servlet三种方式实现servlet接口

简单介绍

Servlet接口实现类

1、Servlet接口SUN公司定义了两个默认实现类，分别为：GenericServlet、HttpServlet。

2、HttpServlet指能够处理HTTP请求的servlet，它在原有Servlet接口上添加了一些与HTTP协议处理方法，它比Servlet接口的功能更为强大。因此开发人员在编写Servlet时，通常应继承这个类，而避免直接去实现Servlet接口。

3、HttpServlet在实现Servlet接口时，覆写了service方法，该方法体内的代码会自动判断用户的请求方式，如为GET请求，则调用HttpServlet的doGet方法，如为Post请求，则调用doPost方法。因此，开发人员在编写Servlet时，通常只需要覆写doGet或doPost方法，而不要去覆写service方法。

接口分离原则：

将一个包含多功能的单一接口根据函数功能分离称多个独立的接口，同时每个接口都有自己特有的、有意义的、具有实际哦那功能的修饰器。
    多重接口修饰：
         每个修饰器设计时都需要首先实现他要修饰的接口，可以配合应用 适配器和修饰器这两个模式来构建多重修饰器，同时也能保证代码量最少。
客户端构建：
    为客户端提供接口实例的方式一定程度上取决于接口实现的数目。如果每个接口都有自己特有的实现，那就需要构造所有实现的实例并提供给客户端。或者如果所有的接口的实现都包含在单个类中，那么只需要构建该类的实例就能够满足客户端的所有依赖。
    1、多接口、多实例：
        控制器类：将所有方法整合到控制器类中（构造函数传入参数为各个接口）。控制器类中的每个方法都需要一个不同的接口来执行他的功能。
    2、单接口、单实例：
        在单个类中继承并实现所有接口。这样将每个接口的实现方法都整合到一个类中。（一般用于叶子实现类）

Servlet的作用：

1.接收请求数据

2.处理请求

3.完成响应

Servlet接口中的五大方法：

其中有三个是生命周期方法：

1.void init(ServletConfig).当Servlet对象被服务器创建时调用，仅调用一次。

2.void service(ServletRequest, ServletResponse).可被服务器调用多次。

3.void destroy().服务器关闭连接之前调用，仅一次。

另外的两个：

4.ServletConfig getServletConfig()

5.String getServletInfo()

Servlet代码源

package javax.servlet;

import java.io.IOException;

public interface Servlet {
    void init(ServletConfig var1) throws ServletException;

    ServletConfig getServletConfig();

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

    String getServletInfo();

    void destroy();
}

HttpServlet代码源

package javax.servlet.http;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
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");

    public HttpServlet() {
    }

    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())) {
            this.doGet(req, resp);
        } else {
            NoBodyResponse response = new NoBodyResponse(resp);
            this.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;
        } else {
            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(this.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;
        Class clazz = null;

        try {
            clazz = Class.forName("org.apache.catalina.connector.RequestFacade");
            Method getAllowTrace = clazz.getMethod("getAllowTrace", (Class[])null);
            ALLOW_TRACE = (Boolean)getAllowTrace.invoke(req, (Object[])null);
        } catch (NoSuchMethodException | SecurityException | IllegalAccessException | IllegalArgumentException | InvocationTargetException | ClassNotFoundException var14) {
            ;
        }

        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 = allow + ", HEAD";
            }
        }

        if (ALLOW_POST) {
            if (allow == null) {
                allow = "POST";
            } else {
                allow = allow + ", POST";
            }
        }

        if (ALLOW_PUT) {
            if (allow == null) {
                allow = "PUT";
            } else {
                allow = allow + ", PUT";
            }
        }

        if (ALLOW_DELETE) {
            if (allow == null) {
                allow = "DELETE";
            } else {
                allow = allow + ", DELETE";
            }
        }

        if (ALLOW_TRACE) {
            if (allow == null) {
                allow = "TRACE";
            } else {
                allow = allow + ", TRACE";
            }
        }

        if (ALLOW_OPTIONS) {
            if (allow == null) {
                allow = "OPTIONS";
            } else {
                allow = allow + ", OPTIONS";
            }
        }

        resp.setHeader("Allow", allow);
    }

    protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String CRLF = "\r\n";
        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();
        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);
        }

    }

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

        }
    }

    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        HttpServletRequest request;
        HttpServletResponse response;
        try {
            request = (HttpServletRequest)req;
            response = (HttpServletResponse)res;
        } catch (ClassCastException var6) {
            throw new ServletException("non-HTTP request or response");
        }

        this.service(request, response);
    }
}

GenericServlet代码源

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();
    }
}

实现接口代码：

package com.wbg;

import javax.servlet.*;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

@WebServlet("/MyServlet")
public class MyServlet implements Servlet {

    @Override
    public void init(ServletConfig servletConfig) throws ServletException {
        System.out.println("初始化servlet");
    }

    @Override
    public ServletConfig getServletConfig() {
        System.out.println("getServletConfig()");
        return null;
    }

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException, IOException {
        System.out.println("正在销毁");
    }

    @Override
    public String getServletInfo() {
        System.out.println("getServletInfo()");
        return null;
    }

    @Override
    public void destroy() {
        System.out.println("正在销毁");
    }
}

浏览器访问servlet时，需要将servlet与路径绑定，需要在web.xml中对servlet进行配置：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <servlet>

 <servlet-name>testservlet</servlet-name>

        <servlet-class>com.wbg.testservlet</servlet-class>
    </servlet>
</web-app>

实现接口的意义:

1、接口和实现分离给软件系统提供了无限的想像空间，才使得软件世界如此丰富多彩。

2、接口和实现分离使得多态变为可能

3、接口和实现分离使得解耦变为可能

4、接口和实现分离使得组合更有意义

5、接口和实现分离使得设计模式变为可能，才能衍生出23种设计模式。

简单的说接口和实现分离给软件系统提供了无限的想象空间、多态变为可能、解耦变为可能、组合更有意义、设计模式变为可能，才能衍生出23钟设计模式(创建型、结构型、行为型)