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Spring实战系列(三)-BeanPostProcessor的妙用

时间:2020-07-03 15:42:11      阅读:51      评论:0      收藏:0      [点我收藏+]

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"对于Spring框架,现实公司使用的非常广泛,但是由于业务的复杂程度不同,了解到很多小伙伴们利用Spring开发仅仅是利用了Spring的IOC,即使是AOP也很少用,但是目前的Spring是一个大家族,形成了一个很大的生态,覆盖了我们平时开发的方方面面,抛开特殊的苛刻要求之外,Spring的生态其实已经很全面了,所以在此开个系列来研究下Spring提供给我们的一些平时不太却又很实用的内容。"

说明:

对于Spring开发时,我们有时会遇到同一个接口有多个实现类,为了避免错误,我们通常在具体调用的地方通过ApplicationContext根据业务的需要来选择不同的接口实现类,虽然可以在抽象出一个工厂方法,但是还是感觉不够优雅,如果通过@Autowired直接引入接口,则需要在某个实现类上标注@Primary,否则会报错。那么书归正传如何优雅的解决上述的问题呢,此处就介绍一种利用Spring的BeanPostProcessor来处理。话不多说先上接口

示例:

1、声明接口

public interface HelloService {
    public void sayHello();
}

 

 

2、对应的接口实现类1:

@Service
public class HelloServiceImpl1 implements HelloService{
    @Override
    public void sayHello() {
        System.out.println("你好我是HelloServiceImpl1");
    }
}

 

3、对应接口实现类2:

@Service
public class HelloServiceImpl2 implements HelloService{
    @Override
    public void sayHello() {
        System.out.println("你好我是HelloServiceImpl2");
    }
}

 

4、自定义注解:

  

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface RountingInjected {
    String value() default "helloServiceImpl1";
}

 

5、自定义BeanPostProcessor实现类:

@Component
public class HelloServiceInjectProcessor implements BeanPostProcessor {
 
    @Autowired
    private ApplicationContext applicationContext;
 
    @Override
    public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        return bean;
    }
 
    @Override
    public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        Class<?> targetCls = bean.getClass();
        Field[] targetFld = targetCls.getDeclaredFields();
        for (Field field : targetFld) {
            //找到制定目标的注解类
            if (field.isAnnotationPresent(RountingInjected.class)) {
                if (!field.getType().isInterface()) {
                    throw new BeanCreationException("RoutingInjected field must be declared as an interface:" + field.getName()
                            + " @Class " + targetCls.getName());
                }
                try {
                    this.handleRoutingInjected(field, bean, field.getType());
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }
        return bean;
    }
 
    /**
     * @param field
     * @param bean
     * @param type
     * @throws IllegalAccessException
     */
    private void handleRoutingInjected(Field field, Object bean, Class type) throws IllegalAccessException {
        Map<String, Object> candidates = this.applicationContext.getBeansOfType(type);
        field.setAccessible(true);
        if (candidates.size() == 1) {
            field.set(bean, candidates.values().iterator().next());
        } else if (candidates.size() == 2) {
            String injectVal = field.getAnnotation(RountingInjected.class).value();
            Object proxy = RoutingBeanProxyFactory.createProxy(injectVal, type, candidates);
            field.set(bean, proxy);
        } else {
            throw new IllegalArgumentException("Find more than 2 beans for type: " + type);
        }
    }

 

6、对应的代理实现类:

public class RoutingBeanProxyFactory {
 
    private final static String DEFAULT_BEAN_NAME = "helloServiceImpl1";
 
    public static Object createProxy(String name, Class type, Map<String, Object> candidates) {
        ProxyFactory proxyFactory = new ProxyFactory();
        proxyFactory.setInterfaces(type);
        proxyFactory.addAdvice(new VersionRoutingMethodInterceptor(name, candidates));
        return proxyFactory.getProxy();
    }
 
    static class VersionRoutingMethodInterceptor implements MethodInterceptor {
        private Object targetObject;
 
        public VersionRoutingMethodInterceptor(String name, Map<String, Object> beans) {
            this.targetObject = beans.get(name);
            if (this.targetObject == null) {
                this.targetObject = beans.get(DEFAULT_BEAN_NAME);
            }
        }
 
        @Override
        public Object invoke(MethodInvocation invocation) throws Throwable {
            return invocation.getMethod().invoke(this.targetObject, invocation.getArguments());
        }
    }
}

 

7、结果测试类

@Component
public class HelloServiceTest {
 
    @RountingInjected(value = "helloServiceImpl2")
    private HelloService helloService;
 
    public void testSayHello() {
        helloService.sayHello();
    }
 
    public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext("colin.spring.basic.advanced.bbp");
        HelloServiceTest helloServiceTest = applicationContext.getBean(HelloServiceTest.class);
        helloServiceTest.testSayHello();
    }

 

上述是整个解决方案的示例流程,其核心思想就是根据自定义注解拦截要注入的接口实现类,运用java反射和代理的知识点来进行有效的实现类注入。

再次补充下BeanPostProcessor的一些知识点,

 

BeanPostProcessor接口作用:

     如果我们想在Spring容器中完成bean实例化、配置以及其他初始化方法前后要添加一些自己逻辑处理。我们需要定义一个或多个BeanPostProcessor接口实现类,然后注册到Spring IoC容器中。

Spring中Bean的实例化过程图示:

技术图片

注意:

1、接口中的两个方法都要将传入的bean返回,而不能返回null,如果返回的是null那么我们通过getBean方法将得不到目标。

 

2、BeanFactory和ApplicationContext对待bean后置处理器稍有不同。ApplicationContext会自动检测在配置文件中实现了BeanPostProcessor接口的所有bean,并把它们注册为后置处理器,然后在容器创建bean的适当时候调用它,因此部署一个后置处理器同部署其他的bean并没有什么区别。而使用BeanFactory实现的时候,bean 后置处理器必须通过代码显式地去注册,在IoC容器继承体系中的ConfigurableBeanFactory接口中定义了注册方法


    /**  
     * Add a new BeanPostProcessor that will get applied to beans created  
     * by this factory. To be invoked during factory configuration.  
     * <p>Note: Post-processors submitted here will be applied in the order of  
     * registration; any ordering semantics expressed through implementing the  
     * {@link org.springframework.core.Ordered} interface will be ignored. Note  
     * that autodetected post-processors (e.g. as beans in an ApplicationContext)  
     * will always be applied after programmatically registered ones.  
     * @param beanPostProcessor the post-processor to register  
     */    
    void addBeanPostProcessor(BeanPostProcessor beanPostProcessor);

 


另外,不要将BeanPostProcessor标记为延迟初始化。因为如果这样做,Spring容器将不会注册它们,自定义逻辑也就无法得到应用。假如你在<beans />元素的定义中使用了‘default-lazy-init‘属性,请确信你的各个BeanPostProcessor标记为‘lazy-init="false"‘。

 

InstantiationAwareBeanPostProcessor

InstantiationAwareBeanPostProcessor是BeanPostProcessor的子接口,可以在Bean生命周期的另外两个时期提供扩展的回调接口,即实例化Bean之前(调用postProcessBeforeInstantiation方法)和实例化Bean之后(调用postProcessAfterInstantiation方法),该接口定义如下:
package org.springframework.beans.factory.config;    
    
import java.beans.PropertyDescriptor;    
    
import org.springframework.beans.BeansException;    
import org.springframework.beans.PropertyValues;    
    
public interface InstantiationAwareBeanPostProcessor extends BeanPostProcessor {    
    
    Object postProcessBeforeInstantiation(Class<?> beanClass, String beanName) throws BeansException;    
    
    boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException;    
    
    PropertyValues postProcessPropertyValues(    
            PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName)    
            throws BeansException;    
    
}  

 


其使用方法与上面介绍的BeanPostProcessor接口类似,只时回调时机不同。
 
如果是使用ApplicationContext来生成并管理Bean的话则稍有不同,使用ApplicationContext来生成及管理Bean实例的话,在执行BeanFactoryAware的setBeanFactory()阶段后,若Bean类上有实现org.springframework.context.ApplicationContextAware接口,则执行其setApplicationContext()方法,接着才执行BeanPostProcessors的ProcessBeforeInitialization()及之后的流程。

 

Spring实战系列(三)-BeanPostProcessor的妙用

标签:led   地方   cte   order   ica   declare   体系   efault   advance   

原文地址:https://www.cnblogs.com/xuxinstyle/p/13230482.html

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