Servlet基本介紹
阿新 • • 發佈:2018-05-02
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基本概念
Servlet又稱為Java Servlet是一個基於java技術的web組件,運行在服務器端,用於生成動態的內容。Servlet是平臺獨立的java類,編寫一個Servlet實際上就是按照Servlet規範編寫的java類。
Servlet運行需要一個運行環境,及需要一個Servlet容器,這裏我們以tomcat為例,tomcat作為一個web服務器,具有處理HTML頁面的功能,另外它還是一個Servlet和jsp容器。
實現方式
第一種:實現servlet接口,需要實現接口中定義的5個方法
package day_052102; import java.io.IOException;import java.io.PrintWriter; import javax.servlet.Servlet; import javax.servlet.ServletConfig; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; /* 這裏通過實現Servlet接口來編寫一個簡單的Servlet */ public class ServletDemo implements Servlet { publicvoid init(ServletConfig config) throws ServletException { } public ServletConfig getServletConfig() { return null; } public String getServletInfo() { return null; } public void destroy() { } public void service(ServletRequest req, ServletResponse res)throws ServletException, IOException { PrintWriter out=res.getWriter(); out.print("hello World!"); out.close(); } }
第二種:繼承抽象類GenericServlet,該類定義了一個通用的Servlet
public abstract class GenericServlet implements Servlet, ServletConfig, java.io.Serializable
public class GenericServletDemo extends GenericServlet { @Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { PrintWriter out=res.getWriter(); out.println("hello World!"); out.close(); } }
第三種:通過繼承抽象的HttpServlet類,此類繼承於GenericServlet類
HttpServlet中service方法的實現代碼:
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) //當請求方式為GET時,調用doGet方法 { long lastModified = getLastModified(req); if (lastModified == -1) { // servlet doesn‘t support if-modified-since, no reason // to go through further expensive logic doGet(req, resp); } else { long ifModifiedSince; try { ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); } catch (IllegalArgumentException iae) { // Invalid date header - proceed as if none was set ifModifiedSince = -1; } if (ifModifiedSince < (lastModified / 1000 * 1000)) { // If the servlet mod time is later, call doGet() // Round down to the nearest second for a proper compare // A ifModifiedSince of -1 will always be less maybeSetLastModified(resp, lastModified); doGet(req, resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } }
如果想在自己的類中重寫service方法,那麽servlet容器就會把請求交給我們自己重寫的service方法處理
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) //當請求方式為GET時,調用doGet方法 { long lastModified = getLastModified(req); if (lastModified == -1) { // servlet doesn‘t support if-modified-since, no reason // to go through further expensive logic doGet(req, resp); } else { long ifModifiedSince; try { ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); } catch (IllegalArgumentException iae) { // Invalid date header - proceed as if none was set ifModifiedSince = -1; } if (ifModifiedSince < (lastModified / 1000 * 1000)) { // If the servlet mod time is later, call doGet() // Round down to the nearest second for a proper compare // A ifModifiedSince of -1 will always be less maybeSetLastModified(resp, lastModified); doGet(req, resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } }
在web中的應用
首先java類的實現代碼和“實現方式”中的代碼類似,不再累述。
除此之外,還需要在web.xml中配置響應的servlet標簽,分為兩部分:
<servlet> <servlet-name>ServletDemo</servlet-name> <servlet-class>day_052102.ServletDemo</servlet-class> </servlet> <servlet-mapping> <servlet-name>ServletDemo</servlet-name> <url-pattern>/ServletDemo</url-pattern> </servlet-mapping>
註意:servlet必須放在servlet-mapping之前
匹配原則
1.精確匹配
2.最長路徑匹配
3.擴展匹配
註意事項
1.一個servlet可以對應多個servlet-mapping
2.servlet標簽必須寫在對應的servlet-mapping之前(這個不太確定)
Servlet基本介紹