Spring MVC Fast Tutorial: Dependency Injection

Use singletons of CarManager and BrandManager instead of creating multiple instances.

Dependency injection (also called inversion of control) is basically giving an object what it needs instead of letting this object get it by itself.

For the moment, CarListController gets its instance of CarManager by itself:

CarManager carManager = new CarManager();

Let's change that by making it an attribute:

package springmvc.web;
 
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.Controller;
 
import springmvc.service.CarManager;
 
public class CarListController implements Controller {
	private CarManager carManager;
 
	public ModelAndView handleRequest(HttpServletRequest arg0,
			HttpServletResponse arg1) throws Exception {
 
		ModelAndView modelAndView = new ModelAndView("carList");
		modelAndView.addObject("carList", this.carManager.getCarList());
 
		return modelAndView;
	}
 
	public CarManager getCarManager() {
		return carManager;
	}
 
	public void setCarManager(CarManager carManager) {
		this.carManager = carManager;
	}
}

We now need to instantiate a CarManager somewhere else and pass it to this controller. It's done in '/WEB-INF/springmvc-servlet.xml':

<bean id="carManager" class="springmvc.service.CarManager"/>
 
<bean name="/list_cars.html" class="springmvc.web.CarListController">
  <property name="carManager" ref="carManager"/>
</bean>

• a bean 'carManager' is created
• it's used to initialize 'carManager' of CarListController.

We build (ant), relaunch Tomcat: http://localhost:8180/springmvc/list_cars.html

Do it by yourself, it's exactly the same. In the configuration file, use the CarManager instance you've already declared for '/new_car.html'.

Since we now use a single instance of CarManager for our whole application, we don't need its carList attribute to be static anymore:

package springmvc.service;
 
import java.math.BigDecimal;
import java.util.LinkedList;
import java.util.List;
 
import springmvc.model.Brand;
import springmvc.model.Car;
 
public class CarManager {
 
	private List<Car> carList;
 
	public CarManager() {
		Brand brand1 = new Brand();
		brand1.setId((long)1);
		brand1.setName("Mercedes");
		brand1.setCountry("Germany");		
 
		Brand brand2 = new Brand();
		brand2.setId((long)2);
		brand2.setName("Peugeot");
		brand2.setCountry("France");		
 
		Car car1 = new Car();
		car1.setId((long)1);
		car1.setBrand(brand1);
		car1.setModel("SL 500");
		car1.setPrice(new BigDecimal(40000));
 
		Car car2 = new Car();
		car2.setId((long)2);
		car2.setBrand(brand2);
		car2.setModel("607");
		car2.setPrice(new BigDecimal(35000));
 
		carList = new LinkedList<Car>();
		carList.add(car1);
		carList.add(car2);	
	}
 
	public List<Car> getCarList() {
		return carList;
	}	
 
	public Car createCar(Car c) {
		Car car = new Car();
		car.setId((long)carList.size() + 1);
		car.setBrand(c.getBrand());
		car.setModel(c.getModel());
		car.setPrice(c.getPrice());
 
		carList.add(car);
 
		return car;
	}
}

Let's do the same for BrandManager:

• define an instance of BrandManager in the configuration file
• use it for CarNewController (add a private attribute, remove instantiations)
• make its brandList attribute not static

We can go even further: initialize carList (CarManager) and brandList (BrandManager) in Spring configuration file:

    <bean id="carManager" class="springmvc.service.CarManager">
       <property name="carList">
         <list>
           <ref bean="car1"/>
           <ref bean="car2"/>
        </list>
        </property>
    </bean>    
 
    <bean id="brandManager" class="springmvc.service.BrandManager">
      <property name="brandList">
        <list>
          <ref bean="brand1"/>
          <ref bean="brand2"/>
        </list>
      </property>
    </bean>
 
    <bean id="brand1" class="springmvc.model.Brand">
        <property name="id" value="1"/>
        <property name="name" value="Mercedes"/>
        <property name="country" value="Germany"/>
    </bean>
 
    <bean id="brand2" class="springmvc.model.Brand">
        <property name="id" value="2"/>
        <property name="name" value="Peugeot"/>
        <property name="country" value="France"/>
    </bean>
 
    <bean id="car1" class="springmvc.model.Car">
        <property name="id" value="1"/>
        <property name="brand" ref="brand1"/>
        <property name="model" value="SL 500"/>
        <property name="price" value="40000"/>
    </bean>
 
    <bean id="car2" class="springmvc.model.Car">
        <property name="id" value="2"/>
        <property name="brand" ref="brand2"/>
        <property name="model" value="607"/>
        <property name="price" value="35000"/>
    </bean>

• we define brands
• we define cars using the brands
• we initialize CarManager and BrandManager lists
• notice the difference between the 'value' attribute, where you put a primary type and 'ref', where you give a bean name.

To make it work, we just need to add a setter method for CarManager and BrandManager's lists and remove the now useless initialization code. In 'WEB-INF/src/springmvc/service/BrandManager.java':

package springmvc.service;
 
import java.util.List;
 
import springmvc.model.Brand;
 
public class BrandManager {
 
	private List<Brand> brandList;
 
	public List<Brand> getBrandList() {
		return brandList;
	}	
 
	public void setBrandList(List<Brand> brandList) {
		this.brandList = brandList;
	}
 
	public Brand getBrandById(Long id) {
		for (Brand brand : brandList) {
			if (brand.getId().equals(id))
				return brand;
		}
		return null;
	}
}

In 'WEB-INF/src/springmvc/service/CarManager.java':

package springmvc.service;
 
import java.util.List;
 
import springmvc.model.Car;
 
public class CarManager {
 
	private List<Car> carList;
 
	public List<Car> getCarList() {
		return carList;
	}	
 
	public void setCarList(List<Car> carList) {
		this.carList = carList;
	}
 
	public Car createCar(Car c) {
		Car car = new Car();
		car.setId((long)carList.size() + 1);
		car.setBrand(c.getBrand());
		car.setModel(c.getModel());
		car.setPrice(c.getPrice());
 
		carList.add(car);
 
		return car;
	}
}

Much cleaner, isn't it?

We rebuild (ant), relaunch Tomcat: http://localhost:8180/springmvc/list_cars.html

Everything should still work.. You can download the project here.

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