Is default constructor required in Spring injection?

Question

I'm trying to inject a constructor that takes some arguments. After compiling Spring complains it couldn't find a default constructor (I haven't defined it) and throws BeanInstatiationException and NoSuchMethodException.

After defining a default constructor the exceptions don't appear anymore, however my object is never initialized with the argument constructor, only the default one is called. Does Spring really require a default constructor in this case? And if yes, how can I make it use the argument constructor instead of the default one?

This is how I wire everything:

public class Servlet {

  @Autowired
  private Module module;

  (code that uses module...)
}

@Component
public class Module {

  public Module(String arg) {}
  ...
}

Bean configuration:

<beans>
  <bean id="module" class="com.client.Module">
    <constructor-arg type="java.lang.String" index="0">
    <value>Text</value>
    </constructor-arg>
  </bean>

  ...
</beans>

Stack trace:

WARNING: Could not get url for /javax/servlet/resources/j2ee_web_services_1_1.xsd
ERROR  initWebApplicationContext, Context initialization failed
[tomcat:launch] org.springframework.beans.factory.BeanCreationException: Error
creating bean with name 'module' defined in URL [...]: Instantiation of bean failed;  
nested exception is org.springframework.beans.BeanInstantiationException: Could not 
instantiate bean class [com.client.Module]: No default constructor found; nested 
exception is java.lang.NoSuchMethodException: com.client.Module.<init>()

Solution 1

Spring only "requires" a default constructor if you plan on instantiating it without any arguments.

for example, if your class is like this;

public class MyClass {

  private String something; 

  public MyClass(String something) {
    this.something = something;
  }

  public void setSomething(String something) {
    this.something = something;
  }

}

and you set it up in Spring like this;

<bean id="myClass" class="foo.bar.MyClass">
  <property name="something" value="hello"/>
</bean>

you're going to get an error. the reason is that Spring instantiates your class new MyClass() then tries to set call setSomething(..).

so instead, the Spring xml should look like this;

<bean id="myClass" class="foo.bar.MyClass">
  <constructor-arg value="hello"/>
</bean>

so have a look at your com.client.Module and see how its configured in your Spring xml

Solution 2

Most probably you are using component-scanning and since you define annotation @Component for class Module it tries to instantiate the bean. You do not need @Component annotation if You are using XML for bean definition.

Solution 3

Just faced the same problem, i guess till now you might have solved the problem.
Below is what you could have changed your bean configuration to,

<bean id="module" class="com.client.Module">
        <constructor-arg value="Text"/>
</bean>

Comments

[agerrr]

I think I did everything correctly, I included my implementation in the question. If I don't specify the default constructor, Spring complains and the code doesn't compile, if I do, only the default constructor is being used, the argument constructor is ignored. I don't understand why.

[leonidos79]

Thank you @ikettu! You saved my day. Faced the same problem where Spring was throwing a NoSuchMethodException telling me there is no default constructor in the bean class. And indeed, there is none, on purpose. Never would have figured out it is component-scanning that tried to instantiate the bean first. Removing @Component annotation from the bean class instantly solved the error, keeping everything the same.