Invoke Syscalls from Java

You need to use a native method, but you don't need to implement it yourself. Java has a variation on JNI called JNA (Java Native Access), which lets you access shared libraries directly without needing a JNI interface wrapped around them, so you can use that to interface directly with glibc:

import com.sun.jna.Library;
import com.sun.jna.Native;

public class Test {
    public interface CStdLib extends Library {
        int syscall(int number, Object... args);
    }

    public static void main(String[] args) {
        CStdLib c = (CStdLib)Native.loadLibrary("c", CStdLib.class);

        // WARNING: These syscall numbers are for x86 only
        System.out.println("PID: " + c.syscall(20));
        System.out.println("UID: " + c.syscall(24));
        System.out.println("GID: " + c.syscall(47));
        c.syscall(39, "/tmp/create-new-directory-here");
    }
}

It is necessary to use a native method, or a library that does so for you.