org.apache.reef.tang

Class InjectionFuture<T>

java.lang.Object

org.apache.reef.tang.InjectionFuture<T>

All Implemented Interfaces:

Future<T>

public final class InjectionFuture<T>
extends Object
implements Future<T>

A future-based mechanism for cyclic object injections. Since Tang is a constructor-based dependency injector, there is no way to directly create cycles of objects.

In situations where you need to have two objects that point at each other, you can use an InjectionFuture to break the cycle. Simply ask Tang to inject an InjectionFuture<T> into your constructor. Later (after your constructor returns) invoke the get() method of the injection future to get a reference to an injected object of type T.

Note that InjectorFutures and singletons interact in subtle ways.

Normally, Tang never reuses a reference to an injected object unless the object is a singleton or a volatile instance. If InjectionFutures followed this convention, then a cyclic injection of two non-singleton classes would result in an an infinite chain of objects of the two types. Tang addresses this issue as follows:

1) In the first pass, it injects a complete object tree, making note of InjectionFuture objects that will need to be populated later. The injection of this tree respects standard Tang singleton and volatile semantics.

2) In a second pass, Tang populates each of the InjectionFutures with the reference to the requested class that was instantiated in the first pass. If this reference does not exist (or is non-unique) then an InjectionException is thrown.

Note: The semantics of complex cyclic injections may change over time.

We haven't seen many complicated injections that involve cycles in practice. A second approach would be to establish some scoping rules, so that each InjectionFuture binds to the innermost matching parent in the InjectionPlan. This would allow plans to inject multiple cycles involving distinct objects of the same type.

Constructor Summary

Constructors

Constructor and Description
InjectionFuture()
InjectionFuture(Injector injector, Class<? extends T> iface)
InjectionFuture(T instance)

Method Summary

Methods

Modifier and Type	Method and Description
boolean	cancel(boolean mayInterruptIfRunning)
T	get()
T	get(long timeout, TimeUnit unit)
boolean	isCancelled()
boolean	isDone()

Methods inherited from class java.lang.Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

InjectionFuture

public InjectionFuture()

InjectionFuture

public InjectionFuture(Injector injector,
               Class<? extends T> iface)

InjectionFuture

public InjectionFuture(T instance)

Method Detail

cancel

public boolean cancel(boolean mayInterruptIfRunning)

Specified by:

cancel in interface Future<T>

isCancelled

public boolean isCancelled()

Specified by:

isCancelled in interface Future<T>

isDone

public boolean isDone()

Specified by:

isDone in interface Future<T>

get

public T get()

Specified by:

get in interface Future<T>

get

public T get(long timeout,
    TimeUnit unit)

Specified by:

get in interface Future<T>