Guice AssistedInject won't inject the factory

问题:

I am trying to use Guice 3.0 AssistedInject, and it won't instantiate the factory.

SSCCE Code:

Parent class

public class ParentClass() {
  @Inject private MyFactory myFactory;
  private final Foo foo;
  private final Bar bar;
  public ParentClass() {
    if(myFactory == null) System.err.println("Error: I should have injected by now!");
    foo = myFactory.create(new Map<String, Object>());
    // etc.
  }
}

Factory Interface

public interface MyFactory {
  Foo create(Map<String, Object> mapA);
  Bar create(Map<String, Object> mapB, Map<String, Object> mapC);
}

Module

public class ParentModule extends AbstractModule {
  @Override
  protected void configure() {
    install(new FactoryModuleBuilder()
        .implement(Foo.class, FooImpl.class)
        .implement(Bar.class, BarImpl.class)
        .build(MyFactory.class));
  }

FooImpl

public class FooImpl implements Foo {
  private final Map<String, Object> mapA;
  @AssistedInject
  public FooImpl(@Assisted Map<String, Object> mapA) {
    this.mapA = mapA;
  }
}

BarImpl is very similar to FooImpl. What is going wrong here? Note also that I tried both @AssistedInject and @Inject here, both cause the error.

Output:

Error: I should have injected by now!
Exception in thread "main" com.google.inject.ProvisionException: Guice provision errors:

1) Error injecting constructor, java.lang.NullPointerException
  at ParentClass.<init>(ParentClass.java:7)
  while locating com.my.package.ParentClass

1 error
        at com.google.inject.internal.InjectorImpl$4.get(InjectorImpl.java:987)
        at com.google.inject.internal.InjectorImpl.getInstance(InjectorImpl.java:1013)
        at com.my.package.ParentMain.main(ParentMain.java:16)
Caused by: java.lang.NullPointerException
        at com.my.package.ParentClass.<init>(ParentClass.java:9)
        at com.my.package.ParentClass$$FastClassByGuice$$d4b3063a.newInstance(<generated>)
        at com.google.inject.internal.cglib.reflect.$FastConstructor.newInstance(FastConstructor.java:40)
        ... 8 more

Note that line 9 is the line of the first call to myFactory.create()

回答1:

According to Guice's javadoc, field injection is performed after constructor injection.

I assume your ParentClass instance is created by Guice. When your ParentClass's constructor is executed, its myFactory field has not been injected yet.

回答2:

Two things. To use an injection in a constructor, you must use a constructor injection:

public class ParentClass {

    private final Foo foo;
    private final Bar bar;

    @Inject
    public ParentClass(MyFactory myFactory) {
        if(myFactory == null) System.err.println("Error: I should have injected by now!");
        this.foo = myFactory.create(new HashMap<String, Object>());
        this.bar = myFactory.create(new HashMap<String, Object>(), new HashMap<String, Object>());
    }
}

Also, since your factory has two parameters of the same type, you need to name them:

public interface MyFactory {
    Foo create(Map<String, Object> mapA);
    Bar create(@Assisted("B") Map<String, Object> mapB, @Assisted("C") Map<String, Object> mapC);
}

and

public class BarImpl implements Bar {

    private final Map<String, Object> mapA;
    private final Map<String, Object> mapB;

    @AssistedInject
    public BarImpl(@Assisted("B") Map<String, Object> mapA, @Assisted("C")  Map<String, Object> mapB) {
        this.mapA = mapA;
        this.mapB =mapB;
    }
}