Compile Java to WebAssembly (with GraalVM)

Question

I am trying to compile Java to WebAssembly. Is this possible? My initial approach was to take the example described in this blog post

The approach described in the blog post is as follows:

1. Install GraalVM and add native-image and LLVM toolchain to the installation
2. Use GraalVM's javac to compile a simple program (into a class file)
3. Use GraalVM's native-image program to process the class file into a native binary (with options to preprocess using llvm)
4. Use the LLVM IR bit code files to compile as WASM binary.

I get to step 4 but after that I can't compile the LLVM IR bit code to wasm.

I get the following error from LLVM:

LLVM ERROR: unsupported GC: compressed-pointer

Has anyone tried this and succeeded?

Answer 1

With early access build of GraalVM 25 you can compile java to wasm which also requires binaryen, I downloaded windows package from https://github.com/WebAssembly/binaryen/releases and put it into Path. And for GraalVM I used https://github.com/graalvm/oracle-graalvm-ea-builds/releases

ozkan@HP-ENVY-2021-I7 C:\tmp
$ native-image --tool:svm-wasm HelloWasm
============================================================================================================

GraalVM Native Image: Generating 'hellowasm.exe' (executable)...
============================================================================================================

For detailed information and explanations on the build output, visit:
https://github.com/oracle/graal/blob/master/docs/reference-manual/native-image/BuildOutput.md
------------------------------------------------------------------------------------------------------------

[1/8] Initializing...                                                                       (29.1s @ 0.15GB)

 Java version: 25+14-LTS, vendor version: Oracle GraalVM 25-dev+14.1
 Graal compiler: optimization level: 2, target machine: unknown, PGO: off
 C compiler: cl.exe (microsoft, x64, 19.43.34809)
 Garbage collector: JS-runtime-provided GC (max heap size: unknown)
 1 user-specific feature(s):
 - com.oracle.svm.thirdparty.gson.GsonFeature
------------------------------------------------------------------------------------------------------------

Build resources:
 - 12.79GB of memory (75.6% of system memory, less than 8GB of memory available)
 - 8 thread(s) (100.0% of 8 available processor(s), determined at start)
Warning: We could not find any version info for the following classes:
org.graalvm.aotjs.api.JSError
org.graalvm.aotjs.api.JSBoolean
org.graalvm.aotjs.api.JSNumber
org.graalvm.aotjs.api.JSUndefined
org.graalvm.aotjs.api.JSObject
org.graalvm.aotjs.api.JSSymbol
org.graalvm.aotjs.api.JSString
HelloWasm
org.graalvm.aotjs.api.JSValue
org.graalvm.aotjs.api.JSBigInt
[2/8] Performing analysis...  [***]                                                         (12.1s @ 0.18GB)

    1,018 reachable types   (43.5% of    2,340 total)
      745 reachable fields  (29.4% of    2,534 total)
    3,111 reachable methods (20.1% of   15,456 total)
      408 types,    31 fields, and   111 methods registered for reflection
[3/8] Building universe...                                                                   (2.2s @ 0.23GB)

[4/8] Parsing methods...      [*]                                                            (1.0s @ 0.19GB)

[5/8] Inlining methods...     [***]                                                          (0.7s @ 0.21GB)

[6/8] Compiling methods...    [**]                                                           (3.0s @ 0.23GB)

[7/8] Laying out methods...   [*]                                                            (0.3s @ 0.24GB)

[8/8] Creating image...       [***]                                                          (9.3s @ 0.26GB)

    0.00B ( 0.00%) for code area:     1,630 compilation units
 672.71kB (60.25%) for image heap:    9,174 objects and 9 resources
 443.90kB (39.75%) for other data
   1.12MB in total
------------------------------------------------------------------------------------------------------------

Top 10 origins of code area:                          Top 10 object types in image heap:
    0.00B org.graalvm.collections                      130.88kB byte[] for java.lang.String
    0.00B org.graalvm.extraimage.builder               130.30kB com.oracle.svm.core.hub.DynamicHubCompanion
    0.00B com.oracle.svm.svm_enterprise                107.00kB java.lang.Class
    0.00B svm.jar (Native Image)                        70.09kB char[]
    0.00B org.graalvm.webimage.api                      66.82kB java.lang.String
    0.00B jdk.graal.compiler                            21.17kB c.o.s.c.hub.DynamicHub$ReflectionMetadata
    0.00B jdk.proxy3                                    15.52kB byte[] for general heap data
    0.00B jdk.proxy1                                    15.05kB byte[] for reflection metadata
    0.00B jdk.proxy2                                    13.06kB c.o.s.c.configure.ConditionalRuntimeValue
    0.00B java.base                                     12.35kB java.lang.String[]
    0.00B for 1 more packages                           90.48kB for 187 more object types
                      Use '--emit build-report' to create a report with more details.
------------------------------------------------------------------------------------------------------------

Security report:
 - Binary includes Java deserialization.
 - CycloneDX SBOM with 6 component(s) is embedded in binary (486.00B).
------------------------------------------------------------------------------------------------------------

Recommendations:
 PGO:  Use Profile-Guided Optimizations ('--pgo') for improved throughput.
 HEAP: Set max heap for improved and more predictable memory usage.
 CPU:  Enable more CPU features with '-march=native' for improved performance.
 QBM:  Use the quick build mode ('-Ob') to speed up builds during development.
------------------------------------------------------------------------------------------------------------

                 2.1s (3.2% of total time) in 60 GCs | Peak RSS: 874.91MB | CPU load: 3.87
------------------------------------------------------------------------------------------------------------

Build artifacts:
 C:\tmp\hellowasm.js (executable)
 C:\tmp\hellowasm.js.wasm (executable)
 C:\tmp\hellowasm.js.wat (build_info)
============================================================================================================

Finished generating 'hellowasm' in 59.9s.

ozkan@HP-ENVY-2021-I7 C:\tmp
$ node hellowasm.js
Özkan was here :) Hello from WebAssembly generated with GraalVM!

ozkan@HP-ENVY-2021-I7 C:\tmp
$ type HelloWasm.java
  public class HelloWasm {
      public static void main(String[] args) {
          System.out.println("Özkan was here :) Hello from WebAssembly generated with GraalVM!");
      }
  }
ozkan@HP-ENVY-2021-I7 C:\tmp
$ dir
 Volume in drive C is Windows
 Volume Serial Number is BC27-F52E

 Directory of C:\tmp

Sat 29/03/2025  08:24    <DIR>          .
Sat 29/03/2025  08:34               477 HelloWasm.class
Sat 29/03/2025  08:34               188 HelloWasm.java
Sat 29/03/2025  08:35            97,261 hellowasm.js
Sat 29/03/2025  08:35         1,019,354 hellowasm.js.wasm
Sat 29/03/2025  08:35        25,440,609 hellowasm.js.wat
               5 File(s)     26,557,889 bytes
               1 Dir(s)  91,167,571,968 bytes free