Asked  7 Months ago    Answers:  5   Viewed   29 times

I am reading the spring 3.0.x reference documentation to understand Spring Autowired annotation:

3.9.2 @Autowired and @Inject

I am not able to understand the below examples. Do we need to do something in the XML for it to work?

EXAMPLE 1

public class SimpleMovieLister {

    private MovieFinder movieFinder;

    @Autowired
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }

    // ...
}

EXAMPLE 2

public class MovieRecommender {

    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(MovieCatalog movieCatalog,
                    CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }

    // ...
}

How can the two classes be autowired implementing the same interface and using the same class?

Example:

class Red implements Color
class Blue implements Color

class myMainClass{
    @Autowired 
    private Color color;

    draw(){
        color.design(); 
    } 
}

Which design method will be called? How do I make sure the design method of Red class will be called and not Blue?

 Answers

98

TL;DR

The @Autowired annotation spares you the need to do the wiring by yourself in the XML file (or any other way) and just finds for you what needs to be injected where and does that for you.

Full explanation

The @Autowired annotation allows you to skip configurations elsewhere of what to inject and just does it for you. Assuming your package is com.mycompany.movies you have to put this tag in your XML (application context file):

<context:component-scan base-package="com.mycompany.movies" />

This tag will do an auto-scanning. Assuming each class that has to become a bean is annotated with a correct annotation like @Component (for simple bean) or @Controller (for a servlet control) or @Repository (for DAO classes) and these classes are somewhere under the package com.mycompany.movies, Spring will find all of these and create a bean for each one. This is done in 2 scans of the classes - the first time it just searches for classes that need to become a bean and maps the injections it needs to be doing, and on the second scan it injects the beans. Of course, you can define your beans in the more traditional XML file or with an @Configuration class (or any combination of the three).

The @Autowired annotation tells Spring where an injection needs to occur. If you put it on a method setMovieFinder it understands (by the prefix set + the @Autowired annotation) that a bean needs to be injected. In the second scan, Spring searches for a bean of type MovieFinder, and if it finds such bean, it injects it to this method. If it finds two such beans you will get an Exception. To avoid the Exception, you can use the @Qualifier annotation and tell it which of the two beans to inject in the following manner:

@Qualifier("redBean")
class Red implements Color {
   // Class code here
}

@Qualifier("blueBean")
class Blue implements Color {
   // Class code here
}

Or if you prefer to declare the beans in your XML, it would look something like this:

<bean id="redBean" class="com.mycompany.movies.Red"/>

<bean id="blueBean" class="com.mycompany.movies.Blue"/>

In the @Autowired declaration, you need to also add the @Qualifier to tell which of the two color beans to inject:

@Autowired
@Qualifier("redBean")
public void setColor(Color color) {
  this.color = color;
}

If you don't want to use two annotations (the @Autowired and @Qualifier) you can use @Resource to combine these two:

@Resource(name="redBean")
public void setColor(Color color) {
  this.color = color;
}

The @Resource (you can read some extra data about it in the first comment on this answer) spares you the use of two annotations and instead, you only use one.

I'll just add two more comments:

  1. Good practice would be to use @Inject instead of @Autowired because it is not Spring-specific and is part of the JSR-330 standard.
  2. Another good practice would be to put the @Inject / @Autowired on a constructor instead of a method. If you put it on a constructor, you can validate that the injected beans are not null and fail fast when you try to start the application and avoid a NullPointerException when you need to actually use the bean.

Update: To complete the picture, I created a new question about the @Configuration class.

Tuesday, June 1, 2021
 
sassy_geekette
answered 7 Months ago
16

For a long time I believed that there was a value in having a "centralized, declarative, configuration" like the xml files we all used to use. Then I realized that most of the stuff in the files wasn't configuration - it was never changed anywhere after development, ever. Then I realized that "centralized" only has value in quite small systems - only in small systems will you ever be able to grok a configuration file as a whole. And what is really the value of understanding the wiring as a whole, when the same "wirings" are mostly duplicated by dependencies in the code? So the only thing I've kept is meta-data (annotations), which is still kind-of declarative. These never change at runtime and they're never "configuration" data that someone will change on the fly - so I think keeping it in the code is nice.

I use full auto-wiring as much as I can. I love it. I won't go back to old-style spring unless threatened at gun-point. My reasons for preferring fully @Autowired have changed over time.

Right now I think the most important reason for using autowiring is that there's one less abstraction in your system to keep track of. The "bean name" is effectively gone. It turns out the bean name only exists because of xml. So a full layer of abstract indirections (where you would wire bean-name "foo" into bean "bar") is gone. Now I wire the "Foo" interface into my bean directly, and implementation is chosen by run-time profile. This allows me to work with code when tracing dependencies and implementations. When I see an autowired dependency in my code I can just press the "go to implementation" key in my IDE and up comes the list of known implementations. In most cases there's just one implementation and I'm straight into the class. Can't be much simpler than that, and I always know exactly what implementation is being used (I claim that the opposite is closer to the truth with xml wiring - funny how your perspective changes!)

Now you could say that it's just a very simple layer, but each layer of abstraction that we add to our systems increase complexity. I really don't think the xml ever added any real value to any system I've worked with.

Most systems I've ever work with only have one configuration of the production runtime environment. There may be other configurations for test and so on.

I'd say that full autowiring is the ruby-on-rails of spring: It embraces the notion that there's a normal and common usage pattern that most use cases follow. With XML configuration you permit a lot of consistent/inconsistent configuration usage that may/may not be intended. I've seen so much xml configuration go overboard with inconsistencies - does it get refactored together with the code ? Thought not. Are those variations there for a reason? Usually not.

We hardly use qualifiers in our configuration, and found other ways to solve these situations. This is a clear "disadvantage" we encounter: We've slightly changed the way we code to make it interact smoother with autowiring: A customer repository no longer implements the generic Repository<Customer> interface but we make an interface CustomerRepository that extends Repository<Customer>. Sometimes there's also a trick or two when it comes to subclassing. But it usually just points us in the direction of stronger typing, which I find is almost always a better solution.

But yes, you're tying to a particular style of DI that mostly spring does. We don't even make public setters for dependencies any more (So you could argue that we're +1 in the encapsulation/information hiding department) We still have some xml in our system, but the xml basically only contains the anomalies. Full autowiring integrates nicely with xml.

The only thing we need now is for the @Component, @Autowired and the rest to be included in a JSR (like JSR-250), so we don't have to tie in with spring. This is the way things have been happening in the past (the java.util.concurrent stuff springs to mind), so I wouldn't be entirely surprised if this happened again.

Thursday, July 8, 2021
 
Joegramming
answered 5 Months ago
18

You're wanting an EntityManager injected, but you don't have any injection annotations on the field, and "factory" is the wrong qualifier in any case. Annotate your field with @PersistenceContext, using unitName if needed to distinguish between multiple persistence units.

Answer to edited question: Your XML file isn't getting included, because you start from the Application class, and Spring just follows the @ComponentScan from there. Add this import to your Application class (or any scanned @Configuration class):

@ImportResource("classpath:spring-config.xml")

Alternately, you can migrate your XML configuration to the JavaConfig format, which Spring will component-scan.

Sunday, August 1, 2021
 
rkeet
answered 4 Months ago
24

Then I found this: http://www.mkyong.com/mongodb/spring-asm-dependency-issue-in-spring-data/ with a similar problem in spring-data. The exclusion in the pom.xml works just as well:

    <dependency>
        <groupId>org.springframework.security</groupId>
        <artifactId>spring-security-web</artifactId>
        <version>3.1.4.RELEASE</version>
        <exclusions>
            <exclusion>
                <groupId>org.springframework</groupId>
                <artifactId>spring-asm</artifactId>
            </exclusion>
        </exclusions>
    </dependency>

I share the solution for others who get the same error!

Monday, August 2, 2021
 
Markol
answered 4 Months ago
50

When you call the init method from the constructor of class App, Spring has not yet autowired the dependencies into the App object. If you want to call this method after Spring has finished creating and autowiring the App object, then add a method with a @PostConstruct annotation to do this, for example:

@SpringBootApplication
public class App {
    @Autowired
    public Starter starter;

    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }

    public App() {
        System.out.println("constructor of App");
    }

    @PostConstruct
    public void init() {
        System.out.println("Calling starter.init");
        starter.init();
    }
}
Wednesday, August 11, 2021
 
Seibar
answered 4 Months ago
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