本文是學(xué)習(xí)了小馬哥在慕課網(wǎng)的課程的《Spring Boot 2.0深度實(shí)踐之核心技術(shù)篇》的內(nèi)容結(jié)合自己的需要和理解做的筆記。

所謂的視圖內(nèi)容協(xié)商，就是讓W(xué)eb客戶端根據(jù)不同的請求策略，實(shí)現(xiàn)服務(wù)端響應(yīng)對應(yīng)視圖的內(nèi)容輸出屈呕。 接下來讓我們深入的了解一下到底Spring是如何視圖內(nèi)容協(xié)商的。

核心組件

• 視圖解析器:ContentNegotiatingViewResolver
• 內(nèi)容協(xié)商管理器:ContentNegotiationManager
• 內(nèi)容協(xié)商策略:ContentNegotiationStrategy

源碼解讀前置工作

在我們要理解Spring的視圖內(nèi)容協(xié)調(diào)流程圖之前，我們需要新建一個spring-boot項(xiàng)目下面，然后進(jìn)行必要的配置來啟動視圖內(nèi)容協(xié)商。我們新建一個模塊名為 springboot-restful 之所以起這個名字,是因?yàn)橐晥D內(nèi)容協(xié)商不僅是對客戶端視圖渲染的協(xié)商操作绩聘，也是針對restful形式的內(nèi)容的請求和響應(yīng)的協(xié)商操作沥割。

1.新建Model-- springboot-restful

目錄.PNG

2.pom.xml

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>javax.servlet-api</artifactId>
    </dependency>

    <dependency>
        <groupId>javax.servlet</groupId>
        <artifactId>jstl</artifactId>
    </dependency>

    <!-- Provided -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-tomcat</artifactId>
    </dependency>
    <dependency>
        <groupId>org.apache.tomcat.embed</groupId>
        <artifactId>tomcat-embed-jasper</artifactId>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
        </plugin>
    </plugins>
</build>

3.其他代碼

/**
 * Spring 攔截器 配置
 */
@Configuration   //配置
public class WebMvcConfig implements WebMvcConfigurer {

    /**
     * 配置視圖內(nèi)容協(xié)商
     * @param configurer
     */
    @Override
    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
        configurer.favorParameter(true).favorPathExtension(true);

    }


    /**
     * 解決在IDEA下maven多模塊使用spring-boot跳轉(zhuǎn)JSP 404問題
     * @return
     */
    @Bean
    public WebServerFactoryCustomizer<TomcatServletWebServerFactory> customizer() {
        return (factory -> {
            factory.addContextCustomizers(context -> {
                //當(dāng)前webapp路徑
                String relativePath = "springboot-restful/src/main/webapp";
                File docBaseFile = new File(relativePath);
                if(docBaseFile.exists()) {
                    context.setDocBase(new File(relativePath).getAbsolutePath());
                }
            });
        });
    }

}

/**
 * Spring-boot 啟動引導(dǎo)類
 */
@SpringBootApplication
public class SpringBootRestfulBootStrap {


    public static void main(String[] args) {
        SpringApplication.run(SpringBootRestfulBootStrap.class,args);

    }
}

流程圖

流程圖.PNG

源碼解讀

我們可以根據(jù)上面的流程圖來一起閱讀源碼，這樣能讓我們有個初步的理解凿菩。

步驟一

首先我們來看一下Spring-boot是什么時(shí)候開始初始化聲明ContentNegotiationConfigurer机杜。

org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.EnableWebMvcConfiguration#mvcContentNegotiationManager

在這個方法中,我們可以看到在容器初始化時(shí)，Spring-boot的自動裝配就把ContentNegotiationManager裝在到容器了衅谷。

@Bean
@Override
public ContentNegotiationManager mvcContentNegotiationManager() {
   ContentNegotiationManager manager = super.mvcContentNegotiationManager();
   List<ContentNegotiationStrategy> strategies = manager.getStrategies();
   ListIterator<ContentNegotiationStrategy> iterator = strategies.listIterator();
   while (iterator.hasNext()) {
      ContentNegotiationStrategy strategy = iterator.next();
      if (strategy instanceof PathExtensionContentNegotiationStrategy) {
         iterator.set(new OptionalPathExtensionContentNegotiationStrategy(
               strategy));
      }
   }
   return manager;
}

super.mvcContentNegotiationManager()源碼:

/**
 * Return a {@link ContentNegotiationManager} instance to use to determine
 * requested {@linkplain MediaType media types} in a given request.
 */
@Bean
public ContentNegotiationManager mvcContentNegotiationManager() {
   if (this.contentNegotiationManager == null) {
      ContentNegotiationConfigurer configurer = new ContentNegotiationConfigurer(this.servletContext);
      configurer.mediaTypes(getDefaultMediaTypes());
      configureContentNegotiation(configurer);
      this.contentNegotiationManager = configurer.buildContentNegotiationManager();
   }
   return this.contentNegotiationManager;
}

在這里通過ContentNegotiationConfigurer來創(chuàng)建ContentNegotiationManager 對象椒拗，我們先來看一下ContentNegotiationConfigurer中都有哪些關(guān)鍵的方法。

public class ContentNegotiationConfigurer {

   private final ContentNegotiationManagerFactoryBean factory = new ContentNegotiationManagerFactoryBean();
    .......
            /**
     * Build a {@link ContentNegotiationManager} based on this configurer's settings.
     * @since 4.3.12
     * @see ContentNegotiationManagerFactoryBean#getObject()
     */
    protected ContentNegotiationManager buildContentNegotiationManager() {
        this.factory.addMediaTypes(this.mediaTypes);
        return this.factory.build();
    }

}

在這里 我們可以看到ContentNegotiationConfigurer 類中获黔，聲明了一個ContentNegotiationManagerFactoryBean這也如流程圖中的 步驟1 ---- 關(guān)聯(lián)蚀苛。

步驟二

配置策略 則是使用ContentNegotiationConfigurer 的幾個方法來配置，這里我們在自定義的com.web.configuration.WebMvcConfig中只使用了兩種配置肢执。代碼如下

@Override
public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
    configurer.favorParameter(true).favorPathExtension(true);

}

對應(yīng)的方法代碼如下:

    /**
     * Whether a request parameter ("format" by default) should be used to
     * determine the requested media type. For this option to work you must
     * register {@link #mediaType(String, MediaType) media type mappings}.
     * <p>By default this is set to {@code false}.
     * @see #parameterName(String)
     */
//請求參數(shù)
public ContentNegotiationConfigurer favorParameter(boolean favorParameter) {
   this.factory.setFavorParameter(favorParameter);
   return this;
}
    /**
     * Whether the path extension in the URL path should be used to determine
     * the requested media type.
     * <p>By default this is set to {@code true} in which case a request
     * for {@code /hotels.pdf} will be interpreted as a request for
     * {@code "application/pdf"} regardless of the 'Accept' header.
     */
//URL后綴
public ContentNegotiationConfigurer favorPathExtension(boolean favorPathExtension) {
    this.factory.setFavorPathExtension(favorPathExtension);
    return this;
}

這里就不多做解釋了枉阵，感興趣的小伙伴可以看一下英文注釋，也就輕輕松松明白了预茄。

步驟三/步驟四

添加策略和創(chuàng)建ContentNegotiationManager 放在一起講兴溜。

我們可以從步驟一的源碼中看到 最后是調(diào)用ContentNegotiationConfigurer中的buildContentNegotiationManager()方法來創(chuàng)建ContentNegotiationManager的侦厚。

那么我們來進(jìn)一步看一下代碼。

org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer#buildContentNegotiationManager

protected ContentNegotiationManager buildContentNegotiationManager() {
   this.factory.addMediaTypes(this.mediaTypes);
   return this.factory.build();
}

這個方法在步驟一中我就有貼出拙徽，因?yàn)榉浅Ｖ匾俾伲谶@我們可以看到，其實(shí)是使用ContentNegotiationManagerFactoryBean來創(chuàng)建ContentNegotiationManager的膘怕。我們來看一下build()方法想诅。

/**
 * Actually build the {@link ContentNegotiationManager}.
 * @since 5.0
 */
public ContentNegotiationManager build() {
   List<ContentNegotiationStrategy> strategies = new ArrayList<>();

   if (this.strategies != null) {
      strategies.addAll(this.strategies);
   }
   else {
       //是否配置 URL后綴策略
      if (this.favorPathExtension) {
          //聲明并配置 PathExtensionContentNegotiationStrategy 策略
         PathExtensionContentNegotiationStrategy strategy;
         if (this.servletContext != null && !useRegisteredExtensionsOnly()) {
            strategy = new ServletPathExtensionContentNegotiationStrategy(this.servletContext, this.mediaTypes);
         }
         else {
            strategy = new PathExtensionContentNegotiationStrategy(this.mediaTypes);
         }
         strategy.setIgnoreUnknownExtensions(this.ignoreUnknownPathExtensions);
         if (this.useRegisteredExtensionsOnly != null) {
            strategy.setUseRegisteredExtensionsOnly(this.useRegisteredExtensionsOnly);
         }
         strategies.add(strategy);
      }

       //是否配置了參數(shù)策略
      if (this.favorParameter) {
          //聲明并配置ParameterContentNegotiationStrategy策略
         ParameterContentNegotiationStrategy strategy = new ParameterContentNegotiationStrategy(this.mediaTypes);
         strategy.setParameterName(this.parameterName);
         if (this.useRegisteredExtensionsOnly != null) {
            strategy.setUseRegisteredExtensionsOnly(this.useRegisteredExtensionsOnly);
         }
         else {
            strategy.setUseRegisteredExtensionsOnly(true);  // backwards compatibility
         }
         strategies.add(strategy);
      }

      if (!this.ignoreAcceptHeader) {
         strategies.add(new HeaderContentNegotiationStrategy());
      }

      if (this.defaultNegotiationStrategy != null) {
         strategies.add(this.defaultNegotiationStrategy);
      }
   }

   this.contentNegotiationManager = new ContentNegotiationManager(strategies);
   return this.contentNegotiationManager;
}

一下貼出這么多代碼可能有些懵，但是我們對照著流程圖一步一步的看岛心，由于我們已經(jīng)配置了 <u>參數(shù)策略</u>以及<u>URL后綴策略</u>来破。所以 上面的if else 就很好懂了⊥牛看注釋就可以明白了徘禁。

步驟五

關(guān)聯(lián)ContentNegotiatingViewResolver通過org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration.WebMvcAutoConfigurationAdapter#viewResolver方法，來初始化Bean ContentNegotiatingViewResolver 髓堪∷椭欤可以看到方法第二行就是關(guān)聯(lián)ContentNegotiationManager的地方。

@Bean
@ConditionalOnBean(ViewResolver.class)
@ConditionalOnMissingBean(name = "viewResolver", value = ContentNegotiatingViewResolver.class)
public ContentNegotiatingViewResolver viewResolver(BeanFactory beanFactory) {
   ContentNegotiatingViewResolver resolver = new ContentNegotiatingViewResolver();
    //關(guān)聯(lián) ContentNegotiationManager
   resolver.setContentNegotiationManager(
         beanFactory.getBean(ContentNegotiationManager.class));
   // ContentNegotiatingViewResolver uses all the other view resolvers to locate
   // a view so it should have a high precedence
   resolver.setOrder(Ordered.HIGHEST_PRECEDENCE);
   return resolver;
}

步驟六

ViewResolver Bean 關(guān)聯(lián)

這一步驟非常的繞干旁，看了小馬哥的視頻后驶沼，debugger了很久，不明白是在何時(shí)ContentNegotiatingViewResolver調(diào)用方法關(guān)聯(lián)其他ViewResolver的争群。

org.springframework.web.servlet.view.ContentNegotiatingViewResolver#initServletContext

我們先來看源碼:

@Override
protected void initServletContext(ServletContext servletContext) {
    //獲取到所有 ViewResolvers
   Collection<ViewResolver> matchingBeans =
         BeanFactoryUtils.beansOfTypeIncludingAncestors(obtainApplicationContext(), ViewResolver.class).values();
    //關(guān)聯(lián)他們 
   if (this.viewResolvers == null) {
      this.viewResolvers = new ArrayList<>(matchingBeans.size());
      for (ViewResolver viewResolver : matchingBeans) {
         if (this != viewResolver) {
            this.viewResolvers.add(viewResolver);
         }
      }
   }
   else {
      for (int i = 0; i < this.viewResolvers.size(); i++) {
         ViewResolver vr = this.viewResolvers.get(i);
         if (matchingBeans.contains(vr)) {
            continue;
         }
         String name = vr.getClass().getName() + i;
         obtainApplicationContext().getAutowireCapableBeanFactory().initializeBean(vr, name);
      }

   }
   if (this.viewResolvers.isEmpty()) {
      logger.warn("Did not find any ViewResolvers to delegate to; please configure them using the " +
            "'viewResolvers' property on the ContentNegotiatingViewResolver");
   }
    //排序
   AnnotationAwareOrderComparator.sort(this.viewResolvers);
   this.cnmFactoryBean.setServletContext(servletContext);
}

從代碼中我們可以很清晰的看到 首先 先獲取到所有的ViewResolver 然后遍歷關(guān)聯(lián)回怜。

但是，到底是什么時(shí)候關(guān)聯(lián)的呢换薄。

經(jīng)過debugger是在

org.springframework.boot.SpringApplication#run(java.lang.String...)啟動方法中的 refreshContext(context);這個調(diào)用后就可以關(guān)聯(lián)上了鹉戚。那么為什么會這樣，我們可以看到

public class ContentNegotiatingViewResolver extends WebApplicationObjectSupport
      implements ViewResolver, Ordered, InitializingBean {
      ...
      ｝
    

ContentNegotiatingViewResolver繼承了 WebApplicationObjectSupport 专控。

這個類中有這么一個方法

org.springframework.web.context.support.WebApplicationObjectSupport#initApplicationContext

@Override
protected void initApplicationContext(ApplicationContext context) {
   super.initApplicationContext(context);
   if (this.servletContext == null && context instanceof WebApplicationContext) {
      this.servletContext = ((WebApplicationContext) context).getServletContext();
      if (this.servletContext != null) {
         initServletContext(this.servletContext);
      }
   }
}

那么是在什么時(shí)候觸發(fā)的這個方法呢。

由于過程是在太復(fù)雜遏餐，只把最后幾步貼出來伦腐。

1.org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean
2.org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInitialization
3.org.springframework.context.support.ApplicationContextAwareProcessor#postProcessBeforeInitialization
4.org.springframework.context.support.ApplicationContextAwareProcessor#invokeAwareInterfaces

最后在invokeAwareInterfaces這個方法里調(diào)用setApplicationContext
如果感興趣的可以自己打好斷點(diǎn)去Debugger跟一下,對于整體spring的裝載機(jī)制都會有一個比較基礎(chǔ)的理解。

if (bean instanceof ApplicationContextAware) {
   ((ApplicationContextAware) bean).setApplicationContext(this.applicationContext);
}

其實(shí)這么復(fù)雜的過程失都，用一句簡單的話總結(jié)就是Spring-boot的一個關(guān)于在訪問 ServletContext的一個回調(diào)接口柏蘑，來自定義初始化。

總結(jié)

以上的內(nèi)容其實(shí)有點(diǎn)難懂粹庞，感興趣的小伙伴可以仔細(xì)研究一下咳焚，畢竟理解Spring比使用API接口要難得多。共勉加油庞溜。