Handle JPype's macOS java_home fallback path

When JPype finds no JVM library via JAVA_HOME or its search locations,
it falls back to /usr/libexec/java_home, whose output it returns as the
bytes of a Java home directory rather than a JVM library path. This
broke jvm_version() with a TypeError, and jpype.startJVM() cannot start
from that directory either.

Decode the path and look for bin/java when it is a directory, and point
JAVA_HOME at that directory before starting the JVM, so that JPype then
finds the JVM library inside it.

Fixes #95.

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

Author: ctrueden

src/scyjava/_jvm.py

@@ -70,6 +70,9 @@ def jvm_version() -> tuple[int, ...]:
     default_jvm_path = jpype.getDefaultJVMPath()
     if not default_jvm_path:
         raise RuntimeError("Cannot glean the default JVM path")
+    # Note: On macOS, JPype may fall back to /usr/libexec/java_home, which
+    # yields a Java home directory as bytes, rather than a JVM library path.
+    default_jvm_path = os.fsdecode(default_jvm_path)
     _logger.debug(f"Default JVM path from JPype: {default_jvm_path}")
 
     # Good ol' macOS! Nothing beats macOS.
@@ -104,6 +107,13 @@ def jvm_version() -> tuple[int, ...]:
         if java is None:
             raise RuntimeError(f"Invalid default JVM path: {p}")
 
+    if java is None and p.is_dir():
+        java = p / "bin" / "java"
+        if os.name == "nt":
+            java = java.with_suffix(".exe")
+        if not java.is_file():
+            raise RuntimeError(f"No bin/java found inside: {p}")
+
     if java is None:
         for _ in range(3):  # The bin folder is always <=3 levels up from libjvm.
             p = p.parent
@@ -181,6 +191,17 @@ def start_jvm(options: Sequence[str] | None = None) -> None:
     # download Java as appropriate
     resolve_java()
 
+    # Note: When JPype finds no JVM library via JAVA_HOME (e.g. JAVA_HOME=/usr
+    # from macOS's /usr/bin/java stub), it falls back to /usr/libexec/java_home,
+    # which yields a Java home directory rather than a JVM library path, and
+    # from which JPype cannot start the JVM. But given that directory as
+    # JAVA_HOME, JPype finds the JVM library inside it.
+    default_jvm_path = jpype.getDefaultJVMPath()
+    if default_jvm_path and os.path.isdir(default_jvm_path):
+        java_home = os.fsdecode(default_jvm_path)
+        _logger.debug(f"Detected JAVA_HOME: {java_home}")
+        os.environ["JAVA_HOME"] = java_home
+
     # Fail fast if Java version is too old. JPype 1.6+ dropped Java 8 support.
     try:
         ver = jvm_version()

tests/test_jvm_version.py

@@ -2,7 +2,13 @@
 Tests for functions in _versions submodule.
 """
 
-from scyjava._jvm import _jvm_version_str_to_tuple
+import os
+import subprocess
+from unittest import mock
+
+import jpype
+
+from scyjava._jvm import _jvm_version_str_to_tuple, jvm_version
 
 
 def test_jvm_version():
@@ -20,3 +26,20 @@ def test_jvm_version():
     )
     assert _jvm_version_str_to_tuple(' version "1.8.0_312"', "java") == (1, 8, 0)
     assert _jvm_version_str_to_tuple(' version "25"', "java") == (25,)
+
+
+def test_jvm_version_from_java_home_bytes(tmp_path):
+    # JPype's macOS java_home fallback yields a Java home directory as bytes.
+    java = tmp_path / "bin" / ("java.exe" if os.name == "nt" else "java")
+    java.parent.mkdir()
+    java.touch()
+    output = b'openjdk version "25.0.1" 2025-10-21'
+    with (
+        mock.patch.object(jpype, "getJVMVersion", return_value=(0, 0, 0)),
+        mock.patch.object(
+            jpype, "getDefaultJVMPath", return_value=os.fsencode(tmp_path)
+        ),
+        mock.patch.object(subprocess, "check_output", return_value=output) as co,
+    ):
+        assert jvm_version() == (25, 0, 1)
+    assert co.call_args.args[0] == [str(java), "-version"]