example java function invocation from C

Author: Andrea Peruffo

graalwasm/graalwasm-embed-c-code-guide/README.md

@@ -185,8 +185,8 @@ public class App {
 Compile and run this Java application with Maven:
 
 ```shell
-mvw package
-mvn exec:exec
+./mvnw package
+./mvnw exec:exec
 ```
 
 The expected output should contain the first 10 lines of [Floyd's triangle](https://en.wikipedia.org/wiki/Floyd%27s_triangle), printed using the C function:

graalwasm/graalwasm-embed-c-code-guide/src/main/c/floyd.c

@@ -1,11 +1,17 @@
 #include <stdio.h>
 
+extern int javaInc(int number)
+__attribute__((
+    __import_module__("env"),
+    __import_name__("java-increment"),
+));
+
 void floyd(int rows) {
     int number = 1;
     for (int i = 1; i <= rows; i++) {
         for (int j = 1; j <= i; j++) {
             printf("%d ", number);
-            ++number;
+            number = javaInc(number);
         }
         printf(".\n");
     }

graalwasm/graalwasm-embed-c-code-guide/src/main/java/com/example/App.java

@@ -8,22 +8,36 @@
 
 import java.io.IOException;
 import java.net.URL;
+import java.util.Map;
 
 import org.graalvm.polyglot.Context;
 import org.graalvm.polyglot.Source;
 import org.graalvm.polyglot.Value;
+import org.graalvm.polyglot.proxy.ProxyExecutable;
+import org.graalvm.polyglot.proxy.ProxyObject;
 
 public class App {
+
+    public static Object javaIncrement(Value... v) {
+        return Value.asValue(v[0].asInt() + 1);
+    }
+
     public static void main(String[] args) throws IOException {
         // Find the WebAssembly module resource
         URL wasmFile = App.class.getResource("floyd.wasm");
         Source source = Source.newBuilder("wasm", wasmFile).name("example").build();
 
         // Create Wasm context
-        try (Context context = Context.newBuilder("wasm").option("wasm.Builtins", "wasi_snapshot_preview1").build()) {
-            // Compile and instantiate the module
+        try (Context context = Context.newBuilder("wasm")
+                .option("wasm.Builtins", "wasi_snapshot_preview1")
+                .build()) {
+            // Compile and instantiate the module with host function
             Value module = context.eval(source);
-            Value instance = module.newInstance();
+            Value instance = module.newInstance(ProxyObject.fromMap(
+                Map.of("env", ProxyObject.fromMap(
+                        Map.of("java-increment", (ProxyExecutable) App::javaIncrement)
+                ))
+            ));
 
             // Get the exports member from the module instance
             Value exports = instance.getMember("exports");