SpringMVC中的ConversionServiceExposingInterceptor工具类解析

// WebMvcConfigurationSupport 代码片段
// 
	/**
	 * Provide access to the shared handler interceptors used to configure
	 * {@link HandlerMapping} instances with.
	 * <p>This method cannot be overridden; use {@link #addInterceptors} instead.
	 */
	protected final Object[] getInterceptors() {
		if (this.interceptors == null) {
			InterceptorRegistry registry = new InterceptorRegistry();
			addInterceptors(registry);
			registry.addInterceptor(new ConversionServiceExposingInterceptor(mvcConversionService()));
			registry.addInterceptor(new ResourceUrlProviderExposingInterceptor(mvcResourceUrlProvider()));
			this.interceptors = registry.getInterceptors();
		}
		return this.interceptors.toArray();
	}
// requestMappingHandlerMapping() bean 定义中使用所有 getInterceptors() 定义的通用拦截器
// 		mapping.setInterceptors(getInterceptors());
// viewControllerHandlerMapping() bean 定义中使用所有 getInterceptors() 定义的通用拦截器
// 		handlerMapping.setInterceptors(getInterceptors());
// beanNameHandlerMapping() bean 定义中使用所有 getInterceptors() 定义的通用拦截器
// 		mapping.setInterceptors(getInterceptors());
// resourceHandlerMapping() bean 定义中使用所有 getInterceptors() 定义的通用拦截器
// 		handlerMapping.setInterceptors(getInterceptors());

// HandlerExecutionChain 代码片段
    /**
	 * Apply preHandle methods of registered interceptors.
	 * @return {@code true} if the execution chain should proceed with the
	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
	 * that this interceptor has already dealt with the response itself.
	 */
	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HandlerInterceptor[] interceptors = getInterceptors();
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = 0; i < interceptors.length; i++) {
				HandlerInterceptor interceptor = interceptors[i];
				if (!interceptor.preHandle(request, response, this.handler)) {
					triggerAfterCompletion(request, response, null);
					return false;
				}
				this.interceptorIndex = i;
			}
		}
		return true;
	}

package org.springframework.web.servlet.handler;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.core.convert.ConversionService;
import org.springframework.util.Assert;
/**
 * Interceptor that places the configured {@link ConversionService} in request scope
 * so it's available during request processing. The request attribute name is
 * "org.springframework.core.convert.ConversionService", the value of
 * {@code ConversionService.class.getName()}.
 *
 * <p>Mainly for use within JSP tags such as the spring:eval tag.
 *
 * @author Keith Donald
 * @since 3.0.1
 */
public class ConversionServiceExposingInterceptor extends HandlerInterceptorAdapter {
	private final ConversionService conversionService;
	/**
	 * Creates a new {@link ConversionServiceExposingInterceptor}.
	 * @param conversionService the conversion service to export to request scope when this interceptor is invoked
	 */
	public ConversionServiceExposingInterceptor(ConversionService conversionService) {
		Assert.notNull(conversionService, "The ConversionService may not be null");
		this.conversionService = conversionService;
	}
	@Override
	public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
			throws ServletException, IOException {
       // 请求处理前向请求添加属性 
       // 属性名称使用 ConversionService.class.getName()      
		request.setAttribute(ConversionService.class.getName(), this.conversionService);
		return true;
	}
}