Technical requirements

yGuard requires JDK 1.7.x or greater and Ant 1.5 or greater installed on your system. It may work with earlier versions of these pieces of software as well, however this has not been tested thoroughly. yGuard 1.3.x and upwards works together with Ant 1.6.

Java 11 - Java 13 Compatibility

Beginning with version 2.10, yGuard supports obfuscation of Java class files that contain nesthost or nestmembers attributes which were introduced with the Java 11 .class file format. Further yGuard does support obfuscating dynamic instructions which were introduced with the Java 11 .class file format.

Please read the notes regarding 3rd party JVM support if you intend to use yGuard with something other than Java.

Java 9 / Java 10 Compatibility

Beginning with version 2.7, yGuard supports obfuscation of Java class files that contain module information which was introduced with the Java 9 .class file format. yGuard does not change module names, though.

yGuard does not support obfuscating multi-release Java archives which were introduced with Java 9.

Java 7 / Java 8 Compatibility

Beginning with version 2.5, yGuard supports obfuscation of Java class files that contain the invokedynamic instruction, which was introduced with the Java 7 .class file format. JDK 7 does not contain any means of issuing this instruction, with JDK 8 it is being issued when using lambda expressions or default methods.

While yGuard does fully support obfuscating invokedynamic instructions and therefore default methods and lambda expressions, shrinking of Java class files that contain this instruction is not supported yet.

Compatibility to 3rd party JVM

Obfuscating dynamic and invokedynamic instructions is a task that is theoretically infeasible. An obfuscation program cannot determine the type and parameters of such instructions in a generic way. A trade-off solution for this is supporting known MetaFactory objects by their signature. The JRE makes this task quite trivial. yGuard supports the built-in LambdaMetafactory and StringConcatFactory.

This trade-off however means yGuard offers only limited support for instruction sets based on invokedynamic or dynamic. In particular, supporting new JVM targets, such as Scala, might require manual work. As we currently do not have the expertise, nor do we have the resources for this project, this is a chore left for the community.

Below is a documentation on the design process involved in supporting the LambdaMetafactory. It should serve as a base for anyone deciding to add more support for e.g Scala or Groovy.

How LambdaMetafactory is covered

To check that JVM compatibility is ensured in new releases, we verified that there are no differences in the class file format in JVM >= 11. This can be checked in the documentation of the class file format. The JRE ships two targets for the invokedynamic and dynamic instruction sets. These are:

  • LambdaMetafactory
  • StringConcatFactory

We can recognise these factories in the obfuscation and shrinking steps using their signature. Looking at the documentation tells us that we should cover two signatures for LambdaMetaFactory:

  • java/lang/invoke/LambdaMetafactory#metafactory
  • java/lang/invoke/LambdaMetafactory#altMetafactory

yFiles recognises these methods during renaming. In order to obfuscate lambdas, these steps are performed:

In the LambdaMetafactoryTest example, this will remap com/yworks/yguard/obf/LambdaMetaFactoryTest$MyInterface; to La/a/a/a/a$_a;. Most instances of lambda invocations will do a remapping on the JRE functional interface.

However, this process must be carefully tested with actual Java byte code. Even though StringConcatFactory uses similar code, its semantics is completely different. Implementing such a factory requires in-depth knowledge of the underlying mechanism. Even in the case of the JDK it is not always perfectly clear which case will be mapped by the compiler to which construct.