code: public void visitFieldInsn(int opcode, String owner, String name,
              String desc) {
          if (mv != null) {
              mv.visitFieldInsn(opcode, owner, name, desc);
          }
      }
docs: Visits a field instruction. A field instruction is an instruction that
  loads or stores the value of a field of an object.
  
  @param opcode
  the opcode of the type instruction to be visited. This opcode
  is either GETSTATIC, PUTSTATIC, GETFIELD or PUTFIELD.
  @param owner
  the internal name of the field's owner class (see
  {@link Type#getInternalName() getInternalName}).
  @param name
  the field's name.
  @param desc
  the field's descriptor (see {@link Type Type}).