slint-ui/slint · error

Could not generate the java compiler command

Error message

Could not generate the java compiler command: {e}

What it means

The android-activity build script builds a javac command via the android-build helper and fails when `JavaBuild::command()` cannot construct the compiler invocation — typically because a JDK/javac toolchain cannot be located or configured. The build aborts before even attempting compilation.

Solutions

1. Install a full JDK (not a JRE) so javac exists (e.g. apt install openjdk-17-jdk or sdkman)
2. Set JAVA_HOME to the JDK root and ensure it's exported in the build environment
3. Verify `javac -version` works in the shell running cargo
4. Check the android SDK/platform setup (android_jar lookup precedes this) matches requirements

Example fix

// before
export JAVA_HOME=/usr/lib/jvm/java-17-jre
// after
export JAVA_HOME=/usr/lib/jvm/java-17-jdk
export PATH="$JAVA_HOME/bin:$PATH"

import shutil, os
assert shutil.which("javac"), "javac not found; install a JDK"
assert "jdk" in os.environ.get("JAVA_HOME", "").lower() or os.path.exists(os.path.join(os.environ.get("JAVA_HOME", ""), "bin", "javac")), "JAVA_HOME must point to a JDK"

When it happens

Trigger: `JavaBuild::new()...command()` returns Err — no JAVA_HOME / JDK found, javac missing from PATH resolution, or invalid configuration passed to JavaBuild.

Common situations: Docker/CI images without a JDK installed; JAVA_HOME pointing at a JRE (no javac); broken Android SDK/NDK setup altering tool discovery.

Related errors

• Cannot create output directory
• Could not run the java compiler
• Java compilation failed
• create
• write

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/d677ea3abb0b3c9d. Report an issue: GitHub.

Appendix: source

Thrown at internal/backends/android-activity/build.rs:49

    std::fs::create_dir_all(&out_class_dir)
        .unwrap_or_else(|e| panic!("Cannot create output directory {out_class_dir:?} - {e}"));

    let android_jar = android_build::android_jar(None).expect("No Android platforms found");

    // Compile the Java file into .class files
    let o = JavaBuild::new()
        .file(&java_src_path)
        .class_path(&android_jar)
        .classes_out_dir(&out_class_dir)
        .java_source_version(8)
        .java_target_version(8)
        .debug_info(android_build::DebugInfo {
            line_numbers: !release_mode,
            variables: !release_mode,
            source_files: !release_mode,
        })
        .command()
        .unwrap_or_else(|e| panic!("Could not generate the java compiler command: {e}"))
        .args(["-encoding", "UTF-8"])
        .output()
        .unwrap_or_else(|e| panic!("Could not run the java compiler: {e}"));

    if !o.status.success() {
        panic!("Java compilation failed: {}", String::from_utf8_lossy(&o.stderr));
    }

    let o = Dexer::new()
        .android_jar(&android_jar)
        .class_path(&out_class_dir)
        .collect_classes(&out_class_dir)
        .unwrap()
        .release(release_mode)
        .android_min_api(20) // disable multidex for single dex file output
        .out_dir(out_dir)
        .command()
        .unwrap_or_else(|e| panic!("Could not generate the D8 command: {e}"))

View on GitHub (pinned to bb937076de)