Package javassist.util

Class HotSwapAgent

java.lang.Object

javassist.util.HotSwapAgent

public class HotSwapAgent
extends Object

A utility class for dynamically adding a new method or modifying an existing method body. This class provides redefine(Class, CtClass) and redefine(Class[], CtClass[]), which replace the existing class definition with a new one. These methods perform the replacement by java.lang.instrument.Instrumentation. For details of acceptable modification, see the Instrumentation interface.

Before calling the redefine methods, the hotswap agent has to be deployed.

To create a hotswap agent, run createAgentJarFile(String). For example, the following command creates an agent file named hotswap.jar.

 $ jshell --class-path javassist.jar
 jshell> javassist.util.HotSwapAgent.createAgentJarFile("hotswap.jar") 
 

Then, run the JVM with the VM argument -javaagent:hotswap.jar to deploy the hotswap agent.

If the -javaagent option is not given to the JVM, HotSwapAgent attempts to automatically create and start the hotswap agent on demand. This automated deployment may fail. If it fails, manually create the hotswap agent and deploy it by -javaagent.

The HotSwapAgent requires tools.jar as well as javassist.jar.

The idea of this class was given by Adam Lugowski. Shigeru Chiba wrote this class by referring to his RedefineClassAgent. For details, see this discussion.

Since:

3.22

See Also:

redefine(Class, CtClass), redefine(Class[], CtClass[])

Constructor Summary

Constructors

Constructor	Description
HotSwapAgent()

Method Summary

Modifier and Type	Method	Description
static void	agentmain(String agentArgs, Instrumentation inst)	The entry point invoked when this agent is started after the JVM starts.
static File	createAgentJarFile(String fileName)	Creates an agent file for using HotSwapAgent.
Instrumentation	instrumentation()	Obtains the Instrumentation object.
static void	premain(String agentArgs, Instrumentation inst)	The entry point invoked when this agent is started by -javaagent.
static void	redefine(Class<?>[] oldClasses, CtClass[] newClasses)	Redefines classes.
static void	redefine(Class<?> oldClass, CtClass newClass)	Redefines a class.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

HotSwapAgent

public HotSwapAgent()

Method Detail

instrumentation

public Instrumentation instrumentation()

Obtains the Instrumentation object.

Returns:

null when it is not available.

premain

public static void premain(String agentArgs,
                           Instrumentation inst)
                    throws Throwable

The entry point invoked when this agent is started by -javaagent.

Throws:

Throwable

agentmain

public static void agentmain(String agentArgs,
                             Instrumentation inst)
                      throws Throwable

The entry point invoked when this agent is started after the JVM starts.

Throws:

Throwable

redefine

public static void redefine(Class<?> oldClass,
                            CtClass newClass)
                     throws NotFoundException,
                            IOException,
                            CannotCompileException

Redefines a class.

Throws:

NotFoundException

IOException

CannotCompileException

redefine

public static void redefine(Class<?>[] oldClasses,
                            CtClass[] newClasses)
                     throws NotFoundException,
                            IOException,
                            CannotCompileException

Redefines classes.

Throws:

NotFoundException

IOException

CannotCompileException

createAgentJarFile

public static File createAgentJarFile(String fileName)
                               throws IOException,
                                      CannotCompileException,
                                      NotFoundException

Creates an agent file for using HotSwapAgent.

Throws:

IOException

CannotCompileException

NotFoundException