GenericServlet是Sevlet的默认实现，主要做了三件事：1、实现了ServletConfig接口，我们可以直接调用ServletConfig里的方法；2、提供了无参的init方法；3、提供了log方法。
1、实现了ServletConfig接口
GenericServlet实现了ServletConfig接口，我们在需要调用ServletConfig中方法的时候可以直接调用，而不再需要获取ServletConfig了。比如：获取ServletContext的时候可以直接调用getServletContext方法，而无需调用getServletConfig.getServletContext()了，不过其底层实现其实是在内部调用了。getServletContext的代码如下：
public ServletContext getServletContext() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletContext();
}
public ServletConfig getServletConfig() {
return config;
}2、提供了无参的init方法
GenericServlet实现了Servlet的init(ServletConfig config)方法，在里面将config设置给了内部变量config，然后调用了无参的init()方法，这个方法是个模板方法，在子类中可以通过覆盖它来完成自己的初始化工作，代码如下：
private transient ServletConfig config;
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
public void init() throws ServletException {
}
3、提供了log方法
GenericServlet 提供了2个log方法，一个记录日志，一个记录异常。具体实现是通过传给ServletContext的日志实现的。
public void log(String message, Throwable t) {
getServletContext().log(getServletName() + ": " + message, t);
}
public void log(String msg) {
getServletContext().log(getServletName() + ": "+ msg);
}一般我们都要自己的日志处理方式，所以这个用的不是很多。摘自：看透SpingMvc
GenericServlet源码：
public abstract class GenericServlet
implements Servlet, ServletConfig, java.io.Serializable
{
private static final String LSTRING_FILE = "javax.servlet.LocalStrings";
private static ResourceBundle lStrings =
ResourceBundle.getBundle(LSTRING_FILE);
private transient ServletConfig config;
/**
*
* Does nothing. All of the servlet initialization
* is done by one of the <code>init</code> methods.
*
*/
public GenericServlet() { }
/**
* Called by the servlet container to indicate to a servlet that the
* servlet is being taken out of service. See {@link Servlet#destroy}.
*
*
*/
public void destroy() {
}
/**
* Returns a <code>String</code> containing the value of the named
* initialization parameter, or <code>null</code> if the parameter does
* not exist. See {@link ServletConfig#getInitParameter}.
*
* <p>This method is supplied for convenience. It gets the
* value of the named parameter from the servlet's
* <code>ServletConfig</code> object.
*
* @param name a <code>String</code> specifying the name
* of the initialization parameter
*
* @return String a <code>String</code> containing the value
* of the initialization parameter
*
*/
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);
}
/**
* Returns the names of the servlet's initialization parameters
* as an <code>Enumeration</code> of <code>String</code> objects,
* or an empty <code>Enumeration</code> if the servlet has no
* initialization parameters. See {@link
* ServletConfig#getInitParameterNames}.
*
* <p>This method is supplied for convenience. It gets the
* parameter names from the servlet's <code>ServletConfig</code> object.
*
*
* @return Enumeration an enumeration of <code>String</code>
* objects containing the names of
* the servlet's initialization parameters
*
*/
public Enumeration getInitParameterNames() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getInitParameterNames();
}
/**
* Returns this servlet's {@link ServletConfig} object.
*
* @return ServletConfig the <code>ServletConfig</code> object
* that initialized this servlet
*
*/
public ServletConfig getServletConfig() {
return config;
}
/**
* Returns a reference to the {@link ServletContext} in which this servlet
* is running. See {@link ServletConfig#getServletContext}.
*
* <p>This method is supplied for convenience. It gets the
* context from the servlet's <code>ServletConfig</code> object.
*
*
* @return ServletContext the <code>ServletContext</code> object
* passed to this servlet by the <code>init</code>
* method
*
*/
public ServletContext getServletContext() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletContext();
}
/**
* Returns information about the servlet, such as
* author, version, and copyright.
* By default, this method returns an empty string. Override this method
* to have it return a meaningful value. See {@link
* Servlet#getServletInfo}.
*
*
* @return String information about this servlet, by default an
* empty string
*
*/
public String getServletInfo() {
return "";
}
/**
*
* Called by the servlet container to indicate to a servlet that the
* servlet is being placed into service. See {@link Servlet#init}.
*
* <p>This implementation stores the {@link ServletConfig}
* object it receives from the servlet container for later use.
* When overriding this form of the method, call
* <code>super.init(config)</code>.
*
* @param config the <code>ServletConfig</code> object
* that contains configutation
* information for this servlet
*
* @exception ServletException if an exception occurs that
* interrupts the servlet's normal
* operation
*
*
* @see UnavailableException
*
*/
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
/**
*
* A convenience method which can be overridden so that there's no need
* to call <code>super.init(config)</code>.
*
* <p>Instead of overriding {@link #init(ServletConfig)}, simply override
* this method and it will be called by
* <code>GenericServlet.init(ServletConfig config)</code>.
* The <code>ServletConfig</code> object can still be retrieved via {@link
* #getServletConfig}.
*
* @exception ServletException if an exception occurs that
* interrupts the servlet's
* normal operation
*
*/
public void init() throws ServletException {
}
/**
*
* Writes the specified message to a servlet log file, prepended by the
* servlet's name. See {@link ServletContext#log(String)}.
*
* @param msg a <code>String</code> specifying
* the message to be written to the log file
*
*/
public void log(String msg) {
getServletContext().log(getServletName() + ": "+ msg);
}
/**
* Writes an explanatory message and a stack trace
* for a given <code>Throwable</code> exception
* to the servlet log file, prepended by the servlet's name.
* See {@link ServletContext#log(String, Throwable)}.
*
*
* @param message a <code>String</code> that describes
* the error or exception
*
* @param t the <code>java.lang.Throwable</code> error
* or exception
*
*
*/
public void log(String message, Throwable t) {
getServletContext().log(getServletName() + ": " + message, t);
}
/**
* Called by the servlet container to allow the servlet to respond to
* a request. See {@link Servlet#service}.
*
* <p>This method is declared abstract so subclasses, such as
* <code>HttpServlet</code>, must override it.
*
*
*
* @param req the <code>ServletRequest</code> object
* that contains the client's request
*
* @param res the <code>ServletResponse</code> object
* that will contain the servlet's response
*
* @exception ServletException if an exception occurs that
* interferes with the servlet's
* normal operation occurred
*
* @exception IOException if an input or output
* exception occurs
*
*/
public abstract void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException;
/**
* Returns the name of this servlet instance.
* See {@link ServletConfig#getServletName}.
*
* @return the name of this servlet instance
*
*
*
*/
public String getServletName() {
ServletConfig sc = getServletConfig();
if (sc == null) {
throw new IllegalStateException(
lStrings.getString("err.servlet_config_not_initialized"));
}
return sc.getServletName();
}
}