Constructor-based DI is effected by invoking a constructor with a number of arguments, each representing a dependency. Additionally, calling a static factory method with specific arguments
to construct the bean, can be considered almost equivalent, and the rest of this text will consider arguments to a constructor and arguments to a staticfactory method similarly. Find below an example of a class that could only
be dependency injected using constructor injection. Notice that there is nothing special about this class.

public class SimpleMovieLister {

    // the SimpleMovieLister has a dependency on a MovieFinder

    private MovieFinder movieFinder;

    // a constructor so that the Spring container can 'inject' a MovieFinder

    public SimpleMovieLister(MovieFinder movieFinder) {

        this.movieFinder = movieFinder;

    }

    // business logic that actually 'uses' the injected MovieFinder is omitted...

}

There is no potential for ambiguity here (assuming of course that Bar and Baz classes are not related in an inheritance hierarchy). Thus the following configuration will work just fine, and you do
not need to specify the constructor argument indexes and / or types explicitly.

<beans>

    <bean name="foo" class="x.y.Foo">

        <constructor-arg>

            <bean class="x.y.Bar"/>

        </constructor-arg>

        <constructor-arg>

            <bean class="x.y.Baz"/>

        </constructor-arg>

    </bean>

</beans>

When another bean is referenced, the type is known, and matching can occur (as was the case with the preceding example). When a simple type is used, such as<value>true<value>, Spring cannot determine the type of the value, and so
cannot match by type without help. Consider the following class:

package examples;

public class ExampleBean {

    // No. of years to the calculate the Ultimate Answer

    private int years;

    // The Answer to Life, the Universe, and Everything

    private String ultimateAnswer;

    public ExampleBean(int years, String ultimateAnswer) {

        this.years = years;

        this.ultimateAnswer = ultimateAnswer;

    }

}

<?xml version="1.0" encoding="UTF-8"?>

<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">

<!-- udaoimpl is a UserDAOImpl Object-->

  <bean id="udaoimpl" class="com.bjsxt.dao.impl.UserDAOImpl">

  </bean>

<!-- userService is a UserService Object-->

  <bean id="userService" class="com.bjsxt.service.UserService">

  <!-- 最常用的setter方法注入 -->

 	<!--userService's setUserDAO method DI(注入) 之前产生的udaoimpl对象-->

   <!--  <property name="userDAO"><ref bean="udaoimpl"/></property>

   -->

   <!-- 不推荐，可以忘记‘’‘’ userService的构造方法注入 -->

    <constructor-arg>

            <ref bean="udaoimpl"/>

   </constructor-arg>

   <!--<property name="userDAO" ref="udaoimpl"/>  -->

  </bean>

</beans>