Spring Auto-Wiring Using Annotations
Typically in Spring, dependency injection is achieved using
However, in large applications, the number of beans will increase and the corresponding XML written to configure the numerous beans will become very large and unwieldly.
Spring provides a feature called ‘Auto-Wiring’ that minimizes the XML to be written provided that certain assumptions are made about the nomenclature of beans and properties. Using ‘Auto-Wiring’ feature, the number of tags like
is significantly minimized and the size of XML is reduced considerably for large applications.
However, in ‘Auto-wiring’ the bean definitions using
tag still need to be defined within spring-config.xml.
Spring provides a way to avoid declaring even the bean definitions within spring-config.xml thereby automatically discovering beans to be injected.
Automatic discovery of beans is based on the following rules:
- Use context:annotation-config tag in spring-config.xml to let Spring use Annotations
- Use context:component-scan tag in spring-config.xml and tell Spring the package in which to look for auto-discovering beans
- Use @Component annotation to mark a class as a Spring auto-discoverable bean
annotation is used, then the bean declarations need not be declared in spring-config.xml and this minimizes XML to a great extent.
Sample Program Overview
The sample program showcases the automatic discovery of beans by Spring framework using
We will create the Person class with members as name and email. We will then annotate the Person class with
We will also create the spring-config.xml and use the
tag in spring-context.xml to let Spring know that annotation based auto-wiring will be used.. We will also use the
tag and provide the name of the package for Spring to look into for auto-discovery.
Finally, we will test our setup using TestAnnotationAutoDiscovery class which will load Spring context and get a reference to Person class. We will print name and email of the Person bean on the console to verify that automatic discovery of beans using
annotation has occured successfully.
Create the Person (see sample code below).
annotation on Person class (see line 5 below).
Create members name and email (see lines 9-10 below)
Create accessor methods for name and email (see lines 11-22 below).
Create the spring-config.xml file (see below).
Let Spring know that annotation based auto-wiring will be used by declaring the tag
tag(see line 14 below)
Declare the tag
tag and provide the name of the package that Spring should look into for automatic discovery of beans(see line 16 below)
Note that there are no bean declarations in spring-config.xml file thereby highlighting the automatic bean discovery mechanism of Spring.
Finally, we need a java program to test the our setup.This is done by TestAnnotationAutoDiscovery (see source code below).
We need to tell Spring framework to use the ‘spring-config.xml’ to load our beans (see line 11 below).
We get the reference to Person class through Spring using the bean name ‘person’ (see line 12 below).
We access the getName() and getEmail() methods of Person class and print the output to verify that automatic discovery of beans using
annotation has occured successfully (see lines 13-14 below).
This sample program has been packaged as a jar installer which will copy the source code (along with all necessary dependencies) on your machine and automatically run the program for you as shown in the steps below. To run the sample program, you only need Java Runtime Environment (JRE) on your machine and nothing else.
(Alternatively you can go the folder containing the springannotationautodiscovery-installer.jar and execute the jar using
java -jar springannotationautodiscovery-installer.jar
This source code for this program is downloaded in the folder specified by you (say, C:\Temp) as an eclipse project called
. All the required libraries have also been downloaded and placed in the same location. You can open this project from Eclipe IDE and directly browse the source code. See below for details of the project structure.