quarkusio/quarkus · error · UncheckedIOException

Failed to collect project's classes in a temporary dir

Error message

Failed to collect project's classes in a temporary dir

What it means

QuarkusGradleUtils.mergeClassesDirs copies the project's compiled classes directories into a single merged temporary directory; if any IOException occurs during copying it wraps it in an UncheckedIOException with the message 'Failed to collect project's classes in a temporary dir'. This blocks dev mode because Quarkus needs one merged output directory for the application classes.

Source

Thrown at devtools/gradle/gradle-application-plugin/src/main/java/io/quarkus/gradle/tasks/QuarkusGradleUtils.java:76

        try {
            Path mergedClassesDir = tmpDir.toPath().resolve("quarkus-app-classes" + (test ? "-test" : ""));

            if (!populated) {
                return mergedClassesDir;
            }

            if (Files.exists(mergedClassesDir)) {
                IoUtils.recursiveDelete(mergedClassesDir);
            }

            for (Path classesDir : existingClassesDirs) {
                IoUtils.copy(classesDir, mergedClassesDir);
            }

            return mergedClassesDir;
        } catch (IOException e) {
            throw new UncheckedIOException(ERROR_COLLECTING_PROJECT_CLASSES, e);
        }
    }

}

View on GitHub (pinned to e1c734241f)

Solutions

1. Check available disk space and write permissions on java.io.tmpdir (or the configured temp location).
2. Stop duplicate Gradle daemons: ./gradlew --stop, then re-run the dev-mode task.
3. Set the java.io.tmpdir system property to a writable directory: GRADLE_OPTS=-Djava.io.tmpdir=/path/writable.
4. Rebuild the project (./gradlew classes) to ensure the classes dirs exist before dev mode.
5. Exclude the project directory from antivirus/indexing tools that lock files during copying.

Example fix

// before: full or unwritable default temp dir
// after: point Gradle at a writable temp dir
// in gradle.properties or environment
org.gradle.jvmargs=-Djava.io.tmpdir=/home/user/tmp

// Pre-check writable temp space before running dev mode
import java.nio.file.*;
Path tmp = Paths.get(System.getProperty("java.io.tmpdir"));
Files.createTempFile("quarkus-check", ".tmp");
if (tmp.toFile().getUsableSpace() < 100_000_000L) {
  throw new IllegalStateException("Insufficient space in " + tmp + "; set -Djava.io.tmpdir");
}

try {
  ./gradlew quarkusDev
} catch (UncheckedIOException e) {
  if (e.getMessage().contains("Failed to collect project's classes")) {
    // free temp space, stop extra daemons (./gradlew --stop), rerun with GRADLE_OPTS=-Djava.io.tmpdir=<writable>
  }
}

When it happens

Trigger: Calling QuarkusGradleUtils.mergeClassesDirs (via merged) when creating or copying into the temporary merged-classes dir fails: missing read permissions on source classes dirs, unwritable temp directory, or file locks held by another process.

Common situations: Read-only or full TMPDIR (/tmp full or noexec/quota limits); two Gradle daemons or an IDE build running concurrently and holding file locks; classes directory deleted mid-build by a parallel clean; sandboxed CI environments restricting temp file creation.

Related errors

• Failed to write Quarkus build configuration settings
• Failed to get source file mtime for
• Failed to delete file:
• Failed to create output directory for generated sources: %s
• IOException (wrapped)

AI-assisted analysis of quarkusio/quarkus@e1c734241f (2026-09-05). Data as JSON: /api/errors/a2d69ad33cd38479. Report an issue: GitHub.