Request administration is the aliment and adulate of Java web appliance development. In adjustment to acknowledge to requests from the network, a Java web appliance charge aboriginal actuate what cipher will acknowledge to the appeal URL, again align a response. Every technology assemblage has a way of accomplishing request-response handling. In Java, we use servlets (and the Java Servlet API) for this purpose. Think of a servlet as a tiny server whose job is to acquire requests and affair responses.
As I explained in my accession to the Java Runtime Environment, we can appearance software as a alternation of layers. Each band in a software arrangement holds assertive capabilities that are adapted by the layers aloft it. As an example, the accouterments band sits beneath the firmware layer, acknowledging its functionality. Likewise, the firmware band (BIOS on a PC or EFI on a Mac) is bare to run the operating system. Figure 1 shows these three apparatus in a layered diagram.
Figure 1. Software as layers
You could additionally attending at a software arrangement as a alternation of containers, area the lower layers act as containers for the college ones. Each band acts as a ambience for active the aing akin of functionality: accouterments contains firmware, and firmware contains the operating system.
A server-side Java appliance is one of a ample chic of applications that serve as arrangement endpoints, accepting HTTP requests from a accustomed URL and sending aback abstracts in an altering architecture like HMTL or JSON. Server-side Java consists of both affiliated Java servers and technologies for interacting with those servers. The Java Servlet API is the accepted acclimated to collaborate with a Java server.
In Java-based systems, the operating arrangement (OS) contains the JVM, which provides a constant ambiance for active Java applications. A Java server sits on top of the JVM. Aloof as the JVM is the agent amid the OS and your Java application, the Java server provides consistent, automated admission to the operating system’s networking and processing capabilities. A Java appliance runs central the server, appliance the Java Servlet API to admission the server’s capabilities.
Figure 2 shows a software assemblage for server-side Java.
Figure 2. Server-side Java stack
The Java Servlet blueprint provides the basal analogue for a Java server and accompanying components. It defines how the server will accelerate requests and responses during arrangement interactions over HTTP. All Java servers charge be accordant with the Java Servlet specification. Best Java servers today are accordant with Servlet 4.0.
For a applied compassionate of how the Servlet blueprint works, accede the commodity you are currently reading. Somewhere in the belly of JavaWorld’s infrastructure, this commodity was formatted and submitted for publication. It was assigned a URL, baffled through the network, and accustomed at a server. The server affiliated the antiquity (article) with the URL and bent that aback a GET appeal accustomed for that URL, it would serve aback this commodity as HTML.
When you actualize a Java web application, you are creating software that runs central the Java server. The appliance utilizes the accessories provided by the server context, and one of those accessories is the Servlet API. For this reason, a Java server that accouterments the Servlet blueprint is sometimes alleged a servlet container.
To actualize a servlet, you apparatus the Servlet interface and arrange it central a servlet container. Figure 3 shows how your appliance relies on the servlet.
Figure 3. Java servlet detail
Now that you’ve got a conceptual overview, let’s get bottomward to the business of autograph a Java servlet.
The aboriginal affair you’ll charge is a servlet container, contrarily accepted as a Java appliance server. Tomcat and Jetty are two of the best accepted servlet containers. We’ll use Tomcat because it’s one of the best abiding appliance servers for Java. Tomcat is chargeless and contains basal accretion and whistles, which will accumulate things simple for our example. (“Bells and whistles” is a abstruse term, by the way.)
If you don’t already accept Tomcat, alpha by aperture the Tomcat download page. There, you can baddest either the Windows installer or the best adapted ZIP download for your computer (for example, I’m downloading the 64-bit Windows zip).
That’s it: you’ve aloof added the Java server band to your operating system!
Before activity further, let’s accomplish abiding you can run Tomcat. Either alpha the Windows account or run startup.sh or startup.bat book from the command-line.
If you now accessible a web browser and go to localhost:8080, you should be greeted by the afterward screen:
Figure 4. Tomcat acceptable screen
If you appointment any problems active Tomcat, you can appointment the Tomcat affidavit for troubleshooting.
Now let’s get a attending at a Java servlet. Conveniently, Tomcat has included some simple examples.
Click on the Examples articulation you see in the Developer Quick Alpha area of the Tomcat acceptable page. Once you’re there, bang the Servlet Examples link.
Now you can see a simple Servlet in activity by beat the Hello World sample’s Execute link. That will accompany your browser to the http://localhost:8080/examples/servlets/servlet/HelloWorldExample URL, area you will see the abiding programmer’s salutation.
Click the Aback arrow in your browser, again bang the antecedent articulation for the HelloWorld app. The antecedent is apparent in Advertisement 1.
This actual simple cipher advertisement contains the basal apparatus of a Java servlet. Let’s accede it footfall by step.
The aboriginal band calls the accepted Java imports. After that, the affairs defines a new class, which extends the HttpServlet class. This is analytical because servlets charge apparatus the Servlet interface in adjustment to run central a servlet container.
Next, the HelloWorld chic defines a adjustment alleged doGet(). This is a accepted adjustment on servlets: it tells the server to avenue HTTP GET requests to this method. Other HTTP methods, like POST, are handled by analogously alleged methods, like doPost.
Notice that that doGet() has two parameters: (HttpServletRequest request, HttpServletResponse response). These two altar represent the appeal and response. They accommodate admission to aggregate your cipher needs to accord with the appeal and affair a response. In the HelloWorld.doGet servlet method, for example, the acknowledgment commodity is acclimated to acquaint the server what agreeable blazon attack to issue. In this case, it’s response.setContentType(“text/html”);.
Finally, the affairs obtains a Java Writer commodity from the acknowledgment response.getWriter(). The Writer is again acclimated to actualize a simple HTML acknowledgment to acknowledgment to the browser.
The anatomy and cipher breeze in Advertisement 1 are adequately intuitive, but there is a audacious omission. How does the server apperceive to accessory the http://localhost:8080/examples/servlets/servlet/HelloWorldExample URL to the HelloWorld.doGet method?
You will acquisition the acknowledgment to this abstruseness in the appliance meta-data. Every Java web appliance includes a accepted meta-data file, alleged web.xml, which tells the server how to map URLs to servlets.
In the Tomcat archetype apps, the web.xml is begin at apache-tomcat-9.0.11webappsexamplesWEB-INFweb.xml. The WEB-INFweb.xml is the accepted area of the meta-data book for servlets. If you accessible this file, you’ll see how the server is configured.
The abbreviated web.xml in Advertisement 2 has aloof the advice we charge for our discussion.
Listing 2 shows a archetypal XML book attack referencing the action for a Java web appliance descriptor. This is followed by two entries: <servlet> and<servlet-mapping>.
The alarm to <servlet> assigns a analytic name, HelloWorldExample, to the HelloWorldExample class, via the <servlet-name> and <servlet-class> fields.
The alarm to <servlet-mapping> assigns that analytic name to the <url-pattern> value, thereby advertence the cipher to the URL.
Note that the <url-pattern> acreage supports agrarian cards in adjustment to handle a array of adjustable URL mappings.
In accession to URL mapping, servlets accommodate added capabilities for clarification and authentication. Filters are acclimated for processing requests, and affidavit is acclimated to accredit simple users and roles to URL patterns. The JavaServer Pages (JSP) blueprint provides abutment for breeding HTML in a abundant added able way.
This commodity has been a conceptual overview of Java servlets, including URL appeal and acknowledgment administration central a Java server. Compassionate these basal elements of server-side Java will advice you accommodate added avant-garde concepts such as server advance and runtime analysis of URL mappings, which are new in Servlet 4.0.
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