Strange "!*" entry in LocalVariableTypeTable when compiling with Eclipse compiler

Question

Let's compile the following code with ECJ compiler from Eclipse Mars.2 bundle:

import java.util.stream.*;

public class Test {
    String test(Stream<?> s) {
        return s.collect(Collector.of(() -> "", (a, t) -> {}, (a1, a2) -> a1));
    }
}

The compilation command is the following:

$ java -jar org.eclipse.jdt.core_3.11.2.v20160128-0629.jar -8 -g Test.java

After the successful compilation let's check the resulting class file with javap -v -p Test.class. The most interesting is the synthetic method generated for the (a, t) -> {} lambda:

  private static void lambda$1(java.lang.String, java.lang.Object);
    descriptor: (Ljava/lang/String;Ljava/lang/Object;)V
    flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC
    Code:
      stack=0, locals=2, args_size=2
         0: return
      LineNumberTable:
        line 5: 0
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       1     0     a   Ljava/lang/String;
            0       1     1     t   Ljava/lang/Object;
      LocalVariableTypeTable:
        Start  Length  Slot  Name   Signature
            0       1     1     t   !*

I was quite surprised to see this !* entry in LocalVariableTypeTable. JVM specification covers LocalVariableTypeTable attribute and says:

The constant_pool entry at that index must contain a CONSTANT_Utf8_info structure (§4.4.7) representing a field signature which encodes the type of a local variable in the source program (§4.7.9.1).

§4.7.9.1 defines a grammar for field signatures which, if I understand correctly, does not cover anything similar to !*.

It should also be noted that neither javac compiler, nor older ECJ 3.10.x versions generate this LocalVariableTypeTable entry. Is !* some non-standard Eclipse extension or I'm missing something in JVM spec? Does this mean that ECJ does not conform to JVM spec? What !* actually mean and are there any other similar strings which could appear in LocalVariableTypeTable attribute?

Answer 1

The token ! is used by ecj to encode a capture type in generic signatures. Hence !* signifies a capture of an unbounded wildcard.

Internally, ecj uses two flavours of CaptureBinding, one to implement, what JLS 18.4 calls "fresh type variables", the other to implement captures a la JLS 5.1.10 (which uses the same lingo of "free type variables"). Both produce a signature using !. At a closer look, in this example we have an "old-style" capture: t has type capture#1-of ?, capturing the <T> in Stream<T>.

The problem is: JVMS 4.7.9.1. doesn't seem to define an encoding for such fresh type variables (which among other properties have no correspondence in source code and hence no name).

I couldn't get javac to emit any LocalVariableTypeTable for the lambda, so they might simply avoid answering this question.

Given that both compilers agree on inferring t to a capture, why does one compiler generate a LVTT, where the other does not? JVMS 4.7.14 has this

This difference is only significant for variables whose type uses a type variable or parameterized type.

According to JLS, captures are fresh type variables, so an LVTT entry is significant, and it is an omission in JVMS not to specify a format for this type.

Consequences

The above only describes and explains the status quo, demonstrating that no specification tells a compiler to behave differently from current status. Obviously, this is not an entirely desirable situation.

1. Someone may want to contact Oracle, mentioning that Java 8 introduces a situation that is not covered by parts of the JVMS. This situation may become even more relevant once also local variables become subject to type inference
2. Anybody observing negative impact of the current situation is invited to chime in in rfe 494198 (ecj), which otherwise has low priority.

Update: Meanwhile someone has reported an example where a regular Signature attribute (which cannot be opportunistically omitted) is required to encode a type which cannot be encoded according to JVMS. In that case also javac creates unspecified byte code. According to a follow-up no variable should ever have such a type, but I don't think that this discussion is over, yet (and admittedly JLS doesn't yet ensure this goal).

Update 2: After receiving advice from a spec author I see three parts to the ultimate solution:

(1) Every type signature in any bytecode attribute must adhere to the grammar in JVMS 4.7.9.1. Neither ecj's ! nor javac's <captured wildcard> is legal.

(2) Compilers should approximate type signatures where no legal encoding exists, e.g., by using the erasure instead of a capture. For an LVTT entry, such approximation should be considered as legitimate.

(3) JLS must ensure that only types encodable using JVMS 4.7.9.1 appear in positions where generating a Signature attribute is mandatory.

For future versions of ecj items (1) and (2) have been resolved. I cannot speak about schedules when javac and JLS will be fixed accordingly.