Spring_1_Deal with the ambiguity of automatic assembly（处理自动装配的歧义问题）

Deal with the ambiguity of automatic assembly（处理自动装配的歧义问题）

It’s first time for me to use this platform.And as u see,I choose to write this in English. So many reason,mostly because I like it.

Ok,time to get into business.
When first met this problem,When the Spring told me :

NoUniqueBeanDefinitionException:
nested exception is org.springframework.beans.factory.NoUniqueBeanDefinitionException:
No qualifying bean of type [********] is defined:
expected single matching bean but found X:.....

“Do i know you?” Puzzled. All I remember is I used much time to figure anwser out.Looked for information about it on the internet and asked my friends. Of course got what I want, but soon forgot. Can’t find a better method, i bought 《Spring IN ACTION》and read it about 3 times（bad memory？NO! Just because write them down in my notebook,and only write down in notebook…so, I wanna tell you DO NOT RELY TOO MUCH ON NOTEBOOK BUT YOUR MIND）.
Ok,time to get into business again.
HOW TO DEAL WITH IT?
Ⅰ、Mark the preferred bean
What it means？Like u like to fruit，among thousands of fruit，which one do u like best?Watermelons?Pitaya?Juicy peach?Grapes…Make a mark to tell me.
Just tell Spring which bean you need in this way.Now u need to use @Primary.

@Conponent
@Primary
public class classNae implements interface{....}

But make sure there is only one thing u like best or u want more problems.
Ⅱ、Qualify the auto-assembled bean ----- qualifiers
Method Ⅰ got A big problem : Its limitation, ProMonkey can only make one choice.In opposite terms,qualifiers can let monkeys have many choices.
@Qualifier is the main way to use qualifiers.It can work with @Autowired.

@Autowired
@Qualifier("beanID")
public void methodName(parameters)｛Method block
｝

It’s a simple example.Remember one thing:beanID is a class name start with lowercase.If you change the class name, do not forget to change the qualifier.
Ⅲ、Create a custom qualifier
We can create a unique qualifier for a class when we create a class.

@Component
@Qualifier("specificName")
public class className implements interface{....}

The advantage of this method is that there is no coupling. It means if you change the class name and you forget to change qualifier, it’s ok, still works.

Ⅳ、Use custom qualifier annotations
There is one thing you need to know:Java does not allow multiple annotations of the same type to be repeated on the same entry.Like:

   @Autowired//beanID1 and beanID2 are same type@Qualifier("beanID1")@Qualifier("beanID2")public void methodName(parameters)｛Method block｝

If you do that,compiler will tell you : “error!”.And the problem is what if we cannot find a way to choose the specific-bean in a group because there are many bean with same properties,what do we do?
Yes,use custom qualifier annotations.
HOW?

@Target({ElementType.CONSTRUCTOR,ElementType.FIELD,ElementType.METHOD,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Qualifier
public @interface className{ }

Now you can use a custom qualifier annotations : @className

If there are any shortcomings, please point them out,thanks!

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Spring_1_Deal with the ambiguity of automatic assembly（处理自动装配的歧义问题）

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