Interface ClassFileTransformer

public interface ClassFileTransformer
A transformer of class files. An agent registers an implementation of this interface using the addTransformer method so that the transformer's transform method is invoked when classes are loaded, redefined, or retransformed. The implementation should override one of the transform methods defined here. Transformers are invoked before the class is defined by the Java virtual machine.

There are two kinds of transformers, determined by the canRetransform parameter of Instrumentation.addTransformer(ClassFileTransformer,boolean):

Once a transformer has been registered with addTransformer, the transformer will be called for every new class definition and every class redefinition. Retransformation capable transformers will also be called on every class retransformation. The request for a new class definition is made with ClassLoader.defineClass or its native equivalents. The request for a class redefinition is made with Instrumentation.redefineClasses or its native equivalents. The request for a class retransformation is made with Instrumentation.retransformClasses or its native equivalents. The transformer is called during the processing of the request, before the class file bytes have been verified or applied. When there are multiple transformers, transformations are composed by chaining the transform calls. That is, the byte array returned by one call to transform becomes the input (via the classfileBuffer parameter) to the next call.

Transformations are applied in the following order:

  • Retransformation incapable transformers
  • Retransformation incapable native transformers
  • Retransformation capable transformers
  • Retransformation capable native transformers

For retransformations, the retransformation incapable transformers are not called, instead the result of the previous transformation is reused. In all other cases, this method is called. Within each of these groupings, transformers are called in the order registered. Native transformers are provided by the ClassFileLoadHook event in the Java Virtual Machine Tool Interface).

The input (via the classfileBuffer parameter) to the first transformer is:

  • for new class definition, the bytes passed to ClassLoader.defineClass
  • for class redefinition, definitions.getDefinitionClassFile() where definitions is the parameter to Instrumentation.redefineClasses
  • for class retransformation, the bytes passed to the new class definition or, if redefined, the last redefinition, with all transformations made by retransformation incapable transformers reapplied automatically and unaltered; for details see Instrumentation.retransformClasses

If the implementing method determines that no transformations are needed, it should return null. Otherwise, it should create a new byte[] array, copy the input classfileBuffer into it, along with all desired transformations, and return the new array. The input classfileBuffer must not be modified.

In the retransform and redefine cases, the transformer must support the redefinition semantics: if a class that the transformer changed during initial definition is later retransformed or redefined, the transformer must insure that the second class output class file is a legal redefinition of the first output class file.

If the transformer throws an exception (which it doesn't catch), subsequent transformers will still be called and the load, redefine or retransform will still be attempted. Thus, throwing an exception has the same effect as returning null. To prevent unexpected behavior when unchecked exceptions are generated in transformer code, a transformer can catch Throwable. If the transformer believes the classFileBuffer does not represent a validly formatted class file, it should throw an IllegalClassFormatException; while this has the same effect as returning null. it facilitates the logging or debugging of format corruptions.

Note the term class file is used as defined in chapter 4 The class File Format of The Java Virtual Machine Specification, to mean a sequence of bytes in class file format, whether or not they reside in a file.

API Note:
Great care must be taken when transforming core JDK classes which are at the same time required during the transformation process as this can lead to class circularity or linkage errors.

If for example the invocation of transform() for a class C requires loading or resolving the same class C, an error is thrown that is an instance of LinkageError (or a subclass). If the LinkageError occurs during reference resolution (see section 5.4.3 Resolution of The Java Virtual Machine Specification) for a class D, the resolution of the corresponding reference in class D will permanently fail with the same error at any subsequent attempt. This means that a LinkageError triggered during transformation of C in a class D not directly related to C can repeatedly occur later in arbitrary user code which uses D.

Since:
1.5
See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    default byte[]
    transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer)
    Transforms the given class file and returns a new replacement class file.
    default byte[]
    transform(Module module, ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer)
    Transforms the given class file and returns a new replacement class file.

  • Method Details

    • transform

      default byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException
      Transforms the given class file and returns a new replacement class file. This method is invoked when the Module bearing transform is not overridden.
      Implementation Requirements:
      The default implementation returns null.
      Parameters:
      loader - the defining loader of the class to be transformed, may be null if the bootstrap loader
      className - the name of the class in the internal form of fully qualified class and interface names as defined in The Java Virtual Machine Specification. For example, "java/util/List".
      classBeingRedefined - if this is triggered by a redefine or retransform, the class being redefined or retransformed; if this is a class load, null
      protectionDomain - the protection domain of the class being defined or redefined
      classfileBuffer - the input byte buffer in class file format - must not be modified
      Returns:
      a well-formed class file buffer (the result of the transform), or null if no transform is performed
      Throws:
      IllegalClassFormatException - if the input does not represent a well-formed class file

    • transform

      default byte[] transform(Module module, ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException
      Transforms the given class file and returns a new replacement class file.
      Implementation Requirements:
      The default implementation of this method invokes the transform method.
      Parameters:
      module - the module of the class to be transformed
      loader - the defining loader of the class to be transformed, may be null if the bootstrap loader
      className - the name of the class in the internal form of fully qualified class and interface names as defined in The Java Virtual Machine Specification. For example, "java/util/List".
      classBeingRedefined - if this is triggered by a redefine or retransform, the class being redefined or retransformed; if this is a class load, null
      protectionDomain - the protection domain of the class being defined or redefined
      classfileBuffer - the input byte buffer in class file format - must not be modified
      Returns:
      a well-formed class file buffer (the result of the transform), or null if no transform is performed
      Throws:
      IllegalClassFormatException - if the input does not represent a well-formed class file
      Since:
      9