Develop Reference

Java 10 Panama Project - port JNI solutions to Panama

I have been reading about the Panama Project recently.
I understand that it will be the next generation replacement to JNI - it will allow java developers to code on the native layer using Java (which is amazing IMHO).
The usage is simple from what I can tell looking at jnr-posix, for example:
public class FileTest {
private static POSIX posix;
#BeforeClass
public static void setUpClass() throws Exception {
posix = POSIXFactory.getPOSIX(new DummyPOSIXHandler(), true);
}
#Test
public void utimesTest() throws Throwable {
// FIXME: On Windows this is working but providing wrong numbers and therefore getting wrong results.
if (!Platform.IS_WINDOWS) {
File f = File.createTempFile("utimes", null);
int rval = posix.utimes(f.getAbsolutePath(), new long[]{800, 200}, new long[]{900, 300});
assertEquals("utimes did not return 0", 0, rval);
FileStat stat = posix.stat(f.getAbsolutePath());
assertEquals("atime seconds failed", 800, stat.atime());
assertEquals("mtime seconds failed", 900, stat.mtime());
// The nano secs part is available in other stat implementations. We really just want to verify that the
// nsec portion of the timeval is passed through to the POSIX call.
// Mac seems to fail this test sporadically.
if (stat instanceof NanosecondFileStat && !Platform.IS_MAC) {
NanosecondFileStat linuxStat = (NanosecondFileStat) stat;
assertEquals("atime useconds failed", 200000, linuxStat.aTimeNanoSecs());
assertEquals("mtime useconds failed", 300000, linuxStat.mTimeNanoSecs());
}
f.delete();
}
}
// ....
// ....
// ....
}
My question is this - having worked with JNI, and knowing how cumbersome it is, will there be a solution for porting existing JNI solutions to the Panama format?
IE - go over the generated (via the deprecated javah) C header file and given implementation in C of the header file, identify functions which can be replaced by the Panama API, then generate a java output file?
Or will existing JNI solutions need to be refactored by hand?
Additional links :
OpenJDK: Panama
Working with Native Libraries in Java
JEP 191: Foreign Function Interface thanks to a comment made by Holger

The JNI format is as follows:
Java -> JNI glue-code library -> Native code
One of the goals of project panama is to remove this middle layer and get:
Java -> Native code
The idea is that you can use a command line tool to process a native header (.h) file to generate a Java interface for calling the native code, and the JDK code will do the rest at runtime as far as connecting the 2 together goes.
If your current JNI code does a lot of stuff in this glue-code layer, then that might have to be re-written on the Java side when porting to panama. (this depends on how much could be done automatically by the used interface extraction tool).
But if you are using something like JNA or JNR then moving to panama should be relatively easy, since those 2 have very similar APIs, where you bind an interface to a native library as well.
But questions like:
will there be a solution for porting existing JNI solutions to the Panama format?
Are difficult to answer, since nobody can predict the future. I feel that there are enough differences between panama and JNI that an automatic 1-to-1 conversion between the 2 will probably not be possible. Though if your glue-code is not doing much besides forwarding arguments then the interface extraction tool will probably be able to do all the work for you.
If you're interested you could take a look at the early access builds of panama that started shipping recently: https://jdk.java.net/panama/
Or watch a recent talk about it: https://youtu.be/cfxBrYud9KM

Getting java.lang.UnsatisfiedLinkError at runtime

I'm trying to use JNI to access C++ methods from a Java class. I'm able to compile (both in Eclipse or on command line) my Java class fine, but on executing the class at runtime, I'm getting:
Exception in thread "main" java.lang.UnsatisfiedLinkError: com.domain.services.CallServiceAPIS.createSession()I
at com.domain.services.CallServiceAPIS.createSession(Native Method)
at com.domain.services.CallServiceAPIS.main(CallServiceAPIS.java:18)
Java code is as follows:
package com.domain.services;
public class CallServiceAPIS {
static {
System.loadLibrary("service.client");
}
public native int createSession();
public static void main(String[] args) {
System.out.println(System.getProperty("java.library.path"));
new CallServiceAPIS().createSession();
}
}
I included the printout of the java.library.path just to make sure it's pointing to the correct location of the C++ library - and it is. I also tried setting the LD_LIBRARY_PATH in my Eclipse environment. But neither worked.
Note that the System.loadLibrary call IS working since 1) the code compiles and 2) the error occurs on line 18, which is the new CallServiceAPIs call.
C++ code:
int createSession(const PosServiceInfo info, const SessionArgs& args, Domain::UUID& uuidSession)
{
return int::undefined;
}
Any ideas?

Never mind. I realized that I was using the JNI interface incorrectly. I was thinking you could load an EXISTING C++ library using EXISTING C++ source. But you basically have to rewrite the existing code to make use of the JNI interface.

Loading java code at runtime

I got a little project where I have to compute a list. The computation depends on serveal factors.
The point is that these factors change from time to time and the user should be allowed to change this by it's self.
Up to now, the factors are hard-coded and no changes can be done without recompiling the code.
At the moment the code looks like this:
if (someStatement.equals("someString")) {
computedList.remove("something");
}
My idea is to make an editable and human readable textfile, configfile, etc. which is loaded at runtime/ at startup? This file should hold the java code from above.
Any ideas how to do that? Please note: The targeted PCs do not have the JDK installed, only an JRE.

An effective way of going about this is using a static initializer. Static Block in Java A good and concise explanation can be found under this link.

One option here that would allow this would be to use User Input Dialogs from the swing API - then you could store the users answer's in variables and export them to a text file/config file, or just use them right in the program without saving them. You would just have the input dialogs pop up at the very beginning of the program before anything else happens, and then the program would run based off those responses.

You could use Javascript for the configuration file language, instead of java. Java 7 SE and later includes a javascript interpreter that you can call from Java. it's not difficult to use, and you can inject java objects into the javascript environment.
Basically, you'd create a Javascript environment, insert the java objects into it which the config file is expected to configure, and then run the config file as javascript.

Okay, here we go... I found an quite simple solution for my problem.
I am using Janino by Codehaus (Link). This library has an integrated Java compiler and seems to work like the JavaCompiler class in Java 7.
BUT without having the JDK to be installed.
Through Janino you can load and compile *.java files(which are human readable) at runtime, which was exactly what I needed.
I think the examples and code-snippets on their homepage are just painful, so here's my own implementation:
Step one is to implement an interface with the same methods your Java file has which is loaded at runtime:
public interface ZuordnungInterface {
public ArrayList<String> Zuordnung(ArrayList<String> rawList);}
Then you call the Janino classloader when you need the class:
File janinoSourceDir = new File(PATH_TO_JAVAFILE);
File[] srcDir = new File[] { janinoSourceDir };
String encoding = null;
ClassLoader parentClassLoader = this.getClass().getClassLoader();
ClassLoader cl = new JavaSourceClassLoader(parentClassLoader, srcDir,
encoding);
And create an new instance
ZuordnungsInterface myZuordnung = (ZuordnungInterface) cl.loadClass("zuordnung")
.newInstance();
Note: The class which is loaded is named zuordnung.java, so there is no extension needed in the call cl.loadClass("zuordnung").
And finaly the class I want to load and compile at runtime of my program, which can be located wherever you want it to be (PATH_TO_JAVAFILE):
public class zuordnung implements ZuordnungInterface {
public ArrayList<String> Zuordnung(ArrayList<String> rawList){
ArrayList<String> computedList = (ArrayList<String>) rawList.clone();
if (Model.getSomeString().equals("Some other string")) {
computedList.add("Yeah, I loaded an external Java class");
}
return computedList;
}}
That's it. Hope it helps others with similar problems!

Create iOS static library from robovm project (BAD_ACCESS in JNI)

I have a large amount of Java code (only calculation functions, no UI) that I want to reuse as a static library in iOS. My approach was to use robovm and follow the unofficial way to create a static library described in the two articles in the robovm forum: 1 Basic way and 2 Refined version
Trying to follow the steps exactly as described I got stuck unfortunately after creating the shared library with the script, linking the library (.a) in Xcode and building the project successfully.
During runtime I see that my C++ bridge code is called but the JNI calls back to the library fail with a BAD_ACCESS. For example the following line crashes:
jclass myJavaClass = jniEnv->FindClass("com/test/robovm/bridge/MyJavaRoboCode");
in this method:
void callSomethingInJava(const char* arg) {
// To call into java from your native app, use JNI
Env* rvmEnv = rvmGetEnv();
JNIEnv* jniEnv = &(rvmEnv->jni);
jclass myJavaClass = jniEnv->FindClass("com/test/robovm/bridge/MyJavaRoboCode");
jmethodID myJavaMethod = jniEnv->GetStaticMethodID(myJavaClass, "callJava", "(Ljava/lang/String;)V");
jstring argAsJavaString = jniEnv->NewStringUTF(arg);
jniEnv->CallStaticVoidMethod(myJavaClass, myJavaMethod, argAsJavaString);
}
The same is true if I try to use the rvmXX methods directly instead of JNI and try to access something in my "Java" classes. It looks like the rvmEnv is not fully initialized. (I double checked for package name errors or typos).
It would be great if someone already succeeded with the creation of a shared static library from a robovm project and could share the experience here or point me in the right direction to resolve the issue.

As you mentioned, you probably haven't finished initialising robovm.
You'll need to create a method, say initRoboVM(), to somewhat mirror bc.c's main method. This will be called by your code when you want to initialise robovm. You'll need to pass the app path in, which you can hardcode when you're testing.
initRoboVM() will need some modifications, namely it should not call your Java app's main method, well, at least, that's what well behaving libraries should not do IMO. It should also not call rvmShutdown.

JNI Use native method

I'm working on a project where there's the need to call some methods from dll files.
These two dlls are
EasySign.dll
EasySignJNI
EasySignJNI depends on EasySign.
I wrote the class to load EasySignJNI as follows:
package easysign;
class EasySign {
EasySign(){}
public native String EasyHashFile(String filename);
public native int EasySign(String pkcs11_driver,String pin, int type, String file_data, int out_format, String signed_file, String cert_out, int cert_format);
public native int EasyVerify(String cert_user, String file_data, String signed_file, String crl_file, String ca_file, String out_document);
static {
System.loadLibrary("EasySignJNI");
}
}
Now I would call these method from my main method like this:
public class Test {
public static void main(String[] args) throws IOException{
EasySign es = new EasySign();
System.out.println("EasyHashFile : " + es.EasyHashFile("test.txt"));
}
}
What I have to specify in the -Djava.library.path ? Only the path where my EasySignJNI.dll is located? It is possible to call native method in this way?
I'm using NetBeans for completeness.
EDIT:
I have noticed that the third party dll provided to me (the JNI dll in particular) defines method names without any package, so I'm forced to put the class that loads the dll in the default package. Is there any way to change only the dll method names including my own package name?
EDIT 2:
What I mean is that both EasySign.dll and EasySignJNI.dll are provided me as they are and I can't modify them (I have not the source code). The EasySignJNI is the JNI portion but inspecting it I have noticed thath the method sign is in the form: _java_EasySign_MethodName. When I load the dll in Java from my Easysign class (this class must reside in the "mypackage" package), I receive the jni unsatisfiedlinkerror because, if I understood right, I'm calling the "_java_mypackage_EasySign_MethodName" method, i.e the sign is different from the dll's one. So the only way to make it work is to rewrite the JNI part and build it to have the correct sign of the JNI method?

What I have to specify in the -Djava.library.path ? Only the path where my EasySignJNI.dll is located?
Correct, the operating system will locate the dependent EasySign.dll for you as long as it is available where the OS expects it to be.
It is possible to call native method in this way? I'm using NetBeans for completeness.
I read through your edit and you have successfully lost me. What default package are you referring to? (Remember that none of us know what EasySign.dll is) So, I am going to provide some info about how I do what you originally described and hopefully it will help.
First start by compiling EasySign if you have the src. Do not build a DLL or shared object, instead build a static library. If you do not have the src code for EasySign, or a prebuilt static library, you will be stuck with the dll and can continue to the next step.
Now you are ready to compile the jni portion. All of your JNI C code should basically translate your Java input/output to their JVM/Native types and call the appropriate functions in the DLL library. You want to keep this layer and thin and simple as possible because it is incredibly difficult to debug. Your C++ package names shouldn't really matter here and you can use what ever package name you want for your Java classes. You should be able to compile the JNI code and preferably static link to the EasySign.dll file so you don't need to worry about distributing it. If you must dynamically link, make sure EasySign.dll gets installed to a location that is on the DLL PATH / LDPATH because the OS will need to locate and load that file right after the JVM loads the JNI DLL.
At this point you should just be able to point -Djava.library.path at your JNI DLL's path and all should work.