SpringMVC的DispatcherServlet的默认策略

落日映苍穹つ 2022-03-11 05:15 179阅读 0赞

# SpringMVC的DispatcherServlet的默认策略 #

在使用SpringMVC的时候，我们知道需要HandlerMapping定义请求路径与处理器之间的映射，需要HandlerAdapter来调用处理器方法并返回一个ModelAndView对象，需要ViewResolver来解析视图。这些是SpringMVC中最基本的接口。通常我们都需要在SpringMVC的配置文件中定义好需要使用的HandlerMapping、HandlerAdapter和ViewResolver，基于注解的SpringMVC配置也是类似的。所以一般我们的SpringMVC配置文件会是如下这样：

<mvc:annotation-driven/>
        
    <mvc:view-resolvers>
        <mvc:jsp prefix="/WEB-INF/views/" suffix=".jsp"/>
    </mvc:view-resolvers>

mvc命名空间是Spring提供的一个简化SpringMVC配置的命名空间，`<mvc:annotation-driven/>`会自动在Spring的bean容器中注册HandlerMapping接口实现RequestMappingHandlerMapping类型的bean和HandlerAdapter接口实现类RequestMappingHandlerAdapter类型的bean。`<mvc:view-resolvers/>`是用来快速定义ViewResolver实现的，其中`<mvc:jsp/>`会自动定义一个InternalResourceViewResolver类型的ViewResolver。

针对这些比较基本接口，即使我们不定义它们的实现，SpringMVC内部也给了默认的定义，它管这些叫策略。SpringMVC把这些默认策略都定义在一个叫`DispatcherServlet.properties`的文件中，它与DispatcherServlet在同一个包中。以下是笔者在使用的4.1.0版本中的DispatcherServlet.properties文件中的定义。

org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver
    
    org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver
    
    org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
    	org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping
    
    org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
    	org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
    	org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter
    
    org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver,\
    	org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver,\
    	org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver
    
    org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator
    
    org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver
    
    org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager

从上面的定义中我们可以看出来，在使用SpringMVC时，即使你不定义HandlerMapping，SpringMVC默认也会给你定义一个BeanNameUrlHandlerMapping和DefaultAnnotationHandlerMapping。HandlerAdapter和ViewResolver也是一样的。还有一些其它的默认策略也请参考DispatcherServlet.properties中。如果默认策略不能满足你的要求，那么我们可以在bean容器中定义我们自己对应的实现，这个时候就会应用我们自己的实现了。我们拿初始化HandlerMapping为例，来看一下DispatcherServlet的源码。

/** * Initialize the HandlerMappings used by this class. * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace, * we default to BeanNameUrlHandlerMapping. */
    private void initHandlerMappings(ApplicationContext context) { 
        this.handlerMappings = null;
    
        if (this.detectAllHandlerMappings) { 
            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerMapping> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) { 
                this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                OrderComparator.sort(this.handlerMappings);
            }
        }
        else { 
            try { 
                HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
                this.handlerMappings = Collections.singletonList(hm);
            }
            catch (NoSuchBeanDefinitionException ex) { 
                // Ignore, we'll add a default HandlerMapping later.
            }
        }
    
        // Ensure we have at least one HandlerMapping, by registering
        // a default HandlerMapping if no other mappings are found.
        if (this.handlerMappings == null) { 
            this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
            if (logger.isDebugEnabled()) { 
                logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
            }
        }
    }

从源码中我们可以看到，SpringMVC先会从绑定的ApplicationContext中获取对应的HandlerMapping定义，如果没有取到就会调用`getDefaultStrategies(context, HandlerMapping.class)`从默认策略中获取。它的代码如下所示。

protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) { 
        String key = strategyInterface.getName();
        String value = defaultStrategies.getProperty(key);
        if (value != null) { 
            String[] classNames = StringUtils.commaDelimitedListToStringArray(value);
            List<T> strategies = new ArrayList<T>(classNames.length);
            for (String className : classNames) { 
                try { 
                    Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader());
                    Object strategy = createDefaultStrategy(context, clazz);
                    strategies.add((T) strategy);
                }
                catch (ClassNotFoundException ex) { 
                    throw new BeanInitializationException(
                            "Could not find DispatcherServlet's default strategy class [" + className +
                                    "] for interface [" + key + "]", ex);
                }
                catch (LinkageError err) { 
                    throw new BeanInitializationException(
                            "Error loading DispatcherServlet's default strategy class [" + className +
                                    "] for interface [" + key + "]: problem with class file or dependent class", err);
                }
            }
            return strategies;
        }
        else { 
            return new LinkedList<T>();
        }
    }

其中的defaultStrategies就是对应的`DispatcherServlet.properties`文件中的内容。

private static final String DEFAULT_STRATEGIES_PATH = "DispatcherServlet.properties";
    
    private static final Properties defaultStrategies;
    
    static { 
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try { 
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) { 
            throw new IllegalStateException("Could not load 'DispatcherServlet.properties': " + ex.getMessage());
        }
    }

（完）  
（注：本文是基于SpringMVC4.1.0所写）