java_library: output JNI headers generated by javac

rules/java_rules.build_defs

@@ -4,7 +4,10 @@ def java_library(name:str, srcs:list=None, src_dir:str=None, resources:list=[],
                  deps:list=[], modular:bool=False, exported_deps:list=None, visibility:list=None,
                  test_only:bool&testonly=False, javac_flags:list&javacopts=None, labels:list=[],
                  toolchain:str=CONFIG.JAVA_TOOLCHAIN):
-    """Compiles Java source to a .jar which can be collected by other rules.
+    """Compiles Java source to a .jar which can be collected by other rules. If the source makes use of
+    the Java Native Interface (JNI), and the target is included as a dependency of a C/C++ rule, this
+    rule instead outputs the JNI headers generated by the Java compiler to allow the source of the
+    native-code component to be built.
 
     Args:
       name (str): Name of the rule
@@ -84,6 +87,33 @@ def java_library(name:str, srcs:list=None, src_dir:str=None, resources:list=[],
 
         src_dir_label = [] if srcs else ['src_dir:'+src_dir]
 
+        def _jni_header_outputs(name, output):
+            for hdr_path in output:
+                if hdr_path != "":
+                    add_out(name, hdr_path)
+                    add_label(name, "cc:inc:" + join_path(package_name(), dirname(hdr_path)))
+
+        jni_hdrs = build_rule(
+            name = name,
+            tag = "jni_hdrs",
+            srcs = {
+                "WORKER": srcs or [src_dir],
+            },
+            cmd = " && ".join([
+                "mkdir -p _jni",
+                f"{javac_cmd} -h _jni >/dev/null 2>&1",
+                "find _jni -type f -name '*.h' | sort",
+            ]),
+            post_build = _jni_header_outputs,
+            labels = labels,
+            tools = {
+                "javac": [javac],
+            },
+            test_only = test_only,
+            visibility = visibility,
+            deps = deps,
+        )
+
         return build_rule(
             name=name,
             srcs={
@@ -99,6 +129,9 @@ def java_library(name:str, srcs:list=None, src_dir:str=None, resources:list=[],
             cmd=cmd,
             building_description="Compiling...",
             requires=['java'],
+            provides={
+                "cc_hdrs": jni_hdrs,
+            },
             labels = labels + ['rule:java_test_library' if test_only else 'rule:java_library'] + src_dir_label,
             test_only=test_only,
             tools={