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Effective Dependency
Injection
Nicholas Behrens and Jerome Dochez
Google 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

DI = Dependency Injection

Declare what you need, not how to get it

PaymentProcessor(
CreditCardService creditCardService) {
this.creditCardService = creditCardService;
this.riskAssessor =
new RiskAssessorImpl(...);
}

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

(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

(cont'd)
@Inject ContactsFormatter(

DataStoreProvider<Contacts> contactsProvider) {

this.contactsProvider = contactsProvider;

}

String formatContacts() throws DataStoreException {

Contacts contacts = contactsProvider.get(); // throws

...

}

Guice-specific

(cont'd)
Problems
● More complex, verbose bindings
● More setup work for tests
● Business logic creep
● Viral

Guice-specific

Avoid direct deps on Injector

● No longer declaring specific deps
● Injector allows dep on anything
● Easy to start using Injector, hard to stop

Avoid binding very general types

Bad
bindConstant().to(8080);
@Inject int port;

Better
bindConstant().annotatedWith(ForHttp.class)
.to(8080);
@Inject @ForHttp int port;

Avoid parameterized binding
annotations
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.

Using parameterized binding
annotations safely
Consistently use constants (enums!) for parameters:

@ForPort(Ports.HTTP) int httpPort;

@FeatureEnabled(Features.X) boolean xEnabled;

Keep modules fast and side-effect
free

Make it easy to compose modules

Avoid
● bind().toInstance(...)
● requestStaticInjection(...)
● Serial eager initialization of objects

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 context
public 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...
void visit(TypeToVisit typeToVisit, User user);
}
void visit(TypeToVisit typeToVisit,
User user, Logger logger);
}
void visit(TypeToVisit typeToVisit,
User user, Logger logger, RequestParams params);
}

and so on...

Ah yes, the Context to the rescue
void visit(PipelineContext context);
}
public Class PipelineContext {
TypeToVisit typeToVisit;
User user;
Logging logging;
... and many more ...
}

and adapts the Pipeline itself.
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 context
public 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-Binder
class VisitorsModule extend AbstractModule {
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 Pipeline
private final Set<Provider<PipelineVisitor.class>>
visitors

@Inject
Pipeline(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 Pipeline
public 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

Modular Java: Declare API and Impl
API: (interfaces, enums, binding annotations)
public interface SpamChecker {
void checkIsSpam(...);
}

Implementation: (package private)
class SpamCheckerImpl implements SpamChecker {
void checkIsSpam(...) { ... }
}

Modular Java: Colocate bindings
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: Document deps
Document what your module requires:

class SpamCheckerModule ... {
requireBinding(Clock.class);
requireBinding(CryptoService.class);
...
}
}

Modular Java: Package it all up
1. Maven
a. All public interfaces in -api.jar module
b. All implementations and modules in -impl.jar
c. api module should only be used for compiling
dependent modules, impl module should be used for
assembling the application

2. OSGi
a. Export interfaces and Module only
b. Avoid versioning at runtime

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...

Testing bindings
Module.override to overcome problematic
bindings for testing:

Module testModule = Modules
.override(new AcmeModule())
.with(new AbstractModule() {
bind(AuthChecker.class)
.toInstance(new StubAuthChecker());
}
});

Avoid overriding too many bindings

System tests
System (end-to-end) tests a must with DI

Discover runtime failures in tests or your users
will discover them for you

5. Applying DI to existing code

Migrating to DI
Rewiring an existing code base to use DI will
take time

Don't need to use everywhere (but consistency
is good)

Techniques for 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

Things that make DI difficult: deep
inheritance
class 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
methods
class 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

Avoid @ImplementedBy

@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

Guice-specific

Constructor injection promotes
thread-safe code
@ThreadSafe
class Greeter {
private final Greetings greetings;
private final User user;

@Inject
Greeter(Greetings greetings, User user) {
this.greetings = greetings;
this.user = user;
}

String getGreeting(GreetingId greetingId) { ... }
}

Minimize Custom Scopes
● 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, ...?

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() { ... }
}

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Devoxx 2012 (v2)

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