Develop Reference

JNI: dealing with a complicated C++ class

I have a 3rd party Win32 dll, written in C++, which I need to load in my eclipse java project and call some methods from it. What is problematic - all the classes which I really need from this dll have virtual functions. I've created a dummy C++ project, which received all the stuff via static linking and via my self-defined C++ class. Everything is working in my C++ world. Now I want to load my dummy dll (which in turn loads all other stuff I need) in my java project. The code in cpp world looks like this (I've mixed up .h and .cpp for simplicity):
class Dummy : public VirtualClassFromLib
{
public:
//...
//some standard stuff here
//...
void method1()
{
VirtualClassFromLib::function1();
}
void method2()
{
//a header is loaded with #include
AdditionalStructureFromHeader object;
VirtualClassFromLib::function2(object);
}
void virtual_function()
{
std::cout << "Cool, now you are not virtual";
}
//...
}
where function1 and function2 are not virtual in the base class and virtual_function is.
I'm a newbie in JNI and as far as I understood, I should write a "mirror" class with the same method's and other stuff's names in my java project. Question: how to deal with this VirtualClassFromLib? Like, this VirtualClassFromLib has a very complicated structure with a wide usage of self-defined structures in the original dll. Or don't I need to "mirror" that completely to java project? I don't get it from examples and tutorials in the Internet.

How to implement Java Interface methods in C

Let's take the following code
public class SomeClass {
public OtherClass method(final String param1,final String param2){
AnotherClass obj1 = AnotherClass.getInstance();
return obj.instanceMethod(new YetAnotherClass<OtherClass>() {
#Override
public OtherClass run() {
return OtherClass.get(param1, param2);
}
});
}
}
My question is there any way that I can implement the interface in C/C++ through JNI, without creating a native method in Java?

One option is the Java Native Access (JNA) library. Have a look at its project page at JNA. I quote from its project site:
JNA provides Java programs easy access to native shared libraries without writing anything but Java code - no JNI or native code is required. This functionality is comparable to Windows' Platform/Invoke and Python's ctypes.
The following example from the project's toturial page demonstrates how it is used to call the printf function from the native library that the function is defined in:
package com.sun.jna.examples;
import com.sun.jna.Library;
import com.sun.jna.Native;
import com.sun.jna.Platform;
/** Simple example of JNA interface mapping and usage. */
public class HelloWorld {
// This is the standard, stable way of mapping, which supports extensive
// customization and mapping of Java to native types.
public interface CLibrary extends Library {
CLibrary INSTANCE = (CLibrary)
Native.loadLibrary((Platform.isWindows() ? "msvcrt" : "c"),
CLibrary.class);
void printf(String format, Object... args);
}
public static void main(String[] args) {
CLibrary.INSTANCE.printf("Hello, World\n");
for (int i=0;i < args.length;i++) {
CLibrary.INSTANCE.printf("Argument %d: %s\n", i, args[i]);
}
}
}
On Windows, the printf function is defined in the msvcrt.dll and the sample loads that DLL and calls the function from it.
The JNA project is mature and according to its web page, it has some very famous users.
It should be pointed out that JNA itself uses the JNI under the hood but in most cases, you do not need to use JNI yourself. Therefore, you can focus on implementing your native code in C (create your own DLL or Shared Library files) and then load them in Java with JNA.

UnsatisfiedLinkError: Native method not found (NOT caused by the common cause for this error)

I am trying to use a library that uses JNI.
I've tried the sample app provided by the developers and it works. So I know it's not a bug in the library.
I assume I'm doing something wrong in the process of importing the library:
copy the .so file into my libs folder (its called libjniRTSP.so)
copy the the jniRTSP.java (summarized below) into my project:
public class jniRTSP {
private volatile static jniRTSP libRTSP = null;
public static jniRTSP getInstance() {
if(null == libRTSP) {
synchronized(jniRTSP.class) {
if(null == libRTSP) {
libRTSP = new jniRTSP();
libRTSP.InitProductList();
// DEBUG
libRTSP.SetDebugView(1);
}
}
}
return libRTSP;
}
static {
try {
System.loadLibrary("jniRTSP");
} catch (Exception e) {
e.printStackTrace();
}
}
public native int GetBrandEnableRecorder();
public native int GetBrandEnableLocal();
public native int GetBrandEnableRemote();
...
then in my onCreate() I try to call one of the methods:
jniRTSP.getInstance().Init(.....)
Which returns the error:
UnsatisfiedLinkError: Native method not found com.myuniquepackage.jniRTSP.InitProductList:()I
UPDATE (FIX): instead of just copying the jniRTSP java file, I copied the whole package that contained it, keeping the same package name. I'm not sure if this fixed it because the package name was the issue, or if because it needed one of the other java files that were in that package. Although I'm pretty sure if it was a missing file, it would complain at compile time.

Fairly certain the package declarations have to be the same inside the C code as the Java code.
So the class jniRTSP should be in the com.myuniquepackage package in Java and have the native method InitProductList declared and the C code should have method declared as Java_com_myuniquepackage_jniRTSP_InitProductList
By moving the class you are probably breaking the link, change the package declaration in Java to match the demo project and see if it works, if it does you can change it back and then change it in the C code which is a bit more time consuming but easy enough.

How to access DLL methods in Java code using JNA?

By running System.loadLibrary("myAPI"), I verified that the DLL file "myAPI.dll" can be successfully loaded into my Eclipse Java project. Now I need to call methods specified inside this DLL file from my Java code. To do this, I added JNA to my Java project. Then I wrote the below-given code snippet that should be able to get instances of classes IProject and ProjectFactory (specified in the DLL file).
I still don't understand how to properly implement this with JNA. I checked different threads, e.g. this one, but the ones I checked don't provide an answer. Any help is highly appreciated. Thanks.
import com.sun.jna.Library;
import com.sun.jna.Native;
public class MyClass {
public interface myAPI extends Library {
//...
}
void LoadProj() {
myAPI api = (myAPI) Native.loadLibrary("myAPI",myAPI.class);
String fileName = "xxx.sp";
IProject project; // this is wrong but shows what I am trying to do
try {
project = ProjectFactory.LoadProject(fileName);
}
catch (Exception ex) {
MessageBox.Show(this, ex.Message, "Load failure");
}
}
}

Not sure what problem you are facing but as a practice your myAPI interface should declare all the methods verbatim with appropriate parameter mapping. I don't see any methods inside your interface.
Please checkout the this link as well as the link mentioned above by #Perception

If there are no Java classes or Java source hidden inside this DLL (which would be ... strange), then it will never work this way. You can't instantiate C# classes or use C# interfaces. MessageBox.Show( isn't Java either, it is Windows Forms code.

Code for running the method declared in DLL

I created a Java code that tries to access the method LoadProject of the class IProjectFactory defined in myAPI.dll. The description of the DLL file says: IProjectFactory is used to load a project file into memory. IProjectFactory is a static class in the myAPI.dll assembly. It exposes the LoadProject method that takes a string containing the path to the file to load, and returns a reference to the resulting IProject. Using the IProject interface you can then manipulate the loaded project in various ways.
import com.sun.jna.Library;
import com.sun.jna.Native;
public class MyClass {
public interface IProjectFactory extends Library {
public Object LoadProject(String fileName);
}
public MyClass() {
//System.loadLibrary("myAPI");
load();
}
void load() {
String fileName = "xxx.sp";
IProjectFactory api = (IProjectFactory) Native.loadLibrary("myAPI",IProjectFactory.class);
try {
Object project = api.LoadProject(fileName);
}
catch (Exception e) {
e.printStackTrace();
}
}
}
After running this code, the following error message has been generated:
Exception in thread "AWT-EventQueue-0" java.lang.UnsatisfiedLinkError: Error looking up function 'LoadProject': The specified procedure could not be found.
at com.sun.jna.Function.<init>(Function.java:179)
at com.sun.jna.NativeLibrary.getFunction(NativeLibrary.java:350)
at com.sun.jna.NativeLibrary.getFunction(NativeLibrary.java:330)
at com.sun.jna.Library$Handler.invoke(Library.java:203)
at $Proxy0.LoadProject(Unknown Source)
Does it mean that myAPI does not contain the class IProjectFactory with the method LoadProject?

You can call functions in native libraries with JNI or JNA. Primitive types are mapped. Certain structures are possible also. Even callback functions are possible. See JNA's mapping table.
What is impossible, by design: Getting a class or interface that was designed for a different runtime environment (like CLR) and use it seamless within the JVM.
So if you have a native procedural/functional library that just returns pointers or primitive types then you can use it quite well.
If you need to work with objects that are returned then you are out of luck. You need to run them in their native environment and find some way of interprocess communication.