JLink function not producing result

Question

I have a simple JLink function as follows:

Needs["JLink`"]
sha256Signature[bytesList_] :=
JavaBlock[
  Module[{sig, mdc, mdo},
  InstallJava[];
  mdc = LoadJavaClass["java.security.MessageDigest"];
  mdo = mdc.getInstance["SHA-256"];
  mdo[update[bytesList]];
  sig = mdo[digest];
  ReleaseJavaObject[mdo];
  Return[sig]
 ]
];
sha256Signature[byteList]

It produces the following output

(JLink`JavaClass["java.security.MessageDigest", 0, {JLink`JVM["vm1"]}, 1,
java`security`MessageDigest`", False, True].getInstance["SHA-256"])[digest]

There is no error indication, but the code doesn't seem to do anything. I must be missing something simple.

Making suggested modifications (see below)

Needs["JLink`"];
InstallJava[];
sha256Signature[bytesList_] :=
  JavaBlock[
   Module[{sig, mdc, mdo},
    mdc = LoadJavaClass["java.security.MessageDigest"];
    mdo = MessageDigest`getInstance["SHA-256"];
    mdo[update[bytesList]];
    sig = mdo[digest];
    ReleaseJavaObject[mdo];
    Return[sig]
  ]
];
sha256Signature[byteList]

Produces

Java::fldx: Attempting to set field named digest defined in class
java.security.MessageDigest$Delegate to an incorrect type of value:
    JavaObject[java.security.MessageDigest$Delegate]».
$Failed

Answer 1

Try like this:

sha256Signature[bytesList_] := 
 JavaBlock@Module[{mdo}, 
   LoadJavaClass["java.security.MessageDigest"];
   mdo = MessageDigest`getInstance["SHA-256"];
   mdo@update[bytesList];
   mdo@digest[]
 ]

sha256Signature[Range[10]]

(* ==> {-56, 72, -31, 1, 63, -97, 4, -87, -42, 63, -92, 60, -25, -3, 74,
        -16, 53, 21, 44, 124, 102, -102, 74, 64, 75, 103, 16, 124, -18, 95,
         46, 78} *)

It's much clearer if you use @ instead of [ ... ] because it (almost) directly corresponds to the . in Java.

You do not need to use ReleaseJavaObject if you are already using JavaBlock.

You do not need to us an explicit Return at the end of the function in Mathematica. Just don't suppress returning the value by using ;.