servlet 、GenericServlet 、HttpServlet 詳解

今天突然想研究下servlet，總結下新的，希望對大家有幫助：

 

 

首先，看看怎樣才能實現一個servlet，方法有3種：

1、實現servlet接口

2、繼承GenericServlet（實現service 方法 ）

3、繼承HttpServlet類（重寫doGet，doPost方法就行）

 

servlet 接口源代碼

public abstract interface Servlet { public abstract void init(ServletConfig paramServletConfig) throws ServletException; public abstract ServletConfig getServletConfig(); public abstract void service(ServletRequest paramServletRequest, ServletResponse paramServletResponse) throws ServletException, IOException; public abstract String getServletInfo(); public abstract void destroy(); }

 

GenericServlet源代碼

package javax.servlet; import java.io.IOException; import java.io.Serializable; import java.util.Enumeration; import java.util.ResourceBundle; public abstract class GenericServlet implements Servlet, ServletConfig, Serializable { private static final String LSTRING_FILE = "javax.servlet.LocalStrings"; private static ResourceBundle lStrings = ResourceBundle.getBundle("javax.servlet.LocalStrings"); private transient ServletConfig config; public void destroy() { } public String getInitParameter(String name) { ServletConfig sc = getServletConfig(); if (sc == null) { throw new IllegalStateException(lStrings.getString("err.servlet_config_not_initialized")); } return sc.getInitParameter(name); } public Enumeration getInitParameterNames() { ServletConfig sc = getServletConfig(); if (sc == null) { throw new IllegalStateException(lStrings.getString("err.servlet_config_not_initialized")); } return sc.getInitParameterNames(); } public ServletConfig getServletConfig() { return this.config; } public ServletContext getServletContext() { ServletConfig sc = getServletConfig(); if (sc == null) { throw new IllegalStateException(lStrings.getString("err.servlet_config_not_initialized")); } return sc.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() { ServletConfig sc = getServletConfig(); if (sc == null) { throw new IllegalStateException(lStrings.getString("err.servlet_config_not_initialized")); } return sc.getServletName(); } }

 

 

HttpServlet源代碼

 

package javax.servlet.http; import java.io.IOException; import java.io.Serializable; import java.lang.reflect.Method; import java.text.MessageFormat; import java.util.Enumeration; import java.util.ResourceBundle; 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 implements Serializable { 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 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 { 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 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(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 == null)) 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"; String responseString = "TRACE " + req.getRequestURI() + " " + req.getProtocol(); Enumeration reqHeaderEnum = req.getHeaderNames(); while (reqHeaderEnum.hasMoreElements()) { String headerName = (String)reqHeaderEnum.nextElement(); responseString = responseString + CRLF + headerName + ": " + req.getHeader(headerName); } responseString = responseString + CRLF; int responseLength = responseString.length(); resp.setContentType("message/http"); resp.setContentLength(responseLength); ServletOutputStream out = resp.getOutputStream(); out.print(responseString); out.close(); } protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { long lastModified; String method = req.getMethod(); if (method.equals("GET")) { lastModified = getLastModified(req); if (lastModified == -1L) { doGet(req, resp); } else { long ifModifiedSince = req.getDateHeader("If-Modified-Since"); if (ifModifiedSince < lastModified / 1000L * 1000L) { maybeSetLastModified(resp, lastModified); doGet(req, resp); } else resp.setStatus(304); } } else if (method.equals("HEAD")) { 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 { HttpServletRequest request; HttpServletResponse response; try { request = (HttpServletRequest)req; response = (HttpServletResponse)res; } catch (ClassCastException e) { throw new ServletException("non-HTTP request or response"); } service(request, response); } }

 

 

GenericServlet實現了接口：Servlet , ServletConfig

HttpServlet ：繼承了GenericServlet 

 

 

咱先看下訪問頁面是怎樣的一個流程：

 

當你訪問一個servlet的時候(也可以說是訪問一個jsp頁面)，訪問的方式有（get，post，delete），調用service（）方法處理你的請求，你可以在裏面寫你想要幹嘛。

 

在GenericServlet中，service是一個抽象方法，也就是當你以繼承GenericServlet的方式創建一個servlet時，必須實現service方法，在裏面處理前端發過來的request，和response。

 

在HttpServlet中，大家可以看它的原代碼，有2個service方法，一個是保護的（主要看裏面的實現），一個是公有的，公有的service接收到請求時，會拋給保護的那個service，它在這只是起一箇中轉的作用。當前端有請求過來時，會默認找到這2個service方法，具體先找哪個我不清楚，反正最後請求都會在保護的那個service那裏，在那個service中，會根據前端是哪種方式的請求，再拋給相應的處理方法，如：get則交給doGet處理。所以，在以繼承GenericServlet的方式創建一個servlet時，你只要重寫相應的doGet或者doPost等處理方法就行了，然後再在裏面寫你的代碼就行了。

 

 

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還有一個就是關於servlet的生命週期，它開始於init，而且容器中一個servlet只有一個實例，多個請求過來時，是共享一個servlet的，每次請求都是調用service，它銷燬與容器關閉的時候，還有人說可以調用destory來銷燬一個實例，我做了實驗，不行。可能是不同容器不同的處理吧，你們可以看下源代碼，destory在GenericServlet中是空實現，不知道爲什麼。求高手解答。

 

 

測試銷燬servlet的代碼

 

public class LoginServlet extends HttpServlet { private Integer a = 0; // 一個servlet將會一直在容器中存在，除非它重啓服務器或者強制 /** * */ private static final long serialVersionUID = 1L; @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doPost(req, resp); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { PrintWriter pw = resp.getWriter(); pw.write("這是一個servlet"); System.out.println(a++); if (a > 5) { destroy(); //如果銷燬的話，下次輸出a就又從0開始了，否則就是繼續遞增 } } }

 

 

 

其實，當你reload一些服務器或者關閉服務器或者重啓服務器時，纔會調用destory，這個方法有點像垃圾回收裏面那個finalize（），在執行Gc前，會調用finalize來做一些清理。