Package javassist.util

Class HotSwapper

java.lang.Object
javassist.util.HotSwapper
public class HotSwapper extends Object
A utility class for dynamically reloading a class by the Java Platform Debugger Architecture (JPDA), or HotSwap. It works only with JDK 1.4 and later.

Note: The new definition of the reloaded class must declare the same set of methods and fields as the original definition. The schema change between the original and new definitions is not allowed by the JPDA.

To use this class, the JVM must be launched with the following command line options:

For Java 1.4,

java -Xdebug -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=8000

For Java 5,

java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8000

Note that 8000 is the port number used by HotSwapper. Any port number can be specified. Since HotSwapper does not launch another JVM for running a target application, this port number is used only for inter-thread communication.

Furthermore, JAVA_HOME/lib/tools.jar must be included in the class path.

Using HotSwapper is easy. See the following example: 

 CtClass clazz = ...
 byte[] classFile = clazz.toBytecode();
 HotSwapper hs = new HostSwapper(8000);  // 8000 is a port number.
 hs.reload("Test", classFile);

reload() first unload the Test class and load a new version of the Test class. classFile is a byte array containing the new contents of the class file for the Test class. The developers can repatedly call reload() on the same HotSwapper object so that they can reload a number of classes.

HotSwap depends on the debug agent to perform hot-swapping but it is reported that the debug agent is buggy under massively multithreaded environments. If you encounter a problem, try HotSwapAgent.

Since:
3.1
See Also:

  • Constructor Details

    • HotSwapper

      public HotSwapper(int port) throws IOException, com.sun.jdi.connect.IllegalConnectorArgumentsException
      Connects to the JVM.
      Parameters:
      port - the port number used for the connection to the JVM.
      Throws:
      IOException
      com.sun.jdi.connect.IllegalConnectorArgumentsException

    • HotSwapper

      public HotSwapper(String port) throws IOException, com.sun.jdi.connect.IllegalConnectorArgumentsException
      Connects to the JVM.
      Parameters:
      port - the port number used for the connection to the JVM.
      Throws:
      IOException
      com.sun.jdi.connect.IllegalConnectorArgumentsException

  • Method Details

    • reload

      public void reload(String className, byte[] classFile)
      Reloads a class.
      Parameters:
      className - the fully-qualified class name.
      classFile - the contents of the class file.

    • reload

      public void reload(Map<String,byte[]> classFiles)
      Reloads a class.
      Parameters:
      classFiles - a map between fully-qualified class names and class files. The type of the class names is String and the type of the class files is byte[].