Package javassist.util

Class HotSwapAgent

  • public class HotSwapAgent
extends java.lang.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

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

      • Methods inherited from class java.lang.Object

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

    • Constructor Detail

      • HotSwapAgent

        public HotSwapAgent()

    • Method Detail

      • instrumentation

        public java.lang.instrument.Instrumentation instrumentation()
        Obtains the Instrumentation object.
        Returns:
        null when it is not available.

      • premain

        public static void premain​(java.lang.String agentArgs,
                           java.lang.instrument.Instrumentation inst)
                    throws java.lang.Throwable
        The entry point invoked when this agent is started by -javaagent.
        Throws:
        java.lang.Throwable

      • agentmain

        public static void agentmain​(java.lang.String agentArgs,
                             java.lang.instrument.Instrumentation inst)
                      throws java.lang.Throwable
        The entry point invoked when this agent is started after the JVM starts.
        Throws:
        java.lang.Throwable