Experimental feature in GraalVM


GraalWasm is an open source WebAssembly runtime. It runs WebAssembly programs in the binary format and can be used to embed and leverage WebAssembly modules in Java applications. GraalWasm is under active development and is tracking a number of WebAssembly extensions.

Running WebAssembly Embedded in Java #

Compiled WebAssembly binary code can be accessed programmatically with GraalVM SDK Polyglot API, which allows embedding WebAssembly into user applications.

The example below demonstrates how to compile a C function to WebAssembly and run it embedded in a Java application. To run the demo, you need the following:

Demo Part #

  1. Put the following C program in a file named floyd.c:
     #include <stdio.h>
     void floyd() {
         int number = 1;
         int rows = 10;
         for (int i = 1; i <= rows; i++) {
             for (int j = 1; j <= i; j++) {
                 printf("%d ", number);
     int main() {
         return 0;

    Note that floyd is defined as a separate function and can be exported.

  2. Compile the C code using the most recent version of the Emscripten compiler frontend:
     emcc --no-entry -s EXPORTED_FUNCTIONS=_floyd -o floyd.wasm floyd.c

    The exported functions must be prefixed by _. If you reference that function in, for example, the Java code, the exported name should not contain the underscore.

    It produces a standalone file floyd.wasm in the current working directory.

  3. Add dependencies. The GraalVM SDK Polyglot API is not available by default, but can be easily added as a Maven dependency to your Java project. The GraalWasm artifact should be on the Java module or classpath too. Add the following set of dependencies to the project configuration file (pom.xml in case of Maven).

    • To enable the GraalVM polyglot runtime:
    • To enable Wasm:
  4. Now you can embed this WebAssembly function in a Java application, for example:

     import org.graalvm.polyglot.*;
     import org.graalvm.polyglot.io.ByteSequence;
     // Load the WebAssembly contents into a byte array
     byte[] binary = Files.readAllBytes(Path.of("path", "to", "wasm", "file", "floyd.wasm"));
     // Setup context
     Context.Builder contextBuilder = Context.newBuilder("wasm").option("wasm.Builtins", "wasi_snapshot_preview1");
     Source.Builder sourceBuilder = Source.newBuilder("wasm", ByteSequence.create(binary), "example");
     Source source = sourceBuilder.build();
     Context context = contextBuilder.build();
     // Evaluate the WebAssembly module
     // Execute the floyd function
     Value mainFunction =context.getBindings("wasm").getMember("example").getMember("floyd");
  5. Compile and run this Java application with Maven as usual.

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