BeanPostProcessor的用法

BeanPostProcessor也稱為Bean后置處理器，它是Spring中定義的接口凑保，在Spring容器的創(chuàng)建過程中（具體為Bean初始化前后）會回調(diào)BeanPostProcessor中定義的兩個方法。BeanPostProcessor的源碼如下:

public interface BeanPostProcessor {
    Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException;    
    Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException;
}

其中postProcessBeforeInitialization方法會在每一個bean對象的初始化方法調(diào)用之前回調(diào)犯助；postProcessAfterInitialization方法會在每個bean對象的初始化方法調(diào)用之后被回調(diào)。

提出的問題

在Spring開發(fā)過程中剂买，存在同一個接口有多個實現(xiàn)類的情況癌蓖，根據(jù)不同的應用場景瞎颗，通常在具體調(diào)用的地方來選擇不同的接口實現(xiàn)類疯特，雖然可以在抽象出一個工廠方法蜘腌，但是還是感覺不夠優(yōu)雅。如果通過注解的方式解決這個問題呢责循，Spring的BeanPostProcessor是個好的選擇。

具體實現(xiàn)

聲明接口
public interface HelloService{
    public void sayHello();
}

接口實現(xiàn)類1

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

接口實現(xiàn)類2

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

自定義注解

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

自定義的BeanPostProcessor類

@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) {
            //找到制定目標的注解類
            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);
        }
    }
}

代理實現(xiàn)類

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());
        }
    }
}

SpringBoot入口類

@SpringBootApplication
@MapperScan("com.lx.mapper")
public class MlxcApplication {
    public static void main(final String[] args) {
        try (ConfigurableApplicationContext applicationContext = SpringApplication.run(MlxcApplication.class, args)) {
            HelloServiceTest helloService = applicationContext.getBean(HelloServiceTest.class);
            helloService.testSayHello();
        }
    }
}

總結(jié)
上述是整個解決方案的示例流程歹垫，其核心思想就是根據(jù)自定義注解攔截要注入的接口實現(xiàn)類，運用java反射和代理的知識點來進行有效的實現(xiàn)類注入吭敢。