/*

 * Copyright (c) 2003, 2019, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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/*

 * Copyright 2003 Wily Technology, Inc.

 */

/**

 * Provides services that allow Java programming language agents to instrument

 * programs running on the JVM. The mechanism for instrumentation is modification

 * of the byte-codes of methods.

 *

 * <p> An agent is deployed as a JAR file. An attribute in the JAR file manifest

 * specifies the agent class which will be loaded to start the agent. Agents can

 * be started in several ways:

 *

 * <ol>

 *   <li><p> For implementations that support a command-line interface, an agent

 *   can be started by specifying an option on the command-line. </p></li>

 *

 *   <li><p> An implementation may support a mechanism to start agents some time

 *   after the VM has started. For example, an implementation may provide a

 *   mechanism that allows a tool to <i>attach</i> to a running application, and

 *   initiate the loading of the tool's agent into the running application. </p></li>

 *

 *   <li><p> An agent may be packaged with an application in an executable JAR

 *   file.</p></li>

 * </ol>

 *

 * <p> Each of these ways to start an agent is described below.

 *

 *

 * <h2>Starting an Agent from the Command-Line Interface</h2>

 *

 * <p> Where an implementation provides a means to start agents from the

 * command-line interface, an agent is started by adding the following option

 * to the command-line:

 *

 * <blockquote>{@code

 *     -javaagent:<jarpath>[=<options>]

 * }</blockquote>

 *

 * where <i>{@code <jarpath>}</i> is the path to the agent JAR file and

 * <i>{@code <options>}</i> is the agent options.

 *

 * <p> The manifest of the agent JAR file must contain the attribute {@code

 * Premain-Class} in its main manifest. The value of this attribute is the

 * name of the <i>agent class</i>. The agent class must implement a public

 * static {@code premain} method similar in principle to the {@code main}

 * application entry point. After the Java Virtual Machine (JVM) has

 * initialized, the {@code premain} method will be called, then the real

 * application {@code main} method. The {@code premain} method must return

 * in order for the startup to proceed.

 *

 * <p> The {@code premain} method has one of two possible signatures. The

 * JVM first attempts to invoke the following method on the agent class:

 *

 * <blockquote>{@code

 *     public static void premain(String agentArgs, Instrumentation inst)

 * }</blockquote>

 *

 * <p> If the agent class does not implement this method then the JVM will

 * attempt to invoke:

 * <blockquote>{@code

 *     public static void premain(String agentArgs)

 * }</blockquote>

 * <p> The agent class may also have an {@code agentmain} method for use when

 * the agent is started after VM startup (see below). When the agent is started

 * using a command-line option, the {@code agentmain} method is not invoked.

 *

 * <p> Each agent is passed its agent options via the {@code agentArgs} parameter.

 * The agent options are passed as a single string, any additional parsing

 * should be performed by the agent itself.

 *

 * <p> If the agent cannot be started (for example, because the agent class

 * cannot be loaded, or because the agent class does not have an appropriate

 * {@code premain} method), the JVM will abort. If a {@code premain} method

 * throws an uncaught exception, the JVM will abort.

 *

 * <p> An implementation is not required to provide a way to start agents

 * from the command-line interface. When it does, then it supports the

 * {@code -javaagent} option as specified above. The {@code -javaagent} option

 * may be used multiple times on the same command-line, thus starting multiple

 * agents. The {@code premain} methods will be called in the order that the

 * agents are specified on the command line. More than one agent may use the

 * same <i>{@code <jarpath>}</i>.

 *

 * <p> There are no modeling restrictions on what the agent {@code premain}

 * method may do. Anything application {@code main} can do, including creating

 * threads, is legal from {@code premain}.

 *

 *

 * <h2>Starting an Agent After VM Startup</h2>

 *

 * <p> An implementation may provide a mechanism to start agents sometime after

 * the the VM has started. The details as to how this is initiated are

 * implementation specific but typically the application has already started and

 * its {@code main} method has already been invoked. In cases where an

 * implementation supports the starting of agents after the VM has started the

 * following applies:

 *

 * <ol>

 *

 *   <li><p> The manifest of the agent JAR must contain the attribute {@code

 *   Agent-Class} in its main manfiest. The value of this attribute is the name

 *   of the <i>agent class</i>. </p></li>

 *

 *   <li><p> The agent class must implement a public static {@code agentmain}

 *   method. </p></li>

 *

 * </ol>

 *

 * <p> The {@code agentmain} method has one of two possible signatures. The JVM

 * first attempts to invoke the following method on the agent class:

 *

 * <blockquote>{@code

 *     public static void agentmain(String agentArgs, Instrumentation inst)

 * }</blockquote>

 *

 * <p> If the agent class does not implement this method then the JVM will

 * attempt to invoke:

 *

 * <blockquote>{@code

 *     public static void agentmain(String agentArgs)

 * }</blockquote>

 *

 * <p> The agent class may also have a {@code premain} method for use when the

 * agent is started using a command-line option. When the agent is started after

 * VM startup the {@code premain} method is not invoked.

 *

 * <p> The agent is passed its agent options via the {@code agentArgs}

 * parameter. The agent options are passed as a single string, any additional

 * parsing should be performed by the agent itself.

 *

 * <p> The {@code agentmain} method should do any necessary initialization

 * required to start the agent. When startup is complete the method should

 * return. If the agent cannot be started (for example, because the agent class

 * cannot be loaded, or because the agent class does not have a conformant

 * {@code agentmain} method), the JVM will not abort. If the {@code agentmain}

 * method throws an uncaught exception it will be ignored (but may be logged

 * by the JVM for troubleshooting purposes).

 *

 *

 * <h2>Including an Agent in an Executable JAR file</h2>

 *

 * <p> The JAR File Specification defines manifest attributes for standalone

 * applications that are packaged as <em>executable JAR files</em>. If an

 * implementation supports a mechanism to start an application as an executable

 * JAR then the main manifest may include the {@code Launcher-Agent-Class}

 * attribute to specify the class name of an agent to start before the application

 * {@code main} method is invoked. The Java virtual machine attempts to

 * invoke the following method on the agent class:

 *

 * <blockquote>{@code

 *     public static void agentmain(String agentArgs, Instrumentation inst)

 * }</blockquote>

 *

 * <p> If the agent class does not implement this method then the JVM will

 * attempt to invoke:

 *

 * <blockquote>{@code

 *     public static void agentmain(String agentArgs)

 * }</blockquote>

 *

 * <p> The value of the {@code agentArgs} parameter is always the empty string.

 *

 * <p> The {@code agentmain} method should do any necessary initialization

 * required to start the agent and return. If the agent cannot be started, for

 * example the agent class cannot be loaded, the agent class does not define a

 * conformant {@code agentmain} method, or the {@code agentmain} method throws

 * an uncaught exception or error, the JVM will abort.

 *

 *

 * <h2> Loading agent classes and the modules/classes available to the agent

 * class </h2>

 *

 * <p> Classes loaded from the agent JAR file are loaded by the

 * {@linkplain ClassLoader#getSystemClassLoader() system class loader} and are

 * members of the system class loader's {@linkplain ClassLoader#getUnnamedModule()

 * unnamed module}. The system class loader typically defines the class containing

 * the application {@code main} method too.

 *

 * <p> The classes visible to the agent class are the classes visible to the system

 * class loader and minimally include:

 *

 * <ul>

 *

 *   <li><p> The classes in packages exported by the modules in the {@linkplain

 *   ModuleLayer#boot() boot layer}. Whether the boot layer contains all platform

 *   modules or not will depend on the initial module or how the application was

 *   started. </p></li>

 *

 *   <li><p> The classes that can be defined by the system class loader (typically

 *   the class path) to be members of its unnamed module. </p></li>

 *

 *   <li><p> Any classes that the agent arranges to be defined by the bootstrap

 *   class loader to be members of its unnamed module. </p></li>

 *

 * </ul>

 *

 * <p> If agent classes need to link to classes in platform (or other) modules

 * that are not in the boot layer then the application may need to be started in

 * a way that ensures that these modules are in the boot layer. In the JDK

 * implementation for example, the {@code --add-modules} command line option can

 * be used to add modules to the set of root modules to resolve at startup. </p>

 *

 * <p> Supporting classes that the agent arranges to be loaded by the bootstrap

 * class loader (by means of {@link Instrumentation#appendToBootstrapClassLoaderSearch

 * appendToBootstrapClassLoaderSearch} or the {@code Boot-Class-Path} attribute

 * specified below), must link only to classes defined to the bootstrap class loader.

 * There is no guarantee that all platform classes can be defined by the boot

 * class loader.

 *

 * <p> If a custom system class loader is configured (by means of the system property

 * {@code java.system.class.loader} as specified in the {@link

 * ClassLoader#getSystemClassLoader() getSystemClassLoader} method) then it must

 * define the {@code appendToClassPathForInstrumentation} method as specified in

 * {@link Instrumentation#appendToSystemClassLoaderSearch appendToSystemClassLoaderSearch}.

 * In other words, a custom system class loader must support the mechanism to

 * add an agent JAR file to the system class loader search.

 *

 * <h2>Manifest Attributes</h2>

 *

 * <p> The following manifest attributes are defined for an agent JAR file:

 *

 * <blockquote><dl>

 *

 * <dt>{@code Premain-Class}</dt>

 * <dd> When an agent is specified at JVM launch time this attribute specifies

 * the agent class. That is, the class containing the {@code premain} method.

 * When an agent is specified at JVM launch time this attribute is required. If

 * the attribute is not present the JVM will abort. Note: this is a class name,

 * not a file name or path. </dd>

 *

 * <dt>{@code Agent-Class}</dt>

 * <dd> If an implementation supports a mechanism to start agents sometime after

 * the VM has started then this attribute specifies the agent class. That is,

 * the class containing the {@code agentmain} method. This attribute is required

 * if it is not present the agent will not be started. Note: this is a class name,

 * not a file name or path. </dd>

 *

 * <dt>{@code Launcher-Agent-Class}</dt>

 * <dd> If an implementation supports a mechanism to start an application as an

 * executable JAR then the main manifest may include this attribute to specify

 * the class name of an agent to start before the application {@code main}

 * method is invoked. </dd>

 *

 * <dt>{@code Boot-Class-Path}</dt>

 * <dd> A list of paths to be searched by the bootstrap class loader. Paths

 * represent directories or libraries (commonly referred to as JAR or zip

 * libraries on many platforms). These paths are searched by the bootstrap class

 * loader after the platform specific mechanisms of locating a class have failed.

 * Paths are searched in the order listed. Paths in the list are separated by one

 * or more spaces. A path takes the syntax of the path component of a hierarchical

 * URI. The path is absolute if it begins with a slash character ('/'), otherwise

 * it is relative. A relative path is resolved against the absolute path of the

 * agent JAR file. Malformed and non-existent paths are ignored. When an agent is

 * started sometime after the VM has started then paths that do not represent a

 * JAR file are ignored. This attribute is optional. </dd>

 *

 * <dt>{@code Can-Redefine-Classes}</dt>

 * <dd> Boolean ({@code true} or {@code false}, case irrelevant). Is the ability

 * to redefine classes needed by this agent. Values other than {@code true} are

 * considered {@code false}. This attribute is optional, the default is {@code

 * false}. </dd>

 *

 * <dt>{@code Can-Retransform-Classes}</dt>

 * <dd> Boolean ({@code true} or {@code false}, case irrelevant). Is the ability

 * to retransform classes needed by this agent. Values other than {@code true}

 * are considered {@code false}. This attribute is optional, the default is

 * {@code false}. </dd>

 *

 * <dt>{@code Can-Set-Native-Method-Prefix}</dt>

 * <dd> Boolean ({@code true} or {@code false}, case irrelevant). Is the ability

 * to set native method prefix needed by this agent. Values other than {@code

 * true} are considered {@code false}. This attribute is optional, the default

 * is {@code false}. </dd>

 *

 * </dl></blockquote>

 *

 * <p> An agent JAR file may have both the {@code Premain-Class} and {@code

 * Agent-Class} attributes present in the manifest. When the agent is started

 * on the command-line using the {@code -javaagent} option then the {@code

 * Premain-Class} attribute specifies the name of the agent class and the {@code

 * Agent-Class} attribute is ignored. Similarly, if the agent is started sometime

 * after the VM has started, then the {@code Agent-Class} attribute specifies

 * the name of the agent class (the value of {@code Premain-Class} attribute is

 * ignored).

 *

 *

 * <h2>Instrumenting code in modules</h2>

 *

 * <p> As an aid to agents that deploy supporting classes on the search path of

 * the bootstrap class loader, or the search path of the class loader that loads

 * the main agent class, the Java virtual machine arranges for the module of

 * transformed classes to read the unnamed module of both class loaders.

 *

 * @since 1.5

 * @revised 1.6

 * @revised 9

 */

package java.lang.instrument;