Add example of using JNI to call into Rust

examples/.bazelrc

@@ -21,6 +21,11 @@ common --noenable_bzlmod
 # This isn't currently the defaut in Bazel, but we enable it to test we'll be ready if/when it flips.
 build --incompatible_disallow_empty_glob
 
+build --java_runtime_version=remotejdk_21
+build --java_language_version=21
+build --tool_java_runtime_version=remotejdk_21
+build --tool_java_language_version=21
+
 # This import should always be last to allow users to override
 # settings for local development.
 try-import %workspace%/user.bazelrc

examples/WORKSPACE.bazel

@@ -34,6 +34,17 @@ load("@rules_rust//bindgen:transitive_repositories.bzl", "rust_bindgen_transitiv
 
 rust_bindgen_transitive_dependencies()
 
+load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
+
+# We need to load rules_java before anything proto-related happens because otherwise it will pull in its own rules_java which isn't compatible with rules_jvm_external.
+http_archive(
+    name = "rules_java",
+    sha256 = "f8ae9ed3887df02f40de9f4f7ac3873e6dd7a471f9cddf63952538b94b59aeb3",
+    urls = [
+        "https://github.com/bazelbuild/rules_java/releases/download/7.6.1/rules_java-7.6.1.tar.gz",
+    ],
+)
+
 load("@rules_rust//proto/protobuf:repositories.bzl", "rust_proto_protobuf_dependencies", "rust_proto_protobuf_register_toolchains")
 
 rust_proto_protobuf_dependencies()
@@ -58,8 +69,6 @@ load("@rules_rust//wasm_bindgen/rules_js:repositories.bzl", "js_rust_wasm_bindge
 
 js_rust_wasm_bindgen_dependencies()
 
-load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
-
 ###############################################################################
 # Workspace examples
 ###############################################################################
@@ -161,6 +170,39 @@ http_archive(
     ],
 )
 
+http_archive(
+    name = "rules_jvm_external",
+    sha256 = "08ea921df02ffe9924123b0686dc04fd0ff875710bfadb7ad42badb931b0fd50",
+    strip_prefix = "rules_jvm_external-6.1",
+    url = "https://github.com/bazelbuild/rules_jvm_external/releases/download/6.1/rules_jvm_external-6.1.tar.gz",
+)
+
+load("@rules_jvm_external//:repositories.bzl", "rules_jvm_external_deps")
+
+rules_jvm_external_deps()
+
+load("@rules_jvm_external//:setup.bzl", "rules_jvm_external_setup")
+
+rules_jvm_external_setup()
+
+load("@rules_jvm_external//:defs.bzl", "maven_install")
+
+maven_install(
+    name = "maven",
+    artifacts = [
+        "net.java.dev.jna:jna:5.14.0",
+        "org.hamcrest:hamcrest:2.2",
+    ],
+    maven_install_json = "@//:maven_install.json",
+    repositories = [
+        "https://repo1.maven.org/maven2",
+    ],
+)
+
+load("@maven//:defs.bzl", "pinned_maven_install")
+
+pinned_maven_install()
+
 ###############################################################################
 
 http_archive(

examples/ffi/java_calling_rust/com/example/rustjni/BUILD.bazel

@@ -0,0 +1,21 @@
+load("@rules_java//java:defs.bzl", "java_library", "java_test")
+
+java_library(
+    name = "rustjni",
+    srcs = ["RustStringLength.java"],
+    data = ["//ffi/java_calling_rust/rust-crate:rstrlen"],
+    deps = [
+        "@bazel_tools//tools/java/runfiles",
+        "@maven//:net_java_dev_jna_jna",
+    ],
+)
+
+java_test(
+    name = "rustjni_test",
+    srcs = ["RustJniTest.java"],
+    test_class = "com.example.rustjni.RustJniTest",
+    deps = [
+        ":rustjni",
+        "@maven//:org_hamcrest_hamcrest",
+    ],
+)

examples/ffi/java_calling_rust/com/example/rustjni/RustJniTest.java

@@ -0,0 +1,15 @@
+package com.example.rustjni;
+
+import org.junit.Test;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.equalTo;
+
+public class RustJniTest {
+    @Test
+    public void testCallsJniToRust() throws Exception {
+        final String s = "hello";
+        long result = RustStringLength.loadNativeLibrary().calculate_string_length_from_rust(s);
+        assertThat(result, equalTo(5L));
+    }
+}

examples/ffi/java_calling_rust/com/example/rustjni/RustStringLength.java

@@ -0,0 +1,29 @@
+package com.example.rustjni;
+
+import com.google.devtools.build.runfiles.AutoBazelRepository;
+import com.google.devtools.build.runfiles.Runfiles;
+
+import com.sun.jna.Library;
+import com.sun.jna.Native;
+
+import java.io.IOException;
+
+@AutoBazelRepository
+public interface RustStringLength extends Library {
+    long calculate_string_length_from_rust(String s);
+
+    static RustStringLength loadNativeLibrary() throws IOException {
+        String prefix = "lib";
+        String extension = "so";
+        if ("Mac OS X".equals(System.getProperty("os.name"))) {
+            extension = "dylib";
+        } else if (System.getProperty("os.name").contains("Windows")) {
+            prefix = "";
+            extension = "dll";
+        }
+        Runfiles.Preloaded runfiles = Runfiles.preload();
+        String dylibPath = runfiles.withSourceRepository(AutoBazelRepository_RustStringLength.NAME).rlocation("examples/ffi/java_calling_rust/rust-crate/" + prefix + "rstrlen." + extension);
+
+        return Native.load(dylibPath, RustStringLength.class);
+    }
+}

examples/ffi/java_calling_rust/rust-crate/BUILD.bazel

@@ -0,0 +1,8 @@
+load("@rules_rust//rust:defs.bzl", "rust_shared_library")
+
+rust_shared_library(
+    name = "rstrlen",
+    srcs = ["rust_string_length.rs"],
+    edition = "2021",
+    visibility = ["//visibility:public"],
+)

examples/ffi/java_calling_rust/rust-crate/rust_string_length.rs

@@ -0,0 +1,16 @@
+use std::ffi::CStr;
+use std::os::raw::c_char;
+
+/// Calculates the length of a string.
+///
+/// # Safety
+///
+/// The argument must null-terminated.
+#[no_mangle]
+pub unsafe extern "C" fn calculate_string_length_from_rust(s: *const c_char) -> i64 {
+    let slice = unsafe { CStr::from_ptr(s).to_bytes() };
+    std::str::from_utf8(slice).map_or(-1, |s| {
+        let l = s.len();
+        l.try_into().unwrap_or(-1)
+    })
+}

examples/maven_install.json

@@ -0,0 +1,49 @@
+{
+  "__AUTOGENERATED_FILE_DO_NOT_MODIFY_THIS_FILE_MANUALLY": "THERE_IS_NO_DATA_ONLY_ZUUL",
+  "__INPUT_ARTIFACTS_HASH": -152875169,
+  "__RESOLVED_ARTIFACTS_HASH": 61617868,
+  "artifacts": {
+    "net.java.dev.jna:jna": {
+      "shasums": {
+        "jar": "34ed1e1f27fa896bca50dbc4e99cf3732967cec387a7a0d5e3486c09673fe8c6"
+      },
+      "version": "5.14.0"
+    },
+    "org.hamcrest:hamcrest": {
+      "shasums": {
+        "jar": "5e62846a89f05cd78cd9c1a553f340d002458380c320455dd1f8fc5497a8a1c1"
+      },
+      "version": "2.2"
+    }
+  },
+  "dependencies": {},
+  "packages": {
+    "net.java.dev.jna:jna": [
+      "com.sun.jna",
+      "com.sun.jna.internal",
+      "com.sun.jna.ptr",
+      "com.sun.jna.win32"
+    ],
+    "org.hamcrest:hamcrest": [
+      "org.hamcrest",
+      "org.hamcrest.beans",
+      "org.hamcrest.collection",
+      "org.hamcrest.comparator",
+      "org.hamcrest.core",
+      "org.hamcrest.internal",
+      "org.hamcrest.io",
+      "org.hamcrest.number",
+      "org.hamcrest.object",
+      "org.hamcrest.text",
+      "org.hamcrest.xml"
+    ]
+  },
+  "repositories": {
+    "https://repo1.maven.org/maven2/": [
+      "net.java.dev.jna:jna",
+      "org.hamcrest:hamcrest"
+    ]
+  },
+  "services": {},
+  "version": "2"
+}