Spring Autowiring

1. Introduction

Spring Autowiring is a feature that automatically injects dependencies into a bean.

It helps to:

• Reduce manual configuration

• Make code clean and maintainable

• Support loose coupling

2. What is Autowiring

Autowiring means:

• Spring automatically finds and injects dependent objects

• No need to manually configure dependencies

3. Types of Autowiring

Spring supports the following types:

1. no (Default)

• No autowiring

• Dependencies must be set manually

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<bean id="state" class="sample.State">
    <property name="name" value="UP"/>
</bean>

<bean id="city" class="sample.City"/>

2. byName

• Dependency injected using bean name

• Property name must match bean id

• Uses setter method

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<bean id="city" class="sample.City" autowire="byName"/>

3. byType

• Dependency injected based on type (class)

• Bean name does not matter

• Error if multiple same type beans

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<bean id="city" class="sample.City" autowire="byType"/>

4. constructor

• Injection through constructor

• Uses type matching

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<bean id="city" class="sample.City" autowire="constructor"/>

5. autodetect

• First tries constructor, then byType

• Deprecated in modern Spring

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<bean id="city" class="sample.City" autowire="autodetect"/>

4. Example of Autowiring (byName)

Step 1: Create State Class

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package sample;

public class State {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

Step 2: Create City Class

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package sample;

public class City {

    private int id;
    private String name;

    private State state;

    public void setState(State state) {
        this.state = state;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void showCityDetails() {
        System.out.println("City Id : " + id);
        System.out.println("City Name : " + name);
        System.out.println("State : " + state.getName());
    }
}

Step 3: XML Configuration

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<bean id="state" class="sample.State">
    <property name="name" value="UP"/>
</bean>

<bean id="city" class="sample.City" autowire="byName"/>

Step 4: Main Class

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package sample;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class DemoApplication {

    public static void main(String[] args) {

        ApplicationContext context =
            new ClassPathXmlApplicationContext("applicationContext.xml");

        City city = context.getBean("city", City.class);

        city.setId(1);
        city.setName("Varanasi");

        city.showCityDetails();
    }
}

5. Advantages

• Less configuration

• Cleaner code

• Easy dependency management

6. Disadvantages

• Less control

• Issues when multiple beans exist

• Not suitable for primitive and String injection

7. Conclusion

• Autowiring simplifies dependency injection

• byName and byType are commonly used

• Annotation-based autowiring is preferred in Spring Boot