fix: use the JDK version from the JAR when no language version is recorded

src/macaron/build_spec_generator/common_spec/maven_spec.py

@@ -84,7 +84,7 @@ def resolve_fields(self, purl: PackageURL) -> None:
         existing = self.data["language_version"][0] if self.data["language_version"] else None
 
         # Select JDK from jar or another source, with a default of version 8.
-        selected_jdk_version = jdk_from_jar or existing if existing else "8"
+        selected_jdk_version = jdk_from_jar or existing or "8"
 
         major_jdk_version = normalize_jdk_version(selected_jdk_version)
         if not major_jdk_version:

tests/build_spec_generator/common_spec/test_maven_spec.py

@@ -0,0 +1,66 @@
+# Copyright (c) 2026 - 2026, Oracle and/or its affiliates. All rights reserved.
+# Licensed under the Universal Permissive License v 1.0 as shown at https://oss.oracle.com/licenses/upl/.
+
+"""This module tests the Maven build specification field resolution."""
+
+from unittest.mock import patch
+
+import pytest
+from packageurl import PackageURL
+
+from macaron.build_spec_generator.common_spec.base_spec import BaseBuildSpecDict
+from macaron.build_spec_generator.common_spec.maven_spec import MavenBuildSpec
+
+PURL_STRING = "pkg:maven/com.example/demo@1.0.0"
+
+
+def _build_spec_dict(language_version: list[str]) -> BaseBuildSpecDict:
+    """Return a minimal Maven build spec dict carrying the passed language version list."""
+    return BaseBuildSpecDict(
+        ecosystem="maven",
+        purl=PURL_STRING,
+        language="java",
+        build_tools=["maven"],
+        macaron_version="0.0.0",
+        group_id="com.example",
+        artifact_id="demo",
+        version="1.0.0",
+        language_version=list(language_version),
+        build_commands=[
+            {
+                "build_tool": "maven",
+                "build_config_path": "pom.xml",
+                "command": ["mvn", "clean", "package"],
+                "confidence_score": 1.0,
+            }
+        ],
+    )
+
+
+def _resolve(language_version: list[str], jdk_from_jar: str | None) -> list[str]:
+    """Resolve a Maven build spec with the JAR lookup pinned, and return the resulting language version."""
+    data = _build_spec_dict(language_version)
+    with patch(
+        "macaron.build_spec_generator.common_spec.maven_spec.find_jdk_version_from_central_maven_repo",
+        return_value=jdk_from_jar,
+    ):
+        MavenBuildSpec(data).resolve_fields(PackageURL.from_string(PURL_STRING))
+    return data["language_version"]
+
+
+@pytest.mark.parametrize(
+    ("existing", "jdk_from_jar", "expected"),
+    [
+        # The JAR manifest is the strongest evidence and wins whether or not the database
+        # recorded a language version. The empty-list case is the ordinary one: core.py sets
+        # "language_version" to [] whenever the database holds no recorded version.
+        ([], "17", ["17"]),
+        (["11"], "17", ["17"]),
+        # No JAR evidence: fall back to what the database recorded, then to the default of 8.
+        (["11"], None, ["11"]),
+        ([], None, ["8"]),
+    ],
+)
+def test_resolve_fields_selects_jdk_version(existing: list[str], jdk_from_jar: str | None, expected: list[str]) -> None:
+    """The JDK read from the Maven Central JAR must not be discarded when the database recorded none."""
+    assert _resolve(existing, jdk_from_jar) == expected