JavaWeb三大组件之一Servlet【HttpServlet类】

HttpServlet

HttpServlet概述

HttpServlet类是GenericServlet的子类,它提供了对HTTP请求的特殊支持,所以通常我们都会通过继承HttpServlet来完成自定义的Servlet。

HttpServlet覆盖了service()方法

HttpServlet类中提供了service(HttpServletRequest,HttpServletResponse)方法,这个方法是HttpServlet自己的方法,不是从Servlet继承来的。在HttpServlet的service(ServletRequest,ServletResponse)方法中会把ServletRequest和ServletResponse强转成HttpServletRequest和HttpServletResponse,然后调用service(HttpServletRequest,HttpServletResponse)方法,这说明子类可以去覆盖service(HttpServletRequest,HttpServletResponse)方法即可,这就不用自己去强转请求和响应对象了。

其实子类也不用去覆盖service(HttpServletRequest,HttpServletResponse)方法,因为HttpServlet还要做另一步简化操作,下面会介绍。

HttpServlet.java:

public abstract class HttpServlet extends GenericServlet {
    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
        ……
}
    @Override
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {

        HttpServletRequest  request;
        HttpServletResponse response;

        try {
            request = (HttpServletRequest) req;//强转
            response = (HttpServletResponse) res;//强转
         } catch (ClassCastException e) {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response); //调用上面service()方法
}
……
}

doGet()和doPost()

在HttpServlet的service(HttpServletRequest,HttpServletResponse)方法会去判断当前请求是GET还是POST,如果是GET请求,那么会去调用本类的doGet()方法,如果是POST请求会去调用doPost()方法,这说明我们在子类中去覆盖doGet()或doPost()方法即可。

public class AServlet extends HttpServlet {
    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        System.out.println("hello doGet()...");
    }
}
public class BServlet extends HttpServlet {
    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        System.out.println("hello doPost()...");
    }
}

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