Module java.instrument

# Package java.lang.instrument

package java.lang.instrument
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.

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:

1. For implementations that support a command-line interface, an agent can be started by specifying an option on the command-line.

2. 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.

3. 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.

## Starting an Agent from the Command-Line Interface

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:

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

where `<jarpath>` is the path to the agent JAR file and `<options>` is the agent options.

The manifest of the agent JAR file must contain the attribute `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 `premain` method similar in principle to the `main` application entry point. After the Java Virtual Machine (JVM) has initialized, the `premain` method will be called, then the real application `main` method. The `premain` method must return in order for the startup to proceed.

The `premain` method has one of two possible signatures. The JVM first attempts to invoke the following method on the agent class:

> `public static void premain(String agentArgs, Instrumentation inst)`

If the agent class does not implement this method then the JVM will attempt to invoke:

> `public static void premain(String agentArgs)`

The agent class may also have an `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 `agentmain` method is not invoked.

Each agent is passed its agent options via the `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 `premain` method), the JVM will abort. If a `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 `-javaagent` option as specified above. The `-javaagent` option may be used multiple times on the same command-line, thus starting multiple agents. The `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 `<jarpath>`.

There are no modeling restrictions on what the agent `premain` method may do. Anything application `main` can do, including creating threads, is legal from `premain`.

## Starting an Agent After VM Startup

An implementation may provide a mechanism to start agents sometime after the VM has started. The details as to how this is initiated are implementation specific but typically the application has already started and its `main` method has already been invoked. In cases where an implementation supports the starting of agents after the VM has started the following applies:

1. The manifest of the agent JAR must contain the attribute `Agent-Class` in its main manfiest. The value of this attribute is the name of the agent class.

2. The agent class must implement a public static `agentmain` method.

The `agentmain` method has one of two possible signatures. The JVM first attempts to invoke the following method on the agent class:

> `public static void agentmain(String agentArgs, Instrumentation inst)`

If the agent class does not implement this method then the JVM will attempt to invoke:

> `public static void agentmain(String agentArgs)`

The agent class may also have a `premain` method for use when the agent is started using a command-line option. When the agent is started after VM startup the `premain` method is not invoked.

The agent is passed its agent options via the `agentArgs` parameter. The agent options are passed as a single string, any additional parsing should be performed by the agent itself.

The `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 `agentmain` method), the JVM will not abort. If the `agentmain` method throws an uncaught exception it will be ignored (but may be logged by the JVM for troubleshooting purposes).

## Including an Agent in an Executable JAR file

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 `Launcher-Agent-Class` attribute to specify the class name of an agent to start before the application `main` method is invoked. The Java virtual machine attempts to invoke the following method on the agent class:

> `public static void agentmain(String agentArgs, Instrumentation inst)`

If the agent class does not implement this method then the JVM will attempt to invoke:

> `public static void agentmain(String agentArgs)`

The value of the `agentArgs` parameter is always the empty string.

The `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 `agentmain` method, or the `agentmain` method throws an uncaught exception or error, the JVM will abort.

## Loading agent classes and the modules/classes available to the agent class

Classes loaded from the agent JAR file are loaded by the system class loader and are members of the system class loader's unnamed module. The system class loader typically defines the class containing the application `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 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 `--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 `appendToBootstrapClassLoaderSearch` or the `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 `java.system.class.loader` as specified in the `getSystemClassLoader` method) then it must define the `appendToClassPathForInstrumentation` method as specified in `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.

## Manifest Attributes

The following manifest attributes are defined for an agent JAR file:

> `Premain-Class`
>   When an agent is specified at JVM launch time this attribute specifies the agent class. That is, the class containing the `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.
> `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 `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.
> `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 `main` method is invoked.
> `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.
> `Can-Redefine-Classes`
>   Boolean (`true` or `false`, case irrelevant). Is the ability to redefine classes needed by this agent. Values other than `true` are considered `false`. This attribute is optional, the default is `false`.
> `Can-Retransform-Classes`
>   Boolean (`true` or `false`, case irrelevant). Is the ability to retransform classes needed by this agent. Values other than `true` are considered `false`. This attribute is optional, the default is `false`.
> `Can-Set-Native-Method-Prefix`
>   Boolean (`true` or `false`, case irrelevant). Is the ability to set native method prefix needed by this agent. Values other than `true` are considered `false`. This attribute is optional, the default is `false`.

An agent JAR file may have both the `Premain-Class` and `Agent-Class` attributes present in the manifest. When the agent is started on the command-line using the `-javaagent` option then the `Premain-Class` attribute specifies the name of the agent class and the `Agent-Class` attribute is ignored. Similarly, if the agent is started sometime after the VM has started, then the `Agent-Class` attribute specifies the name of the agent class (the value of `Premain-Class` attribute is ignored).

## Instrumenting code in modules

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

- Related Packages
  Module
  Package
  Description
  java.base
  java.lang
  Provides classes that are fundamental to the design of the Java programming language.
- Class
  Description
  ClassDefinition
  This class serves as a parameter block to the `Instrumentation.redefineClasses` method.
  ClassFileTransformer
  A transformer of class files.
  IllegalClassFormatException
  Thrown by an implementation of `ClassFileTransformer.transform` when its input parameters are invalid.
  Instrumentation
  This class provides services needed to instrument Java programming language code.
  UnmodifiableClassException
  Thrown by an implementation of `Instrumentation.redefineClasses` when one of the specified classes cannot be modified.
  UnmodifiableModuleException
  Thrown to indicate that a module cannot be modified.