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Spring Framework: Dependency Injection by Constructor with Example
Injecting primitive and string-based values
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.
package com.springexample;
public class Employee {
private int id;
private String name;
public Employee() {System.out.println("def cons");}
public Employee(int id) {this.id = id;}
public Employee(String name) { this.name = name;}
public Employee(int id, String name) {
this.id = id;
this.name = name;
}
void show(){
System.out.println(id+" "+name);
}
}
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.
<?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"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
<bean id="e" class="com.springexample.Employee">
<constructor-arg value="10" type="int"></constructor-arg>
</bean>
</beans>
Test.java
This class gets the bean from the applicationContext.xml file and calls the show method.
package com.springexample;
import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.*;
public class Test {
public static void main(String[] args) {
Resource r=new ClassPathResource("applicationContext.xml");
BeanFactory factory=new XmlBeanFactory(r);
Employee s=(Employee)factory.getBean("e");
s.show();
}
}
Output:10 null
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.
....
<bean id="e" class="com.springexample.Employee">
<constructor-arg value="10"></constructor-arg>
</bean>
....
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:
....
<bean id="e" class="com.springexample.Employee">
<constructor-arg value="Sonoo"></constructor-arg>
</bean>
....
Output:0 Sonoo
You may pass integer literal and string both as following
....
<bean id="e" class="com.springexample.Employee">
<constructor-arg value="10" type="int" ></constructor-arg>
<constructor-arg value="Sonoo"></constructor-arg>
</bean>
....