Use Servlet filter in Spring Boot [Spring Boot 1.x, 2.x compatible]

environment

• Spring Boot 1.5.10
• Embedded Tomcat 8.5.27
• JDK 8

2018-03-09 Addendum It was the same behavior when I upgraded to Spring Boot 2.0.0.RELEASE.

2018-12-20 postscript Added the explanation of Spring Boot 2.1.

Basic Filter registration method

Use the FilterRegistrationBean class provided by Spring Boot. When you define a FilterRegistrationBean, the Filter is fetched in the Spring Boot Auto Configuration class and registered with the Embedded Server.

@Configuration
public class SomeConfig {

    @Bean
    public FilterRegistrationBean hogeFilter() {
        //New Filter and pass it to the FilterRegistrationBean constructor
        FilterRegistrationBean bean = new FilterRegistrationBean(new HogeFilter());
        //Filter url-Specify pattern (variadic argument allows multiple specifications)
        bean.addUrlPatterns("/*");
        //Filter execution order. Performed for aiming at integer values
        bean.setOrder(Integer.MIN_VALUE);
        return bean;
    }

    @Bean
    public FilterRegistrationBean fugaFilter() {
        FilterRegistrationBean bean = new FilterRegistrationBean(new FugaFilter());
        bean.addUrlPatterns("/*");
        //Executed after HogeFilter
        bean.setOrder(Integer.MIN_VALUE + 1);
        return bean;
    }

    //Any number of Filters can be defined
}

Use other beans in Filter

The @Bean method can accept other beans as arguments. This is a feature that Spring originally has. It is OK if the Bean received as an argument is received by the constructor of Filter.

public class HogeFilter extends OncePerRequestFilter { //Of course, "implements Filter" is also OK
    private final FugaBean fugaBean; //Create field

    public HogeFilter(FugaBean fugaBean) { //Receive the bean in the constructor
        this.fugaBean = fugaBean;
    }

    @Override
    protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain chain) throws ServletException, IOException {
        //Write pre-processing here
        chain.doFilter(request, response);
        //Write post-processing here
    }
}

@Configuration
public class HogeConfig {

    @Bean
    public FilterRegistrationBean hogeFilter(FugaBean fugaBean) {
        //Pass the bean to the Filter constructor
        FilterRegistrationBean bean = new FilterRegistrationBean(new HogeFilter(fugaBean));
        // ...
        return bean;
    }
}

The FugaBean must be bean-defined somewhere else (such as another Config class or component scan).

Note that the Filter itself is not a bean. That is, you cannot DI a bean to a Filter with @ Autowired or apply an AOP to a Filter.

See the list of application order of all filters

It is a class called ServletContextInitializerBeans that extracts Filter from FilterRegistrationBean and registers it in Embedded Server. It is OK if you output the DEBUG log of this class (Note: TRACE log after Spring Boot 2.1). Filter is executed in ascending order of the value of ʻorder`.

application.properties

# Boot 2.After 1 is trace instead of debug
logging.level.org.springframework.boot.web.servlet.ServletContextInitializerBeans=debug

Standard output

...
2018-02-28 11:58:37.611 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Added existing Filter initializer bean 'loggingFilter1'; order=-2147483648, resource=com.example.Application
2018-02-28 11:58:37.611 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Added existing Filter initializer bean 'loggingFilter2'; order=-2147483647, resource=com.example.Application
2018-02-28 11:58:37.612 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Added existing Filter initializer bean 'loggingFilter3'; order=-2147483646, resource=com.example.Application
2018-02-28 11:58:37.612 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Added existing Servlet initializer bean 'dispatcherServletRegistration'; order=2147483647, resource=class path resource [org/springframework/boot/autoconfigure/web/DispatcherServletAutoConfiguration$DispatcherServletRegistrationConfiguration.class]
2018-02-28 11:58:37.638 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Created Filter initializer for bean 'characterEncodingFilter'; order=-2147483648, resource=class path resource [org/springframework/boot/autoconfigure/web/HttpEncodingAutoConfiguration.class]
2018-02-28 11:58:37.638 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Created Filter initializer for bean 'hiddenHttpMethodFilter'; order=-10000, resource=class path resource [org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.class]
2018-02-28 11:58:37.638 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Created Filter initializer for bean 'httpPutFormContentFilter'; order=-9900, resource=class path resource [org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration.class]
2018-02-28 11:58:37.638 DEBUG 4323 --- [ost-startStop-1] o.s.b.w.s.ServletContextInitializerBeans : Created Filter initializer for bean 'requestContextFilter'; order=-105, resource=class path resource [org/springframework/boot/autoconfigure/web/WebMvcAutoConfiguration$WebMvcAutoConfigurationAdapter.class]
...

What if you define the Filter itself as a Bean?

It is OK to define the Filter itself as a bean, but I cannot specify the url-pattern or execution order. Even if you specify the @ Order annotation in the @ Bean method, it seems to be ignored (regardless of the value of @ Order, the value of order output to the log will be 2147483647).

    @Bean
    @Order(Integer.MIN_VALUE) //This value seems to be ignored (not sure if it's a spec)
    public HogeFilter hogeFilter() {
        return new HogeFilter();
    }

So let's use FilterRegistrationBean.

Reference material

Spring Boot Reference https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#boot-features-embedded-container-servlets-filters-listeners-beans Javadoc https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/web/servlet/FilterRegistrationBean.html