为什么在找不到默认构造函数时Spring引发异常

Question

A simple test Shopping Application where i have two classes Clothing and Offers. Now on calling formalshirt bean, it throws the following exception

Caused by: org.springframework.beans.factory.BeanCreationException: 
Error creating bean with name 'DiwaliOffer' defined in class path resource [spring.xml]: Instantiation of bean failed; 
nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [Offers]: No default constructor found; 
nested exception is java.lang.NoSuchMethodException: Offers.<init>()

Now if i comment the Offers constructor, the app runs successfully. My query is Why does Spring looks for default constructor only when there is another constructor?

Clothing Class

public class Clothing {
    private int price;
    private List<Offers> offer;

    public void setOffer(List<Offers> offer) {
        this.offer = offer;
    }

    public void setPrice(int price) {
        this.price = price;
    }
}

.

Offers Class

public class Offers {
    private int discount;
    private String promocode;

    public Offers(int val1, String val2)
    {
        this.discount=val1;
        this.promocode=val2;
    }

    //public Offers(){} /*Default Constructor added due to Spring Exception as in below */  
    /* Caused by: org.springframework.beans.BeanInstantiationException: Could not instantiate bean class No default constructor found */
    /* Caused by: java.lang.NoSuchMethodException: com.test.Shopping.Offers.<init>() */

    public void setDiscount(int discount) {
        this.discount = discount;
    }

    public void setPromocode(String promocode) {
        this.promocode = promocode;
    }
}

Spring.xml

<bean id="formalshirt" class="com.test.Shopping.Clothing">
    <property name="price" value="800"></property>              
    <property name="offer">
        <list>
            <ref bean="DiwaliOffer" />
        </list>
    </property>
</bean>

<bean id="DiwaliOffer" class="com.test.Shopping.Offers">
    <property name="discount" value="10"></property>
    <property name="promocode" value="diwali"></property>
</bean>

Answer 1

You need to change your Spring XML configuration of Offers to:

<bean id="DiwaliOffer" class="com.test.Shopping.Offers">
    <constructor-arg value="10"></constructor-arg>
    <constructor-arg value="diwali"></constructor-arg>
</bean>

The way you have configured it, Spring first attempts to call the default constructor and then call the setters. But of course there is no default constructor, and there fore Spring reports the exception.

Another option if you are using Spring 3+ is to use Java Config, instead of XML config. You would just have

@Configuration
public class AppConfig {

   //add other beans

   @Bean
   public Offers DiwaliOffer() {
      return new Offers(10, diwali);
   }
}

In your case Java Config has the benefit that your configuration would not even compile if you didn't call the constructor, ie. it would fail early instead of fail late as with the XML configuration

Spring is extremely flexible with how it creates beans, but you need to declare how it will be done

Answer 2

This is because when you add parametrize constructor , you need to tell spring to intanitate by giving constructor argument...

The concept is straight forward:

in default constriuctor case you will simply create object like new Test() ;

where as when you don't have default rather have parameterize constructor you have to create object like new Test1("test") ;

Class Test{

}

Class Test1{
      Test1(String a){

}
}

Spring way will be:

<bean id="test1" class="Test1">

<constructor-arg value="Zara"/>
</bean

Please have look at Spring Doc for more details

Answer 3

You need to use constructor-args for your bean properties so that it picks the non-default constructor. Otherwise Spring creates the object first, and without any parameters, it must use the zero-arg constructor, then sets properties with setters.

To pass in constructor args, change property to constructor-arg like so:

<bean id="DiwaliOffer" class="com.test.Shopping.Offers">

    <constructor-arg index="0" value="10"/>
    <constructor-arg index="1" value="diwali" />

</bean>

Answer 4

If there are any constructor available in the class then java compiler does not provide default constructor.

And we can see com.test.Shopping.Clothing can able to create instance and com.test.Shopping.Offers can't.

To create the instance you need to pass constructor-arg of discount and promocode .

<bean id="DiwaliOffer" class="com.test.Shopping.Offers">
    <constructor-arg value="10"></property>
    <constructor-arg value="diwali"></property>
</bean>

Answer 5

You can do :

<bean id="DiwaliOffer" class="com.test.Shopping.Offers" >
    <constructor-arg type="int" value="10" />
    <constructor-arg type="java.lang.String" value="Diwali"/>
</bean>

The constructor-arg element within the bean element is used to set the property value thru constructor injection. Since there is only one constructor in the User bean class, this code will work fine.