Servlet源码学习二_GenericServlet

  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();
    }
}