DispatcherServlet的继承体系和初始化

最新推荐文章于 2021-04-01 14:11:01 发布

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最新推荐文章于 2021-04-01 14:11:01 发布

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分类专栏： springmvc

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本文解析了Spring MVC中的核心组件DispatcherServlet的工作流程。从Servlet的继承体系出发，详细介绍了GenericServlet和HttpServlet的作用，并深入分析了DispatcherServlet如何处理HTTP请求。

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DispatcherServlet的继承体系如图，

这两个顶层的抽象类属于java提供的servlet层级。

图中的5个servlet，有java提供的servlet，也有springmvc提供的servlet。

java提供的servlet：

GenericServlet抽象类，实现了Servlet接口
HttpServlet抽象类，继承了GenericServlet

spring提供的servlet

HttpServletBean
FrameworkServlet
DispatcherServlet

GenericServlet

GenericServlet主要做了2件事：

1.GenericServlet实现了Servlet接口的init(ServletConfig config)方法，获取了servlet的配置信息，如名称，servlet上下文ServletContext，参数等。

    public void init(ServletConfig config) throws ServletException {
	this.config = config;
	this.init();
    }

2.GenericServlet实现了Servlet接口的service(ServletRequest req, ServletResponse res),将其写为抽象方法，供下层实现。

    @Override
    public abstract void service(ServletRequest req, ServletResponse res)
            throws ServletException, IOException;

HttpServlet

HttpServlet继承了GenericServlet，实现了GenericServlet的service(ServletRequest req, ServletResponse res)抽象方法，

在方法类,将ServletRequest，ServletResponse强转为HttpServletRequest，HttpServletResponse，

    @Override
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {

        HttpServletRequest  request;
        HttpServletResponse response;

        try {
            request = (HttpServletRequest) req;
            response = (HttpServletResponse) res;
        } catch (ClassCastException e) {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response);
    }

然后调用了HttpServlet自身的service(HttpServletRequest req, HttpServletResponse resp)方法。

   protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {

        String method = req.getMethod();

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                } catch (IllegalArgumentException iae) {
                    // Invalid date header - proceed as if none was set
                    ifModifiedSince = -1;
                }
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);

        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);

        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);

        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);

        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);

        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);

            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }

在该方法中，根据请求方法的类型，调用对应的doXXX()方法。