A common misunderstanding among starters is that they think that the call of a forward(), sendRedirect(), or sendError() would magically exit and “jump” out of the method block, hereby ignoring the remnant of the code. For example:

protected void doPost() {
    if (someCondition) {
        sendRedirect();
    }
    forward(); // This is STILL invoked when someCondition is true!
}

This is thus actually not true. They do certainly not behave differently than any other Java methods (expect of System#exit() of course). When the someCondition in above example is true and you’re thus calling forward() after sendRedirect() or sendError() on the same request/response, then the chance is big that you will get the exception:

java.lang.IllegalStateException: Cannot forward after response has been committed

If the if statement calls a forward() and you’re afterwards calling sendRedirect() or sendError(), then below exception will be thrown:

java.lang.IllegalStateException: Cannot call sendRedirect() after the response has been committed

To fix this, you need either to add a return; statement afterwards

protected void doPost() 
{
    if (someCondition) 
    {
        sendRedirect();
        return;
    }
    forward();
}

… or to introduce an else block.

protected void doPost() {
    if (someCondition) {
        sendRedirect();
    } else {
        forward();
    }
}

Another probable cause is that the servlet writes to the response while a forward() will be called, or has been called in the very same method.

protected void doPost() 
{
    out.write("some string");
    // ... 
    forward(); // Fail!
}

The response buffer size defaults in most server to 2KB, so if you write more than 2KB to it, then it will be committed and forward() will fail the same way:

java.lang.IllegalStateException: Cannot forward after response has been committed

Solution is obvious, just don’t write to the response in the servlet. That’s the responsibility of the JSP. You just set a request attribute like so request.setAttribute(“data”, “some string”) and then print it in JSP like so ${data}

The best way to resolve this problem just set the page (where you suppose to forward the request) dynamically according your logic. That is:

protected void doPost(request , response){
String returnPage="default.jsp";
if(condition1){
 returnPage="page1.jsp";
}
if(condition2){
   returnPage="page2.jsp";
}
request.getRequestDispatcher(returnPage).forward(request,response); //at last line
}

What is Event
Changing the state of an object is known as an event.We can perform some important tasks at the occurrence of these exceptions, such as counting total and current logged-in users, creating tables of the database at time of deploying the project, creating database connection object etc.

Event Types

1. ServletRequestEvent – ServletRequestListener
2. ServletContextEvent – ServletContextListener
3. ServletRequestAttributeEvent – ServletRequestAttributeEvent
4. ServletContextAttributeEvent – ServletContextAttributeListener
5. HttpSessionEvent – HttpSessionListener
6. HttpSessionBindingEvent – HttpSessionAttributeListener

Listener Types

What is Filter
A filter is an object that is invoked at the preprocessing and postprocessing of a request.

It is mainly used to perform filtering tasks such as conversion, logging, compression, encryption and decryption, input validation etc.

The servlet filter is pluggable, i.e. its entry is defined in the web.xml file, if we remove the entry of filter from the web.xml file, filter will be removed automatically and we don’t need to change the servlet.

Filter interface
Filter interface contains three methods

1. public void init(FilterConfig config)init() method is invoked only once. It is used to initialize the filter.
2. public void doFilter(HttpServletRequest request,HttpServletResponse response, FilterChain chain)doFilter() method is invoked every time when user request to any resource, to which the filter is mapped.It is used to perform filtering tasks.
3. public void destroy()This is invoked only once when filter is taken out of the service.

How to define a Filter
web.xml

<web-app>        
	<filter>  
		<filter-name>...</filter-name>  
		<filter-class>...</filter-class>  
	</filter>         
	<filter-mapping>  
		<filter-name>...</filter-name>  
		<url-pattern>...</url-pattern>  
	</filter-mapping>        
</web-app>  

Example of Filter
web.xml

<web-app>
        <servlet>
		<servlet-name>AdminServlet</servlet-name>
		<servlet-class>AdminServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>AdminServlet</servlet-name>
		<url-pattern>/servlet1</url-pattern>
	</servlet-mapping>

	<filter>
		<filter-name>f1</filter-name>
		<filter-class>MyFilter</filter-class>
	</filter>
	<filter-mapping>
		<filter-name>f1</filter-name>
		<url-pattern>/servlet1</url-pattern>
	</filter-mapping>
</web-app>  

MyFilter.java

public class MyFilter implements Filter 
{
  public void init(FilterConfig arg0) throws ServletException 
  {
  }

  public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
	throws IOException, ServletException {
  PrintWriter out = resp.getWriter();
  out.print("filter is invoked before");

  chain.doFilter(req, resp);// sends request to next resource
  out.print("filter is invoked after");
}

  public void destroy() 
  {
  }
}

HelloServlet.java

public class HelloServlet extends HttpServlet 
{
	public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
	{
		response.setContentType("text/html");
		PrintWriter out = response.getWriter();
		out.print("<br>welcome to servlet<br>");
	}
}

Whenever the /Servlet1is accessed it will go through the filter before and after processing the actual java page HelloServlet.java

How to access Init param in filter
web.xml

.
<filter>
	<filter-name>f1</filter-name>
	<filter-class>MyFilter</filter-class>
	<init-param>
		<param-name>construction</param-name>
		<param-value>no</param-value>
	</init-param>
</filter>
<filter-mapping>
	<filter-name>f1</filter-name>
	<url-pattern>/servlet1</url-pattern>
</filter-mapping>  
.
.

MyFilter.java

public class MyFilter implements Filter 
{
	FilterConfig config;

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

	public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain)
			throws IOException, ServletException {

		PrintWriter out = resp.getWriter();

		String s = config.getInitParameter("construction");

		if (s.equals("yes")) {
			out.print("This page is under construction");
		} else {
			chain.doFilter(req, resp);// sends request to next resource
		}
	}

	public void destroy() {
	}
}

Context path
The context path is the prefix of a URL path that is used to select the context(s) to which an incoming request is passed. Typically a URL in a Java servlet server is of the format http://hostname.com/contextPath/servletPath/pathInfo, where each of the path elements can be zero or more / separated elements.

How to get ContextPath

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException 
{
  ServletContext servletContext = getServletContext();
  String contextPath = servletContext.getRealPath(File.separator);
  PrintWriter out = response.getWriter();
  out.println("<br/>File system context path (in TestServlet): " + contextPath);
}

ServletConfig

1. ServletConfig is one of the pre-defined interface.
2. ServletConfig object is used for developing flexible servlets.
3. ServletConfig objct exist one per servlet program.
4. An object of ServletConfig created by the container during its initialization phase.
5. An object of ServletConfig is available to the servlet during its execution, once the servlet execution is completed, automatically ServletConfig interface object will be removed by the container.
6. An object of ServletConfig interface contains details at web.xml, of a particular servlet.
7. The moment when we are using an object of ServletConfig, we need to configure the web.xml by writing tag under tag of web.xml.
8. When ever compiler executes init() mehod then the ServletConfig will be created in general.
9. An object of ServletConfig contain the data in the form of key,value pairs, here the keys represents init param names and values are its values, which are represented in the web.xml file

How to Get ServletConfig Object into Servelt
Method1

ServletConfig conf = getServletConfig();

Method2
ServletConfig object will be available in init() method of the servlet.

public void init(ServletConfig config)
{
// …………………
}

How to Retrieve Data from ServletConfig Interface Object

public String getInitParameter(“param name”);
public Enumeration getInitParameterNames();

web.xml

<web-app> 
  <servlet>
     <servlet-name>onServletConfig</servlet-name>
     <servlet-class>java4s.OnServletConfig</servlet-class> 
     <init-param>
        <param-name> n1 </param-name>
        <param-value> 100 </param-value>
     </init-param>
    .
    .
</web-app>

Test.java

ServletConfig conf=getServletConfig(); 
String s1=conf.getInitParameter("n1");
 

ServletContext
An object of ServletContext is created by the web container at time of deploying the project. This object can be used to get configuration information from web.xml file. There is only one ServletContext object per web application.

Methods of ServletContext interface

1. public String getInitParameter(String name):Returns the parameter value for the specified parameter name.
2. public Enumeration getInitParameterNames():Returns the names of the context’s initialization parameters.
3. public void setAttribute(String name,Object object):sets the given object in the application scope.
4. public Object getAttribute(String name):Returns the attribute for the specified name.
5. public Enumeration getInitParameterNames():Returns the names of the context’s initialization parameters as an Enumeration of String objects.

web.xml

    <web-app>  
     ......  
          
      <context-param>  
        <param-name>parametername</param-name>  
        <param-value>parametervalue</param-value>  
      </context-param>  
     ......  
    </web-app>  

Test.java

  ServletContext context=getServletContext();    

  //Getting the value of the initialization parameter and printing it  
  String driverName=context.getInitParameter("parametername");  

The method getServletContext() is undefined for the type HttpServletRequest

  ServletContext context = request.getServletContext();

getServletContext() is available from HttpServlet class that your servlet extended. You can invoke the method as if it were defined in your own servlet class

  ServletContext context = getServletContext();