Use /usr/libexec/java_home as a fallback for JAVA_HOME

docs/targets.md

@@ -43,7 +43,7 @@ runtime when its first symbol is accessed. Concretely, it does the following in
     1. Depend on `@fmeum_rules_jni//jni:libjvm` directly from your top-level `cc_binary`.
     2. Add `#include <rules_jni.h>`.
     3. Call `rules_jni_init(const char* argv0)` from your `main` function, providing `argv[0]` as the argument.
-   
+
    If you want this lookup to succeed also for binaries executed by other binaries that are themselves run from Bazel,
    [set the environment variables required for runfiles discovery](https://github.com/bazelbuild/bazel/blob/e8a066e9e625a136363338d10f03ed14c26dedfa/tools/cpp/runfiles/runfiles_src.h#L58).
 
@@ -52,7 +52,9 @@ runtime when its first symbol is accessed. Concretely, it does the following in
 3. If `JAVA_HOME` is set, find the `jvm` shared library relative to it at well-known locations, exiting if it cannot be
    found.
 
-4. If `PATH` is set, find the Java binary (`java` or `java.exe`) on it and try to load the `jvm` shared library from
+4. (macOS only) Execute `/usr/libexec/java_home` and use its output as a replacement for `JAVA_HOME`.
+
+5. If `PATH` is set, find the Java binary (`java` or `java.exe`) on it and try to load the `jvm` shared library from
    well-known locations relative to it, exiting if it cannot be found.
 
 To get detailed runtime logs from this location procedure, set the environment variable `RULES_JNI_TRACE` to a non-empty

jni/tools/libjvm_stub/libjvm_stub.c

@@ -189,6 +189,7 @@ static char* find_java_executable() {
 /* The returned string has to be freed by the caller. */
 static char* find_java_home() {
   const char* java_home_env;
+  char* java_home_fallback = NULL;
   char* java_executable_path = NULL;
   char* pos;
   size_t separator_count_from_end;
@@ -205,6 +206,12 @@ static char* find_java_home() {
     return res;
   }
 
+  java_home_fallback = get_java_home_fallback();
+  if (java_home_fallback != NULL) {
+    trace("get_java_home_fallback() returned: %s", java_home_fallback);
+    return java_home_fallback;
+  }
+
   java_executable_path = find_java_executable();
   if (java_executable_path == NULL) {
     return NULL;

jni/tools/libjvm_stub/unix/utils.h

@@ -18,6 +18,10 @@
 #include <stddef.h>
 #include <stdio.h>
 #include <stdlib.h>
+#ifdef __APPLE__
+#include <string.h>
+#include <sys/syslimits.h>
+#endif
 #include <unistd.h>
 
 static const char* JAVA_EXECUTABLE = "/java";
@@ -41,6 +45,44 @@ static const size_t MAX_CANDIDATE_PATH_LENGTH = 22;
 
 static int executable_exists(const char* path) { return access(path, X_OK); }
 
+#ifdef __APPLE__
+static char* our_strdup(const char* src);
+#endif
+
+/* The returned string has to be freed by the caller. */
+static char* get_java_home_fallback() {
+#ifdef __APPLE__
+  char* res = NULL;
+  FILE* pipe = NULL;
+  char buffer[PATH_MAX];
+
+  /* java_home prints the JAVA_HOME of the default installation to stdout.
+   * Silence warnings by redirecting stderr to /dev/null. */
+  pipe = popen("/usr/libexec/java_home 2> /dev/null", "r");
+  if (pipe == NULL) {
+    goto cleanup;
+  }
+
+  res = fgets(buffer, sizeof(buffer), pipe);
+  if (res == NULL || strcmp(res, "") == 0) {
+    goto cleanup;
+  }
+
+  /* The output of java_home is terminated by a newline. Skip over it. */
+  res[strlen(res) - 1] = '\0';
+  res = strdup(res);
+
+cleanup:
+  if (pipe != NULL) {
+    pclose(pipe);
+  }
+
+  return res;
+#else
+  return NULL;
+#endif
+}
+
 static void* load_library(const char* path) {
   return dlopen(path, RTLD_LAZY | RTLD_LOCAL);
 }

jni/tools/libjvm_stub/windows/utils.h

@@ -41,6 +41,8 @@ static int executable_exists(const char* path) {
   return 0;
 }
 
+static char* get_java_home_fallback() { return NULL; }
+
 static void* load_library(const char* path) { return LoadLibrary(path); }
 
 static void* load_symbol(void* library_handle, const char* symbol) {

tests/libjvm_stub/BUILD.bazel

@@ -11,6 +11,12 @@ cc_test(
         "PATH_OVERRIDE": "",
         "RULES_JNI_TRACE": "1",
     },
+    target_compatible_with = select({
+        # Skip on macOS since the /usr/libexec/java_home fallback means that
+        # a JDK will still be found without PATH and JAVA_HOME.
+        "@platforms//os:macos": ["@platforms//:incompatible"],
+        "//conditions:default": [],
+    }),
     deps = [
         "@fmeum_rules_jni//jni:libjvm_lite",
     ],