对于基于任何条件和spring引导中的任何列表的过滤，查询语言更加灵活，并允许我们向下过滤到我们需要的确切资源。 例如，如果我们有一个简单的实体，如User，并像这样定义它。

@Entity
public class User {
@Id
@GeneratedValue(strategy = 
GenerationType.AUTO)
private Long id;

private String firstName;
private String lastName;
private String email;

private int age;

//Setter and getter as usual

然后我们创建一个实现Specification接口的UserSpecification，我们将传递我们自己的约束来构造实际的查询。在本部分中，与标准和构建器相关的方法非常有用和灵活。

public class UserSpecification implements Specification<User> {

private SearchCriteria criteria;

@Override
public Predicate toPredicate
  (Root<User> root, CriteriaQuery<?> query, CriteriaBuilder builder) {

    if (criteria.getOperation().equalsIgnoreCase(">")) {
        return builder.greaterThanOrEqualTo(
          root.<String> get(criteria.getKey()), criteria.getValue().toString());
    } 
    else if (criteria.getOperation().equalsIgnoreCase("<")) {
        return builder.lessThanOrEqualTo(
          root.<String> get(criteria.getKey()), criteria.getValue().toString());
    } 
    else if (criteria.getOperation().equalsIgnoreCase(":")) {
        if (root.get(criteria.getKey()).getJavaType() == String.class) {
            return builder.like(
              root.<String>get(criteria.getKey()), "%" + criteria.getValue() + "%");
        } else {
            return builder.equal(root.get(criteria.getKey()), criteria.getValue());
        }
    }
    return null;
}
}

此外，我们还创建了一个基于以下SearchCriteria类中表示的一些简单约束的规范，任何条件都可以像这样创建:

public class SearchCriteria {
private String key;
private String operation;
private Object value;
 }

最后定义UserRepository和扩展JpaSpecificationExecutor，

public interface UserRepository 
extends JpaRepository<User, Long>, 
JpaSpecificationExecutor<User> {}

没有特殊的注释来表示servlet筛选器。您只需声明一个Filter类型的@Bean(或FilterRegistrationBean)。在Boot自己的EndpointWebMvcAutoConfiguration中有一个例子(为所有响应添加自定义头);

如果你只声明一个过滤器，它将被应用到所有的请求。如果您还添加了一个FilterRegistrationBean，您可以另外指定要应用的单个servlet和url模式。

注意:

从Spring Boot 1.4开始，FilterRegistrationBean不再弃用，而是简单地将包从org.springframework. Boot .context. embedd.filterregistrationbean移动到org.springframework.boot.web.servlet.FilterRegistrationBean

使用@WebFilter注释，可以这样做:

@WebFilter(urlPatterns = {"/*" })
public class AuthenticationFilter implements Filter{

    private static Logger logger = Logger.getLogger(AuthenticationFilter.class);

    @Override
    public void destroy() {
        // TODO Auto-generated method stub

    }

    @Override
    public void doFilter(ServletRequest arg0, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {

         logger.info("checking client id in filter");
        HttpServletRequest request = (HttpServletRequest) arg0;
        String clientId = request.getHeader("clientId");
        if (StringUtils.isNotEmpty(clientId)) {
            chain.doFilter(request, response);
        } else {
            logger.error("client id missing.");
        }
    }

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub

    }

}

你可以在实现javax.servlet.Filter的类上使用@WebFilter javax.servlet.annotation.WebFilter:

@WebFilter(urlPatterns = "/*")
public class MyFilter implements Filter {
}

然后使用@ServletComponentScan注册。

此过滤器还将帮助您允许跨源访问

@Component
@Order(Ordered.HIGHEST_PRECEDENCE)
public class SimpleCORSFilter implements Filter {

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {

            HttpServletResponse response = (HttpServletResponse) res;
            HttpServletRequest request = (HttpServletRequest) req;
            response.setHeader("Access-Control-Allow-Origin", "*");
            response.setHeader("Access-Control-Allow-Methods", "POST, GET, OPTIONS, DELETE");
            response.setHeader("Access-Control-Max-Age", "20000");
            response.setHeader("Access-Control-Allow-Headers", "x-requested-with, authorization, Content-Type, Authorization, credential, X-XSRF-TOKEN");

            if("OPTIONS".equalsIgnoreCase(request.getMethod())) {
                response.setStatus(HttpServletResponse.SC_OK);
            } else {
                chain.doFilter(req, res);
            }
    }


    public void destroy() {}

    @Override
    public void init(FilterConfig arg0) throws ServletException {
        // TODO Auto-generated method stub

    }

}

过滤器，顾名思义，用于对资源的请求或资源的响应执行过滤，或同时对两者执行过滤。Spring Boot提供了一些选项来在Spring Boot应用程序中注册自定义过滤器。让我们看看不同的选项。

1. 定义Spring Boot过滤器和调用顺序

实现Filter接口，在Spring Boot中创建一个新的过滤器。

@Configuration
@Order(Ordered.HIGHEST_PRECEDENCE)
public class CustomFilter implements Filter {

    private static final Logger LOGGER = LoggerFactory.getLogger(CustomFilter.class);

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        LOGGER.info("########## Initiating Custom filter ##########");
    }

    @Override
    public void doFilter(ServletRequest servletRequest,
                         ServletResponse servletResponse,
                         FilterChain filterChain)
                         throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        LOGGER.info("Logging Request  {} : {}", request.getMethod(), request.getRequestURI());

        // Call next filter in the filter chain
        filterChain.doFilter(request, response);

        LOGGER.info("Logging Response :{}", response.getContentType());
    }

    @Override
    public void destroy() {
        // TODO: 7/4/2018
    }
}

让我们快速看一下上面代码中的一些要点

@Component注释注册的过滤器。 为了以正确的顺序触发过滤器，我们需要使用@Order注释。 @ component @Order (1) 公共类CustomFirstFilter实现了Filter { ｝ @ component @Order (2) 公共类CustomSecondFilter实现了Filter { ｝

在上面的代码中，CustomFirstFilter将在CustomSecondFilter之前运行。

数字越低，优先级越高

2. URL模式

如果基于约定的映射不够灵活，我们可以使用FilterRegistrationBean对应用程序进行完全控制。在这里，不要为过滤器类使用@Component注释，而是使用FilterRegistrationBean注册过滤器。

public class CustomURLFilter implements Filter {

    private static final Logger LOGGER = LoggerFactory.getLogger(CustomURLFilter.class);

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        LOGGER.info("########## Initiating CustomURLFilter filter ##########");
    }

    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {

        HttpServletRequest request = (HttpServletRequest) servletRequest;
        HttpServletResponse response = (HttpServletResponse) servletResponse;

        LOGGER.info("This Filter is only called when request is mapped for /customer resource");

        // Call the next filter in the filter chain
        filterChain.doFilter(request, response);
    }

    @Override
    public void destroy() {

    }
}

使用FilterRegistrationBean注册自定义过滤器。

@Configuration
public class AppConfig {

    @Bean
    public FilterRegistrationBean < CustomURLFilter > filterRegistrationBean() {
        FilterRegistrationBean < CustomURLFilter > registrationBean = new FilterRegistrationBean();
        CustomURLFilter customURLFilter = new CustomURLFilter();

        registrationBean.setFilter(customURLFilter);
        registrationBean.addUrlPatterns("/greeting/*");
        registrationBean.setOrder(2); // Set precedence
        return registrationBean;
    }
}