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Xtend Maven issues after upgrading to Java 9 [message #1830829]

Wed, 05 August 2020 01:06

Mirko Raner

Messages: 124
Registered: July 2009
Location: New York City, NY

Senior Member

I recently upgraded an Xtend project from Java 8 to Java 9, i.e. in the parent POM I replaced

  <maven.compiler.source>1.8</maven.compiler.source>
  <maven.compiler.target>1.8</maven.compiler.target>

with

  <maven.compiler.release>9</maven.compiler.release>

This project previously built like a charm with Maven, but after upgrading to Java 9 I'm seeing a whole bunch of strange errors, both in the Xtend-to-Java compilation step, as well as during the translation of the generated Java code. I've tried coming up with a smaller project to demonstrate the issue, but this turned out to be non-straightforward, so I'm hoping maybe someone will recognize some of the problems I'm seeing and can point out what I'm missing.
The project is built on a CI system running Java 11 for a while; the problems were introduced when changing the target Java release from 8 to 9.
The project itself is not yet open-sourced, but there are a few self-contained source files that I can share, which hopefully shines a light on what's going here.
For example, consider the following Xtend class:

package fxxx.runtime

import fxxx.lang.Function
import java.lang.reflect.ParameterizedType
import java.util.NoSuchElementException
import java.util.stream.Stream

class Dispatcher<T>
{
  Stream.Builder<Function<?, ?, T>> cases

  new()
  {
    this(Stream.builder)
  }

  private new(Stream.Builder<Function<?, ?, T>> cases)
  {
    this.cases = cases
  }

  def Dispatcher<T> with(Function<?, ?, T> ^case)
  {
    new Dispatcher(cases.add(^case))
  }

  def T ^dispatch(Object dispatchObject)
  {
    cases.build
      .filter[parameterType.isAssignableFrom(dispatchObject.class)]
      .findFirst()
      .map[(it as Function<?, Object, T>).apply(dispatchObject)]
      .orElseThrow[new NoSuchElementException]
  }

  private def Class<?> parameterType(extension Function<?, ?, ?> function)
  {
    val ParameterizedType type = function.class.genericInterfaces.get(0) as ParameterizedType
    type.actualTypeArguments.get(1) as Class<?>
  }
}

After the Java 9 upgrade this class now fails with the following errors:

[ERROR] Failed to execute goal org.eclipse.tycho:tycho-compiler-plugin:1.7.0:compile (default-compile) on project fxxx.runtime: Compilation failure: Compilation failure: 
[ERROR] /Users/mirko/git/fxxx/fxxx.runtime/xtend-gen/fxxx/runtime/Dispatcher.java:[89] 
[ERROR] 	Type _get = function.getClass().getGenericInterfaces()[0];
[ERROR] 	            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[ERROR] Type mismatch: cannot convert from Type to Type
[ERROR] /Users/mirko/git/fxxx/fxxx.runtime/xtend-gen/faxx/runtime/Dispatcher.java:[92] 
[ERROR] 	_xblockexpression = ((Class<?>) _get_1);
[ERROR] 	                    ^^^^^^^^^^^^^^^^^^^
[ERROR] Cannot cast from Type to Class<?>
[ERROR] 2 problems (2 errors)

The generated Java code, looks perfectly fine in Eclipse:

  ...
  import java.lang.reflect.Type;
  ...
  private Class<?> parameterType(@Extension final Function<?, ?, ?> function) {
    Class<?> _xblockexpression = null;
    {
      Type _get = function.getClass().getGenericInterfaces()[0];
      final ParameterizedType type = ((ParameterizedType) _get);
      Type _get_1 = type.getActualTypeArguments()[1];
      _xblockexpression = ((Class<?>) _get_1);
    }
    return _xblockexpression;
  }

The only way to work around this issue is to set

<maven.compiler.release>8</maven.compiler.release>

for the affected sub-project.
Other sub-projects even fail already during the Xtend translation step, somehow the upgrade to Java 9 interfered with the handling of default methods:

package fxxx.generator

import java.util.function.Function

interface State<S, A> extends Function<S, Pair<S, A>>
{
  def <B> State<S, B> map(Function<A, B> function)
  {
  	val State<S, A> that = this
    return
    [
      val Pair<S, A> value = that.apply(it)
      value.key -> function.apply(value.value)
    ]
  }

  def <B> State<S, B> flatMap(Function<A, State<S, B>> function)
  {
  	val State<S, A> that = this
    return
    [
      val Pair<S, A> value = that.apply(it)
      function.apply(value.value).apply(value.key)
    ]
  }
}

The default methods now cause an error:

[INFO] --- xtend-maven-plugin:2.22.0:compile (default) @ fxxx ---
[ERROR] 
ERROR: 	State.xtend - /Users/mirko/git/fxxx/fxxx/src/fxxx/generator/State.xtend
7: Abstract methods do not specify a body
[ERROR] 
ERROR: 	State.xtend - /Users/mirko/git/fxxx/fxxx/src/fxxx/generator/State.xtend
17: Abstract methods do not specify a body

These two different issue are pretty representative of the other problems I'm seeing. Is there anything that I'm missing for upgrading an Xtend project to Java 9?

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