[JAVA] [Spring] Autowired multiple beans. (With bonus)

Can be used in such cases

When you want to use multiple (0 ~ n) beans registered in the container by @Autowired.

Premise

Two classes that implement the MyService interface (where doSomething is defined) are defined and registered in the container.

@Component("myServiceImpl1")
public class MyServiceImpl1 implements MyService {

	@Override
	public void doSomething() {
		System.out.println("myServiceImpl1!!")
	}

}

@Component("myServiceImpl2")
public class MyServiceImpl2 implements MyService {

	@Override
	public void doSomething() {
		System.out.println("myServiceImpl2!!")
	}

}

The user wants to use both of them @Autowired. However, if you normally @Autowired, you will get angry with a duplicate bean error, and it is nonsense (depending on your requirements) to add @Qualifier and @Autowired one by one.

Nonsense injection example

@Qualifier("myServiceImpl1")
@Autowired
private MyService service1;

@Qualifier("myServiceImpl2")
@Autowired
private MyService service2;

manner

To @Autowired multiple beans, use the List class on the @ Autowired side.

MyServiceExecuter.java

@Autowired
private List<MyService> services;

public void execServices(){
	services.forEach(s -> s.doSomething());
}

output

myServiceImpl1!!
myServiceImpl2!!

Bonus: Besides List ...

Looking at it in this way, it seems that there are various classes other than List that can be done using Generics.

Map class

If you enter @Autowired in the Map class, you can get a Map with Key as Bean Name and Value as Bean.

@Autowired
private Map<String,MyService> services;

public void printServices(){
	services.get("myServiceImpl1").doSomething();
	services.get("myServiceImpl2").doSomething();
}

output

myServiceImpl1!!
myServiceImpl2!!

Optional class

I don't know if the target bean is registered in the container. If it is registered, I want to use that bean, and if not, I don't have to inject it. In that case

@Autowired(required=false)
private MyService service;

Many of you know that you can avoid the NoSuchBeanDefinitionException by writing required = false. But if you want to write null-safe code, you can use *** Optional. *** ***

@Autowired
private Optional<MyService> service;

If you write this, @Autowired does not need required = false, and above all, it is null safe.

Summary

Spring DI is deep.

I did not find any information about the feeling of a quick search, so I summarized it.

Environment ↓

springBootVersion = '2.0.5.RELEASE'

[Addition] I changed the expression. 2018.09.24