Printing debug info on errors with java 8 lambda expressions

debugging, java, java-8, lambda, reflection

Solution

In case you expect method references as the only input, you can debug them to printable names with the following trick:

public static void main(String[] args) {
  Person p = new Person();
  Supplier<String> nameSupplier1 = () -> "MyName";
  Supplier<String> nameSupplier2 = () -> { throw new RuntimeException(); };
  set(p, Person::setName, nameSupplier1);
  System.out.println(p.getName()); // prints MyName
  set(p, Person::setName, nameSupplier2); // throws exception with message
  System.out.println(p.getName()); // Does not execute
}

interface DebuggableBiConsumer<A, B> extends BiConsumer<A, B>, Serializable {}

private static <E, V> void set(
    E o, DebuggableBiConsumer<E, V> setter, Supplier<V> valueSupplier) {
  try {
    setter.accept(o, valueSupplier.get());
  } catch (RuntimeException e) {
    throw new RuntimeException("Failed to set the value of "+name(setter), e);
  }
}

private static String name(DebuggableBiConsumer<?, ?> setter) {
  for (Class<?> cl = setter.getClass(); cl != null; cl = cl.getSuperclass()) {
    try {
      Method m = cl.getDeclaredMethod("writeReplace");
      m.setAccessible(true);
      Object replacement = m.invoke(setter);
      if(!(replacement instanceof SerializedLambda))
        break;// custom interface implementation
      SerializedLambda l = (SerializedLambda) replacement;
      return l.getImplClass() + "::" + l.getImplMethodName();
    }
    catch (NoSuchMethodException e) {}
    catch (IllegalAccessException | InvocationTargetException e) {
      break;
    }
  }
  return "unknown property";
}

The limitations are that it will print not very useful method references for lambda expressions (references to the synthetic method containing the lambda code) and `"unknown property"` for custom implementations of the interface.

Problem

I want to use a static method as setter helper that catch exceptions and print debug info about the operation that failed. I don't want the exception details only. I want to show what property was being set so that detail help to debug the problem quickly. I am working with Java 8. How should I provide or detect the property being set? What I wish is to remove the "name" string in the example and get the same result. I know I can't use reflection over the supplied setter method supplied that is transformed to lambda expression and then to BiConsumer. I got this but the property name needs to be provided. ``` /** setter helper method **/ private static <E, V> void set(E o, BiConsumer<E, V> setter, Supplier<V> valueSupplier, String propertyName) { try { setter.accept(o, valueSupplier.get()); } catch (RuntimeException e) { throw new RuntimeException("Failed to set the value of " + propertyName, e); } } ``` Example: ``` Person p = new Person(); Supplier<String> nameSupplier1 = () -> "MyName"; Supplier<String> nameSupplier2 = () -> { throw new RuntimeException(); }; set(p, Person::setName, nameSupplier1, "name"); System.out.println(p.getName()); // prints MyName set(p, Person::setName, nameSupplier2, "name"); // throws exception with message Failed to set the value of name System.out.println(p.getName()); // Does not execute ``` EDIT: I know reflection does not help with the lambdas. I know AOP and I know this can be made with pure reflection too but I want to know if there a better way to get this done with Java 8 that didn't exist with Java 7. It seems it should to me. Now it is possible to do things like to pass a setter method to another one.

Original source