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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
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Copyright 2003 Wily Technology, Inc.
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-->
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</head>
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<body bgcolor="white">
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Provides services that allow Java programming language agents to instrument programs running on the JVM.
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The mechanism for instrumentation is modification of the byte-codes of methods.
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<h2>Package Specification</h2>
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<P>
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An agent is deployed as a JAR file. An attribute in the JAR file manifest specifies the
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agent class which will be loaded to start the agent. For implementations that support a command-line
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interface, an agent is started by specifying an option on the command-line.
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Implementations may also support a mechanism to start agents some time after the VM has
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started. For example, an implementation may provide a mechanism that allows a tool to
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<i>attach</i> to a running application, and initiate the loading of the tool's agent into
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the running application. The details as to how the load is initiated, is implementation
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dependent.
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<h3>Command-Line Interface</h3>
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<P>
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On implementations with a command-line interface, an agent is started by
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adding this option to the command-line:
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<blockquote>
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<code><b>-javaagent:</b></code><i>jarpath[</i><code><b>=</b></code><i>options]</i>
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</blockquote>
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<i>jarpath</i> is the path to the agent JAR file.
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<i>options</i> is the agent options.
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This switch may be used multiple times on the same command-line,
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thus creating multiple agents.
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More than one agent may use the same <i>jarpath</i>.
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An agent JAR file must conform to the JAR file specification.
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<P>
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The manifest of the agent JAR file must contain the attribute <code>Premain-Class</code>. The
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value of this attribute is the name of the <i>agent class</i>. The agent class must implement a
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public static <code>premain</code> method similar in principle to the <code>main</code> application
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entry point. After the Java Virtual Machine (JVM) has initialized, each <code>premain</code> method
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will be called in the order the agents were specified, then the real application
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<code>main</code> method will be called.
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Each <code>premain</code> method must return in order for the startup sequence to proceed.
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<P>
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The <code>premain</code> method has one of two possible signatures. The JVM first attempts to
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invoke the following method on the agent class:
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<blockquote>
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<code>public static void
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premain(String agentArgs, Instrumentation inst);
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</code>
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</blockquote>
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<P>
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If the agent class does not implement this method then the JVM will attempt to invoke:
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<blockquote>
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<code>public static void
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premain(String agentArgs);
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</code>
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</blockquote>
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<P>
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The agent class may also have an <code>agentmain</code> method for use when the agent is started
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after VM startup. When the agent is started using a command-line option, the <code>agentmain</code>
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method is not invoked.
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<P>
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The agent class will be loaded by the system class loader
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(see {@link java.lang.ClassLoader#getSystemClassLoader ClassLoader.getSystemClassLoader}). This is
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the class loader which typically loads the class containing the application <code>main</code> method.
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The <code>premain</code> methods will be run under the same security and classloader
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rules as the application <code>main</code> method.
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There are no modeling restrictions on what the agent <code>premain</code> method may do.
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Anything application <code>main</code> can do, including creating threads, is legal from <code>premain</code>.
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<P>
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Each agent is passed its agent options via the <code>agentArgs</code> parameter.
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The agent options are passed as a single string,
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any additional parsing should be performed by the agent itself.
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<P>
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If the agent cannot be resolved
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(for example, because the agent class cannot be loaded,
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or because the agent class does not have an appropriate <code>premain</code> method), the JVM will abort.
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If a <code>premain</code> method throws an uncaught exception, the JVM will abort.
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<h3>Starting Agents After VM Startup</h3>
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<p>
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An implementation may provide a mechanism to start agents sometime after the
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the VM has started. The details as to how this is initiated are implementation
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specific but typically the application has already started and its <code>
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main</code> method has already been invoked. In cases where an implementation
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supports the starting of agents after the VM has started the following applies:
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<ol>
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<li><p>The manifest of the agent JAR must contain the attribute <code>Agent-Class</code>.
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The value of this attribute is the name of the <i>agent class</i>. </p></li>
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<li><p>The agent class must implement a public static <code>agentmain</code> method. </p></li>
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<li><p>The system class loader (
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{@link java.lang.ClassLoader#getSystemClassLoader ClassLoader.getSystemClassLoader}) must
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support a mechanism to add an agent JAR file to the system class path. <p></li>
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</ol>
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<P>
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The agent JAR is appended to the system class path. This is the class loader that typically loads
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the class containing the application <code>main</code> method. The agent class is loaded and the
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JVM attempts to invoke the <code>agentmain</code> method. The JVM first attempts to invoke
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the following method on the agent class:
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<blockquote>
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<code>public static void
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agentmain(String agentArgs, Instrumentation inst);
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</code>
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</blockquote>
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<P>
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If the agent class does not implement this method then the JVM will attempt to invoke:
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<blockquote>
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<code>public static void
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agentmain(String agentArgs);
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</code>
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</blockquote>
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<P>
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The agent class may also have an <code>premain</code> method for use when the agent is started
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using a command-line option. When the agent is started after VM startup the <code>premain</code>
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method is not invoked.
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<P>
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The agent is passed its agent options via the <code>agentArgs</code> parameter.
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The agent options are passed as a single string,
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any additional parsing should be performed by the agent itself.
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<P>
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The <code>agentmain</code> method should do any necessary initialization
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required to start the agent. When startup is complete the method should
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return. If the agent cannot be started
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(for example, because the agent class cannot be loaded,
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or because the agent class does not have a conformant <code>agentmain</code> method), the JVM will
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not abort. If the <code>agentmain</code> method throws an uncaught exception it will be ignored.
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<h3>Manifest Attributes</h3>
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The following manifest attributes are defined for an agent JAR file:
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<blockquote>
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<dl>
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<dt><code>Premain-Class</code></dt>
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<dd>
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When an agent is specified at JVM launch time this attribute
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specifies the agent class.
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That is, the class containing the <code>premain</code> method.
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When an agent is specified at JVM launch time this attribute
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is required. If the attribute is not present the JVM will abort.
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Note: this is a class name, not a file name or path.
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</dd>
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<dt><code>Agent-Class</code></dt>
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<dd>
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If an implementation supports a mechanism to start agents
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sometime after the VM has started then this attribute specifies
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the agent class.
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That is, the class containing the <code>agentmain</code> method.
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This attribute is required, if it is not present the agent
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will not be started.
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Note: this is a class name, not a file name or path.
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</dd>
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<dt><code>Boot-Class-Path</code></dt>
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<dd>
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A list of paths to be searched by the bootstrap class
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loader. Paths represent directories or libraries
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(commonly referred to as JAR or zip libraries on
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many platforms).
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These paths are searched by the
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bootstrap class loader after the platform specific
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mechanisms of locating a class have failed.
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Paths are searched in the order listed.
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Paths in the list are separated by one or more spaces.
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A path takes the syntax of the path component of a
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hierarchical URI. The path is
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absolute if it begins with a slash character ('/'),
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otherwise it is relative. A relative path is resolved
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against the absolute path of the agent JAR file.
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Malformed and non-existent paths are ignored.
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When an agent is started sometime after the VM has
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started then paths that do not represent a JAR file
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are ignored.
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This attribute is optional.
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</dd>
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<dt><code>Can-Redefine-Classes</code></dt>
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<dd>
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Boolean (<code>true</code> or <code>false</code>, case irrelevant).
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Is the ability to redefine classes
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needed by this agent.
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Values other than <code>true</code> are considered <code>false</code>.
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This attribute is optional, the default is <code>false</code>.
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</dd>
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<dt><code>Can-Retransform-Classes</code></dt>
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<dd>
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Boolean (<code>true</code> or <code>false</code>, case irrelevant).
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Is the ability to retransform classes
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needed by this agent.
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Values other than <code>true</code> are considered <code>false</code>.
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This attribute is optional, the default is <code>false</code>.
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</dd>
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<dt><code>Can-Set-Native-Method-Prefix</code></dt>
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<dd>
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Boolean (<code>true</code> or <code>false</code>, case irrelevant).
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Is the ability to set native method prefix needed by this agent.
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Values other than <code>true</code> are considered <code>false</code>.
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This attribute is optional, the default is <code>false</code>.
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</dd>
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</dl>
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</blockquote>
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<p>
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An agent JAR file may have both the <code>Premain-Class</code> and <code>Agent-Class</code>
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attributes present in the manifest. When the agent is started on the command-line using
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the <code>-javaagent</code> option then the <code>Premain-Class</code> attribute
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specifies the name of the agent class and the <code>Agent-Class</code> attribute is
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ignored. Similarly, if the agent is started sometime after the VM has started, then
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the <code>Agent-Class</code> attribute specifies the name of the agent class
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(the value of <code>Premain-Class</code> attribute is ignored).
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<h2>Related Documentation</h2>
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For tool documentation, please see:
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<ul>
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<li><a href="{@docRoot}/../technotes/tools/index.html">JDK Tools and Utilities</a>
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</ul>
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@since JDK1.5
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@revised 1.6
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</body>
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</html>
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