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Servlets are the pure Java solution to handle web requests. Many web application will use servlets instead of JSP and others will use servlets in conjunction with JSP. Experienced JSP programmers use servlets in conjunction with JSP to create clearer and simpler applications. The servlets handle Java processing: form handing, calculation and database queries. JSP formats the results.

REST web-applications will also use servlets. The REST output will typically use XML or JSON instead of HTML, but will use the same servlet container as for web pages.

Servlets belong in WEB-INF/classes. On this machine, the source is in Java source in /var/resin/webapps/ROOT/WEB-INF/classes. WEB-INF/classes is the standard location for servlets and other Java classes. Resin automatically reloads and recompiles servlets, beans, and classes placed in WEB-INF/classes. You should make some changes and add errors to become familiar with Resin's recompilation and the error reporting.

Files in the Tutorial

WEB-INF/classes/test/HelloServlet.java

There is one file that you need to create for the tutorial, the Java class implements the servlet itself.

Creating the Servlet class

Create the following servlet in WEB-INF/classes/test/HelloServlet.java with your favorite editor: eclipse, notepad, emacs, vi, or whatever.

WEB-INF/classes/test/HelloServlet.java

 package test;

 import java.io.*;

 import javax.servlet.annotation.*;
 import javax.servlet.http.*;
 import javax.servlet.*;

 @WebServlet("/hello")
 public class HelloServlet extends HttpServlet {
   public void doGet (HttpServletRequest req,
                      HttpServletResponse res)
     throws ServletException, IOException
   {
     PrintWriter out = res.getWriter();

     out.println("Hello, world!");
   }
 }

Now browse the servlet at http://localhost:8080/hello. Resin will automatically compile the servlet for you. Browsing servlets differs from page browsing because you're executing a servlet class, not looking at a page. The /hello URL is configured for the hello, world servlet below.

installing and deploying a servlet application

To test this tutorial, you'll need to have a servlet engine like Resin installed on your computer. The Resin servlet engine download is at http://resin.caucho.com.

Using Eclipse and the Resin Servlet Container

First go here to get Eclipse for Java EE: Install Guide for Eclipse for Java EE Indigo or higher.

Using Eclipse Indigo or higher

Install Resin (lightweight, fast, easy to use Java EE 6 servlet container) plugin:

Go to File menu -> New Project -> Dynamic Web Project
In New Dynamic Web Project Dialog-> New Runtime...
In New Runtime Dialog -> Download Additional Server Adapters -> Select Resin (Java EE Web Profile) 4.0.x
(Click Next and Ok until it installs Resin runtime)
(Eclipse needs to restart)

Setup new Web Project in Eclipse

File -> New Project -> Dynamic Web Project
In New Dynamic Web Project Dialog-> New Runtime...->Select Resin->Check create local server checkbox
Step 2 of New Runtime...->Click Download and Install (you only have to do this once)
Fill out project name etc. (bookstore).
(Click Next and Ok until you are done)

Ubuntu install of Resin servlet container

On ubuntu system, you can install Resin servlet container directly with the following three steps:

 unix# add-apt-repository http://resin.caucho.com/download/debian
 unix# apt-get update
 unix# apt-get install resin

The Resin directory for your application will be in /var/resin/webapps/ROOT, and the default port will be port 8080, as in this tutorial.

Windows install of Resin servlet container

On windows, download the Resin zip from http://resin.caucho.com.
unzip Resin into something like resin-4.0.32
resin-4.0.32/resin start-all
resin-4.0.32/resin deploy ROOT.war

You can also deploy the example in resin-4.0.32/webapps/ROOT. The "ROOT" means the top-level URL.

deploying an application

You can also bundle up your application into a .war archive, which is just a zip file with a java ".war" extension that you create with the "jar" command (like zip). You'll deploy the .war archived application using Resin's command-line tool:

 unix> cd myapp; jar -cf ../ROOT.war *
 unix> resinctl deploy ROOT.war

The "deploy" will copy the ROOT.war to the webapps directory and deploy your application. Most servlet applications are bundled into .war files and deployed like this.

Review

The servlet is a Java class that handles HTTP web requests by implementing the Servlet interface, usually by extending HttpServlet
The servlet classes belong in WEB-INF/classes, or in .jar files in WEB-INF/lib
The servlet to URL mapping is configured in WEB-INF/web.xml
The servlet doGet method serves a standard HTTP request
The servlet text output is generated from ServletResponse.getWriter
Servlet applications are usually bundled in *.war files and deployed in webapps directories
Applications bundled as war files are deploy using resinctl deploy myapp.war

Programming Patterns in the Servlet Hello World Tutorial

Since all servlets use the same basic structure as this tutorial, it's a good idea to understand this most basic example completely.

While a cookbook example like the hello, world servlet is primarily useful to get you started, you can use it more effectively by studying the patterns it introduces to develop your programming skills. Expert programmers can recognize a huge number of patterns, which is why they are effective. Beginning programmers only know a few patterns and can't always recognize when the pattern should be used. So although you can just take the cookbook above and work with it, you will gain more benefit from the discussion below. Like all expertise, it's also worth coming back as you learn more.

The fractal diagram on the right shows why patterns are valuable not just as a cookbook, but also for building complex applications. A fractal uses a simple rule or pattern repeated at finer levels of detail. The fractal on the right cuts each box in half. The key insight of fractals is to use the same patterns when you're looking at a high abstraction as when you're looking at the detail. This hello servlet tutorial is at the highest level possible, since it's the entire application, but the principles also apply to details of complex applications.

Servlets are some of the fastest web application, from web servers, to JSON/REST web services, to high-performance websocket mobile messaging applications, web sites, and even fast PHP servers written in Java. For example, the Resin web server uses servlets to implement a web server that's faster than the best C-based web servers, including Apache httpd and nginx. If you understand these tutorials, you'll be on the path to developing the best applications on the internet.

Focus and encapsulation is the key general pattern for server applications, and for most programming. People can only focus on a few ideas at a time, and cannot multitask effectively. You might be able to manage three ideas at a time, but not twenty. To use focus in your programs, each part should have its own job, and it should be ignorant of the details of the internals of other parts. Ignorance is good in programs. Each part should be as ignorant as possible, and should keep its own details hidden from others to avoid distracting them. Encapsulation is the programming principle of keeping the details hidden from other components. The better you understand encapsulation and how to use it effectively, the better your programs will be.

Service and Dispatch Pattern

All servlets follow the service and dispatch pattern, which is why they are called servlets. In a sense, every servlet application is an example of a service-oriented architecture on the internet. There are two key parts of the service pattern: the address of the service and the general server interface. For servlets, the address is the HTTP URL, and the interface is the servlet interface, where the HttpServletRequest and HttpServletResponse are the most important part. For the browser client on the internet, the address is the URL, and the interface is the HTTP protocol. The same service pattern is used for REST applications, where the address is the URL again, and the interface is often a JSON-encoded HTTP response. All of these use the same basic service pattern.

address of the service (examples: the URL is the address for most internet applications)
interface of the service (examples: the servlet request/response, or HTTP, or a custom Java interface, or JSON/REST)

Following the general encapsulation pattern, the service pattern has two characters or actors, the dispatcher and the service each of which knows as little as possible about the other. For this hello tutorial, the service is hello servlet itself and it focuses on implementing the hello service. The dispatcher is the Resin servlet engine. The hello servlet knows about writing "hello", but does not know or care how the dispatching works. Most importantly, the service does not know or care about any other services in the system; it is isolated from them. The Resin servlet engine knows how to take the URL address and dispatch to the hello servlet, but does not know or care how the servlet processes the servlet. The dispatcher knows all the services and their addresses, but doesn't know what they do. The dispatcher and service characters only need to know about the common interface, which acts like a contract.

service implementation (examples: servlet, REST service, bean in a dependency-injection system)
dispatcher (examples: servlet engine, CDI engine for JavaEE standard dependency-injection)

The service and dispatch pattern has two phases: a registration phase where the service tells the dispatcher its address, and a dispatch phase where the dispatcher sends a request to the service. In the servlet tutorial, the @WebServlet is the key to the registration phase. When the servlet container starts, it looks for all classes with a @WebServlet annotation. For each one that it finds, it registers the URL with the dispatcher. When a new request comes in, it will use the URL to dispatch to the servlet. The dispatcher essentially works like a post office or a phone service. When you move to a new house, or get a phone, you need to register your address or phone number with the post office or phone company. Once you're registered, you'll receive new messages automatically.

the servlet registers with the dispatcher at a URL address
the dispatcher sends matching requests to the servlet

registration of servlet with servlet container dispatcher

 @WebServlet("/hello")
 public class HelloServlet extends HttpServlet {

Interfaces and Abstract classes

The hello tutorial is a good example of using both interfaces and abstract classes. Interfaces are used to communicate between different code, for example the Servlet and HttpServletRequest and HttpServletResponse are used by the servlet container and the servlet to pass information back and forth. Abstract classes, like HttpServlet let implementations like the HelloServlet focus on the important behavior, leaving the standard shared code to the common code.

Abstract classes are good for managing upgrades and version changes when you use interfaces and abstract classes in your own code. When you add a new method to an interface, you can add a stub method to the abstract class, and your implementations do not need to change.

The following example is like the servlet API, where MyInterface is javax.servlet.Servlet and MyAbstract is HttpServlet, and MyImpl is HelloServlet.

MyInterface.java

 public interface MyInterface {
   public String hello();  // original method
 
   public String newHello(); // new method
 }

MyAbstract

 abstract public class MyAbstract implements MyInterface {
   abstract public String hello();  // original method

   public String newHello() { return "default-value"; }
 }

MyImpl.java

 public class MyImpl extends MyAbstract {
   public String hello() { return "hello, world"; }
 }

In general, it's a good idea when you use interfaces in your own code to also provide abstract classes to let you upgrade your application as you develop it.

Multithreading and Concurrency

To get the highest performance, servlets are multithreaded, which means multiple HTTP requests will run through your servlet at the same time. While multithreading is a huge topic, there are a few key concepts that can help understand the issue.

data specific to a request is thread safe
multithreading is only a problem for shared data.
shared data needs to be protected or managed to be thread safe

In the servlet API, the HttpServletRequest and HttpServletResponse objects are specific to the request thread, so they are thread safe. Any local variables or objects you create for a thread will also be thread safe. The servlet class itself is shared, so it is not thread safe.

The following bad example shows a broken, unsafe servlet. Because multiple requests might run through the servlet at the same time, the _badSharedVariableXXX can get an incorrect value. For example, the first request 'A' might see an output "1" and the second request B might also see an output "1" and even though two requests came through, the counter was only incremented once.

Bad, non-thread safe code

public class BadServlet extends HttpServlet {

   private int _badSharedCounterXXX;
 
   public void doGet (HttpServletRequest req,
                               HttpServletResponse res)
     throws ServletException, IOException
   {
     PrintWriter out = res.getWriter();

     int badValue = _badSharedCounterXXX++;
 
     out.println("Bad counter: " + badValue);
   }

How do you manage the multithreading problem? There's no simple answer, but here are some general things to keep in mind:

emphasize request-based data, except where necessary (like actual shared resources), and pass the current request state as arguments
know which data is shared and which is per-request
use frameworks that manage some of the multithreading, for example JPA for persistence and JCache for caching and MVC frameworks
in some cases, use messaging systems like JMS to convert multithreaded problems into single-threaded queues

You should know that multithreading and concurrency is not as simple as using synchronized methods or ConcurrentXXX data structures like the ConcurrentHashMap.

Cookbooks and Tutorials

Hello World Servlet Tutorial

From Resin 4.0 Wiki

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