feat: update main class to run WASM code

Adapted from wasmtime-java example at:
https://github.com/kawamuray/wasmtime-java/blob/v0.14.0/examples/src/main/java/examples/HelloWorld.java

Author: ctrueden

packages/core/java/src/main/java/org/itk/wasm/Main.java

@@ -19,8 +19,73 @@
  */
 package org.itk.wasm;
 
+import io.github.kawamuray.wasmtime.Engine;
+import io.github.kawamuray.wasmtime.Extern;
+import io.github.kawamuray.wasmtime.Func;
+import io.github.kawamuray.wasmtime.Instance;
+import io.github.kawamuray.wasmtime.Module;
+import io.github.kawamuray.wasmtime.Store;
+import io.github.kawamuray.wasmtime.WasmFunctions;
+
+import java.io.ByteArrayOutputStream;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Arrays;
+import java.util.Collection;
+
 public class Main {
-  public static void main(String... args) {
-    System.out.println("Hello world");
+  public static void main(String... args) throws IOException {
+    // Configure the initial compilation environment, creating the global
+    // `Store` structure. Note that you can also tweak configuration settings
+    // with a `Config` and an `Engine` if desired.
+    System.err.println("Initializing...");
+    try (Store<Void> store = Store.withoutData()) {
+      // Compile the wasm binary into an in-memory instance of a `Module`.
+      System.err.println("Compiling module...");
+      try (Engine engine = store.engine();
+          Module module = new Module(engine, readWAT("hello.wat")))
+      {
+        // Here we handle the imports of the module, which in this case is our
+        // `HelloCallback` type and its associated implementation of `Callback.
+        System.err.println("Creating callback...");
+        try (Func helloFunc = WasmFunctions.wrap(store, () -> {
+          System.err.println("CB!! Calling back...");
+          System.err.println("CB!! > Hello World!");
+        })) {
+          // Once we've got that all set up we can then move to the instantiation
+          // phase, pairing together a compiled module as well as a set of imports.
+          // Note that this is where the wasm `start` function, if any, would run.
+          System.err.println("Instantiating module...");
+          Collection<Extern> imports = Arrays.asList(Extern.fromFunc(helloFunc));
+          try (Instance instance = new Instance(store, module, imports)) {
+            // Next we poke around a bit to extract the `run` function from the module.
+            System.err.println("Extracting export...");
+            try (Func f = instance.getFunc(store, "run").get()) {
+              WasmFunctions.Consumer0 fn = WasmFunctions.consumer(store, f);
+
+              // And last but not least we can call it!
+              System.err.println("Calling export...");
+              fn.accept();
+
+              System.err.println("Done.");
+            }
+          }
+        }
+      }
+    }
+  }
+
+  private static byte[] readWAT(String filename) throws IOException {
+    try (InputStream is = new FileInputStream("/home/curtis/code/kitware/itk-wasm/packages/core/java/src/main/resources/org/itk/wasm/" + filename)) {
+    //try (InputStream is = Main.class.getResourceAsStream(filename)) {
+      ByteArrayOutputStream buffer = new ByteArrayOutputStream();
+      int nRead;
+      byte[] buf = new byte[16384];
+      while ((nRead = is.read(buf, 0, buf.length)) != -1) {
+        buffer.write(buf, 0, nRead);
+      }
+      return buffer.toByteArray();
+    }
   }
 }

packages/core/java/src/main/resources/org/itk/wasm/hello.wat

@@ -0,0 +1,4 @@
+(module
+  (func $hello (import "" "hello"))
+  (func (export "run") (call $hello))
+)