| author | sspitsyn |
| Mon, 12 Sep 2016 15:00:58 -0700 | |
| changeset 40818 | 9c2079cba606 |
| parent 36511 | 9d0388c6b336 |
| child 42338 | a60f280f803c |
| permissions | -rw-r--r-- |
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<!-- |
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Copyright (c) 2003, 2016, Oracle and/or its affiliates. All rights reserved. |
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DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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This code is free software; you can redistribute it and/or modify it |
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under the terms of the GNU General Public License version 2 only, as |
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published by the Free Software Foundation. Oracle designates this |
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particular file as subject to the "Classpath" exception as provided |
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by Oracle in the LICENSE file that accompanied this code. |
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This code is distributed in the hope that it will be useful, but WITHOUT |
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ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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version 2 for more details (a copy is included in the LICENSE file that |
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accompanied this code). |
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You should have received a copy of the GNU General Public License version |
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2 along with this work; if not, write to the Free Software Foundation, |
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Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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or visit www.oracle.com if you need additional information or have any |
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questions. |
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--> |
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"> |
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<html> |
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<head> |
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<!-- |
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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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An implementation is not required to provide a way to start agents from the |
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command-line interface. On implementations that do provide a way to start agents |
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from the command-line interface, an agent is started by adding this option to |
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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 system class loader must support a mechanism to add an agent JAR file to the system class path. |
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If it is a custom system class loader then it must define the |
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<code>appendToClassPathForInstrumentation</code> method as specified in |
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{@link Instrumentation#appendToSystemClassLoaderSearch appendToSystemClassLoaderSearch}.
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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. |
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If it is a custom system class loader then it must define the |
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<code>appendToClassPathForInstrumentation</code> method as specified in |
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{@link Instrumentation#appendToSystemClassLoaderSearch appendToSystemClassLoaderSearch}.</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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<h3>Instrumenting code in modules</h3> |
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Agents that instrument code in named modules may need to arrange for the |
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modules to read other modules. If code is instrumented to invoke a method |
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in a support class in another module, then the module of the instrumented |
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code should read the module of the supporting class. Furthermore, the |
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supporting class will only be accessible to the instrumented code if |
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it is <code>public</code> and in a package that is exported by its module. |
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Agents can use {@link Instrumentation#addModuleReads addModuleReads} to update
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a module to read another. |
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<p> |
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As an aid to agents that deploy supporting classes on the search path of the |
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bootstrap class loader, or the search path of the class loader that loads |
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the main agent class, the Java virtual machine arranges for the module of |
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transformed classes to read the unnamed module of both class loaders. |
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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 1.5 |
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@revised 1.6 |
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</body> |
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</html> |