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. For implementations that support a command-line interface, an agent is started by specifying an option on the command-line. Implementations may also 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. The details as to how the load is initiated, is implementation dependent.
An implementation is not required to provide a way to start agents from the command-line interface. On implementations that do provide a way to start agents from the command-line interface, an agent is started by adding this option to the command-line:
jarpath is the path to the agent JAR file. options is the agent options. This switch may be used multiple times on the same command-line, thus creating multiple agents. More than one agent may use the same jarpath. An agent JAR file must conform to the JAR file specification.-javaagent:
jarpath[=
options]
The manifest of the agent JAR file must contain the attribute Premain-Class
. 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, each premain
method
will be called in the order the agents were specified, then the real application
main
method will be called.
Each premain
method must return in order for the startup sequence 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. When the agent is started using a command-line option, the agentmain
method is not invoked.
The agent class will be loaded by the system class loader
(see ClassLoader.getSystemClassLoader
). This is
the class loader which typically loads the class containing the application main
method.
The system class loader must support a mechanism to add an agent JAR file to the system class path.
If it is a custom system class loader then it must define the
appendToClassPathForInstrumentation
method as specified in
appendToSystemClassLoaderSearch
.
The premain
methods will be run under the same security and classloader
rules as the application main
method.
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
.
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 resolved
(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 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
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 Agent-Class
.
The value of this attribute is the name of the agent class.
The agent class must implement a public static agentmain
method.
The system class loader (
ClassLoader.getSystemClassLoader
) must
support a mechanism to add an agent JAR file to the system class path.
If it is a custom system class loader then it must define the
appendToClassPathForInstrumentation
method as specified in
appendToSystemClassLoaderSearch
.
The agent JAR is appended to the system class path. This is the class loader that typically loads
the class containing the application main
method. The agent class is loaded and the
JVM attempts to invoke the agentmain
method. 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 an 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.
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.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
orfalse
, case irrelevant). Is the ability to redefine classes needed by this agent. Values other thantrue
are consideredfalse
. This attribute is optional, the default isfalse
.Can-Retransform-Classes
- Boolean (
true
orfalse
, case irrelevant). Is the ability to retransform classes needed by this agent. Values other thantrue
are consideredfalse
. This attribute is optional, the default isfalse
.Can-Set-Native-Method-Prefix
- Boolean (
true
orfalse
, case irrelevant). Is the ability to set native method prefix needed by this agent. Values other thantrue
are consideredfalse
. This attribute is optional, the default isfalse
.
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).
Interface | Description |
---|---|
ClassFileTransformer |
A transformer of class files.
|
Instrumentation |
This class provides services needed to instrument Java
programming language code.
|
Class | Description |
---|---|
ClassDefinition |
This class serves as a parameter block to the
Instrumentation.redefineClasses method. |
Exception | Description |
---|---|
IllegalClassFormatException |
Thrown by an implementation of
ClassFileTransformer.transform
when its input parameters are invalid. |
UnmodifiableClassException |
Thrown by an implementation of
Instrumentation.redefineClasses
when one of the specified classes cannot be modified. |
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For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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