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: * *
For implementations that support a command-line interface, an agent * can be started by specifying an option on the command-line.
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 attach to a running application, and * initiate the loading of the tool's agent into the running application.
An agent may be packaged with an application in an executable JAR * file.
Agents can transform classes in arbitrary ways at load time, transform * modules, or transform the bytecode of methods of already loaded classes. * Developers or administrators that deploy agents, deploy applications that * package an agent with the application, or use tools that load agents into a * running application, are responsible for verifying the trustworthiness of each * agent including the content and structure of the agent JAR file. * *
The three ways to start an agent are described below. * *
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: * *
{@code * -javaagent:* * where {@code[= ] * }
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 agent class. 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. * *
The {@code premain} method has one of two possible signatures. The * JVM first attempts to invoke the following method on the agent class: * *
{@code * public static void premain(String agentArgs, Instrumentation inst) * }* *
If the agent class does not implement this method then the JVM will * attempt to invoke: *
{@code * public static void premain(String agentArgs) * }*
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. * *
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. * *
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. * *
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 {@code
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}. * * *
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: * *
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 agent class.
The agent class must implement a public static {@code agentmain} * method.
The {@code agentmain} method has one of two possible signatures. The JVM * first attempts to invoke the following method on the agent class: * *
{@code * public static void agentmain(String agentArgs, Instrumentation inst) * }* *
If the agent class does not implement this method then the JVM will * attempt to invoke: * *
{@code * public static void agentmain(String agentArgs) * }* *
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. * *
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. * *
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). * * *
The JAR File Specification defines manifest attributes for standalone * applications that are packaged as executable JAR files. 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: * *
{@code * public static void agentmain(String agentArgs, Instrumentation inst) * }* *
If the agent class does not implement this method then the JVM will * attempt to invoke: * *
{@code * public static void agentmain(String agentArgs) * }* *
The value of the {@code agentArgs} parameter is always the empty string. * *
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. * * *
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. * *
The classes visible to the agent class are the classes visible to the system * class loader and minimally include: * *
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.
The classes that can be defined by the system class loader (typically * the class path) to be members of its unnamed module.
Any classes that the agent arranges to be defined by the bootstrap * class loader to be members of its unnamed module.
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.
* *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. * *
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. * *
The following manifest attributes are defined for an agent JAR file: * *
* ** *
- {@code Premain-Class}
*- 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.
* *- {@code Agent-Class}
*- 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.
* *- {@code Launcher-Agent-Class}
*- 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.
* *- {@code Boot-Class-Path}
*- 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.
* *- {@code Can-Redefine-Classes}
*- 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}.
* *- {@code Can-Retransform-Classes}
*- 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}.
* *- {@code Can-Set-Native-Method-Prefix}
*- 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}.
* *
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). * * *
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;