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Spring @Autowired Annotation

The @Autowired annotation provides more fine-grained control over where and how autowiring should be accomplished. The @Autowired annotation can be used to autowire bean on the setter method just like @Required annotation, constructor, a property or methods with arbitrary names and/or multiple arguments.

@Autowired on Setter Methods

You can use @Autowired annotation on setter methods to get rid of the <property> element in XML configuration file. When Spring finds an @Autowired annotation used with setter methods, it tries to perform byType autowiring on the method.

Let us have working Eclipse IDE in place and follow the following steps to create a Spring application −

Here is the content of TextEditor.java file −

Following is the content of another dependent class file SpellChecker.java :

Following is the content of the MainApp.java file −

Following is the configuration file Beans.xml −

Once you are done with creating source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message −

@Autowired on Properties

You can use @Autowired annotation on properties to get rid of the setter methods. When you will pass values of autowired properties using <property> Spring will automatically assign those properties with the passed values or references. So with the usage of @Autowired on properties your TextEditor.java file will become as follows −

Once you are done with the above two changes in source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message −

@Autowired on Constructors

You can apply @Autowired to constructors as well. A constructor @Autowired annotation indicates that the constructor should be autowired when creating the bean, even if no <constructor-arg> elements are used while configuring the bean in XML file. Let us check the following example.

Once you are done with the above two changes in source and bean configuration files, let us run the application. If everything is fine with your application, this will print the following message:

@Autowired with (required = false) option

By default, the @Autowired annotation implies the dependency is required similar to @Required annotation, however, you can turn off the default behavior by using (required=false) option with @Autowired.

The following example will work even if you do not pass any value for age property but still it will demand for name property. You can try this example yourself because this is similar to @Required annotation example except that only Student.java file has been changed.

Spring – @Autowired

Satish Varma

@Autowired is one of the key annotation in annotation based Dependency Injection. Since version 2.5, Spring provides the @Autowired annotation to discover the beans automatically and inject collaborating beans (other associated dependent beans) into our bean.

In this guide we will look into enabling auto-wiring and various ways of autowiring beans using @Autowired annotation in Spring and Spring Boot application.

1.1. Using Spring XML

1.2. using java configuration, 1.3. @autowired in spring boot, 2.1. @autowired on constructors, 2.2. @autowired on setters and on methods, 2.3. @autowired on properties, 2.4. @autowired on parameters, 3. @autowired by type, 4. ambiguity in using @autowired, 5. @autowired by name, 6. @autowired by @qualifier.

7.2. Using @Autowired with Java 8 Optional

8. conclusion, 9. references, 1. enable configuration to use @autowired.

By declaring all the beans in Spring Configuration file, Spring container can autowire relationships between collaborating beans. Following are the various configuration styles can be used to use @Autowired in Spring applications and you can use mixed configurations within same application.

<context:annotation-config /> is used in XML config files to enable annotation based dependency Injection.

Sample Configuration file :

Following is the sample java based configuration to use @Autowired to inject dependencies with in com.javabydeveloper.spring.autowire package.

In a Spring Boot application, auto-wiring is enabled by default. Following is sample code. @SpringBootApplication automatically loads all configuration files and scans the components in the current package and its sub-packages.

2. Using @Autowired

After enabling annotation based injection, now let’s look into how we can use @Autowired annotation. @Autowired can be used on following injection points.

and dependencies can be injected using by type OR by name OR by @Qualifier.

Firstly let’s use @Autowired on constructor. we will see that SampleService injected by Spring to the constructor of AutowireDITestService .

Let’s try adding @Autowired on setter method.

In following example, the setter method is called with ExampleService instance when AutowireDITestService created.

We can also apply @Autowired on methods with any number of arguments.

We can use @Autowired on properties, so that we can avoid setters and getters for the autowired properties.

Since 5.0, Spring supports @Autowired annotation on individual methods and constructor parameters. But the only part of the core Spring Framework that actively supports autowired parameters is the JUnit Jupiter support in the  spring-test  module.

By default, Spring does auto-wire by type. we have a class type UserDao and it’s an implementation of IUserDao interface. Let’s try auto-wire it in service. There should be only one bean configured for implementation class type for IUserDao interface in the spring bean, Spring automatically inject by it’s type.

By default, Spring does auto-wire by type. If spring found more than one bean of same type in Spring container, it leads to ambiguity issues and framework trows no-unique bean exceptions. To resolve ambiguity conflicts we need to tell Spring which bean need to inject by Using @Autowired by name OR by specifying bean with @Qualifier OR @Primary .

Let’s say we have IDao interface CustomerDao and EmployeeDao are its implementation classes. Spring will inspect the container and look for a bean with the exact name as the property to autowire it. Let’s see an example.

@Qualifier is used to identify particular bean when you have several implementations of same type. Let’s see an example.

7. @Autowired with Optional dependencies

In few cases injecting dependency might be optional, but all the autowired dependencies should be available when a bean is being constructed, otherwise Spring throws exception. There are various ways we can make injecting dependency optional using @Autowired.

7.1. Using @Autowired(required = false )

@Autowired annotation required attribute by default value is true , it means dependency should be available while constructing the bean. We can use @Autowired(required = false) to make injecting dependency is optional by Spring.

Alternatively we can use Java 8 Optional instead of required element. Let’s see an example.

In this guide we have seen autowiring beans using @Autowired annotation various ways Spring and Spring Boot applications.

You can get source code from our GitHub repository.

Other Related Tutorials:

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Example of Autowiring

Let's see the simple code to use autowiring in spring. You need to use autowire attribute of bean element to apply the autowire modes.

Let's see the full example of autowiring in spring. To create this example, we have created 4 files.

This class contains a constructor and method only.

This class contains reference of B class and constructor and method.

This class gets the bean from the applicationContext.xml file and calls the display method.

1) byName autowiring mode

In case of byName autowiring mode, bean id and reference name must be same.

It internally uses setter injection.

But, if you change the name of bean, it will not inject the dependency.

Let's see the code where we are changing the name of the bean from b to b1.

2) byType autowiring mode

In case of byType autowiring mode, bean id and reference name may be different. But there must be only one bean of a type.

In this case, it works fine because you have created an instance of B type. It doesn't matter that you have different bean name than reference name.

But, if you have multiple bean of one type, it will not work and throw exception.

Let's see the code where are many bean of type B.

In such case, it will throw exception.

3) constructor autowiring mode

In case of constructor autowiring mode, spring container injects the dependency by highest parameterized constructor.

If you have 3 constructors in a class, zero-arg, one-arg and two-arg then injection will be performed by calling the two-arg constructor.

4) no autowiring mode

In case of no autowiring mode, spring container doesn't inject the dependency by autowiring.

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Spring – Autowiring

Spring is an open-source application development framework of Java that allows you to create robust enterprise applications using Plain Old Java Objects ( POJO in short). The Spring framework can inject dependencies automatically. The Spring container detects those dependencies specified in the configuration file and @ the relationship between the beans. This is referred to as autowiring in Spring. An autowired application requires fewer lines of code comparatively but at the same time, it provides very little flexibility to the programmer.

Modes of Autowiring

There are five modes of autowiring:

This mode tells the framework that autowiring is not supposed to be done. It is the default mode used by Spring.

It uses the name of the bean for injecting dependencies. However, it requires that the name of the property and bean must be the same. It invokes the setter method internally for autowiring.

It injects the dependency according to the type of the bean. It looks up in the configuration file for the class type of the property. If it finds a bean that matches, it injects the property. If not, the program throws an error. The names of the property and bean can be different in this case. It invokes the setter method internally for autowiring.

4. constructor

It injects the required dependencies by invoking the constructor. It works similar to the “byType” mode but it looks for the class type of the constructor arguments. If none or more than one bean are detected, then it throws an error, otherwise, it autowires the “byType” on all constructor arguments.

5. autodetect

The autodetect mode uses two other modes for autowiring – constructor and byType. It first tries to autowire via the constructor mode and if it fails, it uses the byType mode for autowiring. It works in Spring 2.0 and 2.5 but is deprecated from Spring 3.0 onwards.

Sample Program for the byType Mode

File Name: State.java

File Name: City.java

Spring bean configuration file:

Test Program file: DemoApplication.java

Output:  

Advantage of Autowiring

It requires less code because we don’t need to write the code to inject the dependency explicitly.

Disadvantage of Autowiring

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Spring Boot Annotation @Autowired of Service fails

I'm trying to use @Autowired annotation for a Service class in Spring Boot application, but it keeps throwing No qualifying bean of type exception. However, if I change the service class to a bean, then it works fine. This is my code:

And this is my SpringBoot class:

When I try to run this, I get the following exception:

But when I remove the @Service annotation from the SampleService class, and add it as a bean in my AppConfig class as below, it works fine:

The classes are in different packages. I am not using @ComponentScan . Instead, I'm using @SpringBootApplication which does that automatically. However, I tried with ComponentScan as well but that didn't help.

What am I doing wrong here?

drunkenfist's user avatar

You are using two ways to build a Spring's bean. You just need to use one of them.

@Service over the POJO

@Bean in the configuration class which must be annotated with @Configuration

@Autowired is resolved by class type then @Bean(name="sampleService") is not needed is you have only one bean with that class type.

package com.example

package com.example.config

package com.example.repository

package com.example.service

Using this code you can comment the AppConfig class and then you will see how UserRepository and UserService are autowired. After that comment @Service in each class and un-comment AppConfig and classes will be autowired too.

Max von Hippel's user avatar

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COMMENTS

  1. Spring @Autowired Annotation

    Spring @Autowired annotation is used for automatic dependency injection. Spring framework is built on dependency injection and we inject the class dependencies through spring bean configuration file.. Spring @Autowired Annotation. Usually we provide bean configuration details in the spring bean configuration file and we also specify the beans that will be injected in other beans using ref ...

  2. java

    @Autowired is an annotation introduced in Spring 2.5, and it's used only for injection.. For example: class A { private int id; // With setter and getter method } class B { private String name; @Autowired // Here we are injecting instance of Class A into class B so that you can use 'a' for accessing A's instance variables and methods.

  3. Spring @Autowired Annotation

    Spring @Autowired Annotation. The @Autowired annotation provides more fine-grained control over where and how autowiring should be accomplished. The @Autowired annotation can be used to autowire bean on the setter method just like @Required annotation, constructor, a property or methods with arbitrary names and/or multiple arguments.

  4. Spring

    April 29, 2023. @Autowired is one of the key annotation in annotation based Dependency Injection. Since version 2.5, Spring provides the @Autowired annotation to discover the beans automatically and inject collaborating beans (other associated dependent beans) into our bean. In this guide we will look into enabling auto-wiring and various ways ...

  5. annotation in spring boot @autowired

    To me, annotation is to save time for developer to implement less code, by definition online. But this doesn't satisfy my question below. So far, I am experiencing @autowired. What confused me is that @autowired enable dependency injection and tell bean to configure xml (which i dont understand how useful it is yet) For example,

  6. What exactly does @Autowired mean in Java?

    Makes the code organized and modular. Each component ( Class / Module ) is responsible for declaring its dependencies and Container ( Environment where this class is initialized ) provides the required dependencies. @Autowired is an annotation used by spring to denote that required dependency will be fulfilled by Spring.

  7. java

    6. the term "@Autowiring" seems to be misleading here. As you can also do autowiring with XML configuration, the original question should be rephrased as "Pros and cons of using Spring annotations". The provided answer focuses more on the aspects of autowiring and less on aspects of using annotations. - Neeme Praks.

  8. java

    (I mean why they don't recommend to use @Autowired) When we specify for Spring which beans it should embed in other classes, then we for example, use @Autowired. But this annotation is not recommended for use, but Data Binding is recommended why ?. This is the following answer as an additional one(may be related indirectly to the main question)...

  9. Spring pure annotation with autowired

    2. I am trying to do a full annotations (no xml) implementation of Spring. The autowired members are not being populated. From my research, there are 3 things to do: Set up a config file that manages the bean. Use @Autowired to get the bean to the file. Instantiate an application context to get the ball rolling.

  10. Autowiring in Spring

    It is the default autowiring mode. It means no autowiring bydefault. 2) byName: The byName mode injects the object dependency according to name of the bean. In such case, property name and bean name must be same. It internally calls setter method. 3) byType: The byType mode injects the object dependency according to type.

  11. java

    1. @autowired on a method is used for setter-injection. it is not different from field injection besides that the beans is not that dependent on the spring-container, you could instantiate and wire it yourself as well. one reason to use it is if you have circular dependencies.

  12. AutowiredAnnotationBeanPostProcessor (Spring Framework 6.0.9 API)

    The default autowired annotation types are the Spring-provided @Autowired and @Value annotations as well as the common @Inject annotation, if available. ... A 'required' dependency means that autowiring should fail when no beans are found. Otherwise, the autowiring process will simply bypass the field or method when no beans are found. ...

  13. @Autowired Annotation for Autowiring

    In this video we are going to learn how to use @Autowired annotations for automatic wiring in spring framework. This video is practical video .Download Sourc...

  14. Why and When use @Inject instead of @Autowired in Spring?

    Closed 3 years ago. Since Spring 3.0, Spring supports the standard JSR 330: Dependency Injection for Java. In a Spring 3 application, you can use. @Inject instead of Spring's @Autowired to inject a bean. @Named instead of Spring's @Component to declare a bean.

  15. Can we use @Autowired annotation as a constructor argument?

    @Service("idmHelper") public class IdmHelper { @Autowired Environment env; public IdmHelper() { env.getProperty('property-name') ... } } Here it gives a NullPointerException. Becuase env is null. Option 2 - I tried to use @Autowired inside the constructor as an argument, as shown in the following example.

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    Abstract. This solicitation overview provides grant information and resources for entities that may be interested in funding assistance from the Bureau of Justice Assistance (BJA) to receive reimbursement for certain types of costs incurred in connection with undocumented criminal aliens who had been convicted of at least one felony or two ...

  17. Spring

    The Spring framework can inject dependencies automatically. The Spring container detects those dependencies specified in the configuration file and @ the relationship between the beans. This is referred to as autowiring in Spring. An autowired application requires fewer lines of code comparatively but at the same time, it provides very little ...

  18. Apple looks to advance artificial intelligence for iPhones, iPads and

    Used 5/5/2023 VB. The Apple logo is seen at an Apple Store in Brooklyn, New York, U.S. October 23, 2020.

  19. Spring Boot Annotation @Autowired of Service fails

    1 Answer. You are using two ways to build a Spring's bean. You just need to use one of them. @Bean public SampleService sampleService () { return new SampleService (); } @Autowired is resolved by class type then @Bean (name="sampleService") is not needed is you have only one bean with that class type.