Ask the java executable for its java.home

The java executable located by jgo is not necessarily at JAVA_HOME/bin:
it may be a symlink (Linux alternatives) or a launcher stub (macOS's
/usr/bin/java). In such cases, resolve_java() set JAVA_HOME to /usr,
where JPype finds no JVM library.

Instead, query the java.home property, falling back to the parent of
the executable's bin folder if that fails.

See #95.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: ctrueden

src/scyjava/_jdk_fetch.py

@@ -6,15 +6,13 @@
 
 import logging
 import os
-from typing import TYPE_CHECKING
+import subprocess
+from pathlib import Path
 
 from jgo.exec import JavaLocator, JavaSource
 
 import scyjava.config
 
-if TYPE_CHECKING:
-    from pathlib import Path
-
 _logger = logging.getLogger(__name__)
 
 
@@ -59,13 +57,43 @@ def resolve_java(vendor: str | None = None, version: str | None = None) -> None:
 
     # Locate returns path to java executable (e.g., /path/to/java/bin/java)
     java_exe = locator.locate()
-    java_home = java_exe.parent.parent  # Navigate from bin/java to JAVA_HOME
+    java_home = _java_home(java_exe)
 
     _logger.debug(f"java_home -> {java_home}")
     _add_to_path(str(java_home / "bin"), front=True)
     os.environ["JAVA_HOME"] = str(java_home)
 
 
+def _java_home(java_exe: Path) -> Path:
+    """
+    Determine the Java installation directory of the given java executable.
+
+    The executable is asked for its java.home property, because it is not
+    necessarily located at JAVA_HOME/bin/java: it might be a symlink (e.g.
+    /usr/bin/java with Linux alternatives) or a launcher stub (e.g. macOS's
+    /usr/bin/java, which delegates to the registered JDK).
+    """
+    try:
+        output = subprocess.run(
+            [str(java_exe), "-XshowSettings:properties", "-version"],
+            check=False,
+            capture_output=True,
+            text=True,
+            timeout=30,
+        ).stderr
+        for line in output.splitlines():
+            key, sep, value = line.partition("=")
+            if sep and key.strip() == "java.home":
+                java_home = Path(value.strip())
+                if java_home.is_dir():
+                    return java_home
+    except (OSError, subprocess.SubprocessError) as e:
+        _logger.debug(f"Failed to query java.home from {java_exe}: {e}")
+
+    _logger.debug(f"Could not query java.home from {java_exe}; assuming bin/java")
+    return java_exe.parent.parent
+
+
 def _add_to_path(path: Path | str, front: bool = False) -> None:
     """Add a path to the PATH environment variable.
 

tests/test_jdk_fetch.py

@@ -0,0 +1,42 @@
+"""
+Tests for functions in _jdk_fetch submodule.
+"""
+
+import os
+
+import pytest
+
+from scyjava._jdk_fetch import _java_home
+
+posix_only = pytest.mark.skipif(os.name == "nt", reason="uses a shell script")
+
+
+def _fake_java(path, stderr):
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(f"#!/bin/sh\ncat >&2 <<'EOF'\n{stderr}\nEOF\n")
+    path.chmod(0o755)
+    return path
+
+
+@posix_only
+def test_java_home_from_launcher_stub(tmp_path):
+    # E.g. macOS's /usr/bin/java, which is not located inside its Java home.
+    real_home = tmp_path / "jdk" / "Contents" / "Home"
+    real_home.mkdir(parents=True)
+    java = _fake_java(
+        tmp_path / "usr" / "bin" / "java",
+        f"Property settings:\n    java.home = {real_home}\n"
+        '    java.version = 25.0.1\n\nopenjdk version "25.0.1"',
+    )
+    assert _java_home(java) == real_home
+
+
+@posix_only
+def test_java_home_fallback(tmp_path):
+    java = _fake_java(tmp_path / "jdk" / "bin" / "java", "no properties here")
+    assert _java_home(java) == tmp_path / "jdk"
+
+
+def test_java_home_missing_executable(tmp_path):
+    java = tmp_path / "jdk" / "bin" / "java"
+    assert _java_home(java) == tmp_path / "jdk"