Obscure field and modifier in Log4j Logger

Question

When looking into the class file for org.apache.log4j.Logger it defines a synthetic field of type Class with name class$org$apache$log4j$Logger.

From looking at the byte code, it is clear that this field represents the self-referenced class from a time where the constant pool could not yet reference types. What I do however find strange is the modifier of this field which is 0x41008 which indicates a private, synthetic field (that I can follow) but which adds a modifier 0x40000 which I cannot find documented anywhere.

Where does this modifier at bit 19 come from and what does it express? (Log4j is compiled for Java 1).

Answer 1

javap is perfectly happy with that class file:

  static java.lang.Class class$org$apache$log4j$Logger;
    descriptor: Ljava/lang/Class;
    flags: ACC_STATIC
    Synthetic: true

access_flags are supposed to be u2, meaning 2-byte unsigned. It is puzzling to see 0x41008, which is larger than u2. Some tools are known to store access_flags in larger types, and inject auxiliary bits (ASM does it, JVMs do it, etc.) I know you are reading with ASM, so this is probably what you are looking at:

org/objectweb/asm/ClassReader.java:

        } else if ("Synthetic".equals(attrName)) {
            access |= Opcodes.ACC_SYNTHETIC
                    | ClassWriter.ACC_SYNTHETIC_ATTRIBUTE;

org/objectweb/asm/ClassWriter.java:

/**
 * Pseudo access flag to distinguish between the synthetic attribute and the
 * synthetic access flag.
 */
static final int ACC_SYNTHETIC_ATTRIBUTE = 0x40000;

The question is, how does it leak to you...