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Servlet（Server Applet）是J2EE的内容之一，由 Java 编写的服务器端小程序。它是web请求的入口，主要功能在于交互式地（Request 和 Response）浏览和修改数据，生成动态 Web内容。Servlet 运行于支持 Java的应用服务器中，如 Tomcat。从实现上讲，Servlet 可以响应任何类型的请求，但绝大多数情况下 Servlet 只用来扩展基于 HTTP协议 的 Web服务器。servlet 的工作模式如下：
另外，由于各种MVC框架的兴起，现在几乎没人会直接使用 servlet 来处理请求咯，往往都是把 SpringMVC 在 Tomcat 中一配，各种请求都交由 DispatcherServlet 来分发。
简单总结一下就是：只要访问 Servlet，service()方法 就会被调用，init()方法 只有第一次访问 Servlet 的时候才会被调用，destroy()方法 会在 Tomcat 关闭的时候被调用。
在 web.xml 中配置 Servlet 时有个属性 <load-on-startup>1</load-on-startup>。翻译过来就是 “在启动时加载”，其作用如下：
servlet对象 在 Tomcat服务器中 是 单实例-多线程并发访问的，比如 DispatcherServlet对象 只会被创建一次，但多个请求同时过来时，Tomcat线程池 的多个工作线程就会并发地访问该 DispatcherServlet对象。所以，若一个servlet对象中存在被并发修改的共享数据（成员变量 等），又没有加锁控制并发安全，就很可能会出现线程安全问题。
解决方案：
对于每次客户端请求，Web容器 都会创建一个新的 HttpServletRequest请求对象 和 一个新的HttpServletResponse响应对象，然后将这两个对象作为参数传递给相应的 Servlet对象 的 service()方法，service()方法 再根据请求方式分别调用 doGet()/doPost()/doXXX()方法。经过一系列业务层处理，最后将结果封装到 response对象中，交由 Socket对象传输响应给客户端。
javax.servlet 包对 Servlet规范 的一些主要行为和接口进行了定义和简单实现，它是 Servlet规范 的体现，具体的实现交由下游厂商或开发者(如：Tomcat / Jetty)，就像体现了JDBC规范的 java.sql包 一样，主要负责定义标准和规范。其源码和注释如下。
public interface Servlet {
/**
* 初始化servlet
*/
public abstract void init(ServletConfig servletconfig) throws ServletException;
/**
* 提供服务
*/
public abstract void service(ServletRequest servletrequest, ServletResponse servletresponse)
throws ServletException, IOException;
/**
* 销毁本servlet
*/
public abstract void destroy();
public abstract ServletConfig getServletConfig();
public abstract String getServletInfo();
}
/**
* ServletConfig 主要用于保存一些 servlet的配置信息，与 servlet对象 成对出现
*/
public interface ServletConfig {
public abstract String getServletName();
public abstract ServletContext getServletContext();
public abstract String getInitParameter(String s);
public abstract Enumeration getInitParameterNames();
}
/**
* 一个web应用对应一个ServletContext，ServletContext实例包含了所有servlet共享的资源信息
*/
public interface ServletContext {
public abstract ServletContext getContext(String s);
public abstract Servlet getServlet(String s) throws ServletException;
public abstract Enumeration getServlets();
public abstract Enumeration getServletNames();
public abstract int getMajorVersion();
public abstract Set getResourcePaths(String s);
public abstract URL getResource(String s) throws MalformedURLException;
public abstract InputStream getResourceAsStream(String s);
public abstract RequestDispatcher getRequestDispatcher(String s);
public abstract RequestDispatcher getNamedDispatcher(String s);
public abstract String getRealPath(String s);
public abstract String getServerInfo();
public abstract String getInitParameter(String s);
public abstract Enumeration getInitParameterNames();
public abstract Object getAttribute(String s);
public abstract Enumeration getAttributeNames();
public abstract void setAttribute(String s, Object obj);
public abstract void removeAttribute(String s);
public abstract String getServletContextName();
}
/**
*定义了 Servlet 要处理的 Request请求信息，调用 Servlet 的 service()方法 时，会作为参数传入
*/
public interface ServletRequest {
public abstract Object getAttribute(String s);
public abstract Enumeration getAttributeNames();
public abstract String getCharacterEncoding();
public abstract void setCharacterEncoding(String s) throws UnsupportedEncodingException;
public abstract int getContentLength();
public abstract String getContentType();
public abstract ServletInputStream getInputStream() throws IOException;
public abstract String getParameter(String s);
public abstract Enumeration getParameterNames();
public abstract String[] getParameterValues(String s);
public abstract Map getParameterMap();
public abstract String getProtocol();
public abstract String getScheme();
public abstract String getServerName();
public abstract int getServerPort();
public abstract BufferedReader getReader() throws IOException;
public abstract String getRemoteAddr();
public abstract String getRemoteHost();
public abstract void setAttribute(String s, Object obj);
public abstract void removeAttribute(String s);
public abstract Locale getLocale();
public abstract Enumeration getLocales();
public abstract boolean isSecure();
public abstract RequestDispatcher getRequestDispatcher(String s);
}
/**
* 定义了 Servlet 要返回的 Response响应信息，调用Servlet 的 service()方法 时，会作为参数传入
*/
public interface ServletResponse {
public abstract String getCharacterEncoding();
public abstract ServletOutputStream getOutputStream() throws IOException;
public abstract PrintWriter getWriter() throws IOException;
public abstract void setContentLength(int i);
public abstract void setContentType(String s);
public abstract void setBufferSize(int i);
public abstract int getBufferSize();
public abstract void flushBuffer() throws IOException;
public abstract void resetBuffer();
public abstract boolean isCommitted();
public abstract void reset();
public abstract void setLocale(Locale locale);
public abstract Locale getLocale();
}
其主要部分的类图 如下。
下面看一下 javax.servlet.http 包下的内容，它提供了很多 我经常用到的类和接口，比如：HttpServlet、HttpServletRequest、HttpServletResponse。其源码如下。
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");
/**
* 将 请求及响应 转换成 HttpServletRequest 及 HttpServletResponse，
* 再调用 service() 的重载方法
*/
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);
}
/**
* 对HTTP协议 各种类型的请求分别进行处理
*/
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);
}
}
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.getName().equals("javax.servlet.http.HttpServlet"))
return null;
int j = 0;
Method parentMethods[] = getAllDeclaredMethods(c.getSuperclass());
Method thisMethods[] = c.getDeclaredMethods();
if (parentMethods != null) {
Method allMethods[] = new Method[parentMethods.length + thisMethods.length];
for (int i = 0; i < parentMethods.length; i++) {
allMethods[i] = parentMethods[i];
j = i;
}
for (int i = ++j; i < thisMethods.length + j; i++)
allMethods[i] = thisMethods[i - j];
return allMethods;
} else {
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();
for (Enumeration reqHeaderEnum = req.getHeaderNames(); 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();
}
private void maybeSetLastModified(HttpServletResponse resp, long lastModified) {
if (resp.containsHeader("Last-Modified"))
return;
if (lastModified >= 0L)
resp.setDateHeader("Last-Modified", lastModified);
}
}
/**
* 对 HTTP 请求信息进行定义
*/
public interface HttpServletRequest extends ServletRequest {
public static final String BASIC_AUTH = "BASIC";
public static final String FORM_AUTH = "FORM";
public static final String CLIENT_CERT_AUTH = "CLIENT_CERT";
public static final String DIGEST_AUTH = "DIGEST";
public abstract String getAuthType();
public abstract Cookie[] getCookies();
public abstract long getDateHeader(String s);
public abstract String getHeader(String s);
public abstract Enumeration getHeaders(String s);
public abstract Enumeration getHeaderNames();
public abstract int getIntHeader(String s);
public abstract String getMethod();
public abstract String getPathInfo();
public abstract String getPathTranslated();
public abstract String getContextPath();
public abstract String getQueryString();
public abstract String getRemoteUser();
public abstract boolean isUserInRole(String s);
public abstract Principal getUserPrincipal();
public abstract String getRequestedSessionId();
public abstract String getRequestURI();
public abstract StringBuffer getRequestURL();
public abstract String getServletPath();
public abstract HttpSession getSession(boolean flag);
public abstract HttpSession getSession();
public abstract boolean isRequestedSessionIdValid();
public abstract boolean isRequestedSessionIdFromCookie();
public abstract boolean isRequestedSessionIdFromURL();
}
/**
* 对 HTTP 响应信息进行定义
*/
public interface HttpServletResponse extends ServletResponse {
public static final int SC_CONTINUE = 100;
public static final int SC_SWITCHING_PROTOCOLS = 101;
public static final int SC_OK = 200;
public static final int SC_CREATED = 201;
public static final int SC_ACCEPTED = 202;
public static final int SC_NON_AUTHORITATIVE_INFORMATION = 203;
public static final int SC_NO_CONTENT = 204;
public static final int SC_RESET_CONTENT = 205;
public static final int SC_PARTIAL_CONTENT = 206;
public static final int SC_MULTIPLE_CHOICES = 300;
public static final int SC_MOVED_PERMANENTLY = 301;
public static final int SC_MOVED_TEMPORARILY = 302;
public static final int SC_SEE_OTHER = 303;
public static final int SC_NOT_MODIFIED = 304;
public static final int SC_USE_PROXY = 305;
public static final int SC_TEMPORARY_REDIRECT = 307;
public static final int SC_BAD_REQUEST = 400;
public static final int SC_UNAUTHORIZED = 401;
public static final int SC_PAYMENT_REQUIRED = 402;
public static final int SC_FORBIDDEN = 403;
public static final int SC_NOT_FOUND = 404;
public static final int SC_METHOD_NOT_ALLOWED = 405;
public static final int SC_NOT_ACCEPTABLE = 406;
public static final int SC_PROXY_AUTHENTICATION_REQUIRED = 407;
public static final int SC_REQUEST_TIMEOUT = 408;
public static final int SC_CONFLICT = 409;
public static final int SC_GONE = 410;
public static final int SC_LENGTH_REQUIRED = 411;
public static final int SC_PRECONDITION_FAILED = 412;
public static final int SC_REQUEST_ENTITY_TOO_LARGE = 413;
public static final int SC_REQUEST_URI_TOO_LONG = 414;
public static final int SC_UNSUPPORTED_MEDIA_TYPE = 415;
public static final int SC_REQUESTED_RANGE_NOT_SATISFIABLE = 416;
public static final int SC_EXPECTATION_FAILED = 417;
public static final int SC_INTERNAL_SERVER_ERROR = 500;
public static final int SC_NOT_IMPLEMENTED = 501;
public static final int SC_BAD_GATEWAY = 502;
public static final int SC_SERVICE_UNAVAILABLE = 503;
public static final int SC_GATEWAY_TIMEOUT = 504;
public static final int SC_HTTP_VERSION_NOT_SUPPORTED = 505;
public abstract void addCookie(Cookie cookie);
public abstract boolean containsHeader(String s);
public abstract String encodeURL(String s);
public abstract String encodeRedirectURL(String s);
public abstract void sendError(int i, String s) throws IOException;
public abstract void sendError(int i) throws IOException;
public abstract void sendRedirect(String s) throws IOException;
public abstract void setDateHeader(String s, long l);
public abstract void addDateHeader(String s, long l);
public abstract void setHeader(String s, String s1);
public abstract void addHeader(String s, String s1);
public abstract void setIntHeader(String s, int i);
public abstract void addIntHeader(String s, int i);
public abstract void setStatus(int i);
}
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