HttpServlet解读与源码分析

Servlet生命周期分为三个阶段：

　　1，初始化阶段  调用init()方法

　　2，响应客户请求阶段　　调用service()方法

　　3，终止阶段　　调用destroy()方法

学习源码

package javax.servlet;
 
import java.io.IOException;
 
public interface Servlet {
 
    public void init(ServletConfig config) throws ServletException;//初始化
 
    public ServletConfig getServletConfig();//获取ServletConfig对象
 
    public void service(ServletRequest req, ServletResponse res)//调用service方法
	throws ServletException, IOException;
	
    public String getServletInfo();//返回servlet的信息
	
    public void destroy();//销毁
}

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() {
    }


    //get请求
    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);
        }

    }
    //post请求
    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);
        }

    }
    //put请求
    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);
        }

    }
    //delete请求
    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);
        }

    }

 

 
    //service方法，请求来了先执行这个方法，然后通过请求头，去分别判断执行那个具体方法
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();//获取请求头部
        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);
        }

    }

}

@WebServlet(value = "url")//推荐注解开发方便，还有还有就是在web.xml中配置本servlet类
public class ServletTest extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //一般定义一个Servlet执行需要继承HttpServlet重写service
    }

    //下面不建议，重写所有请求方法，代码量太多。
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    }
    @Override
    protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    }
    @Override
    protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

web.xml中配置servlet，注解配置任选一种，个人推荐注解配置（@WebServlet(value = "url")）

<?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>servletTest</servlet-name>
        <servlet-class>ServletTest</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>servletTest</servlet-name>
        <url-pattern>url</url-pattern>
    </servlet-mapping>
</web-app>