一次由HandlerInterceptor进行的深刻思考

HandlerInterceptor

是SpringFramework为咱们提供的拦截器，通常咱们能够用来鉴权或者日志记录等。

它是一个interface，主要方法有：

/**
	 * Intercept the execution of a handler. Called after HandlerMapping determined
	 * an appropriate handler object, but before HandlerAdapter invokes the handler.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can decide to abort the execution chain,
	 * typically sending a HTTP error or writing a custom response.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler chosen handler to execute, for type and/or instance evaluation
	 * @return {@code true} true表示继续流程（如调用下一个拦截器或处理器）；false表示流程中断
	 *（如登陆检查失败），不会继续调用其余的拦截器或处理器，此时咱们须要经过response来产生响应
	 * @throws Exception in case of errors
	 */
	boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
	    throws Exception;

	/**
	 * 后处理回调方法，实现处理器的后处理（但在渲染视图以前），此时咱们能够经过modelAndView（模型和视图对象）对模型数据进行处理或对视图进行处理，modelAndView也可能为null。
     * Intercept the execution of a handler. Called after HandlerAdapter actually
	 * invoked the handler, but before the DispatcherServlet renders the view.
	 * Can expose additional model objects to the view via the given ModelAndView.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can post-process an execution,
	 * getting applied in inverse order of the execution chain.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler handler (or {@link HandlerMethod}) that started async
	 * execution, for type and/or instance examination
	 * @param modelAndView the {@code ModelAndView} that the handler returned
	 * (can also be {@code null})
	 * @throws Exception in case of errors
	 */
	void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
			throws Exception;

	/**
	 * Callback after completion of request processing, that is, after rendering
	 * the view. Will be called on any outcome of handler execution, thus allows
	 * for proper resource cleanup.
	 * <p>Note: Will only be called if this interceptor's {@code preHandle}
	 * method has successfully completed and returned {@code true}!
	 * <p>As with the {@code postHandle} method, the method will be invoked on each
	 * interceptor in the chain in reverse order, so the first interceptor will be
	 * the last to be invoked.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler handler (or {@link HandlerMethod}) that started async
	 * execution, for type and/or instance examination
	 * @param ex exception thrown on handler execution, if any
	 * @throws Exception in case of errors
	 */
	void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
			throws Exception;

• preHandle 预处理回调，在处理器处理以前回调

• postHandle 处理器处理完毕，在渲染以前回调，能够计算处理器处理时间

• afterCompletion 请求渲染结束后回调 这边咱们能够计算渲染时间，或者整个请求的执行时间

拦截器适配器HandlerInterceptorAdapter

有时候咱们可能只须要实现三个回调方法中的某一个，若是实现HandlerInterceptor接口的话，三个方法必须实现，无论你需不须要，此时spring提供了一个HandlerInterceptorAdapter适配器（适配器设计模式的实现），容许咱们只实现须要的回调方法。

public abstract class HandlerInterceptorAdapter implements AsyncHandlerInterceptor {

	/**
	 * This implementation always returns {@code true}.
	 */
	@Override
	public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
		throws Exception {
		return true;
	}

	/**
	 * This implementation is empty.
	 */
	@Override
	public void postHandle(
			HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
			throws Exception {
	}

	/**
	 * This implementation is empty.
	 */
	@Override
	public void afterCompletion(
			HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
			throws Exception {
	}

	/**
	 * This implementation is empty.
	 * 当Controller中有异步请求方法的时候会触发该方法。 楼主作过测试，异步请求先支持preHandle、而后执行afterConcurrentHandlingStarted
	 */
	@Override
	public void afterConcurrentHandlingStarted(
			HttpServletRequest request, HttpServletResponse response, Object handler)
			throws Exception {
	}

}

运行流程总结以下：

一、拦截器执行顺序是按照Spring配置文件中定义的顺序而定的。

二、会先按照顺序执行全部拦截器的preHandle方法，一直遇到return false为止，好比第二个preHandle方法是return false，则第三个以及之后全部拦截器都不会执行。若都是return true，则按顺序加载完preHandle方法。

三、而后执行主方法（本身的controller接口），若中间抛出异常，则跟return false效果一致，不会继续执行postHandle，只会倒序执行afterCompletion方法。

四、在主方法执行完业务逻辑（页面还未渲染数据）时，按倒序执行postHandle方法。若第三个拦截器的preHandle方法return false，则会执行第二个和第一个的postHandle方法和afterCompletion（postHandle都执行完才会执行这个，也就是页面渲染完数据后，执行after进行清理工做）方法。（postHandle和afterCompletion都是倒序执行

接口适配器模式

其实从上面的HandlerInterceptor到HandlerInterceptorAdapter，咱们能够看出这是使用了接口适配器模式，接口Adapter对接口进行实现，可是又不作具体的逻辑实现。

这样咱们在使用的时候只要继承对应的Adapter复写须要的方法便可

Java8 default interface

上面那种在Java8以前常常会用到，但是在Java8新增default方法后，就不多用到了。前提最低支持Java8

Java8的新特性中有一个新特性为接口默认方法，该新特性容许咱们在接口中添加一个非抽象的方法实现，而这样作的方法只须要使用关键字default修饰该默认实现方法便可。该特性又叫扩展方法

public interface Formula {
    double calculate(int a);
    default double sqrt(int a){
        return Math.sqrt(a);
    }
}

经过该特性，咱们将可以很方便的实现接口默认实现类。这个特性在编译器实现的角度来讲更接近于Scala的trait

SpringBoot2.0由于最低支持Java8后一些改变

若是你们有用过SpringBoot2.0会发现有大量的方法或者类被从新设计，固然一部分是对1.0的接口优化或者功能优化。

其中一个很重要的是对JDK的要求，也就是说Spring Boot2.0的最低版本要求为JDK8。从代码中咱们能够看出，不少地方使用了default方法，对于老的接口Adapter进行了舍弃。

好比咱们之前在配置相应的HandlerInterceptor使用的WebMvcConfigurerAdapter，就被标记为Deprecated

/**
 * An implementation of {@link WebMvcConfigurer} with empty methods allowing
 * subclasses to override only the methods they're interested in.
 *
 * @author Rossen Stoyanchev
 * @since 3.1
 * @deprecated as of 5.0 {@link WebMvcConfigurer} has default methods (made
 * possible by a Java 8 baseline) and can be implemented directly without the
 * need for this adapter
 */
@Deprecated
public abstract class WebMvcConfigurerAdapter implements WebMvcConfigurer

接下来咱们来看下WebMvcConfigurer在SpringBoot2.0和SpringBoot1.0中的代码区别：

SpringBoot2.0中使用的SpringFramework5

public interface WebMvcConfigurer {

	default void configurePathMatch(PathMatchConfigurer configurer) {
	}

	
	default void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
	}

	
	default void configureAsyncSupport(AsyncSupportConfigurer configurer) {
	}

	
	default void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
	}

	
	default void addFormatters(FormatterRegistry registry) {
	}

	
	default void addInterceptors(InterceptorRegistry registry) {
	}

	
	default void addResourceHandlers(ResourceHandlerRegistry registry) {
	}


	default void addCorsMappings(CorsRegistry registry) {
	}


	default void addViewControllers(ViewControllerRegistry registry) {
	}

	
	default void configureViewResolvers(ViewResolverRegistry registry) {
	}

	
	default void addArgumentResolvers(List<HandlerMethodArgumentResolver> resolvers) {
	}

	
	default void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> handlers) {
	}


	default void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
	}


	default void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
	}


	default void configureHandlerExceptionResolvers(List<HandlerExceptionResolver> resolvers) {
	}


	default void extendHandlerExceptionResolvers(List<HandlerExceptionResolver> resolvers) {
	}


	@Nullable
	default Validator getValidator() {
		return null;
	}

	
	@Nullable
	default MessageCodesResolver getMessageCodesResolver() {
		return null;
	}

}

SpringBoot1.0中使用的SpringFramework4

public interface WebMvcConfigurer {

	void addFormatters(FormatterRegistry registry);

	
	void configureMessageConverters(List<HttpMessageConverter<?>> converters);


	void extendMessageConverters(List<HttpMessageConverter<?>> converters);

	
	Validator getValidator();

	
	void configureContentNegotiation(ContentNegotiationConfigurer configurer);

	
	void configureAsyncSupport(AsyncSupportConfigurer configurer);

	
	void configurePathMatch(PathMatchConfigurer configurer);


	void addArgumentResolvers(List<HandlerMethodArgumentResolver> argumentResolvers);


	void addReturnValueHandlers(List<HandlerMethodReturnValueHandler> returnValueHandlers);

	
	void configureHandlerExceptionResolvers(List<HandlerExceptionResolver> exceptionResolvers);

	
	void addInterceptors(InterceptorRegistry registry);

	
	MessageCodesResolver getMessageCodesResolver();


	void addViewControllers(ViewControllerRegistry registry);


	void configureViewResolvers(ViewResolverRegistry registry);

	
	void addResourceHandlers(ResourceHandlerRegistry registry);

	
	void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer);

}

其中惟一的区别就是在SpringFramework5中使用了default关键字，致使的结果就是WebMvcConfigurerAdapter能够被舍弃不使用了。

固然在Spring中的例子还有不少，好比HandlerInterceptor和HandlerInterceptorAdapter等，你们在使用的时候除了要看相应的接口变化，更重要的是要去了解为何会有这种变化

结果致使SpringBoot2.0中WebMvcConfigurer配置方式变化

@Configuration
public class WebConf implements WebMvcConfigurer {
    @Override
    public void addInterceptors(InterceptorRegistry registry) {
        registry.addInterceptor(new AsyncInterceptor()).addPathPatterns("/*");
    }
}