is it possible to use Dagger 2.0 Constructor injection and singleton at one time.
I don't find the answer in the documentation.
Example:
@Singleton
public class MyClass {
private final OtherClass member;
@Inject
public MyClass(OtherClass member){
this.member = member;
}
}
Constructor injection does work for sure. But is it guaranteed that MyClass is created as a singleton if I write @Singleton on the class?
Thank you
Yes.
Since Dagger 2 generates the source code for you, it is easy to inspect what is happening. For example, when using the following module in combination with your MyClass
:
@Component
@Singleton
public interface MyComponent {
MyClass myClass();
}
The following implementation is generated:
@Generated("dagger.internal.codegen.ComponentProcessor")
public final class DaggerMyComponent implements MyComponent {
private Provider<MyClass> myClassProvider;
private DaggerMyComponent(Builder builder) {
assert builder != null;
initialize(builder);
}
public static Builder builder() {
return new Builder();
}
public static MyComponent create() {
return builder().build();
}
private void initialize(final Builder builder) {
this.myClassProvider = ScopedProvider.create(MyClass_Factory.create(OtherClass_Factory.create()));
}
@Override
public MyClass myClass() {
return myClassProvider.get();
}
public static final class Builder {
private Builder() {
}
public MyComponent build() {
return new DaggerMyComponent(this);
}
}
}
In initialize(Builder)
, you can see that a ScopedProvider
is used as a Provider
for MyClass
. When calling the myClass()
method, the ScopedProvider
's get()
method is called, which is implemented as a singleton:
public T get() {
// double-check idiom from EJ2: Item 71
Object result = instance;
if (result == UNINITIALIZED) {
synchronized (this) {
result = instance;
if (result == UNINITIALIZED) {
instance = result = factory.get();
}
}
}
return (T) result;
}