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 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 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 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;
}
}