SpringMVC的注解配置原理

- 日理万妓 2023-02-13 11:25 14阅读 0赞

## META-INF\\services\\javax.servlet.ServletContainerInitializer ##

web容器启动的时候，会扫描每个jar包中META-INF\\services\\javax.servlet.ServletContainerInitializer文件， 加载其中指定的ServletContainerInitializer的实现类，并调用onStartup方法。`@HandlesTypes(value = {HelloServlet.class})`用于指定一个接口，所有该接口的实现类会传给`onStartup`方法的第一个参数。

<dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>javax.servlet-api</artifactId>
      <version>4.0.1</version>
      <scope>provided</scope>
    </dependency>

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw_size_16_color_FFFFFF_t_70]

package com.example.demo3
    
    // HelloServlet是个接口，我例子中的实现类就不贴出来了
    @HandlesTypes(value = { HelloServlet.class})
    public class MyServletContainerInitializer implements ServletContainerInitializer { 
    
        /** * 应用启动的时候，会运行onStartup * @param c @HandlesTypes注解中配置的接口的所有实现类 * @param ctx 可以用来注册三大组件 * @throws ServletException */
        @Override
        public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException { 
            System.out.println("onStartup");
            for (Class<?> aClass : c) { 
                System.out.println(aClass);
            }
        }
    }

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw_size_16_color_FFFFFF_t_70 1]

## 配置web三大组件 ##

@HandlesTypes(value = { HelloServlet.class})
    public class MyServletContainerInitializer implements ServletContainerInitializer { 
    
        @Override
        public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException { 
            System.out.println("onStartup");
            for (Class<?> aClass : c) { 
                System.out.println(aClass);
            }
    
            //形式注册三大组件
            
            //注册组件 ServletRegistration
            ServletRegistration.Dynamic servlet = ctx.addServlet("servletDemo4", new ServletDemo4());
            //配置servlet的映射信息
            servlet.addMapping("/demo4");
    
            //注册Listener
            ctx.addListener(MyServletContextListener.class);
    
            //注册Filter FilterRegistration
            FilterRegistration.Dynamic filter = ctx.addFilter("filterDemo1", FilterDemo1.class);
            //配置Filter的映射信息
            filter.addMappingForUrlPatterns(EnumSet.of(DispatcherType.REQUEST), true, "/*");
        }
    }

过滤器：

public class FilterDemo1 implements Filter { 
        @Override
        public void init(FilterConfig filterConfig) throws ServletException { 
            System.out.println("Filter-init");
        }
    
        @Override
        public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException { 
            System.out.println("Filter-doFilter");
            chain.doFilter(request, response);
        }
    
        @Override
        public void destroy() { 
            System.out.println("Filter-destroy");
        }
    }

监听器：

public class MyServletContextListener implements ServletContextListener { 
    
        /** * 监听ServletContext对象创建 * @param sce */
        @Override
        public void contextInitialized(ServletContextEvent sce) { 
            System.out.println("MyServletContextListener-contextInitialized");
        }
    
        @Override
        public void contextDestroyed(ServletContextEvent sce) { 
            System.out.println("contextDestroyed");
        }
    }

Servlet程序：

public class ServletDemo4 extends HttpServlet { 
        @Override
        protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            System.out.println("demo4");
        }
    }

## Spring整合SpringMVC ##

<dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-webmvc</artifactId>
      <version>5.2.2.RELEASE</version>
    </dependency>

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw_size_16_color_FFFFFF_t_70 2]  
从上面加载的spring-web的jar包中可以看到，也配置了javax.servlet.ServletContainerInitializer，内容是`org.springframework.web.SpringServletContainerInitializer`，所以，Spring启动的时候，会加载`org.springframework.web.SpringServletContainerInitializer`类，并调用`onStartup`方法，这个类的源码如下：

// 会加载所有WebApplicationInitializer的实现类给onStartup方法的webAppInitializerClasses参数
    @HandlesTypes({ WebApplicationInitializer.class})
    public class SpringServletContainerInitializer implements ServletContainerInitializer { 
        public SpringServletContainerInitializer() { 
        }
    
        public void onStartup(Set<Class<?>> webAppInitializerClasses, ServletContext servletContext) throws ServletException { 
            List<WebApplicationInitializer> initializers = new LinkedList();
            Iterator var4;
            if (webAppInitializerClasses != null) { 
                var4 = webAppInitializerClasses.iterator();
    
                while(var4.hasNext()) { 
                    Class<?> waiClass = (Class)var4.next();
                    if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) && WebApplicationInitializer.class.isAssignableFrom(waiClass)) { 
                        try { 
                            initializers.add((WebApplicationInitializer)waiClass.newInstance());
                        } catch (Throwable var7) { 
                            throw new ServletException("Failed to instantiate WebApplicationInitializer class", var7);
                        }
                    }
                }
            }
    
            if (initializers.isEmpty()) { 
                servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
            } else { 
                servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
                AnnotationAwareOrderComparator.sort(initializers);
                var4 = initializers.iterator();
    
                while(var4.hasNext()) { 
                    WebApplicationInitializer initializer = (WebApplicationInitializer)var4.next();
                    initializer.onStartup(servletContext);
                }
    
            }
        }
    }

现在来看看`WebApplicationInitializer`的实现类有哪些：  
![在这里插入图片描述][20200528193413434.png]

*  `AbstractAnnotationConfigDispatcherServletInitializer`：注解方式配置的DispatcherServlet初始化器
 *  `AbstractContextLoaderInitializer` ：创建 根容器
 *  `AbstractDispatcherServletInitializer`：  
    创建web ioc容器  
    创建DispatcherServlet  
    将创建的DispatcherServlet添加到ServletContext中

所以，如果我们希望以注解的方式来启动SpringMVC，就继承 `AbstractAnnotationConfigDispatcherServletInitializer`类，实现抽象方法，具体如何配置请看官网（[https://docs.spring.io/spring/docs/][https_docs.spring.io_spring_docs]），这里只是讨论了注解配置的基本原理。

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200528191951648.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/20200528191847803.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw_size_16_color_FFFFFF_t_70 2]: https://img-blog.csdnimg.cn/20200528192500103.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0lUXzEw,size_16,color_FFFFFF,t_70
[20200528193413434.png]: https://img-blog.csdnimg.cn/20200528193413434.png
[https_docs.spring.io_spring_docs]: https://docs.spring.io/spring/docs/