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<?xml version="1.0" encoding="ISO-8859-1"?><!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1" /> <meta http-equiv="Content-Style-Type" content="text/css" /> <title>Distributed Multitiered Applications</title> <link rel="StyleSheet" href="document.css" type="text/css" media="all" /> <link rel="StyleSheet" href="catalog.css" type="text/css" media="all" /> <link rel="Table of Contents" href="J2EETutorialTOC.html" /> <link rel="Previous" href="Overview.html" /> <link rel="Next" href="Overview3.html" /> <link rel="Index" href="J2EETutorialIX.html" /> </head> <body> <table width="550" summary="layout" id="SummaryNotReq1"> <tr> <td align="left" valign="center"> <font size="-1"> <a href="http://java.sun.com/j2ee/1.4/download.html#tutorial" target="_blank">Download</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/faq.html" target="_blank">FAQ</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/history.html" target="_blank">History</a> </td> <td align="center" valign="center"><a accesskey="p" href="Overview.html"><img id="LongDescNotReq1" src="images/PrevArrow.gif" width="26" height="26" border="0" alt="Prev" /></a><a accesskey="c" href="J2EETutorialFront.html"><img id="LongDescNotReq1" src="images/UpArrow.gif" width="26" height="26" border="0" alt="Home" /></a><a accesskey="n" href="Overview3.html"><img id="LongDescNotReq3" src="images/NextArrow.gif" width="26" height="26" border="0" alt="Next" /></a><a accesskey="i" href="J2EETutorialIX.html"></a> </td> <td align="right" valign="center"> <font size="-1"> <a href="http://java.sun.com/j2ee/1.4/docs/api/index.html" target="_blank">API</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/search.html" target="_blank">Search</a> <br> <a href="http://java.sun.com/j2ee/1.4/docs/tutorial/information/sendusmail.html" target="_blank">Feedback</a></font> </font> </td> </tr> </table> <img src="images/blueline.gif" width="550" height="8" ALIGN="BOTTOM" NATURALSIZEFLAG="3" ALT="Divider"> <blockquote><a name="wp81104"> </a><h2 class="pHeading1">Distributed Multitiered Applications</h2><a name="wp81293"> </a><p class="pBody">The J2EE platform uses a multitiered distributed application model for enterprise applications. Application logic is divided into components according to function, and the various application components that make up a J2EE application are installed on different machines depending on the tier in the multitiered J2EE environment to which the application component belongs. <a href="Overview2.html#wp79743">Figure 1-1</a> shows two multitiered J2EE applications divided into the tiers described in the following list. The J2EE application parts shown in <a href="Overview2.html#wp79743">Figure 1-1</a> are presented in <a href="Overview2.html#wp79745">J2EE Components</a>.</p><div class="pSmartList1"><ul class="pSmartList1"><a name="wp79728"> </a><div class="pSmartList1"><li>Client-tier components run on the client machine.</li></div><a name="wp79729"> </a><div class="pSmartList1"><li>Web-tier components run on the J2EE server.</li></div><a name="wp79730"> </a><div class="pSmartList1"><li>Business-tier components run on the J2EE server.</li></div><a name="wp79731"> </a><div class="pSmartList1"><li>Enterprise information system (EIS)-tier software runs on the EIS server.</li></div></ul></div><a name="wp79735"> </a><p class="pBody">Although a J2EE application can consist of the three or four tiers shown in <a href="Overview2.html#wp79743">Figure 1-1</a>, J2EE multitiered applications are generally considered to be three-tiered applications because they are distributed over three different locations: client machines, the J2EE server machine, and the database or legacy machines at the back end. Three-tiered applications that run in this way extend the standard two-tiered client and server model by placing a multithreaded application server between the client application and back-end storage.</p><div align="left"><img src="images/Fig1.gif" height="261" width="360" alt="Multitiered Applications" border="0" hspace="0" vspace="0"/></div><p class="pBody"></p><p> <a name="79743"> </a><strong><font >Figure 1-1 Multitiered Applications</font></strong></p><a name="wp79745"> </a><h3 class="pHeading2">J2EE Components</h3><a name="wp79747"> </a><p class="pBody">J2EE applications are made up of components. A <span style="font-style: italic">J2EE component</span> is a self-contained functional software unit that is assembled into a J2EE application with its related classes and files and that communicates with other components. The J2EE specification defines the following J2EE components:</p><div class="pSmartList1"><ul class="pSmartList1"><a name="wp79749"> </a><div class="pSmartList1"><li>Application clients and applets are components that run on the client.</li></div><a name="wp79750"> </a><div class="pSmartList1"><li>Java Servlet and JavaServer Pages<img src="images/tm.gif" border="0" alt="Trademarked"> (JSP<img src="images/tm.gif" border="0" alt="Trademarked">) technology components are Web components that run on the server. </li></div><a name="wp79751"> </a><div class="pSmartList1"><li>Enterprise JavaBeans<img src="images/tm.gif" border="0" alt="Trademarked"> (EJB<img src="images/tm.gif" border="0" alt="Trademarked">) components (enterprise beans) are business components that run on the server.</li></div></ul></div><a name="wp79752"> </a><p class="pBody">J2EE components are written in the Java programming language and are compiled in the same way as any program in the language. The difference between J2EE components and "standard" Java classes is that J2EE components are assembled into a J2EE application, verified to be well formed and in compliance with the J2EE specification, and deployed to production, where they are run and managed by the J2EE server.</p><a name="wp79753"> </a><h3 class="pHeading2">J2EE Clients</h3><a name="wp79755"> </a><p class="pBody">A J2EE client can be a Web client or an application client.</p><a name="wp79756"> </a><h4 class="pHeading3">Web Clients</h4><a name="wp79758"> </a><p class="pBody">A Web client consists of two parts: dynamic Web pages containing various types of markup language (HTML, XML, and so on), which are generated by Web components running in the Web tier, and a Web browser, which renders the pages received from the server.</p><a name="wp79759"> </a><p class="pBody">A Web client is sometimes called a <span style="font-style: italic">thin client</span>. Thin clients usually do not do things like query databases, execute complex business rules, or connect to legacy applications. When you use a thin client, heavyweight operations like these are off-loaded to enterprise beans executing on the J2EE server where they can leverage the security, speed, services, and reliability of J2EE server-side technologies.</p><a name="wp79761"> </a><h4 class="pHeading3">Applets</h4><a name="wp79763"> </a><p class="pBody">A Web page received from the Web tier can include an embedded applet. An applet is a small client application written in the Java programming language that executes in the Java virtual machine installed in the Web browser. However, client systems will likely need the Java Plug-in and possibly a security policy file in order for the applet to successfully execute in the Web browser.</p><a name="wp79764"> </a><p class="pBody">Web components are the preferred API for creating a Web client program because no plug-ins or security policy files are needed on the client systems. Also, Web components enable cleaner and more modular application design because they provide a way to separate applications programming from Web page design. Personnel involved in Web page design thus do not need to understand Java programming language syntax to do their jobs.</p><a name="wp79765"> </a><h4 class="pHeading3">Application Clients</h4><a name="wp79767"> </a><p class="pBody">A J2EE application client runs on a client machine and provides a way for users to handle tasks that require a richer user interface than can be provided by a markup language. It typically has a graphical user interface (GUI) created from Swing or Abstract Window Toolkit (AWT) APIs, but a command-line interface is certainly possible.</p>⌨️ 快捷键说明
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