1.普通的通过构造函数初始化，没有指定构造函数参数的就是用默认的无参的构造方法

<bean id="exampleBean" class="examples.ExampleBean"/><bean name="anotherExample" class="examples.ExampleBeanTwo"/>

构造函数的几种方式：

1.普通沟通函数注入方式，按照构造函数参数的顺序和个数来注入bean

package x.y;public class Foo {public Foo(Bar bar, Baz baz) {// ...}
}

<beans><bean id="foo" class="x.y.Foo"><constructor-arg ref="bar"/><constructor-arg ref="baz"/></bean><bean id="bar" class="x.y.Bar"/><bean id="baz" class="x.y.Baz"/></beans>

下面几种注入方式先看下示例：

package examples;public class ExampleBean {// No. of years to the calculate the Ultimate Answerprivate int years;// The Answer to Life, the Universe, and Everythingprivate String ultimateAnswer;public ExampleBean(int years, String ultimateAnswer) {this.years = years;this.ultimateAnswer = ultimateAnswer;}
}

2.按照构造函数的参数类型匹配注入

<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg type="int" value="7500000"/>
<constructor-arg type="java.lang.String" value="42"/>
</bean>

3.按照参数索引顺序注入

<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg index="0" value="7500000"/>
<constructor-arg index="1" value="42"/>
</bean>

4. spring3以上还可以通过参数名称进行注入

<bean id="exampleBean" class="examples.ExampleBean">
<constructor-arg name="years" value="7500000"/>
<constructor-arg name="ultimateanswer" value="42"/>
</bean>

5.spring3以上通过annotation注入 @ConstructorProperties

package examples;public class ExampleBean {// Fields omitted@ConstructorProperties({"years", "ultimateAnswer"})public ExampleBean(int years, String ultimateAnswer) {this.years = years;this.ultimateAnswer = ultimateAnswer;}
}

6.spring3.1以上还可以使用简化的c namespace来进行构造函数注入

 <bean id="foo" class="x.y.Foo" c:bar-ref="bar" c:baz-ref="baz" c:email="foo@bar.com">

c:_index方式注入

<-- 'c-namespace' index declaration -->
<bean id="foo" class="x.y.Foo" c:_0-ref="bar" c:_1-ref="baz">

2.通过静态的工厂方法生成bean，这种方式在配置文件中没有指定返回的bean的类型

<bean id="clientService"class="examples.ClientService"factory-method="createInstance"/>

public class ClientService {private static ClientService clientService = new ClientService();private ClientService() {}public static ClientService createInstance() {return clientService;}
}

3.通过实例化的工厂方法生成bean

<!-- the factory bean, which contains a method called createInstance() -->
<bean id="serviceLocator" class="examples.DefaultServiceLocator"><!-- inject any dependencies required by this locator bean -->
</bean><!-- the bean to be created via the factory bean -->
<bean id="clientService"factory-bean="serviceLocator"factory-method="createClientServiceInstance"/>

public class DefaultServiceLocator {private static ClientService clientService = new ClientServiceImpl();private DefaultServiceLocator() {}public ClientService createClientServiceInstance() {return clientService;}
}

当然这个实例化的工厂类也可以生成多个bean

<bean id="serviceLocator" class="examples.DefaultServiceLocator"><!-- inject any dependencies required by this locator bean -->
</bean>
<bean id="clientService"factory-bean="serviceLocator"factory-method="createClientServiceInstance"/><bean id="accountService"factory-bean="serviceLocator"factory-method="createAccountServiceInstance"/>

public class DefaultServiceLocator {private static ClientService clientService = new ClientServiceImpl();private static AccountService accountService = new AccountServiceImpl();private DefaultServiceLocator() {}public ClientService createClientServiceInstance() {return clientService;}public AccountService createAccountServiceInstance() {return accountService;}
}