chap04.html

Inside the java virtualMachine,深入研究java虚拟机

</FONT><P><FONT FACE="Courier New">end</FONT></P>
<P>This &quot;applet&quot; tag provides enough information to enable the browser to display the applet. The <FONT FACE="Courier New">CODE</FONT> attribute indicates the name of the applet韘 starting class file, in this case: <FONT FACE="Courier New">HeapOfFish.class</FONT>. The <FONT FACE="Courier New">CODEBASE</FONT> attribute gives the location of the applet韘 class files relative to the base URL of the web page. The <FONT FACE="Courier New">WIDTH</FONT> and <FONT FACE="Courier New">HEIGHT</FONT> attributes indicate the size in pixels of the applet韘 panel, the visible portion of the applet that is displayed as part of the web page.</P>
<P>When a browser encounters a web page that includes an applet tag, it passes information from the tag to the running Java application. The Java application either creates a new class loader object object, or re-uses an existing one, to download the starting class file for the applet. It then initializes the applet, by invoking the <FONT FACE="Courier New">init()</FONT> method of the applet韘 starting class. The other class files for the applet are downloaded on an as needed basis, by the normal process of dynamic linking. For example, when a new class is first used by the applet韘 starting class, the symbolic reference to the new class must be resolved. During resolution, if the class has not already been loaded, the Java Virtual Machine will ask the same class loader object that loaded the applet韘 starting class to load the new class. If the class loader object is unable to load the class from the local trusted repository through the primordial class loader, the class loader object will attempt to download the class file across the network from the same location it retrieved the applet韘 starting class. Once initialization of the applet is complete, the applet appears as part of the web page inside the browser.</P>
<P>Network-mobile Java can take other forms besides Java applets, but the framework that supports other network-mobile forms will likely look very similar to the framework for applets. Like applets, other forms of network-mobile Java code will in general be implemented as special flavors of Java program running in the context of a host Java application. Network-mobile class files will generally be loaded by class loader objects for two reasons. The first reason is simply that a class loader object can download class files across a network in custom ways that a primordial class loader can韙. But the second reason involves security. Because network-mobile class files are not always known to be trustworthy, the separate name-spaces provided by class loader objects are needed to protect a malicious applet from interfering with applets loaded from other sources. Also because network-mobile class files can韙 always be trusted, there will generally be a security manager establishing a sandbox for the network-mobile code.</P>
<H3><EM><P>The Resources Page</P>
</EM></H3><P>For links to information about other examples of network-mobile Java, such as Marimba Castanet Channels, Jeeves Servlets, and Aglets (Java-based autonomous software agents), see the resource page for this chapter: <FONT FACE="Courier New"><A HREF="http://www.artima.com/insidejvm/mobility.html" tppabs="http://www.artima.com/insidejvm/mobility.html">http://www.artima.com/insidejvm/mobility.html</A></FONT>.</P>
<TABLE BORDER="0" WIDTH="100%">
<TR><TD><A HREF="http://www.pbg.mcgraw-hill.com/betabooks/stores.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/stores.html" target="bottom"><IMG SRC="hotkey.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/hotkey.gif" ALIGN="LEFT" BORDER="0" WIDTH="40" HEIGHT="40" ALT="Orders"></A>
<IMG SRC="order_text.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/order_text.gif" WIDTH="103" HEIGHT="41" ALT="Orders"></TD>
<TD ALIGN="RIGHT"><A HREF="chap03.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/venners/chap03.html"><IMG SRC="backward.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/backward.gif" BORDER="0" ALT="Backward" WIDTH="32" HEIGHT="32"></A>&nbsp;<A HREF="chap05.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/venners/chap05.html"><IMG SRC="forward.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/forward.gif" BORDER="0" ALT="Forward" WIDTH="32" HEIGHT="32"></A></TD></TR>
<TR><TD COLSPAN="2"><A HREF="mailto:[e-mail]"><IMG SRC="hotkey.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/hotkey.gif" ALIGN="LEFT" BORDER="0" WIDTH="40" HEIGHT="40" ALT="Comments"></A>
<IMG SRC="comment_text.gif" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/images/comment_text.gif" WIDTH="73" HEIGHT="39" ALT="Comments"></TD></TR>

</TABLE>
<HR>
<P ALIGN=CENTER>&nbsp;<A HREF="http://www.pbg.mcgraw-hill.com/computing/computing-home.html" tppabs="http://www.pbg.mcgraw-hill.com/computing/computing-home.html" TARGET="_top">COMPUTING
MCGRAW-HILL</A> | <A HREF="http://www.pbg.mcgraw-hill.com/betabooks/betabooks-home.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/betabooks-home.html" TARGET="_top">Beta Books</A>
| <A HREF="http://www.pbg.mcgraw-hill.com/computing/contact.html" tppabs="http://www.pbg.mcgraw-hill.com/computing/contact.html" TARGET="_top">Contact Us</A>
| <A HREF="http://www.pbg.mcgraw-hill.com/betabooks/stores.html" tppabs="http://www.pbg.mcgraw-hill.com/betabooks/stores.html" TARGET="_top">Order Information</A>
| <A HREF="http://mcgraw-hill.inforonics.com/compsearch.shtml" tppabs="http://mcgraw-hill.inforonics.com/compsearch.shtml" TARGET="_top">Online Catalog</A></P>

<P ALIGN=CENTER><FONT SIZE="-1"><A HREF="http://www.pbg.mcgraw-hill.com/computing/computing-home.html" tppabs="http://www.pbg.mcgraw-hill.com/computing/computing-home.html" TARGET="_top">Computing McGraw-Hill</A> is an imprint of the <A HREF="http://www.pbg.mcgraw-hill.com/pbg-home.html" tppabs="http://www.pbg.mcgraw-hill.com/pbg-home.html" TARGET="_top">McGraw-Hill Professional Book Group</A>.</FONT></P>

<!-- begin footer -->
<HR>
<A HREF="http://www.mcgraw-hill.com/" tppabs="http://www.mcgraw-hill.com/" TARGET="_top"><IMG SRC="division-white.gif" tppabs="http://www.pbg.mcgraw-hill.com/images/division-white.gif" WIDTH="350" HEIGHT="44" ALT="A Division of the McGraw-Hill Companies" BORDER="0"></A><BR>
<FONT SIZE="-2">Copyright &copy; 1997 <A HREF="http://www.mcgraw-hill.com/" tppabs="http://www.mcgraw-hill.com/" TARGET="_top">
The McGraw-Hill Companies</A>. All rights reserved. Any use is subject to the 
<A HREF="http://www.mcgraw-hill.com/corporate/news_info/copyrttm.htm" tppabs="http://www.mcgraw-hill.com/corporate/news_info/copyrttm.htm" TARGET="_top">
Terms of Use</A>; the corporation also has a comprehensive <A HREF="http://www.mcgraw-hill.com/corporate/news_info/privacy.html" tppabs="http://www.mcgraw-hill.com/corporate/news_info/privacy.html" TARGET="_top">
Privacy Policy</A> governing information we may collect from our customers.</FONT>
<!-- end footer -->
</BODY>
</HTML>