Spring Container的扩展点

晨曦之光 发布于 2012/04/25 16:18
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Spring在解析完配置文件后,会调用一些callback方法,使用Spring的开发者可以通过提供这些callback方法达到对Spring Container的扩展.
1,通过实现BeanPostProcessor来完成对某些Bean的一些'定制',BeanPostProcessor定义了两个方法,postProcessBeforeInitialization(Object bean, String beanName)和postProcessAfterInitialization(Object bean, String beanName).
postProcessBeforeInitialization会在Spring对bean初始化完成之后,依赖注册(对property指定的成员变量完成了赋值)已经完成,但是Container还没有调用申明的initialization方法(如afterPropertiesSet,init-method)之前被调用.postProcessAfterInitialization会在在Container调用申明的initialization方法(如afterPropertiesSet)之后被调用.如果需要有多个实现了BeanPostProcessor的类,可以通过让其实现Ordered接口来控制这些类的callback被调用的顺序.
这种bean的post-processor一般用来检查bean是否实现了某个接口,或者把bean包装成某个proxy,Spring的AOP某些框架类就是实现了BeanPostProcessor.
例:
public class MyBeanPostProcessor implements BeanPostProcessor,
        InvocationHandler {
    private Object proxyobj;
    public MyBeanPostProcessor() {   
    }
    public MyBeanPostProcessor(Object obj) {
        proxyobj = obj;
    }
    public Object postProcessBeforeInitialization(Object bean, String beanName)
            throws BeansException {
        System.out.println("postProcessBeforeInitialization Bean '" + beanName
                + "' created : " + bean.toString());
        if (bean instanceof Intf) {
            Class cls = bean.getClass();
            return Proxy.newProxyInstance(cls.getClassLoader(), cls
                    .getInterfaces(), new MyBeanPostProcessor(bean));
        } else {
            return bean;
        }
    }
    public Object postProcessAfterInitialization(Object bean, String beanName)
            throws BeansException {
        System.out.println("postProcessAfterInitialization Bean '" + beanName
                + "' created : " + bean.toString());
        return bean;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args)
            throws Throwable {
        System.out.println("proxy is " + proxy.getClass().getName());
        System.out.println("before calling " + method);

        if (args != null) {
            for (int i = 0; i < args.length; i++) {
                System.out.println(args[i] + "");
            }
        }
        Object o = method.invoke(proxyobj, args);
        System.out.println("after calling " + method);
        return o;
    }
}
public interface Intf {
    public String testFunc();
}
public class IntfBean implements Intf {
    private String strVal = "default value";
    @Override
    public String testFunc() {
        // TODO Auto-generated method stub
        return strVal;
    }
}

<bean id="IntfBean" class="com.test.spring.extent.IntfBean" />
<bean class="com.test.spring.extent.MyBeanPostProcessor" />
注意这里实现BeanPostProcessor的类是针对Ioc容器里其他的bean调用这两个方法,不是针对BeanPostProcessor自身和其他BeanPostProcessor调用这两个方法,
如果在配置文件里就只有BeanPostProcessor本身,没有其他的bean,如只有<beanclass="com.test.spring.extent.MyBeanPostProcessor"/>配置,postProcessBeforeInitialization和postProcessAfterInitialization这两个方法是不会被调用的.BeanPostProcessor会在普通bean被初始化之前先被容器初始化.
调用代码:
Intf intfBean = (Intf) ctx.getBean("IntfBean");
System.out.println(intfBean.testFunc());
这样,MyBeanPostProcessor会把实现Intf接口的bean包装成一个proxy.
2,通过实现BeanFactoryPostProcessor接口来操作配置文件,对配置的元数据进行'特制'.Spring IoC容器允许BeanFactoryPostProcessor在容器实际实例化任何其它的bean之前读取配置元数据,并有可能修改它。Spring自身的PropertyPlaceholderConfigurer就实现了这个接口,通过对XML配置文件中使用占位符,PropertyPlaceholderConfigurer从别的property文件中读取值进行替换.
例子:从ext.properties读取值替换${TEST.PROP1},${TEST.PROP2}.
    <bean
        class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
        <property name="locations" value="classpath:conf/ext.properties" />
        <property name="properties">
            <value>TEST.PROP3=inside property</value>
        </property>
    </bean>
    <bean id="PropReplaceBean" class="com.test.spring.extent.PropReplaceBean">
        <property name="strVal" value="${TEST.PROP1}" />
        <property name="intVal" value="${TEST.PROP2}" />
        <property name="insideProp" value="${TEST.PROP3}" />
    </bean>

通常比较有用的场景是数据库的url,用户名,密码的配置,还可以用来动态指定某个bean的类名,把XMl文件和property文件分开维护更容易.
<bean id="xxxBean" class="${com.xxx.class}"/>
自定义的BeanFactoryPostProcessor,通过实现Ordered接口可以改变被callback的顺序.callback方法中的ConfigurableListableBeanFactory beanFactory提供了配置文件的元数据.(*碰到一个困惑的问题是: PropertyPlaceholderConfigurer的order是 Integer.MAX_VALUE,却发现它比自己定义的order 为-1的 processor先执行.)
<bean class="com.test.spring.extent.MyBeanFactoryPostProcessor" />
public class MyBeanFactoryPostProcessor implements BeanFactoryPostProcessor,
        Ordered {
    private int order = -1;
    public void setOrder(int order) {
        this.order = order;
    }
    public int getOrder() {
        return this.order;
    }
    @Override
    public void postProcessBeanFactory(
            ConfigurableListableBeanFactory beanFactory) throws BeansException {
        BeanDefinition beanDefinition = beanFactory
                .getBeanDefinition("PropReplaceBean");
        MutablePropertyValues pvs = beanDefinition.getPropertyValues();
        PropertyValue[] pvArray = pvs.getPropertyValues();
        for (PropertyValue pv : pvArray) {
            System.out.println(pv.getName() + "  " + pv.getValue().getClass());
        }
    }
}
Spring检测到BeanPostProcessor和BeanFactoryPostProcessor后会由容器自动调用它们的callback方法,不用在代码里主动的去调用.

原文链接:http://blog.csdn.net/kkdelta/article/details/5488430
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