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