Guice AssistedInject不会注入工厂

Question

我正在尝试使用Guice 3.0AssistedInject，它不会实例化工厂。

SSCCE代码：

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.
  }
}

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

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));
  }

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

< code>BarImpl与< code>FooImpl非常相似。这里出了什么问题？还要注意，我在这里尝试了< code>@AssistedInject和< code>@Inject,两者都会导致错误。

输出：

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

请注意，第 9 行是对 myFactory.create（） 的第一次调用的行

Answer 1

两件事。要在构造函数中使用注入，必须使用构造函数注入:

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>());
    }
}

此外，由于您的工厂有两个相同类型的参数，因此需要命名它们：

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

和

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;
    }
}

Answer 2

根据Guice的javadoc，字段注入是在构造函数注入之后进行的。

我假设您的 ParentClass 实例是由 Guice 创建的。执行 ParentClass 的构造函数时，尚未注入其 myFactory 字段。