Spring Constructor Injection-Spring Tutorial

We can inject the dependency by constructor. The <constructor-arg> subelement of <bean> is used for constructor injection. Here we are going to inject

1. primitive and String-based values
2. Dependent object (contained object)
3. Collection values etc.

Injecting primitive and string-based values

Let's see the simple example to inject primitive and string-based values. We have created three files here:

• Employee.java
• applicationContext.xml
• Test.java

Employee.java

It is a simple class containing two fields id and name. There are four constructors and one method in this class.

1. package com.javatpoint;  
2.   
3. public class Employee {  
4. private int id;  
5. private String name;  
6.   
7. public Employee() {System.out.println("def cons");}  
8.   
9. public Employee(int id) {this.id = id;}  
10.   
11. public Employee(String name) {  this.name = name;}  
12.   
13. public Employee(int id, String name) {  
14.     this.id = id;  
15.     this.name = name;  
16. }  
17.   
18. void show(){  
19.     System.out.println(id+" "+name);  
20. }  
21.   
22. }  

applicationContext.xml

We are providing the information into the bean by this file. The constructor-arg element invokes the constructor. In such case, parameterized constructor of int type will be invoked. The value attribute of constructor-arg element will assign the specified value. The type attribute specifies that int parameter constructor will be invoked.

1. <?xml version="1.0" encoding="UTF-8"?>  
2. <beans  
3.     xmlns="http://www.springframework.org/schema/beans"  
4.     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
5.     xmlns:p="http://www.springframework.org/schema/p"  
6.     xsi:schemaLocation="http://www.springframework.org/schema/beans  
7.                 http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">  
8.   
9. <bean id="e" class="com.javatpoint.Employee">  
10. <constructor-arg value="10" type="int"></constructor-arg>  
11. </bean>  
12.   
13. </beans>  

Test.java

This class gets the bean from the applicationContext.xml file and calls the show method.

1. package com.javatpoint;  
2.   
3. import org.springframework.beans.factory.BeanFactory;  
4. import org.springframework.beans.factory.xml.XmlBeanFactory;  
5. import org.springframework.core.io.*;  
6.   
7. public class Test {  
8.     public static void main(String[] args) {  
9.           
10.         Resource r=new ClassPathResource("applicationContext.xml");  
11.         BeanFactory factory=new XmlBeanFactory(r);  
12.           
13.         Employee s=(Employee)factory.getBean("e");  
14.         s.show();  
15.           
16.     }  
17. }  

Output:10 null

download this example

---

Injecting string-based values

If you don't specify the type attribute in the constructor-arg element, by default string type constructor will be invoked.

1. ....  
2. <bean id="e" class="com.javatpoint.Employee">  
3. <constructor-arg value="10"></constructor-arg>  
4. </bean>  
5. ....  

If you change the bean element as given above, string parameter constructor will be invoked and the output will be 0 10.

Output:0 10

---

You may also pass the string literal as following:

1. ....  
2. <bean id="e" class="com.javatpoint.Employee">  
3. <constructor-arg value="Sonoo"></constructor-arg>  
4. </bean>  
5. ....  

Output:0 Sonoo

---

You may pass integer literal and string both as following

1. ....  
2. <bean id="e" class="com.javatpoint.Employee">  
3. <constructor-arg value="10" type="int" ></constructor-arg>  
4. <constructor-arg value="Sonoo"></constructor-arg>  
5. </bean>  
6. ....  

Output:10 Sonoo

download this example (developed using Myeclipse IDE)
download this example (developed using Eclipse IDE)