Spring实例化Bean的三种方式及Bean的类型

1.使用类构造器实例化  [默认的类构造器]

< bean  id=“orderService"  class = "cn.itcast.OrderServiceBean" />

2.使用静态工厂方法实例化

< bean  id = "personService"  class = "cn.itcast.service.OrderFactory"  factory-method = "createOrder" />

public  class  OrderFactory {
    // 注意这里的这个方法是 static 的！
    public  static  OrderServiceBean createOrder(){
        return  new  OrderServiceBean();
    }
}

3.使用实例工厂方法实例化:

< bean  id = "personServiceFactory"  class = "cn.itcast.service.OrderFactory" />
< bean  id = "personService"  factory-bean = "personServiceFactory"  factory-method = "createOrder" />

public  class  OrderFactory {
    public  OrderServiceBean createOrder(){
        return  new  OrderServiceBean();
    }
}

在BAMS中，工作流引擎activiti的各个组件就是使用此方式实例化的。

< bean  id = "processEngine"  class = "org.activiti.spring.ProcessEngineFactoryBean" >
    < property  name = "processEngineConfiguration"  ref = "processEngineConfiguration" />
bean >

< bean  id = "repositoryService"  factory-bean = "processEngine"  factory-method = "getRepositoryService" />
< bean  id = "runtimeService"  factory-bean = "processEngine"  factory-method = "getRuntimeService" />
< bean  id = "formService"  factory-bean = "processEngine"  factory-method = "getFormService" />
< bean  id = "identityService"  factory-bean = "processEngine"  factory-method = "getIdentityService" />
< bean  id = "taskService"  factory-bean = "processEngine"  factory-method = "getTaskService" />
< bean  id = "historyService"  factory-bean = "processEngine"  factory-method = "getHistoryService" />
< bean  id = "managementService"  factory-bean = "processEngine"  factory-method = "getManagementService" />

使用工厂实例化的Bean跟普通Bean不同，其返回的对象不是指定类的一个实例，其返回的是该FactoryBean的getObject方法所返回的对象。以下为ProcessEngineFactoryBean源码。

public  class  ProcessEngineFactoryBean  implements  FactoryBean ..{
    ...
    protected  ProcessEngineImpl processEngine;

    public  ProcessEngine getObject()  throws  Exception {
    initializeExpressionManager();
    initializeTransactionExternallyManaged();

    if  (processEngineConfiguration.getBeans()== null ) {
        processEngineConfiguration.setBeans( new  SpringBeanFactoryProxyMap(applicationContext));
    }

    processEngine = (ProcessEngineImpl) processEngineConfiguration.buildProcessEngine();

    return  processEngine;
    }

    ...
}

ProcessEngineFactoryBean最终返回的是processEngine对象，repositoryService、runtimeService、formService等等组件都是通过processEngine里的getXX方法获得的。

总结：Spring中有三种实例化Bean的方式，类构造器实例化、静态工厂方法实例化及实例工厂方法实例化。两种Bean类型，一种是普通Bean，另一种是工厂Bean，即FactoryBean，这两种Bean都被容器管理，但工厂Bean跟普通Bean不同，其返回的对象不是指定类的一个实例，其返回的是该FactoryBean的getObject方法所返回的对象。在Spring框架内部，有很多地方有FactoryBean的实现类，它们在很多应用如(Spring的AOP、ORM、事务管理)及与其它第三框架(ehCache)集成时都有体现。