CDI对@Produces的不明确依赖 - 为什么？ [英] CDI Ambiguous dependency with @Produces - why?

问题描述

我使用的代码如下：

public Configuration {

    private boolean isBatmanCar = someMethod(...);

    @Produces
    public Car getCar(@New Car car) {
        if(isBatmanCar) {
            car.setName("BatmanCar");
        }
        return car;
    }
}

public Car {
    private String name = "NormalCar";

    public void setName(String name) {
        this.name = name;
    }
}

public Demo {
    @Inject
    Car car;

    // rest of code
}

部署时glassfish的应用程序（Java EE 6顺便说一句）我得到了

When I deploy an application to glassfish (Java EE 6 btw) I get

AmbiguousResolutionException：WELD-001318无法解决（...）Car之间的模糊依赖关系与限定符[@Any @Default]（...）制作方法[Car]与限定符[@Any @Default]

我知道当我将 @Alternative 添加到Car类时，它会起作用，但我想知道这是否是正确的方法，为什么我必须这样做？

I know that when I add @Alternative to Car class it will work, but I wonder if this is the proper way to do it, and why do I have to do it?

在这种情况下你能告诉我@Produces的正确用法是什么吗？

Can you tell me what is the correct usage of @Produces in such case?

我正在使用Java EE 6，CDI 1.0，EJB 3.1，Glassfish 3.2

I'm using Java EE 6, CDI 1.0, EJB 3.1, Glassfish 3.2

推荐答案

错误来自于你有2个类型为 Car ，一个是类，另一个是生产者。你有2个明显的解决方案可以解决歧义：

The error comes from the fact that you have 2 beans of type Car, one being the class, the other being the producer. You have 2 obvious solutions to resolve the ambiguity:

首先，你把逻辑放在原来的 isBatmanCar 字段中class（在构造函数或 @PostConstruct 方法中）并删除您的生产者。那只会留下一个 Car bean。

First, you put the logic behind isBatmanCar field in the original class (in a constructor or a @PostConstruct method for instance) and remove your producer. That would left only one Car bean.

或者如果你真的想拥有2个bean或者无法避免你应该为你的生产bean创建一个限定符：

Or if you really want to have 2 bean or can't avoid it you should create a qualifier for your produced bean:

 @Target({ TYPE, METHOD, PARAMETER, FIELD })
 @Retention(RUNTIME)
 @Documented
 @Qualifier
 public @interface BatmanChecked {
 }

并在生产者处使用它，

@Produces
@BatmanChecked
public Car getCar(Car car) {...}

能够注入汽车的类型

@Inject
Car stdCar;

@Inject
@BatmanChecked
Car batCheckedCar;

限定符是解决模糊注入的自然选择。使用 @Alternative 也可以，但这不是一个好习惯。

Qualifier is the natural option to resolve ambiguous injection. Using @Alternative also works but it's more a trick here than a good practice.

最后评论：<$ c这里不需要$ c> @New ，因为你的 Car bean没有范围（所以 @Dependent 作用域）。 @New仅在生产者注入一个范围不是 @Dependent 的bean时才有用。也就是说，如果 Car 类在范围 @Dependent 中，则此代码不是很有用。

Last remark: @New is not necessary here, since your Car bean has no scope (so is @Dependent scoped). @New is only useful when a producer inject a bean with a scope that is not @Dependent. That said, this code is not very useful if your Car class is in scope @Dependent.

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