Hello everyone,
Been an avid user of Guice for a while now, and just ran across a new
situation that has me stumped. I've got a dependency that I need to
be able to change at runtime. The catch is, the piece of code that
needs it is far down the call graph from where it changes. I've
attempted to use assisted inject to let me customize the object at
runtime, but there's a hitch.
An example will make this more clear:
class MyModule extends AbstractModule {
protected void configure() {
bind( BFactory.class ).toProvider( FactoryProvider.newFactory( BFactory.class,
B.class ) );
}
}
interface BFactory {
B create( D d );
}
class A {
@Inject BFactory bFac;
void executeA( D d ) {
bFac.create( d ).executeB();
}
}
class B {
@Inject C c;
void executeB() {
c.executeC();
}
}
class C {
@Inject @Assisted D d;
public void executeC() {
System.out.println( d );
}
}
class D {};
Now I can call A.executeA() with different instances of D (which is
needed by C) at runtime. Executing the above classes with the
following code,two different D objects are written to stdout as
expected:
A a = i.getInstance( A.class );
a.executeA( new D() );
a.executeA( new D() );
This works as long as I only create C through the BFactory, and as
long as everything is concrete. But if I introduce an interface,
there's a problem, as follows:
class MyModule extends AbstractModule {
protected void configure() {
bind( BFactory.class ).toProvider( FactoryProvider.newFactory( BFactory.class,
B.class ) );
bind( C.class ).to( CImpl.class );
}
}
interface C {
void executeC();
}
class CImpl implements C {
@Inject @Assisted D d;
public void executeC() {
System.out.println( d );
}
}
This gives me an error:
1) No implementation for D annotated with
@com.google.inject.assistedinject.Assisted(value=) was bound.
while locating D annotated with
@com.google.inject.assistedinject.Assisted(value=)
for field at CImpl.d(AssistedInjectTest.java:62)
at MyModule.configure(AssistedInjectTest.java:28)
The BFactory binding is still okay, but the C->CImpl binding is what
produces the error. I assume this is because class C could
conceivably be wired into another class as a dependency (in addition
to class B) and Guice wouldn't know what to do with the dependency on
D.
What's the best solution to this? Am I making it needlessly complex?
I need to know if there's a way to supply D to C without B knowing
anything about it, while still allowing A to change what D is.
Thanks,
Ryan
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