Reflection issue from javascript passing Uint8Array argument to a java byte array

[javabully]

I would expect a Uint8Array created in a javascript context to be converted to a java byte array when invoking a context bound java service. This works as expected unless a value > 127 is inserted into the array. The following class demonstrates this.
`
package test;

import java.util.Base64;
import org.graalvm.polyglot.Context;
import org.graalvm.polyglot.EnvironmentAccess;
import org.graalvm.polyglot.HostAccess;

public class BugExample {

public static void main(String[] args) {
	
	Context context = null;
	try {
		
		context = Context.newBuilder("js")
				.allowCreateProcess(false)
				.allowIO(false)
				.allowHostClassLoading(false)
				.allowEnvironmentAccess(EnvironmentAccess.NONE)
				.allowHostAccess(HostAccess.EXPLICIT)
				.allowNativeAccess(false)
				.build();
		
		context.getBindings("js").putMember("base64", new Base64Util());
		// test the working case
		String source = "base64.encodeToString(new Uint8Array([22]));";
		System.err.println("base64: " + context.eval("js", source));
		
		// now test the fail
		source = "base64.encodeToString(new Uint8Array([223]));";
		System.err.println("base64: " + context.eval("js", source));
		
	} catch(Exception e) {
		e.printStackTrace();
	} finally {
		context.close(true);
	}
}

public static class Base64Util {

	@HostAccess.Export
	public String encodeToString(byte[] data) {

		return Base64.getEncoder().encodeToString(data);
		
	}
}

}
`
Executing the class outputs the following:
base64: Fg==
TypeError: invokeMember (encodeToString) on JavaObject[test.BugExample$Base64Util@1ede5e85 (test.BugExample$Base64Util)] failed due to: UnsupportedTypeException
at :program(Unnamed:1:0-43)
at org.graalvm.polyglot.Context.eval(Context.java:370)
at test.BugExample.main(BugExample.java:36)

[wirthi]

Hi @javabully

thanks for your question.

We are wary of lossy conversions, this is why we don't do it by default and you see the error. Note, that since your report, we have improved the error message, it should be much more expessive now:

TypeError: invokeMember (encodeToString) on Main$.... failed due to: Cannot convert '128'(language: Java, type: java.lang.Integer) to Java type 'byte': Invalid or lossy primitive coercion.

There also is a solution to your problem: you can use a custom targetTypeMapping:

HostAccess ha = HostAccess.newBuilder(HostAccess.EXPLICIT)
  //.targetTypeMapping(Integer.class, Byte.class, null,
  //    (i) -> i.byteValue()  )
  .targetTypeMapping(int[].class, byte[].class, null,
  (iArr) -> {
    byte[] bArr = new byte[iArr.length];
    for (int i=0;i<iArr.length;i++) {
      bArr[i] = (byte)iArr[i];
    }
    return bArr;
  } )
  .build();

Set that custom HostAccess on the Context you build (instead of the HostAccess.EXPLICIT) and this target type mapping will be called whenever a conversion is necessary. This basically overrides our own hard-coded conversion rules. See https://www.graalvm.org/sdk/javadoc/org/graalvm/polyglot/HostAccess.Builder.html#targetTypeMapping-java.lang.Class-java.lang.Class-java.util.function.Predicate-java.util.function.Function-

Best,
Christian