Auto-wiring in Spring

The Spring framework can inject dependencies automatically.
The Spring container detects those dependencies specified in the configuration file and the relationship between the beans. This is referred to as Autowiring in Spring.

Key points:

In other words, the Autowiring feature of the Spring framework enables you to inject the object dependency implicitly.
It Internally uses the setter or constructor injection.
Autowiring can't be used to inject primitive and String values.
It works with reference only.

Advantages of Autowiring:

We don't need to write code to inject the dependency explicitly. So it requires less code.

Disadvantages of Autowiring:

It can't be used for primitive and String values.
No control of the programmer.

Modes of Autowiring

There are five modes of auto wiring:

1. No

This mode tells the framework that auto-wiring is not supposed to be done. It is the default mode used by Spring.

2.byName

It uses the name of the bean for injecting dependencies. However, it requires that the name of the property and bean must be the same.
It invokes the setter method internally for auto wiring.

3.byType

It injects the dependency according to the type of bean. It looks up in the configuration file for the class type of the property. If it finds a bean that matches, it injects the property. If not, the program throws an error.

4. constructor

It injects the required dependencies by invoking the constructor.
It works similarly to the “byType” mode but looks for the constructor arguments' class type.

5. autodetect

The autodetect mode uses two other ways for auto wiring – constructor and byType. It first tries to auto-wire via the constructor mode and if it fails, it uses the byType way for auto-wiring.
It works in Spring 2.0 and 2.5 but is deprecated from Spring 3.0 onwards.

Example:

Engine.java

package com.quipoin;

public class Engine {
	private String model;
	private String capacity;
	public String getModel() {
		return model;
	}
	public void setModel(String model) {
		this.model = model;
	}
	public String getCapacity() {
		return capacity;
	}
	public void setCapacity(String capacity) {
		this.capacity = capacity;
	}

}

Vehicle.java

package com.quipoin;

import org.springframework.beans.factory.annotation.Autowired;

public class Vehicle {
	private String vno;
	private String  vname;
	private Engine engine;
	public String getVno() {
		return vno;
	}
	public void setVno(String vno) {
		this.vno = vno;
	}
	public String getVname() {
		return vname;
	}
	public void setVname(String vname) {
		this.vname = vname;
	}
	public Engine getEngine() {
		return engine;
	}
	@Autowired
	public void setEngine(Engine engine) {
		this.engine = engine;
	}
}

MyApplication.java

package com.quipoin;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MyApplication {
	@Bean
	public Engine e1() {
		Engine e1=new Engine();
		e1.setModel("K10 Engine");
		e1.setCapacity("1.2hpw");
		return e1;
	}
	@Bean
	public Vehicle v1() {
		Vehicle v1=new Vehicle();
		v1.setVno("Ka51");
		v1.setVname("Swift");
		return v1;
	}
}

MyProgram.java

package com.quipoin;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class MyProgram {
	public static void main(String[] args) {
		ApplicationContext context=new AnnotationConfigApplicationContext(MyApplication.class);
		Vehicle v1=(Vehicle) context.getBean("v1");

		System.out.println("Vehicale Number: "+v1.getVno());
		System.out.println("Vehicle Name: "+v1.getVname());
		System.out.println("Vehicle Engine Model: "+v1.getEngine().getModel());
		System.out.println("Vehicle Engine Capacity: "+v1.getEngine().getCapacity());
	}
}

Output:

Vehicale Number: Ka51
Vehicle Name: Swift
Vehicle Engine Model: K10 Engine
Vehicle Engine Capacity: 1.2hpw