Mercurial > hg > openjdk > jdk9 > jdk
view src/java.instrument/share/classes/java/lang/instrument/package-info.java @ 17234:f8b19df2115a
8181087: Module system implementation refresh (6/2017)
Reviewed-by: plevart, mchung
Contributed-by: alan.bateman@oracle.com, alex.buckley@oracle.com
author | alanb |
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date | Fri, 16 Jun 2017 09:20:39 -0700 |
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/* * Copyright (c) 2003, 2017, 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 * or visit www.oracle.com if you need additional information or have any * questions. */ /* * 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. * * * <h3>Starting an Agent from the Command-Line Interface</h3> * * <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}. * * * <h3>Starting an Agent After VM Startup</h3> * * <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). * * * <h3>Including an Agent in an Executable JAR file</h3> * * <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. * * * <h3> Loading agent classes and the modules/classes available to the agent * class </h3> * * <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. * * <h3>Manifest Attributes</h3> * * <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). * * * <h3>Instrumenting code in modules</h3> * * <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;