spring 实例化bean的几种方法

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 />

<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 >
      <constructor-arg ref="bar"/>
      <constructor-arg ref="baz"/>
  </bean>

  <bean />
  <bean />

</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 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;
  }
}

2.按照构造函数的参数类型匹配注入

  

<bean >
<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 >
<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 >
<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 >

c:_index方式注入

<-- 'c-namespace' index declaration -->
<bean >

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 
      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 >
  <!-- inject any dependencies required by this locator bean -->
</bean>

<!-- the bean to be created via the factory bean -->
<bean 
      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 >
  <!-- inject any dependencies required by this locator bean -->
</bean>
<bean 
      factory-bean="serviceLocator"
      factory-method="createClientServiceInstance"/>

<bean 
      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;
  }
}