HttpServlet

HttpServlet类

首先看一下HttpServlet类的继承关系

我们发现HttpServlet继承自GenericServlet类，而GenericServlet类是实现的Servlet和ServletConfig接口

所以我们先看一下GenericServlet类的定义

GenericServlet

package javax.servlet;

import java.io.IOException;
import java.io.Serializable;
import java.util.Enumeration;

public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {
    private static final long serialVersionUID = 1L;
    private transient ServletConfig config;

    public GenericServlet() {
    }

    public void destroy() {
    }

    public String getInitParameter(String name) {
        return this.getServletConfig().getInitParameter(name);
    }

    public Enumeration<String> getInitParameterNames() {
        return this.getServletConfig().getInitParameterNames();
    }

    public ServletConfig getServletConfig() {
        return this.config;
    }

    public ServletContext getServletContext() {
        return this.getServletConfig().getServletContext();
    }

    public String getServletInfo() {
        return "";
    }

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

    public void init() throws ServletException {
    }

    public void log(String msg) {
        this.getServletContext().log(this.getServletName() + ": " + msg);
    }

    public void log(String message, Throwable t) {
        this.getServletContext().log(this.getServletName() + ": " + message, t);
    }

    public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

    public String getServletName() {
        return this.config.getServletName();
    }
}

其中的成员和方法有：

　　可见GenericSevlet类是一个抽象类，在其中只有Service是抽象方法用abstarct修饰
　　我们先只看其中的两个方法

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

    public void init() throws ServletException {
    }

因为在创建Servlet对象的时候是先调用的Init方法，所以我们一般在继承GenericServlet(或者HttpServlet）的时候一般重写Init()方法。这样避免了重写Init(ServletConfig config)而忘记对自身的属性ServletConfig成员进行赋值

HttpServlet

由上面的GenericServlet类可知，HttpServlet需要对GenericServlet为实现的Service方法进行实现。下面是HttpServlet程序对Service方法的实现

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {
            lastModified = this.getLastModified(req);
            if (lastModified == -1L) {
                this.doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
                    ifModifiedSince = req.getDateHeader("If-Modified-Since");
                } catch (IllegalArgumentException var9) {
                    ifModifiedSince = -1L;
                }

                if (ifModifiedSince < lastModified / 1000L * 1000L) {
                    this.maybeSetLastModified(resp, lastModified);
                    this.doGet(req, resp);
                } else {
                    resp.setStatus(304);
                }
            }
        } else if (method.equals("HEAD")) {
            lastModified = this.getLastModified(req);
            this.maybeSetLastModified(resp, lastModified);
            this.doHead(req, resp);
        } else if (method.equals("POST")) {
            this.doPost(req, resp);
        } else if (method.equals("PUT")) {
            this.doPut(req, resp);
        } else if (method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if (method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if (method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[]{method};
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(501, errMsg);
        }

    }

分析源码我们得知，service方法request的请求方式进行了判断，根据不同的方法，分发到不同的处理方法对浏览器做出响应。

下面是doGet方法和doPost方法

 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);
        }
    }
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String protocol = req.getProtocol();
        String msg = lStrings.getString("http.method_post_not_supported");
        if (protocol.endsWith("1.1")) {
            resp.sendError(405, msg);
        } else {
            resp.sendError(400, msg);
        }

    }

分析两种方法我们发现，无论其http协议版本如何,其响应都会给页面发送错误。因此，当我们通过继承HttpServlet类编写servlet程序的时候,需要对DoGet和DoPost方法进行重写，并且不能调用父类的方法。否则web页面会出现错误提示