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LambdaTool

Find out which method is used in a lambda expression or method reference. Only works with interfaces.

This is done by creating a proxy for the interface, and capturing the method with a MethodInvocationHandler. LambdaTool provides several implementations of the method whichMethod (T is the type of the interface):

  • one taking a Consumer<T> to process calls to methods that take no arguments;
  • one taking a BiConsumer<T, ?> to support methods with a single argument;
  • one taking MethodCallWithTwoParameters<T, ?, ?> to capture calls with two arguments.

One could keep adding more, but there is little point. For special cases (more arguments or methods taking some primitive types as arguments), one can always use a lambda instead of a method reference, or sometimes use a cast to avoid ambiguity. Check out LambdaToolTest for examples.

CgLib version

CgLibLambdaTool also supports classes; however, with CgLib not being compatible with the latest Java versions (the required methods are not 'opened' to the public), you won't be able to use that with Java 17.

See cglib/cglib#191 for details.

ByteBuddy version

BbLambdaTool, using ByteBuddy, overcomes the JDK issues; but, so far, I have been unable to get it to work with package-private methods (i.e., they need to be protected or public). The reason is that ByteBuddy needs a new classloader that can access the bytecode of the generated class, therefore even though the package names match, the packages (identified by name + classloader) will be different; therefore, the package-private method in the generated class will not override the package-private method in the original one. See https://stackoverflow.com/a/26861830 and a related Mockito issue at mockito/mockito#796 (comment). (Thanks to Péter Verhás for the tip.)