servlet ---- servlet入门

执行过程

Tomcat Server处理一个http请求的过程

假设来自客户的请求为：http://localhost:8080/wsota/wsota_index.jsp

1. 请求被发送到本机端口8080，被在那里侦听的Coyote HTTP/1.1 Connector获得
2. Connector把该请求交给它所在的Service的Engine来处理，并等待来自Engine的回应
3. Engine获得请求localhost/wsota/wsota_index.jsp，匹配它所拥有的所有虚拟主机Host
4. Engine匹配到名为localhost的Host（即使匹配不到也把请求交给该Host处理，因为该Host被定义为该Engine的默认主机）
5. localhost Host获得请求/wsota/wsota_index.jsp，匹配它所拥有的所有Context
6. Host匹配到路径为/wsota的Context（如果匹配不到就把该请求交给路径名为""的Context去处理）
7. path="/wsota"的Context获得请求/wsota_index.jsp，在它的mapping table中寻找对应的servlet
8. Context匹配到URL PATTERN为*.jsp的servlet，对应于JspServlet类
9. 构造HttpServletRequest对象和HttpServletResponse对象，作为参数调用JspServlet的doGet或doPost方法
   10)Context把执行完了之后的HttpServletResponse对象返回给Host
   11)Host把HttpServletResponse对象返回给Engine
   12)Engine把HttpServletResponse对象返回给Connector
   13)Connector把HttpServletResponse对象返回给客户browser

servlet接口关系图

HttpServlet抽象类中实现了service

    @Override
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException
    {
        HttpServletRequest  request;
        HttpServletResponse response;
        
        if (!(req instanceof HttpServletRequest &&
                res instanceof HttpServletResponse)) {
            throw new ServletException("non-HTTP request or response");
        }

        request = (HttpServletRequest) req;
        response = (HttpServletResponse) res;

        service(request, response);
    }



    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 = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < lastModified) {
                    // 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);
        }
    }

所以需要重写doGet和doPost

@WebServlet(name = "ServletDemo2", value = "/ServletDemo2")
public class ServletDemo2 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("hello servlet");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doGet(request,response);
    }
}

Servlet 生命周期

init : 请求第一次到达servlet 时，对象创建并初始化成功。
service : servlet 一直存在，并提供服务
destroy：服务器宕机，停机销毁servlet对象时触发

@WebServlet(name = "ServletDemo03", value = "/ServletDemo03")
public class ServletDemo03 extends HttpServlet {

    @Override
    public void init() throws ServletException {
        System.out.println("servlet 对象创建了，初始化成");
    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        System.out.println("接收到客户端请求");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request,response);
    }

    @Override
    public void destroy() {
        System.out.println("对象销毁了。。。");
    }
}