Spring, Spring Boot and Component Scan


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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 do a Component Scan automatically?
  • How do you define Component Scan with Spring Boot?
  • How do you resolve problems involving Component Scan?

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@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 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

Executive Summary

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

####### Detailed Example

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 on SpringbootIn10StepsApplication class which is package com.in28minutes.springboot.basics.springbootin10steps.

@SpringBootApplication defines an automatic component scan on 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 a package com.in28minutes.springboot.somethingelse

In this case, you would need add the new package into component scan.

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.

####### 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 {

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####### XML Application Context

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

or Specific Multiple Packages

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

URL Not working

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.

Three possible things you would need to look at a. You have not added the right annotation - @Controller, @Repository or @Controller b. You have not added a component scan. c. 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

What is the difference between @Component and @ComponentScan?

@Component and @ComponentScan are for different purposes.

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

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