CODEWITHSUNDEEP
javajava-native-interfacebazel
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Reputation: 173

Bazel: Java app with JNI dependency

I've managed to build my JNI library (the jar, the jni shared cc_library, the wrapped cc_library) but I don't see how to build a Java app that uses it. My BUILD is simple:

java_binary(
    name = "OCFTestServer",
    srcs = glob(["src/main/java/**/*.java"]),
    deps = ["//:OpenOCF-JNI"])

Here OpenOCF-JNI looks like this:

java_library(
    name = "OpenOCF-JNI",
    srcs = glob(["src/main/**/*.java"]),
    deps = ["libopenocf"],
    visibility = ["//visibility:public"])

And libopenocf is:

cc_library(
    name = "libopenocf",
    srcs = glob(["src/c/*.c"]) + glob(["src/c/*.h"])
    + ["@local_jdk//:jni_header",
       "@local_jdk//:jni_md_header-darwin"],
    ... etc ...

These all build successfully. However, building does not cause dependencies to be built, which is what I would expect (i.e. building OCFTestServer should cause OpenOCF-JNI to be built, which should cause libopenocf-jni to be built). Shouldn't that happen?

If I build them all using separate steps and then try to run the application (using the OCFTestServer wrapper in bazel-bin), I get UnsatisfiedLinkError: no libopenocf-jni in java.library.path. But from reading the docs I get the impression that this should all be set up automatically (i.e. the needed jni lib should be put in the java.library.path).

What am I doing wrong? Does anybody have an example of building and using a JNI lib?

Upvotes: 2

Views: 2891

Answers (3)

bluearty
bluearty

Reputation: 24

Actually, you don't need to add bazel local_jdk jni and jni_md headers to your package cc_library target. But, you can copy them to your project package.

https://github.com/bazelbuild/bazel/blob/c97c77333cab2b78dc8ac76834fb873389f520d7/src/main/native/BUILD#L4

I have already submit PR to https://github.com/mhlopko/bazel-jni-example project to fix the jni relative path problem. maybe this could help you.

Upvotes: 0

hlopko
hlopko

Reputation: 3268

I created a simple repo: https://github.com/mhlopko/bazel-jni-example to help you get started.

BUILD:

cc_library(
    name = "main-jni-lib",
    srcs = [
        "@local_jdk//:jni_header",
        "@local_jdk//:jni_md_header-linux",
        "Main.cc"
        ],
    hdrs = [ "Main.h" ],
    includes = [ "external/local_jdk/include", "external/local_jdk/include/linux" ],
)

cc_binary(
    name = "libmain-jni.so",
    deps = [ ":main-jni-lib" ],
    linkshared = 1,
)

java_binary(
    name = "Main",
    srcs = [ "Main.java" ],
    main_class = "Main",
    data = [ ":libmain-jni.so" ],
    jvm_flags = [ "-Djava.library.path=." ],
)

Main.java:

public class Main {
  static {
    System.loadLibrary("main-jni");
  }

  private native int foo();

  public static void main(String[] args) {
    System.out.println(new Main().foo());
  }
}

Main.h:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Main */

#ifndef _Included_Main
#define _Included_Main
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Main
 * Method:    foo
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_Main_foo(JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

Main.cc:

#include <jni.h>
#include <stdio.h>
#include "Main.h"

JNIEXPORT jint JNICALL Java_Main_foo(JNIEnv *, jobject) {
   return 42;
}

Now by running bazel run :Main you should see 42 printed out, that is coming from Main.cc. The example clearly needs more polish so it works on other-than-linux platforms, and so it works with the launcher script. You might end up needing multiple System.loadLibrary calls, like bazel does in its windows loader.

Upvotes: 5

kd8azz
kd8azz

Reputation: 623

I stumbled across your question, while trying to figure out how to link to malmo, which I just got working. It's linking against a static library, so it may or may not help you.

third_party/BUILD:

java_import(
    name = "malmo",
    jars = ["MalmoJavaJar.jar"],
    deps = [":libMalmo"],
)

cc_library(
    name = "libMalmo",
    srcs = ["libMalmoJava.so"],
)

And then in my actual target:

    "//third_party:malmo",

Upvotes: 0

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