service（）源码如下（很简单，，几个if语句，，doGet比较特殊，需要判断传入内容容量大小）：
/** * Receives standard HTTP requests from the public * <code>service</code> method and dispatches * them to the <code>do</code><i>Method</i> methods defined in * this class. This method is an HTTP-specific version of the * {@link javax.servlet.Servlet#service} method. There's no * need to override this method. * * @param req the {@link HttpServletRequest} object that * contains the request the client made of * the servlet * * @param resp the {@link HttpServletResponse} object that * contains the response the servlet returns * to the client * * @exception IOException if an input or output error occurs * while the servlet is handling the * HTTP request * * @exception ServletException if the HTTP request * cannot be handled * * @see javax.servlet.Servlet#service */ 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; 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); } } } 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); } }在web访问中，每多一个用户，会在web容器中多一个用户线程。 多用户同时访问某个servlet的 doget方法时（在doget方法未使用字段，也就是该方法不依赖外界或者不更改外界变量），此时由于方法的执行在线程中是私有的，每个线程执行某个方法不影响。
于是乎，servlet是单例的，经过测试，init（）方法只在servlet加载时执行，destroy（）只在销毁时执行。
这样看来，servlet单例就够了。
测试servlet代码，TestThreadServlet.java：
package xatu.zsl.servlet; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; /** * Created by zsl on 2017/8/7. */ @WebServlet("/TestThreadServlet") public class TestThreadServlet extends HttpServlet { @Override public void init() throws ServletException { System.out.println("执行了 init（）"); super.init(); } @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { System.out.println("doGet请求，线程名：" + Thread.currentThread().getName()); try { Thread.sleep(10000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("执行完毕！！ doGet请求，线程名：" + Thread.currentThread().getName()); super.doGet(req, resp); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { this.doGet(req, resp); } @Override public void destroy() { System.out.println("执行了 destory()"); super.destroy(); } }doGet中做耗时操作，，，并打印当前线程名， 结果如下：
当然最后sotp，服务器关闭执行：
注意：未做耗时操作时，结果如下图，：
