Spring Beans - how to wire null as a constructor arg?

Question

I have the following bean defined:

<bean id="myBean" class="com.me.myapp.Widget">     <constructor-arg name="fizz" value="null"/>     <constructor-arg name="buzz" ref="someOtherBean" /> </bean> 

When I run my app, Spring throws a bean config exception:

[java] Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'myBean' defined in class path resource [spring-config.xml]: Unsatisfied dependency expressed through constructor argument with index 0 of type [com.me.myapp.Widget]: Could not convert constructor argument value of type [java.lang.String] to required type [com.me.myapp.Widget]: Failed to convert value of type 'java.lang.String' to required type 'com.me.myapp.Widget'; nested exception is java.lang.IllegalStateException: Cannot convert value of type [java.lang.String] to required type [com.me.myapp.Widget]: no matching editors or conversion strategy found 

I just want to create a Widget like I would if I wrote the following Java:

Widget w = new Widget(null, someOtherBean); 

How can I do this? Thanks in advance!

Answer 1

You can use Spring's <null> tag:

<bean id="myBean" class="com.me.myapp.Widget">     <constructor-arg name="fizz">             <null />     </constructor-arg>     <constructor-arg name="buzz" ref="someOtherBean" /> </bean> 

Answer 2

At times spring fails to inject the values in the ordered you have mentioned into a constructor. It's better if you try with explicit ordering, i.e.

<constructor-arg index="0" name="fizz"><null/></constructor-arg>  <constructor-arg index="1" name="buzz"><ref bean="someOtherBean"/></constructor-arg>