reference Injector● Constructors do work
Techniques for migrating to DI
Things that make DI difficult: deep inheritanceclass BaseService { }
class MobileService extends BaseService { }
class AndroidService extends MobileService {
@Inject AndroidService(Foo foo, Bar bar) {
super(foo, bar);
}
What if BaseService needs to start depending on Baz? Prefer composition.
Things that make DI difficult: static methodsclass SomeClass {
static void process(Foo foo, Bar bar) {
processFoo(foo); ...
}
static void processFoo(Foo foo) { ... }
}
I want to bind Foo and start injecting it elsewhere. How do I get to Guice?
Thank you!
https://developers.google.com
Contact us:jedo@google.com
nbehrens@google.com
@ImplementedBy(MySqlUserStore.class)
interface UserStore { ... }
● Guice-ism, provided for convenience● Interface should not have compile-time dep
on specific implementation!● Can be overridden by explicit bindings
Avoid @ImplementedBy
Guice-specific
@ThreadSafeclass Greeter { private final Greetings greetings;
private final User user;
@InjectGreeter(Greetings greetings, User user) {
this.greetings = greetings;this.user = user;
}
String getGreeting(GreetingId greetingId) { ... }
}
Constructor injection promotes thread-safe code
● Adds complexity
● Scopes mismatches
○ e.g. request scoped type into singleton scoped type
● More (wrong) choices for developers
○ Should I put this in scopeA, scopeB, scopeC, ...?
Minimize Custom Scopes
Testing providers
If your module contains non-trivial provider methods, unit test them
class FooModuleTest {
void testBarWhenBarFlagDisabled() {
assertNull(
new FooModule().provideBar(
false /* bar flag */);
}
void testBarWhenBarFlagEnabled() { ... }
}