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servlet和jsp学习指南（一）servlet

近期接触了些纯servlet与jsp的编程项目，顺便把《servlet和jsp学习指南》这本书拿出来看看，感悟良多。记下随笔，以便会议。

要编写一个servlet首先要实现Servlet或者继承HttpServlet

package test.com.servlet;

import java.io.IOException;

import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebServlet;

//Servlet类的名称要以Servlet作为后缀
@WebServlet(name = "MyServlet", urlPatterns = { "/my" })
public class MyServlet implements Servlet {

    private transient ServletConfig servletConfig;
    
    public void destroy() {

    }

    public ServletConfig getServletConfig() {
        return null;
    }

    public String getServletInfo() {
        return "My Servlet";
    }

    public void init(ServletConfig servletConfig) throws ServletException {

        this.servletConfig = servletConfig;
    }

    public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {

        String admin = servletConfig.getInitParameter("admin");
        String servletName = servletConfig.getServletName();
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        writer.println("<html><head></head>" + "<body>Hello from " + servletName + admin + servletConfig+ "</body></html>");
    }

}

实现Servlet接口则需实现它的五个方法，当第一次访问MyServlet时先调用init(ServletConfig servletConfig)方法，其中的ServletConfig是servlet容器传进来的，可以用它获得ServletContext。

init()方法供servlet初始化使用，有且只调用一次。一般会利用它将servletConfig传给类属性，或者利用servletConfig获取ServletContext传给类属性。

之后就用访问到service（）方法。service（）方法是每次访问都会调用的方法。所以用它来实现业务方法。

destroy（）方法是在容器关闭时调用的方法。

由此可见servlet的存活时间了，也就是面试中命中率超高的servlet生命周期问题了。

servlet生命周期：

1，初始化阶段调用init()方法

2，响应请求阶段　　调用service()方法

3，终止阶段　　调用destroy()方法

如果是实现Servlet接口，那么就意味着每次都要实现那么多方法，即使你放着空方法看着也很冗余。那么写个通用类实现Servlet供其他功能servlet继承就只需要关注业务实现不用，写无关的方法了。

这时候出现了抽象类javax.servlet.GenericServlet

/*** Eclipse Class Decompiler plugin, copyright (c) 2012 Chao Chen (cnfree2000@hotmail.com) ***/
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 void destroy() {
    }

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

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

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

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

    public String getServletInfo() {
        return "";
    }

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

    public void init() throws ServletException {
    }

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

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

    public abstract void service(ServletRequest paramServletRequest, ServletResponse paramServletResponse)
            throws ServletException, IOException;

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

抽象类GenericServlet 实现了Servlet接口的绝大部分方法除了service（）方法。
后来又出现现在我们平时写servlet时常继承的HttpServlet，其实HttpServlet是继承于GenericServlet，并封装了doGet、doPost等我们常用的方法。

/*** Eclipse Class Decompiler plugin, copyright (c) 2012 Chao Chen (cnfree2000@hotmail.com) ***/
package javax.servlet.http;

import java.io.IOException;
import java.lang.reflect.Method;
import java.text.MessageFormat;
import java.util.Enumeration;
import java.util.ResourceBundle;
import javax.servlet.DispatcherType;
import javax.servlet.GenericServlet;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public abstract class HttpServlet extends GenericServlet {
    private static final long serialVersionUID = 1L;
    private static final String METHOD_DELETE = "DELETE";
    private static final String METHOD_HEAD = "HEAD";
    private static final String METHOD_GET = "GET";
    private static final String METHOD_OPTIONS = "OPTIONS";
    private static final String METHOD_POST = "POST";
    private static final String METHOD_PUT = "PUT";
    private static final String METHOD_TRACE = "TRACE";
    private static final String HEADER_IFMODSINCE = "If-Modified-Since";
    private static final String HEADER_LASTMOD = "Last-Modified";
    private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings";
    private static final ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.http.LocalStrings");

    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 long getLastModified(HttpServletRequest req) {
        return -1L;
    }

    protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        if (DispatcherType.INCLUDE.equals(req.getDispatcherType())) {
            doGet(req, resp);
        } else {
            NoBodyResponse response = new NoBodyResponse(resp);
            doGet(req, response);
            response.setContentLength();
        }
    }

    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);
    }

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

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

    private static Method[] getAllDeclaredMethods(Class<?> c) {
        if (c.equals(HttpServlet.class)) {
            return null;
        }

        Method[] parentMethods = getAllDeclaredMethods(c.getSuperclass());
        Method[] thisMethods = c.getDeclaredMethods();

        if ((parentMethods != null) && (parentMethods.length > 0)) {
            Method[] allMethods = new Method[parentMethods.length + thisMethods.length];

            System.arraycopy(parentMethods, 0, allMethods, 0, parentMethods.length);

            System.arraycopy(thisMethods, 0, allMethods, parentMethods.length, thisMethods.length);

            thisMethods = allMethods;
        }

        return thisMethods;
    }

    protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        Method[] methods = getAllDeclaredMethods(super.getClass());

        boolean ALLOW_GET = false;
        boolean ALLOW_HEAD = false;
        boolean ALLOW_POST = false;
        boolean ALLOW_PUT = false;
        boolean ALLOW_DELETE = false;
        boolean ALLOW_TRACE = true;
        boolean ALLOW_OPTIONS = true;

        for (int i = 0; i < methods.length; ++i) {
            Method m = methods[i];

            if (m.getName().equals("doGet")) {
                ALLOW_GET = true;
                ALLOW_HEAD = true;
            }
            if (m.getName().equals("doPost"))
                ALLOW_POST = true;
            if (m.getName().equals("doPut"))
                ALLOW_PUT = true;
            if (m.getName().equals("doDelete")) {
                ALLOW_DELETE = true;
            }
        }
        String allow = null;
        if (ALLOW_GET)
            allow = "GET";
        if (ALLOW_HEAD)
            if (allow == null)
                allow = "HEAD";
            else
                allow = new StringBuilder().append(allow).append(", HEAD").toString();
        if (ALLOW_POST)
            if (allow == null)
                allow = "POST";
            else
                allow = new StringBuilder().append(allow).append(", POST").toString();
        if (ALLOW_PUT)
            if (allow == null)
                allow = "PUT";
            else
                allow = new StringBuilder().append(allow).append(", PUT").toString();
        if (ALLOW_DELETE)
            if (allow == null)
                allow = "DELETE";
            else
                allow = new StringBuilder().append(allow).append(", DELETE").toString();
        if (ALLOW_TRACE)
            if (allow == null)
                allow = "TRACE";
            else
                allow = new StringBuilder().append(allow).append(", TRACE").toString();
        if (ALLOW_OPTIONS) {
            if (allow == null)
                allow = "OPTIONS";
            else
                allow = new StringBuilder().append(allow).append(", OPTIONS").toString();
        }
        resp.setHeader("Allow", allow);
    }

    protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String CRLF = "
";
        StringBuilder buffer = new StringBuilder("TRACE ").append(req.getRequestURI()).append(" ")
                .append(req.getProtocol());

        Enumeration reqHeaderEnum = req.getHeaderNames();

        while (reqHeaderEnum.hasMoreElements()) {
            String headerName = (String) reqHeaderEnum.nextElement();
            buffer.append(CRLF).append(headerName).append(": ").append(req.getHeader(headerName));
        }

        buffer.append(CRLF);

        int responseLength = buffer.length();

        resp.setContentType("message/http");
        resp.setContentLength(responseLength);
        ServletOutputStream out = resp.getOutputStream();
        out.print(buffer.toString());
        out.close();
    }

    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;
                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);
        }
    }

    private void maybeSetLastModified(HttpServletResponse resp, long lastModified) {
        if (resp.containsHeader("Last-Modified"))
            return;
        if (lastModified >= 0L)
            resp.setDateHeader("Last-Modified", lastModified);
    }

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

虽然我们没有再写service方法，写的是doGet、doPost方法。其实真正访问一个继承HttpServlet的servlet时先调用的是HttpServlet的service方法，再根据你的请求选择使用doGet方法处理，还是用doPost方法处理。

其实还可以不走寻常路，不复写他的doGet等方法。而是直接复写他的service方法

package com.httpservlet;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;

@WebServlet(name = "MyServlet", urlPatterns = { "/formServlet" })
public class FormServlet extends HttpServlet{
    /**
     * 
     */
    private static final long serialVersionUID = 1L;

    @Override
    public void service(ServletRequest req, ServletResponse response) throws ServletException, IOException {
        String servletName = getServletConfig().getServletName();
        response.setContentType("text/html");
        PrintWriter writer = response.getWriter();
        writer.println("<html><head></head>" + "<body>Hello from " + servletName +"</body></html>");
    }

}

访问 http://localhost:8080/servlet-jsp/formServlet

页面输出 Hello from FormServlet

查看全文

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原文地址：https://www.cnblogs.com/vincentren/p/5778170.html