Servlet/JSP-03 HttpServlet

一. GenericServlet

　　GenericServlet本身是一个抽象类，并且实现了Servlet和ServletConfig接口

　　其在内部定义了一个私有的ServletConfig类型的变量config，并在init(ServletConfig config)为其赋值，然后通过config实现其他方法。

　　service(ServletRequest servletrequest, ServletResponse servletresponse)为抽象方法。具体代码如下：

public abstract class GenericServlet implements Servlet, ServletConfig, Serializable

{

    private transient ServletConfig config;

    public GenericServlet() { }


　　 @override

    public void init(ServletConfig config) throws ServletException

    {

        this.config = config;

        init();

    }

    // 此方法供我们实现的Servlet初始化时使用

    public void init() throws ServletException {}


    @override

    public ServletConfig getServletConfig()

    {

        return config;

    }

    
    @override

    public String getInitParameter(String name)

    {

        return getServletConfig().getInitParameter(name);

    }

    public abstract void service(ServletRequest servletrequest, ServletResponse servletresponse)

        throws ServletException, IOException;

    // 其他方法略

    ...

}

二. HttpServlet

为了简化开发，Servlet容器提供了HttpServlet的实现，其继承了GenericServlet.

1. Servlet容器为我们封装了HttpServletRequest, HttpServletResponse类型的参数，传入service方法，在service方法内强转后传入对应的doXXX方法

2. 开发时继承HttpServlet，只需要重写doXXX方法即可，处理相应方式的HTTP请求。

public abstract class HttpServlet extends GenericServlet

{
    // doXXX方法被子类覆盖

    protected void doGet(HttpServletRequest req, HttpServletResponse resp)

        throws ServletException, IOException

    {

        String protocol = req.getProtocol();

        String msg = lStrings.getString("http.method_get_not_supported");

        if(protocol.endsWith("1.1"))

            resp.sendError(405, msg);

        else

            resp.sendError(400, msg);

    }

    //doPost, doPut,  doDelete类似实现， doHead, doTrace, doOption略复杂

    ......

    //通过此方法调用不同HTTP请求方式的处理方法doXXX

    protected void service(HttpServletRequest req, HttpServletResponse resp)

        throws ServletException, IOException

    {

        String method = req.getMethod();

        if(method.equals("GET"))

        {

            long lastModified = getLastModified(req);

            if(lastModified == -1L)

            {

                doGet(req, resp);

            } else

            {

                long ifModifiedSince;

                try

                {

                    ifModifiedSince = req.getDateHeader("If-Modified-Since");

                }

                catch(IllegalArgumentException iae)

                {

                    ifModifiedSince = -1L;

                }

                if(ifModifiedSince < (lastModified / 1000L) * 1000L)

                {

                    maybeSetLastModified(resp, lastModified);

                    doGet(req, resp);

                } else

                {

                    resp.setStatus(304);

                }

            }

        } else

        if(method.equals("HEAD"))

        {

            long lastModified = getLastModified(req);

            maybeSetLastModified(resp, lastModified);

            doHead(req, resp);

        } else

        if(method.equals("POST"))

            doPost(req, resp);

        else

        if(method.equals("PUT"))

            doPut(req, resp);

        else

        if(method.equals("DELETE"))

            doDelete(req, resp);

        else

        if(method.equals("OPTIONS"))

            doOptions(req, resp);

        else

        if(method.equals("TRACE"))

        {

            doTrace(req, resp);

        } else

        {

            String errMsg = lStrings.getString("http.method_not_implemented");

            Object errArgs[] = new Object[1];

            errArgs[0] = method;

            errMsg = MessageFormat.format(errMsg, errArgs);

            resp.sendError(501, errMsg);

        }

    }

    //此为GenericServlet中方法，最终调用HttpServlet内实现的service方法

    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);

    }

}

巴特西

Servlet/JSP-03 HttpServlet

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