How to pass byte arrays between Java and JavaScript

Question

I need to access SecureRandom Java Object from Javascript. My ultimate goal is to grab 4 bytes from PRNG and convert it to Javascript integer variable. According to http://download.oracle.com/javase/1.4.2/docs/api/java/security/SecureRandom.html, the following two lines of Java code are supposed to do grab 4 random bytes:

byte bytes[] = new byte[4];
random.nextBytes(bytes);

My problems is that I don't know how to 1) allocate byte array suitable for passing to Java method 2) parse that array into integer afterwards

So far I have managed to getSeed() method which returns an array of random bytes. When I render HTML code provided below in Firefox it shows "[B@16f70a4", which appears to be a pointer or something.

<script>
var sprng = new java.security.SecureRandom();
random = sprng.getSeed(4);
document.write(random + "<br/>\n");
</script>

This makes me think that I succeed to instantiate and access Java class, but have a problem with type conversion.

Can anyone please help me to write allocateJavaByteArray(N) and convertJavaByteArrayToInt(N) to let the following code work:

var sprng = new java.security.SecureRandom();
var nextBytes = allocateJavaByteArray(4);
srng.nextBytes(nextBytes);
var nextInt = convertJavaByteArrayToInt(4);

Thank you in advance.

Answer 1

You could implement convertJavaByteArrayToInt like this:

function convertJavaByteArrayToInt(bytes) {
  var r = 0;
  for (var i = 0; i < bytes.length; i++) {
    r += (bytes[i] & 0xff) << (8 * i);
  }
  return r;
}

allocateJavaByteArray is difficult to implement, because we cannot get the Class of byte. So it's not possible to use java.lang.reflect.Array.newInstance to create a byte[] instance. But here is a tricky implementation:

function allocateJavaByteArray(n) {
  var r = "";
  for (var i = 0; i < n; i++) {
    r += "0";
  }
  return new java.lang.String(r).getBytes();
}

updated: It seems that above code not worked in FireFox 3.6. Here is another allocateJavaByteArray implementation, have a try:

function allocateJavaByteArray(n) {
  var r = new java.io.ByteArrayOutputStream(4);
  for (var i = 0; i < n; i++) {
    r.write(0);
  }
  return r.toByteArray();
}

Answer 2

Java string is the only thing that will pass Java->JS or JS->Java without headache.

byte[] or any arry will be seen in JS as JSObject.

---

var sprng = new java.security.SecureRandom();

is

var foo= new java.package.SomeClass();

does work in Netscape/Mozilla/FF

It needs access to classes, so any java standard class or you need to load a jar and then access the class.

---

to orginal question:

1. create applet whith utility method:

   public String someStringEncodedValue(){ return 1+"|"+2; }

2. include applet into the page with unique id

3. JS find applet using unique id

4. call method

5. parse string ( split by | )

Answer 3

Normally you'd generate the random number on the server and pass it in the Request to the jsp.

Answer 4

You could simply generate a random integer in the first place, like this:

var nextInt = sprng.nextInt();