nalim

Nalim is a library for linking Java methods to native functions using JVMCI (JVM compiler interface).

Unlike other Java frameworks for native library access, nalim does not use JNI and therefore does not incur JNI related overhead.

When calling a native function with nalim

• a thread does not switch from in_Java to in_native state and back;
• no memory barrier is involved;
• no JNI handles are created;
• exception checks and safepoint checks are omitted;
• native function can access primitive arrays directly in the heap.

As a result, native calls become faster comparing to JNI, especially when a target function is short. In this sense, nalim is similar to JNI Critical Natives, but relies on a standard supported interface. JNI Critical Natives have been deprecated in JDK 16 and obsoleted since JDK 18, so nalim can serve as a replacement.

Examples

1. Basic usage

public class Libc {

    @Link
    public static native int getuid();

    @Link
    public static native int getgid();

    static {
        Linker.linkClass(Libc.class);
    }
}

System.out.println("My user id = " + Libc.getuid());

2. Linking by a different name

public class Mem {

   @Link(name = "malloc")
   public static native long allocate(long size);

   @Link(name = "free")
   public static native void release(long ptr);

   static {
      Linker.linkClass(Mem.class);
   }
}

3. Working with arrays

@Library("crypto")
public class LibCrypto {

    public static byte[] sha256(byte[] data) {
        byte[] digest = new byte[32];
        SHA256(data, data.length, digest);
        return digest;
    }

    @Link
    private static native void SHA256(byte[] data, int len, byte[] digest);
}

4. Accessing object fields

public class Time {
    public long sec;
    public long nsec;

    public static Time current() {
        Time time = new Time();
        clock_gettime(0, time);
        return time;
    }

    @Link
    private static native void clock_gettime(int clk_id, @FieldOffset("sec") Time time);

    static {
        Linker.linkClass(Time.class);
    }
}

5. Inlining raw machine code

public class Cpu {

    // rdtsc
    // shl    $0x20,%rdx
    // or     %rdx,%rax
    // ret
    @Code("0f31 48c1e220 4809d0 c3")
    public static native long rdtsc();

    static {
        Linker.linkClass(Cpu.class);
    }
}

Running

1. As an agent

java -XX:+UnlockExperimentalVMOptions -XX:+EnableJVMCI \
     -javaagent:nalim.jar -cp <classpath> MainClass

This is the simplest way to add nalim to your application, as the agent exports all required JDK internal packages for you.

The agent optionally accepts a list of classes whose native methods will be automatically linked at startup:

-javaagent:nalim.jar=com.example.MyLib,com.example.OtherLib

2. On the classpath

If not adding nalim as an agent, you'll have to add all required --add-exports manually.

java -XX:+UnlockExperimentalVMOptions -XX:+EnableJVMCI                \
     --add-exports jdk.internal.vm.ci/jdk.vm.ci.code=ALL-UNNAMED      \
     --add-exports jdk.internal.vm.ci/jdk.vm.ci.code.site=ALL-UNNAMED \
     --add-exports jdk.internal.vm.ci/jdk.vm.ci.hotspot=ALL-UNNAMED   \
     --add-exports jdk.internal.vm.ci/jdk.vm.ci.meta=ALL-UNNAMED      \
     --add-exports jdk.internal.vm.ci/jdk.vm.ci.runtime=ALL-UNNAMED   \
     -cp nalim.jar:app.jar MainClass 

Performance

JMH benchmark for comparing regular JNI calls with nalim calls is available here.

The following results were obtained on Intel Core i7-1280P CPU with JDK 19.0.1.

Simple native method

static native int add(int a, int b);

Benchmark            Mode  Cnt  Score   Error  Units
JniBench.add_jni     avgt   10  6,718 ± 0,298  ns/op
JniBench.add_nalim   avgt   10  0,821 ± 0,032  ns/op
JniBench.add_panama  avgt   10  6,673 ± 0,307  ns/op

Array processing

static native long max(long[] array, int length);

Benchmark           (length)  Mode  Cnt    Score   Error  Units
JniBench.max_jni          10  avgt   10   24,642 ± 0,741  ns/op
JniBench.max_jni         100  avgt   10   54,626 ± 1,843  ns/op
JniBench.max_jni        1000  avgt   10  433,813 ± 0,864  ns/op
JniBench.max_nalim        10  avgt   10    3,540 ± 0,218  ns/op
JniBench.max_nalim       100  avgt   10   37,211 ± 0,308  ns/op
JniBench.max_nalim      1000  avgt   10  418,057 ± 0,529  ns/op

Supported platforms

• Linux: amd64 aarch64
• macOS: amd64 aarch64
• Windows: amd64

Limitations

A native function called with nalim has certain limitations comparing to a regular JNI function.

1. It must be static.
2. It does not have access to JNIEnv and therefore cannot call JNI functions, in particular, it cannot throw exceptions.
3. Only primitive types, primitive arrays and plain objects with primitive fields can be passed as arguments.
4. A function must return as soon as possible, since it blocks JVM from reaching a safepoint.

Due to JVMCI limitation in HotSpot, nalim works only with Parallel, Serial and G1 GC, and also with ZGC since JDK 21.