1. 問題

最近在項目中添加了會話驗證過濾器，該Filter中使用@Autowired自動裝載了一些從數(shù)據(jù)庫中獲取的系統(tǒng)配置，調(diào)試的時候發(fā)現(xiàn)注入失敗，返回為null。

// RequestAuthFilter只是一個普通類
// 這段代碼處于Application.java啟動類中
@Bean
    public FilterRegistrationBean contextFilterRegistrationBean() {
        FilterRegistrationBean registrationBean = new FilterRegistrationBean();
        registrationBean.setFilter(new RequestAuthFilter());
        registrationBean.addUrlPatterns("/web/*", "/wechat/*", "/core/*");
        registrationBean.setName(GlobalConstants.AUTH_FILTER_NAME);
        registrationBean.setOrder(1);

        return registrationBean;
    }

2. 處理方案

嘗試以下兩種方案均成功，本質(zhì)上都是將Filter變成讓Spring容器中的bean。

2.1 修改filter為Spring中bean

• 修改RequestAuthFilter為Component

@Component
public class RequestAuthFilter implements Filter {

• 自動裝載該類

@Autowired
    RequestAuthFilter filter;

@Bean
    public FilterRegistrationBean contextFilterRegistrationBean() {
        ...
        registrationBean.setFilter(filter);
        ...
        return registrationBean;
    }

2.2 保留Filter為普通類，在Application.java中重新定義一個Bean

@Bean
    public FilterRegistrationBean contextFilterRegistrationBean() {
        ...
        registrationBean.setFilter(getFilterRegistrationBean());
        ...
    }

    @Bean
    public Filter getFilterRegistrationBean() {
        return new RequestAuthFilter();
    }

3. 分析原因

后期通過new產(chǎn)生的實例中無法自動注入Spring容器中裝載的實例。
原理需要了解Spring依賴注入，或者說控制反轉(zhuǎn)，查閱Introduction to the Spring IoC container and beans。

It is a process whereby objects define their dependencies, that is, the other objects they work with, only through constructor arguments, arguments to a factory method, or properties that are set on the object instance after it is constructed or returned from a factory method. The container then injects those dependencies when it creates the bean.

這些Bean被Spring IoC 容器(org.springframework.context.ApplicationContext)管理，Spring容器也就是一個Bean工廠。
所以自動注入失效，一般有以下兩種可能。

3.1 包沒有被掃描到

引用自spring-boot-and-component-scan

• If your other packages hierarchies are below your main app with the @SpringBootApplication annotation, you’re covered by implicit components scan.
• If there are beans/components in other packages which are not sub packages of the main package, you should manually add them as @ComponentScan

Spring Boot項目的Bean裝配默認規(guī)則是根據(jù)Application類(指項目入口類)所在的包位置從上往下掃描。
假設(shè)Application類在包com.comp.appname下，則只會掃描com.comp.appname包及其所有子包，如果需要自動裝載的類所在包不在com.comp.appname及其子包下，則不會被掃描，自然就沒法被注入！

3.2 調(diào)用者是使用new創(chuàng)建的

如果類A中存在成員屬性B, B是通過@Autowired自動注入，而類A的實例是通過new的方式產(chǎn)生的，那么自動注入會失效的，此時通過Spring的上下文獲取所有的Bean的方法來獲取B。

/**
 * Spring上下文工具類，用以讓普通類獲取Spring容器中的Bean
 */
@Component
public class SpringUtil implements ApplicationContextAware {

    private static ApplicationContext applicationContext = null;

    /**
     * 獲取applicationContext
     */
    public static ApplicationContext getApplicationContext() {
        return applicationContext;
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        if (SpringUtil.applicationContext == null) {
            SpringUtil.applicationContext = applicationContext;
        }
    }

    /**
     * 通過name獲取 Bean
     */
    public static Object getBean(String name) {
        return getApplicationContext().getBean(name);
    }
}

然后在A中就可以這樣來獲取Spring容器中的B實例

BclassInterface b = (BclassInterfaceImpl) SpringUtil.getBean("bclassInterfaceImpl");