Message12690

Could indeed be an openjdk thing. More details:

Under OpenJDK 11

C:\Users\Adam\jython\jython5\jython>java -version
openjdk version "11.0.1" 2018-10-16
OpenJDK Runtime Environment 18.9 (build 11.0.1+13)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.1+13, mixed mode)

Before fix:
  public void clear();
    Code:
       0: aload_0
       1: getfield      #5                  // Field buffer:Ljava/nio/ByteBuffer;
       4: invokevirtual #13                 // Method java/nio/ByteBuffer.clear:()Ljava/nio/ByteBuffer;
       7: iconst_0
       8: invokevirtual #11                 // Method java/nio/ByteBuffer.limit:(I)Ljava/nio/ByteBuffer;
      11: pop
      12: return


With fix
  public void clear();
    Code:
       0: aload_0
       1: getfield      #5                  // Field buffer:Ljava/nio/ByteBuffer;
       4: invokestatic  #13                 // Method com/headius/backport9/buffer/Buffers.clearBuffer:(Ljava/nio/Buffer;)Ljava/nio/Buffer;
       7: pop
       8: aload_0
       9: getfield      #5                  // Field buffer:Ljava/nio/ByteBuffer;
      12: iconst_0
      13: invokestatic  #11                 // Method com/headius/backport9/buffer/Buffers.limitBuffer:(Ljava/nio/Buffer;I)Ljava/nio/Buffer;
      16: pop
      17: return

As you can sort of see in the bytecode, the backport9 fix is to invoke statically, using a generic to always match the type signatures.

public class Buffers {
    /**
     * Invoke Buffer.clear always using Buffer as the target, to avoid binary incompatibility on Java 8.
     *
     */
    public static <T extends Buffer> T clearBuffer(T buf) {
        return (T) buf.clear();
    }


Jython 2.7.2a1+ (uncontrolled:+, Oct. 6 2019, 20:17:35)
[OpenJDK 64-Bit Server VM (Oracle Corporation)] on java11.0.1
Type "help", "copyright", "credits" or "license" for more information.

My Java 8 is indeed Oracle

:\Users\Adam\jython>java -version
java version "1.8.0_111"
Java(TM) SE Runtime Environment (build 1.8.0_111-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.111-b14, mixed mode)