Spring, Spring Boot, and Component Scan

This guide will help you understand the most important concept in Spring — Component Scan. Spring Boot does some magic around Component Scan. Let’s understand that in this article.

You Will Learn

• What is Component Scan?
• Why is Component Scan important?
• Which packages does Spring Boot scan automatically?
• How do you define Component Scan with Spring Boot?
• How do you resolve problems involving Component Scan?

@ComponentScan

If you understand Component Scan, you understand Spring.

Spring is a dependency injection framework. It is all about beans and wiring in dependencies.

The first step of defining Spring Beans is by adding the right annotation — @Component or @Service or @Repository.

However, Spring does not know about the bean unless it knows where to search for it.

This part of “telling Spring where to search” is called a Component Scan.

You define the packages that have to be scanned.

Once you define a Component Scan for a package, Spring would search the package and all its sub packages for components/beans.

Defining a Component Scan

• If you are using Spring Boot, check the configuration in Approach 1.
• If you are doing a JSP/Servlet or a Spring MVC application without using Spring Boot, use Approach 2.

Approach 1: Component Scan in a Spring Boot Project

• If your other package hierarchies are below your main app with the @SpringBootApplication annotation, you’re covered by the implicit Component Scan.
• If there are beans/components in other packages that are not sub-packages of the main package, you should manually add them as @ComponentScan

Consider the class below:

package com.in28minutes.springboot.basics.springbootin10steps;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication
public class SpringbootIn10StepsApplication {

    public static void main(String[] args) {
        ApplicationContext applicationContext =
            SpringApplication.run(SpringbootIn10StepsApplication.class, args);

        for (String name: applicationContext.getBeanDefinitionNames()) {
            System.out.println(name);
        }
    }
}

@SpringBootApplication is defined in the SpringbootIn10StepsApplication class which is in the package com.in28minutes.springboot.basics.springbootin10steps

@SpringBootApplication defines an automatic Component Scan on the package com.in28minutes.springboot.basics.springbootin10steps.

You are fine if all your components are defined in the above package or a sub-package of it.

However, let’s say one of the components is defined in package com.in28minutes.springboot.somethingelse

In this case, you would need to add the new package into Component Scan.

You have two options:

• Define @ComponentScan(“com.in28minutes.springboot”) 
  • This would scan the entire parent tree of com.in28minutes.springboot.
• Or define two specific Component Scans by using an array. 
  • @ComponentScan({“com.in28minutes.springboot.basics.springbootin10steps”,”com.in28minutes.springboot.somethingelse”})

Option 1:

@ComponentScan(“com.in28minutes.springboot”)
@SpringBootApplication
public class SpringbootIn10StepsApplication {

Option 2:

@ComponentScan({"com.in28minutes.springboot.basics.springbootin10steps","com.in28minutes.springboot.somethingelse"})
@SpringBootApplication
public class SpringbootIn10StepsApplication {

Approach 2: Non-Spring Boot Project

In a non-Spring Boot Project, we would typically define the component scan explicitly in an XML application context or a Java Application Context.

Option 1:

@ComponentScan(“com.in28minutes)
@Configuration
public class SpringConfiguration {

Option 2:

@ComponentScan({"com.in28minutes.package1","com.in28minutes.package2"})
@Configuration
public class SpringConfiguration {

XML application context:

<context:component-scan base-package="com.in28minutes" />

Specific multiple packages:

<context:component-scan base-package="com.in28minutes.package1, com.in28minutes.package2" />

URL Not Working

The server starts up fine, but:

• My URL is not working
• My login URL is not working
• My todo URL is not working ``` WARNING: No mapping found for HTTP request with URI [/spring-mvc/login] in DispatcherServlet with name ‘dispatcher’ WARNING: No mapping found for HTTP request with URI [/login] in DispatcherServlet with name ‘dispatcher’ WARNING: No mapping found for HTTP request with URI [/list-todos] in DispatcherServlet with name ‘dispatcher’

#### No qualifying bean of type found

No qualifying bean of type [com.in28minutes.springboot.jpa.UserRepository] found for dependency [com.in28minutes.springboot.jpa.UserRepository]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

```

Same root cause for both above problems — the component is not being picked up.

There are three possible things you would need to look at:

• You have not added the right annotation — @Controller, @Repository, or @Controller
• You have not added a Component Scan.
• The package of your component is not defined in Component Scan.

You have two options:

1. Add the annotation or component scan
2. Move the component to a package already under Component Scan

@Component vs. @ComponentScan

@Component and @ComponentScan are for different purposes.

• @Component indicates that a class might be a candidate for creating a bean. It's like putting a hand up.
• @ComponentScan is searching packages for Components. Trying to find out who all put their hands up.