Servlet 生命周期、工作原理

Servlet 生命周期：Servlet初始化-->服务--->销毁。
init（）：在Servlet的生命周期中，仅执行一次init()方法。它是在服务器装入Servlet时执行的，负责初始化Servlet对象。可以配置服务器，以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（）：它是Servlet的核心，负责响应客户的请求。每当一个客户请求一个HttpServlet对象，该对象的Service()方法就要调用，而且传递给这个方法一个“请求”（ServletRequest）对象和一个“响应”（ServletResponse）对象作为参数。在HttpServlet中已存在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。
destroy（）： 仅执行一次，在服务器端停止且卸载Servlet时执行该方法。当Servlet对象退出生命周期时，负责释放占用的资源。一个Servlet在运行service()方法时可能会产生其他的线程，因此需要确认在调用destroy()方法时，这些线程已经终止或完成。

Tomcat 与 Servlet 是如何工作的：



步骤：
Web Client 向Servlet容器（Tomcat）发出Http请求
Servlet容器接收Web Client的请求
Servlet容器创建一个HttpServletRequest对象，将Web Client请求的信息封装到这个对象中。
Servlet容器创建一个HttpServletResponse对象
Servlet容器调用HttpServlet对象的service方法，把HttpServletRequest对象与HttpServletResponse对象作为参数传给
HttpServlet 对象。
HttpServlet调用HttpServletRequest对象的有关方法，获取Http请求信息。
HttpServlet调用HttpServletResponse对象的有关方法，生成响应数据。
Servlet容器把HttpServlet的响应结果传给Web Client。

Servlet工作原理：

1、首先简单解释一下Servlet接收和响应客户请求的过程，首先客户发送一个请求，Servlet是调用service()方法对请求进行响应的，通过源代码可见，service()方法中对请求的方式进行了匹配，选择调用doGet,doPost等这些方法，然后再进入对应的方法中调用逻辑层的方法，实现对客户的响应。在Servlet接口和GenericServlet中是没有doGet（）、doPost（）等等这些方法的，HttpServlet中定义了这些方法，但是都是返回error信息，所以，我们每次定义一个Servlet的时候，都必须实现doGet或doPost等这些方法。

2、每一个自定义的Servlet都必须实现Servlet的接口，Servlet接口中定义了五个方法，其中比较重要的三个方法涉及到Servlet的生命周期，分别是上文提到的init(),service(),destroy()方法。GenericServlet是一个通用的，不特定于任何协议的Servlet,它实现了Servlet接口。而HttpServlet继承于GenericServlet，因此HttpServlet也实现了Servlet接口。所以我们定义Servlet的时候只需要继承HttpServlet即可。

3、Servlet接口和GenericServlet是不特定于任何协议的，而HttpServlet是特定于HTTP协议的类，所以HttpServlet中实现了service()方法，并将请求ServletRequest、ServletResponse 强转为HttpServletRequest和 HttpServletResponse。

Servlet是接口，GenericServlet和HttpServlet是抽象类，GenericServlet实现了Servlet接口，HttpServlet继承于GenericServlet。

自定义的Servlet类继承于HttpServlet，然后重写init,doPost,doGet,destroy方法即可。

创建Servlet对象的时机：
Servlet容器启动时：读取web.xml配置文件中的信息，构造指定的Servlet对象，创建ServletConfig对象，同时将ServletConfig对象作为参数来调用Servlet对象的init方法。
在Servlet容器启动后：客户首次向Servlet发出请求，Servlet容器会判断内存中是否存在指定的Servlet对象，如果没有则创建它，然后根据客户的请求创建HttpRequest、HttpResponse对象，从而调用Servlet 对象的service方法。
Servlet Servlet容器在启动时自动创建Servlet，这是由在web.xml文件中为Servlet设置的<load-on-startup>属性决定的。从中我们也能看到同一个类型的Servlet对象在Servlet容器中以单例的形式存在。

<servlet>
<servlet-name>Init</servlet-name>
<servlet-class>org.xl.servlet.InitServlet</servlet-class>
<load-on-startup>1</load-on-startup>
</servlet>

转自：http://www.cnblogs.com/xuekyo/archive/2013/02/24/2924072.html

public interface Servlet {
void init(ServletConfig var1) throws ServletException;

ServletConfig getServletConfig();

void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

String getServletInfo();

void destroy();
}

public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {
private transient ServletConfig config;

public GenericServlet() {
}

public void destroy() {
}

public String getInitParameter(String name) {
return this.getServletConfig().getInitParameter(name);
}

public Enumeration getInitParameterNames() {
return this.getServletConfig().getInitParameterNames();
}

public ServletConfig getServletConfig() {
return this.config;
}

public ServletContext getServletContext() {
return this.getServletConfig().getServletContext();
}

public String getServletInfo() {
return "";
}

public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}

public void init() throws ServletException {
}

public void log(String msg) {
this.getServletContext().log(this.getServletName() + ": " + msg);
}

public void log(String message, Throwable t) {
this.getServletContext().log(this.getServletName() + ": " + message, t);
}

public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

public String getServletName() {
return this.config.getServletName();
}
}

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");

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);
this.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;
} else {
int j = 0;
Method[] parentMethods = this.getAllDeclaredMethods(c.getSuperclass());
Method[] thisMethods = c.getDeclaredMethods();
if(parentMethods == null) {
return thisMethods;
} else {
Method[] allMethods = new Method[parentMethods.length + thisMethods.length];

int i;
for(i = 0; i < parentMethods.length; j = i++) {
allMethods[i] = parentMethods[i];
}

++j;

for(i = j; i < thisMethods.length + j; ++i) {
allMethods[i] = thisMethods[i - j];
}

return allMethods;
}
}
}

protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
Method[] methods = this.getAllDeclaredMethods(this.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 allow = 0; allow < methods.length; ++allow) {
Method m = methods[allow];
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 var13 = null;
if(ALLOW_GET && var13 == null) {
var13 = "GET";
}

if(ALLOW_HEAD) {
if(var13 == null) {
var13 = "HEAD";
} else {
var13 = var13 + ", HEAD";
}
}

if(ALLOW_POST) {
if(var13 == null) {
var13 = "POST";
} else {
var13 = var13 + ", POST";
}
}

if(ALLOW_PUT) {
if(var13 == null) {
var13 = "PUT";
} else {
var13 = var13 + ", PUT";
}
}

if(ALLOW_DELETE) {
if(var13 == null) {
var13 = "DELETE";
} else {
var13 = var13 + ", DELETE";
}
}

if(ALLOW_TRACE) {
if(var13 == null) {
var13 = "TRACE";
} else {
var13 = var13 + ", TRACE";
}
}

if(ALLOW_OPTIONS) {
if(var13 == null) {
var13 = "OPTIONS";
} else {
var13 = var13 + ", OPTIONS";
}
}

resp.setHeader("Allow", var13);
}

protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String CRLF = "\r\n";
String responseString = "TRACE " + req.getRequestURI() + " " + req.getProtocol();

String out;
for(Enumeration reqHeaderEnum = req.getHeaderNames(); reqHeaderEnum.hasMoreElements(); responseString = responseString + CRLF + out + ": " + req.getHeader(out)) {
out = (String)reqHeaderEnum.nextElement();
}

responseString = responseString + CRLF;
int responseLength = responseString.length();
resp.setContentType("message/http");
resp.setContentLength(responseLength);
ServletOutputStream out1 = resp.getOutputStream();
out1.print(responseString);
out1.close();
}

protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
String method = req.getMethod();
long errMsg;
if(method.equals("GET")) {
errMsg = this.getLastModified(req);
if(errMsg == -1L) {
this.doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader("If-Modified-Since");
if(ifModifiedSince < errMsg / 1000L * 1000L) {
this.maybeSetLastModified(resp, errMsg);
this.doGet(req, resp);
} else {
resp.setStatus(304);
}
}
} else if(method.equals("HEAD")) {
errMsg = this.getLastModified(req);
this.maybeSetLastModified(resp, errMsg);
this.doHead(req, resp);
} else if(method.equals("POST")) {
this.doPost(req, resp);
} else if(method.equals("PUT")) {
this.doPut(req, resp);
} else if(method.equals("DELETE")) {
this.doDelete(req, resp);
} else if(method.equals("OPTIONS")) {
this.doOptions(req, resp);
} else if(method.equals("TRACE")) {
this.doTrace(req, resp);
} else {
String errMsg1 = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[]{method};
errMsg1 = MessageFormat.format(errMsg1, errArgs);
resp.sendError(501, errMsg1);
}

}

private void maybeSetLastModified(HttpServletResponse resp, long lastModified) {
if(!resp.containsHeader("Last-Modified")) {
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 var6) {
throw new ServletException("non-HTTP request or response");
}

this.service(request, response);
}
}

public class MainServlet  extends HttpServlet {

@Override
public void init() throws ServletException {
super.init();
}

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doGet(req, resp);
}

@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
super.doPost(req, resp);
}

@Override
public void destroy() {
super.destroy();
}

}