SpringMVC

本文在基於SpringMVC 4.3.2.RELEASE版本基礎上，對原始碼進行分析，並探討SpringMVC的原理及用法。
springMVC

整體架構

在瞭解Spring整體架構時，這裡盜用開濤部落格的一張圖。

對該架構流程進行簡單的分析如下:
任何一個請求都要經過DispatcherServlet進行分發,DispatcherServlet並不對請求進行處理，而是將請求交給HandlerMapping進行對映;HandlerMapping將請求對映為HandlerExecutionChain物件。該物件包含一個Handler物件和多個HandlerInterceptor物件。這裡就體現了一種責任鏈模式的思想;Handler經過一層包裝後變成HandlerAdapter，從而支援多種型別的處理器(Controller),HandlerAdapter將請求交給具體的處理器進行處理，完成後返回ModelAndView;通過ViewResolver對檢視進行解析，並利用Model資料模型對檢視渲染(Render).

DispatcherServlet1

DispatcherServlet繼承關係如下:
DispatcherServlet—>FrameworkServlet—>HttpServletBean—>HttpServlet—>GenericServlet—>Servlet.
HttpServletBean繼承自javax.servlet.http包下的HttpServlet,那麼就先從該類分析:

/**
 * Simple extension of {@link javax.servlet.http.HttpServlet} which treats
 * its config parameters ({@code
 
 init-param} entries within the
 * {@code servlet} tag in {@code web.xml}) as bean properties.
 *
 * <p>A handy superclass for any type of servlet. Type conversion of config
 * parameters is automatic, with the corresponding setter method getting
 * invoked with the converted value. It is also possible for
 
 subclasses to
 * specify required properties. Parameters without matching bean property
 * setter will simply be ignored.
 *
 * <p>This servlet leaves request handling to subclasses, inheriting the default
 * behavior of HttpServlet ({@code doGet}, {@code doPost}, etc).
 *
 * <p>This generic servlet base class has no dependency on the Spring
 * {@link org.springframework.context.ApplicationContext} concept. Simple
 * servlets usually don't load their own context but rather access service
 * beans from the Spring root application context, accessible via the
 * filter's {@link #getServletContext() ServletContext} (see
 * {@link org.springframework.web.context.support.WebApplicationContextUtils}).
 *
 * <p>The {@link FrameworkServlet} class is a more specific servlet base
 * class which loads its own application context. FrameworkServlet serves
 * as direct base class of Spring's full-fledged {@link DispatcherServlet}.
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see #addRequiredProperty
 * @see #initServletBean
 * @see #doGet
 * @see #doPost
 */
@SuppressWarnings("serial")
public abstract class HttpServletBean extends HttpServlet
        implements EnvironmentCapable, EnvironmentAware {
    }

該類的初始化方法init()

    /**
     * Map config parameters onto bean properties of this servlet, and
     * invoke subclass initialization.
     * @throws ServletException if bean properties are invalid (or required
     * properties are missing), or if subclass initialization fails.
     */
    @Override
    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");
        }
    }

try…catch中的內容暫時略過。
之後，該方法呼叫了initServletBean(),該方法在類中進行宣告，但並沒有實現，而是留給子類FrameworkServlet實現。子類FrameworkServlet實現過程如下:

    /**
     * Overridden method of {@link HttpServletBean}, invoked after any bean properties
     * have been set. Creates this servlet's WebApplicationContext.
     */
    @Override
    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");
        }
    }

該方法中依然呼叫了initFrameworkServlet()，該方法在類中定義，並沒有實現，而是留給子類進行實現，不過這次DispatcherServlet並沒有重寫該方法。

    /**
     * This method will be invoked after any bean properties have been set and
     * the WebApplicationContext has been loaded. The default implementation is empty;
     * subclasses may override this method to perform any initialization they require.
     * @throws ServletException in case of an initialization exception
     */
    protected void initFrameworkServlet() throws ServletException {
    }

如上對DispatcherServlet對請求處理的過程分析，可以看出，DispatcherServlet將請求分發給HandlerMapping.
HandlerMapping是位於org.springframework.web.servlet包下的介面.他就定義了一個函式getHandler(HttpServletRequest request).

    /**
     * Return a handler and any interceptors for this request. The choice may be made
     * on request URL, session state, or any factor the implementing class chooses.
     * <p>The returned HandlerExecutionChain contains a handler Object, rather than
     * even a tag interface, so that handlers are not constrained in any way.
     * For example, a HandlerAdapter could be written to allow another framework's
     * handler objects to be used.
     * <p>Returns {@code null} if no match was found. This is not an error.
     * The DispatcherServlet will query all registered HandlerMapping beans to find
     * a match, and only decide there is an error if none can find a handler.
     * @param request current HTTP request
     * @return a HandlerExecutionChain instance containing handler object and
     * any interceptors, or {@code null} if no mapping found
     * @throws Exception if there is an internal error
     */
    HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;

ContextLoaderListener初始化上下文和DispatcherServlet初始化上下文的關係

DispatcherServlet中有幾個特殊的Bean，這些Bean就是上文中提到的Controller, HandlerMapping, HandlerAdapter, ViewResolver, LocalResolver, ThemeResolver, MultipartResolver.

DispatcherServlet需要在web.xml中進行配置。需要宣告其在容器啟動時初始化,即load-on-startup。同時設定其url-pattern為“/”，表示它攔截所有的請求。
它預設使用WebApplicationContext作為上下文，如果不進行配置，預設會去/WEB-INF/[servlet名字]-servlet.xml中尋找對應的配置檔案。
也可以手動對其進行配置，以覆蓋預設的配置。