Servlet以及HttpServlet的源码分析
说明：在编写 Servlet 时需要用到两个用于所有 Servlet 的基本软件包：javax.servlet 和 javax.servlet.http。下面主要介绍 javax.servlet 提供的 Servlet 以及 javax.servlet.http 提供的 HTTP Servlet 应用编程接口。
Servlet源码：
package javax.servlet;
import java.io.IOException;
// Referenced classes of package javax.servlet:
// ServletException, ServletConfig, ServletRequest, ServletResponse
public interface Servlet
{
public abstract void init(ServletConfig servletconfig)
throws ServletException;
public abstract ServletConfig getServletConfig();
public abstract void service(ServletRequest servletrequest, ServletResponse servletresponse)
throws ServletException, IOException;
public abstract String getServletInfo();
public abstract void destroy();
}
以上便是Servlet的源码，Servlet本身是一个接口，首先介绍一下Java Servlet的基本方法。
init() 方法
在Servlet的生命周期中，仅执行一次 init() 方法，即在服务器装入Servlet时执行。通过配置服务器，可以设定在启动服务器或客户机首次访问Servlet 时装入Servlet。无论有多少客户机访问Servlet，都不会重复执行 init()。
service() 方法
service()方法是Servlet的主体部分。客户对一个HttpServlet对象的每次请求都会调用该对象的service() 方法，并传递给这个方法一个"请求"对象和一个"响应"对象作为参数。"请求"对象提供有关请求的信息，而"响应"对象提供了一个将响应信息返回给浏览器的通信途径。javax.servlet 软件包中的相关类为ServletRequest和ServletResponse，而javax.servlet.http 软件包中的相关类为HttpServletRequest 和 HttpServletResponse。Servlet 通过这些对象与服务器通信并最终与客户机通信。Servlet 能通过调用"请求"对象的方法获知客户机环境、服务器环境的信息和所有由客户机提供的信息；通过调用"响应"对象的方法，Servlet可以向客户机发送响应。
HttpServlet中的service()方法默认的服务功能是调用与 HTTP 请求的方法相应的 do 功能。例如，如果 HTTP 请求方法为 GET，则默认情况下就调用 doGet()。当一个客户通过HTML表单发出一个HTTP POST请求时，doPost()方法被调用。与POST请求相关的参数作为一个单独的HTTP 请求从浏览器发送到服务器。当需要修改服务器端的数据时，应该使用doPost()方法。
Servlet的响应可以是下列几种类型：
一个输出流，浏览器根据它的内容类型（如text/HTML）进行解释。
一个HTTP错误响应，重定向到另一个URL、Servlet和JSP。
destroy() 方法
destroy() 方法仅执行一次，即在服务器停止且卸载Servlet 时执行该方法。当服务器卸载 Servlet 时，将在所有 service() 方法调用完成后，或在指定的时间间隔过后调用 destroy() 方法。一个Servlet 在运行service() 方法时可能会产生其他的线程，因此在调用 destroy() 方法时，必须确认这些线程已终止或完成。
GetServletConfig()方法
GetServletConfig()方法返回一个 ServletConfig 对象，该对象用来返回初始化参数和ServletContext。ServletContext 接口提供有关Servlet 的环境信息。
GetServletInfo()方法
GetServletInfo()方法是一个可选的方法，它提供有关Servlet 的信息，如作者、版本、版权。
下面是HttpServlet的源码：
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.*; // Referenced classes of package javax.servlet.http: // NoBodyResponse, HttpServletRequest, HttpServletResponse public abstract class HttpServlet extends GenericServlet implements Serializable { 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 { 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(javax/servlet/http/HttpServlet)) 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 = (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 = "\r\n"; String responseString = (new StringBuilder()).append("TRACE ").append(req.getRequestURI()).append(" ").append(req.getProtocol()).toString(); for(Enumeration reqHeaderEnum = req.getHeaderNames(); reqHeaderEnum.hasMoreElements();) { String headerName = (String)reqHeaderEnum.nextElement(); responseString = (new StringBuilder()).append(responseString).append(CRLF).append(headerName).append(": ").append(req.getHeader(headerName)).toString(); } responseString = (new StringBuilder()).append(responseString).append(CRLF).toString(); 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 { String method = req.getMethod(); if(method.equals("GET")) { long 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")) { 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 { 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); } 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"); } 通过HttpServlet的源码我们看到，HttpServlet与Servelt没有关联关系，而是继承了GenericServlet，其实GenericServlet是一个抽象类，该类已经实现了Servlet, ServletConfig, Serializable这三个接口，GenericServlet的源码我就不在这里展示了，大家有兴趣可以再去研究一下GenericServlet的源码便知。 通过HttpServlet的源码我们发现，该类有两个service（）方法，下面我们一一说明： 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); } 通过代码我们发现，这里把相应的request和response转换为了基于HTTP协议的相应对象。 Servlet接口中定义了一个service方法，HttpServlet对该方法进行了实现，将ServletRequest和ServletResponse转换为HttpServletRequest和HttpServletResponse。 protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) { 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 = req.getDateHeader(HEADER_IFMODSINCE); 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); } } } else if (method.equals(METHOD_HEAD)) { long lastModified = getLastModified(req); maybeSetLastModified(resp, lastModified); doHead(req, resp); } else if (method.equals(METHOD_POST)) { doPost(req, resp); } else if (method.equals(METHOD_PUT)) { doPut(req, resp); } else if (method.equals(METHOD_DELETE)) { doDelete(req, resp); } else if (method.equals(METHOD_OPTIONS)) { doOptions(req,resp); } else if (method.equals(METHOD_TRACE)) { doTrace(req,resp); } else { // // Note that this means NO servlet supports whatever // method was requested, anywhere on this server. // String errMsg = lStrings.getString("http.method_not_implemented"); Object[] errArgs = new Object[1]; errArgs[0] = method; errMsg = MessageFormat.format(errMsg, errArgs); resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg); } } 通过该service（）方法可以发现 ， 客户端的请求先提交给Service方法时，先获取提交的方法名，根据方法名调用相应的具体方法。 如果重写了该方法，那么就不会根据方法名调用其他具体的方法了。 下面看一下HttpServlet类内部的几个DoXXX（）方法： protected void doXXX(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_xxx_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(405, msg); } else { resp.sendError(400, msg); } } 发现doXXX()方法只是判断协议类型，然后抛出相应的异常，其他什么都没做。所以，在实现自己的Servlet时，需要重写这些方法，以符合自己的需求。一般不需要重写service方法。 这里就对Servlet和HttpServlet就分析这些，如果大家还有什么疑问，可以再行讨论，谢谢！