How to use constructor injection in a main program

Question

my question is verry simple: using the constructor injection, how can I get the attributes of a class ClassA declared as attribut in a class ClassB, knowing that the ClassA has just a constructur (no setter and getter)?

here i will write my java code :

ClassA:

public class ClassA {

  public int x1;
  public String x2;

  ClassA(int x1, String x2) {
    this.x1 = x1;
    this.x2 = x2;
  }
}

ClassB:

public class ClassB {

  private ClassA a;
  private String y1;
  private String y2;

  public ClassA getA() {
    return a;
  }

  public void setA(Class a) {
    this.a = a;
  }

  public String getY1() {
    return y1;
  }

  public void setY1(String y1) {
    this.y1 = y1;
  }

  public String getY2() {
    return y2;
  }

  public void setY2(String y2) {
    this.y2 = y2;
  }
}

the main program:

public class ConstructorInjection {
    public static void main(String args[]){
        Resource xmlResource = new FileSystemResource("applicationContext.xml");
        BeanFactory factory = new XmlBeanFactory(xmlResource);
        ClassB b= (ClassB)factory.getBean("bBean");
        ClassA a= b.getA();
        System.out.println("y1="+b.getY1);
        System.out.println("y2="+b.getY2);
        System.out.println("x1="+a.x1);
        System.out.println("x2="+a.x2);
    }
}

is it the right example of practicing constructor injection?

Xml configuration:

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:p="http://www.springframework.org/schema/p"
       xmlns:aop="http://www.springframework.org/schema/aop"
       xmlns:tx="http://www.springframework.org/schema/tx"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
       http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
       http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.5.xsd">

    <bean id="bBean" class="javabeat.net.spring.ioc.ClassB">
        <property name="y1" value="expy1" />
        <property name="y2" value="expy2" />
        <property name="a" ref="aBean" />
    </bean>

    <bean id="aBean" class="javabeat.net.spring.ioc.ClassA">
        <constructor-arg name="x1" type="java.lang.int" value="0"/>
        <constructor-arg name="x2" type="java.lang.String" value="exp"/>
    </bean>

</beans>

this part is what bothers me:

System.out.println("x1="+a.x1);
System.out.println("x2="+a.x2);

not sure if that's the right way to get the attributes of ClassA! i have read that constructor injection enforce the initialisation of attributes, but where? in the xml configuration? or in the main program?

And thank you so much :)

"would it work?" Try it. But no, it wouldn't compile currently, there are a lot of typos in here. — Andy Turner, May 27 '16 at 09:37
thank you ! i know about the xml configuration :) my question is if this is the right example of using constructor injection — Smahane, May 27 '16 at 09:40
my doubt is in the part : System.out.println("x1="+a.x1); System.out.println("x2="+a.x2); — Smahane, May 27 '16 at 09:43
Yes, It is the right approach to learn constructor injection, you just need to make fields of ClassA public or provide getter method to get value of x1 and x2 at runtime. — Prakash Hari Sharma, May 27 '16 at 09:44
thank you @PrakashHariSharma so much, that's the response i needed ! but in constructor injection the objectif is to enforce the initialisation of all attributes so where should i initialise them ? — Smahane, May 27 '16 at 09:51

score 0 · Accepted Answer · answered May 27 '16 at 09:52

0

Regarding your concern about the following:

System.out.println("x1="+a.x1); System.out.println("x2="+a.x2);

Both x1 are x2 are private variables, so you will not be able to use a.x1 or a.x2 like you did. You need public getter for x1 and x2. Without seeing your application.xml, it is difficult to say anything more. Please give some code which can be run.

answered May 27 '16 at 09:52

Sanjeev Saha

2,632
1
12
19

i've modified my post would you take a look plz :) – Smahane May 27 '16 at 10:15

score 0 · Answer 2 · answered May 27 '16 at 10:46

0

Modify aBean in your applicationContext.xml as

<bean id="aBean" class="javabeat.net.spring.ioc.ClassA">
    <constructor-arg name="x1" type="int" value="0"/>
    <constructor-arg name="x2" type="java.lang.String" value="exp"/>
</bean>

answered May 27 '16 at 10:46

Prakash Hari Sharma

1,414
2
17
22

ok thank you, what about the main program is it correct? is this : System.out.println("x1="+a.x1); System.out.println("x2="+a.x2); the right way to get the attributes in main program – Smahane May 27 '16 at 11:04
Right way is to use getter for x1 and x2, but because these are public you can still use this way :) – Prakash Hari Sharma May 27 '16 at 11:18

How to use constructor injection in a main program

2 Answers2