Spring MVC 学习 -- 创建过程

Spring MVC 学习 -- 创建过程

 

Spring MVC我们使用的时候会在web.xml中配置

  <servlet>
    <servlet-name>SpringMVC</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>classpath:spring/spring-web.xml</param-value>
    </init-param>
  </servlet>
  <servlet-mapping>
    <servlet-name>SpringMVC</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>

1.核心的类结构

　　继承结构主要有五个类，GenericServlet和HttpServlet 是Java的，HttpServletBean、FrameworkServlet和DispatcherServlet是Spring MVC的。

　　　　在图中还有三个接口：EnvironmentCapable、EnvironmentAware和ApplicationContextAware~

　　　　XXXAware 在spring里标识对XXX可以感知 -- 白话的意思就是：某个类如果想要使用spring的一些东西，就可以通过实现XXXAware接口告诉spring，spring看到后会给你送过来，而接收的方式是通过实现接口唯一的setXXX

public interface ApplicationContextAware extends Aware {
    
void setApplicationContext(ApplicationContext applicationContext) throws BeansException;

}

源码如上，使用的话写一个类实现ApplicationContextAware，然后实现setApplicationContext()方法就行，spring为自动调用这个方法将applicationContext传给我们。　

而EnvironmentCapable是为了实现getEnvironment去拿到ServletContext、ServletConfig、JndiProperty、系统环境变量和系统属性。

2.HttpServletBean

    public final void init() throws ServletException {
        if (logger.isDebugEnabled()) {
            logger.debug("Initializing servlet '" + getServletName() + "'");
        }

        // Set bean properties from init parameters.
        try {
            PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            initBeanWrapper(bw);
            bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
            throw ex;
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();

        if (logger.isDebugEnabled()) {
            logger.debug("Servlet '" + getServletName() + "' configured successfully");
        }
    }

主要看init()方法， 使用了BeanWrapper这个类对象，BeanWrapper是操作JavaBean的工具类，主要就是对属性进行操作。

3.FrameworkServlet

这个类是通过HttpServletBean中的initServletBean()方法进入的。

    protected final void initServletBean() throws ServletException {
        getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
        if (this.logger.isInfoEnabled()) {
            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
        }
        long startTime = System.currentTimeMillis();

        try {
            this.webApplicationContext = initWebApplicationContext();
            initFrameworkServlet();
        }
        catch (ServletException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }
        catch (RuntimeException ex) {
            this.logger.error("Context initialization failed", ex);
            throw ex;
        }

        if (this.logger.isInfoEnabled()) {
            long elapsedTime = System.currentTimeMillis() - startTime;
            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
                    elapsedTime + " ms");
        }
    }

核心代码就try里面包含的两句，很显然主要用来初始化WebApplicationContext和初始化FrameworkServlet的。

    protected WebApplicationContext initWebApplicationContext() {
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent -> set
                        // the root application context (if any; may be null) as the parent
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        if (wac == null) {
            // No context instance was injected at construction time -> see if one
            // has been registered in the servlet context. If one exists, it is assumed
            // that the parent context (if any) has already been set and that the
            // user has performed any initialization such as setting the context id
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            // Either the context is not a ConfigurableApplicationContext with refresh
            // support or the context injected at construction time had already been
            // refreshed -> trigger initial onRefresh manually here.
            onRefresh(wac);
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

        return wac;
    }

主要做了三件事：

• 获取spring的根容器rootContext
• 设置webApplicationContext并根据情况调用onRefresh方法
• 将webApplicationContext设置到ServletContext中

4.DispatcherServlet

通过上面onRefresh()方法，调用initStrategies()方法，初始化策略组件。具体的内容就是用来初始化9个组件，每个组件的代码不一一贴出来了， 这几个方法都会去调用getDefaultStrategies()方法。

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>();
        }
    }

5.代码测试

配置内容都不再贴了，启动Tomcat，然后去请求某个controller地址，进入HttpServletBean的init()方法

然后继续，因为WebApplicationContext是null，会进入

然后configureAndRefreshWebApplicationContext方法中会执行refresh()，这部分可以看spring bean加载过程和实例化。