108 lines
9.3 KiB
HTML
108 lines
9.3 KiB
HTML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html lang="en-us" xml:lang="en-us">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="DC.Type" content="concept" />
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<meta name="DC.Title" content="Java virtual machine" />
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<meta name="abstract" content="The Java virtual machine is a runtime environment that you can add into a web browser or any operating system, such as IBM i5/OS. The Java virtual machine runs instructions that a Java compiler generates. It consists of a bytecode interpreter and runtime that allow Java class files to run on any platform, regardless of the platform on which they were originally developed." />
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<meta name="description" content="The Java virtual machine is a runtime environment that you can add into a web browser or any operating system, such as IBM i5/OS. The Java virtual machine runs instructions that a Java compiler generates. It consists of a bytecode interpreter and runtime that allow Java class files to run on any platform, regardless of the platform on which they were originally developed." />
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<meta name="DC.Relation" scheme="URI" content="platform.htm" />
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<meta name="DC.Relation" scheme="URI" content="appsappl.htm" />
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<meta name="DC.Relation" scheme="URI" content="jarfile.htm" />
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<meta name="DC.Relation" scheme="URI" content="threads.htm" />
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<meta name="DC.Relation" scheme="URI" content="sunjdk.htm" />
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<meta name="copyright" content="(C) Copyright IBM Corporation 2006" />
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<meta name="DC.Rights.Owner" content="(C) Copyright IBM Corporation 2006" />
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<meta name="DC.Format" content="XHTML" />
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<meta name="DC.Identifier" content="javavirm" />
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title>Java virtual machine</title>
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</head>
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<body id="javavirm"><a name="javavirm"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Java virtual machine</h1>
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<div><p>The Java™ virtual machine is a runtime environment that you
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can add into a web browser or any operating system, such as IBM<sup>®</sup> <span class="keyword">i5/OS™</span>.
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The Java virtual
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machine runs instructions that a Java compiler generates. It consists of
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a bytecode interpreter and runtime that allow Java class files to run on any platform,
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regardless of the platform on which they were originally developed.</p>
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<p>The class loader and security manager, which is part of the Java runtime,
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insulate code that comes from another platform. They also can restrict which
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system resources each class that is loaded accesses.</p>
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<div class="note"><span class="notetitle">Note:</span> Java applications
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are not restricted; only applets are restricted. Applications can freely access
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system resources and use native methods. Most IBM Developer Kit for Java programs
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are applications.</div>
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<p>You can use the Create Java Program (CRTJVAPGM) command to ensure
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that the code meets the safety requirements that the Java runtime
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imposes to verify the bytecodes. This includes enforcing type restrictions,
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checking data conversions, ensuring that parameter stack overflows or underflows
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do not occur, and checking for access violations. However, you do not need
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to explicitly verify the bytecodes. If you do not use the CRTJVAPGM command
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in advance, then the checks occur during the first use of a class. Once the
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bytecodes are verified, the interpreter decodes the bytecodes and runs the
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machine instructions that are needed to carry out the desired operations.</p>
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<div class="note"><span class="notetitle">Note:</span> The <a href="#javavirm__jinterpreter">Java interpreter</a> is only used if you
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specify OPTIMIZE(*INTERPRET) or INTERPRET(*YES).</div>
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<p>In addition to loading and running the bytecodes, the Java virtual
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machine includes a garbage collector that manages memory. <a href="gc.htm">Garbage
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collection</a> runs at the same time as the loading and interpretation
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of the bytecodes.</p>
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<div class="section" id="javavirm__jruntimeenv"><a name="javavirm__jruntimeenv"><!-- --></a><h4 class="sectiontitle">Java runtime environment</h4><p>The Java runtime
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environment starts whenever you enter the Run Java (RUNJVA) command or JAVA command on
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the iSeries™ command
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line. Because the Java environment is multithreaded, it is necessary to
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run the Java virtual machine in a job that supports threads,
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such as a batch immediate (BCI) job. As illustrated in the following figure,
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after the Java virtual machine starts, additional threads may
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start in the job in which the garbage collector runs.</p>
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<p><strong>Figure 1: The
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typical Java environment when using the RUNJVA or JAVA CL command</strong><br /><a name="javavirm__rzaha502"><!-- --></a><img id="javavirm__rzaha502" src="rzaha502.gif" alt="This graphic shows
an example of starting the Java runtime environment by using the RUNJVA or
JAVA CL command on the iSeries command line." /><br /></p>
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<p>It is also
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possible to start the Java runtime environment by using the <samp class="codeph">java</samp> command
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in Qshell from the Qshell Interpreter. In this environment, the Qshell Interpreter
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is running in a BCI job that is associated with an interactive job. The Java runtime
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environment starts in the job that is running the Qshell Interpreter.</p>
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<p><strong>Figure
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2: The Java environment when using the java command in Qshell</strong><br /><a name="javavirm__rzaha504"><!-- --></a><img id="javavirm__rzaha504" src="rzaha504.gif" alt="This graphic shows
an example of starting the Java runtime environment by using the java command
in Qshell from the Qshell Interpreter." /><br /></p>
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<p>When the Java runtime
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environment starts from an interactive job, the Java Shell Display is shown. This display
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provides an input line for entering data into the System.in stream, as well
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as displaying data that is written to the System.out stream and System.err
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stream.</p>
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</div>
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<div class="section" id="javavirm__jinterpreter"><a name="javavirm__jinterpreter"><!-- --></a><h4 class="sectiontitle">Java interpreter</h4><p>The Java interpreter
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is the part of the Java virtual machine that interprets Java class
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files for a particular hardware platform. The Java interpreter decodes each bytecode and
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runs a series of machine instructions for that bytecode.</p>
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<div class="p">Related topics:<ul><li><a href="jarfile.htm">Java class files</a></li>
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<li></li>
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</ul>
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</div>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="platform.htm" title="The Java platform is the environment for developing and managing Java applets and applications. It consists of three primary components: the Java language, the Java packages, and the Java virtual machine.">Java platform</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="appsappl.htm" title="An applet is a Java program designed to be included in an HTML Web document. You can write your Java applet and include it in an HTML page, much in the same way an image is included. When you use a Java-enabled browser to view an HTML page that contains an applet, the applet's code is transferred to your system and is run by the browser's Java virtual machine.">Java applets and applications</a></div>
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<div><a href="jarfile.htm" title="A Java ARchive (JAR) file is a file format that combines many files into one. The Java environment differs from other programming environments in that the Java compiler does not generate machine code for a hardware-specific instruction set. Instead, the Java compiler converts Java source code into Java virtual machine instructions, which Java class files store. You can use JAR files to store class files. The class file does not target a specific hardware platform, but instead targets the Java virtual machine architecture.">Java JAR and class files</a></div>
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<div><a href="threads.htm" title="A thread is a single independent stream that runs within a program. Java is a multithreaded programming language, so more than one thread may be running within the Java virtual machine at one time. Java threads provide a way for a Java program to perform multiple tasks at the same time. A thread is essentially a flow of control in a program.">Java threads</a></div>
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<div><a href="sunjdk.htm" title="The Java Development Kit (JDK) is software that is distributed by Sun Microsystems, Inc. for Java developers. It includes the Java interpreter, Java classes, and Java development tools: compiler, debugger, disassembler, appletviewer, stub file generator, and documentation generator.">Sun Microsystems, Inc. Java Development Kit</a></div>
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</div>
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</div>
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</body>
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</html> |