Autowiring Dependencies

Autowiring Dependencies

There are four modes of autowiring a bean using an XML configuration:

• no: the default value – this means no autowiring is used for the bean and we have to explicitly name the dependencies.
• byName: autowiring is done based on the name of the property, therefore Spring will look for a bean with the same name as the property that needs to be set.
• byType: similar to the byName autowiring, only based on the type of the property. This means Spring will look for a bean with the same type of the property to set. If there’s more than one bean of that type, the framework throws an exception.
• constructor: autowiring is done based on constructor arguments, meaning Spring will look for beans with the same type as the constructor arguments.

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For example, let’s autowire the item1 bean defined above by type into the store bean:

@Bean(autowire = Autowire.BY_TYPE)
public class Store {
    
    private Item item;

    public setItem(Item item){
        this.item = item;    
    }
}

We can also inject beans using the @Autowired annotation for autowiring by type:

public class Store {
    
    @Autowired
    private Item item;
}

If there’s more than one bean of the same type, we can use the @Qualifier annotation to reference a bean by name:

public class Store {
    
    @Autowired
    @Qualifier("item1")
    private Item item;
}

Now let’s autowire beans by type through XML configuration:

<bean id="store" class="org.store.Store" autowire="byType"> </bean>

Next, let’s inject a bean named item into the item property of store bean by name through XML:

<bean id="item" class="org.store.ItemImpl1" />

<bean id="store" class="org.store.Store" autowire="byName"> </bean>

We can also override the autowiring by defining dependencies explicitly through constructor arguments or setters.

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参考：
Intro to Inversion of Control and Dependency Injection with Spring