In Spring framework, when your class contains multiple constructors with same number of arguments, it will always cause the constructor injection argument type ambiguities issue.
Problem
Let’s see this customer bean example. It contains two constructor methods, both accept 3 arguments with different data type.
package com.mkyong.common;
public class Customer
{
private String name;
private String address;
private int age;
public Customer(String name, String address, int age) {
this.name = name;
this.address = address;
this.age = age;
}
public Customer(String name, int age, String address) {
this.name = name;
this.age = age;
this.address = address;
}
//getter and setter methods
public String toString(){
return " name : " +name + "\n address : "
+ address + "\n age : " + age;
}
}
In Spring bean configuration file, pass a ‘mkyong’ for name, ‘188’ for address and ’28’ for age.
<!--Spring-Customer.xml-->
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
<bean id="CustomerBean" class="com.mkyong.common.Customer">
<constructor-arg>
<value>mkyong</value>
</constructor-arg>
<constructor-arg>
<value>188</value>
</constructor-arg>
<constructor-arg>
<value>28</value>
</constructor-arg>
</bean>
</beans>
Run it, what’s your expected result?
package com.mkyong.common;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class App
{
public static void main( String[] args )
{
ApplicationContext context =
new ClassPathXmlApplicationContext(new String[] {"Spring-Customer.xml"});
Customer cust = (Customer)context.getBean("CustomerBean");
System.out.println(cust);
}
}
Output
name : mkyong
address : 28
age : 188
The result is not what we expected, the second constructor is run, instead of the first constructor. In Spring, the argument type ‘188’ is capable convert to int, so Spring just convert it and take the second constructor, even you assume it should be a String.
In addition, if Spring can’t resolve which constructor to use, it will prompt following error message
constructor arguments specified but no matching constructor
found in bean 'CustomerBean' (hint: specify index and/or
type arguments for simple parameters to avoid type ambiguities)
Solution
To fix it, you should always specify the exact data type for constructor, via type attribute like this :
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">
<bean id="CustomerBean" class="com.mkyong.common.Customer">
<constructor-arg type="java.lang.String">
<value>mkyong</value>
</constructor-arg>
<constructor-arg type="java.lang.String">
<value>188</value>
</constructor-arg>
<constructor-arg type="int">
<value>28</value>
</constructor-arg>
</bean>
</beans>
Run it again, now you get what you expected.
Output
name : mkyong
address : 188
age : 28
It’s always a good practice to explicitly declared the data type for each constructor argument, to avoid constructor injection type ambiguities issue above.
How to deal with this situation since it’s overloaded and Arrays of Colors and Engine ?
public Car(String name, float price,String[] colors) {
super();
this.name = name;
this.price = price;
this.colors=colors;
}
/**
* @param name
* @param price
* @param engine
*/
public Car(String name, float price, Engine[] engine) {
super();
this.name = name;
this.price = price;
this.engine = engine;
}?
Can we use index while using constructor
DI to avoid ambiguity?
index is flexible to use
mkyong can you review my github here thanks its for spring tutorial tell me what you think and if you updated your source to update my github thanks
https://github.com/shareefhiasat/mkyong
First running give me :
name : mkyong
address : 188
age : 28
and not ur result :
name : mkyong
address : 28
age : 188
Am-I rigth or wrong?
I got same as u…. !!!
this page of spring is supposed to align not able to see properly.
https://mkyong.com/spring/constructor-injection-type-ambiguities-in-spring/
And also give the link to download all the program after the explanation in some cases it is not available.
Thanks.
Really nice. Appreciate your efforts on this.
and can i do something like this:
public class person{ private man Man; person(man Mann){ Man =Mann; } } public class man{ }We can also use ‘index’ attribute for specifying the constructor.
<constructor-arg type="java.lang.String" value="Gagan"></constructor-arg> <constructor-arg type="int" value="20" index="2"></constructor-arg> <constructor-arg type="java.lang.String" value="250" index="1"></constructor-arg>each and every time while i run this example First Constructor is executed.
if i change sequence of constructors in POJO than always first constructor is executed.
Same here. No mater what, same output
Just perfect and was looking for. Terima Kasih Yong
Excellent Thank you very much
Good.
Excellent tutorials. Thank you very much!