I came across this figure while studying JVM. I understood all the components except "Native method interface" and "native method libraries". What are those exactly?
Native method interface: Native method interface is an interface that connects native method libraries (implemented in C, C++ etc.) with JVM for executing native methods.
Native method library: Implementation in native code.
Please refer this link for more information.
Native Method Interface (JNI) is part of JDK that connects Java code with native applications and libraries that are written in other programming languages, such as C, C++, and assembly.
Why use JNI?
Use features that are platform-dependent and are not supported in the Java class library.
Increase performance by implementing a time-critical code in a lower-level language.
Access libraries that already written in another programming language.
Native Method Libraries are libraries that are written in other programming languages, such as C, C++, and assembly.
These libraries can be loaded through JNI.
So, the picture you posted is saying that JNI allows access to Native Method Libraries.
Java Native Interface:
The above diagram you could come across when you are studying about java virtual machine functionality. There is nothing like Native library when you are installing and working with java at first time. Those are all added when we are developing our own library , but it should be in other languages.
When you are developing functionality in other languages Java Virtual machine will include those libraries at the execution level(Third level) of the java application.
To add to this, there are also native libraries in jvm $JAVA_HOME/jre/lib/amd64/ and these are core libraries which are loaded by reflection(null/boot classloader) which is why they are available at run-time while compiling and we're able to use native methods of Object class like getClass(). So, not only for custom development using JNI but also, some of the core functionalities of java are written into native.
Related
I would like to get .Net assembly (dll) file information using java.This is my sample .Net class
namespace com.test
{
public class Testing
{
}
}
So I would like to get this class name ('Testing') and other methods that included in this class using java.Is there any ideas or resources links?
Use JNI (Java Native Interface)
Official Guide
From Wikipedia:
The Java Native Interface (JNI) is a programming framework that enables Java code running in a Java Virtual Machine (JVM) to call and be called by native applications (programs specific to a hardware and operating system platform) and libraries written in other languages such as C, C++ and assembly
I am developing a Java application in which I need to call some C++ functions (from Google Talk library libjingle) . The objective is to run it all on Google App Engine (which only supports Python or Java).
How can I do this?
You need to define native methods in your java code for whatever you want to be implemented in C++ and directly access your native code. Then you run javah on your code and it will generate the C header files for you and you'll need to provide C++ implementations.
The native methods you can call from your Java code like any other methods and still they'll have their implementation written in C++ and talking to whatever other native library directly.
You then need to set the java.library.path system property to include the shared C/C++ libraries that you require: the google library and your own JNI implementation library would be required in this case.
If the library has C bindings through a DLL/SO, I usually prefer writing wrappers in Java using Java Native Access (JNA) rather than writing the bindings in C/C++ using the Java Native Interface (JNI). The former is easier to manipulate as the JNI access to Java objects is a real pain in the neck. However, it's not as obvious to wrap C++ classes using that API.
You might also want to look into the Simplified Wrapper and Interface Generator (SWIG) for automating part of this process!
You can't run native code on App Engine - only JRE code. If there's no avoiding the native code, you'll need to run this part of your app on another system, and call it from your App Engine app - or use the built-in XMPP API, in this case.
Do they use something like Mono's PInvoke? Or is it more like internal calls registered before the runtime is started? Does java have a base library for handling native calls like mscorlib.dll? If I want to invoke a JVM in C code will it libraries look for the .so/.dll files? Does it make a difference to Java standard libraries if I statically link all of the JRE natives libraries?
They use JNI, exactly as it is publicly documented, to invoke native shared libraries for the specific platform.
As far as invoking a JVM from C code, the JVM uses shared libraries (DLL, SO, etc). A quick search of the JDK 6 source code does not reveal any System.loadLibrary() for the core native support (like native methods in Object, String, etc). That suggests to me that the native code for these methods, which appears to be in DLL's judging from the contents of the JRE/bin directory, is explicitly linked by java.exe (and javaw.exe in Windows).
When I last looked at this stuff, the requirements for invoking a JVM from C code was a well documented part of JNI - I strongly suggest you refer to that doco to proceed further. We even went so far as successfully writing a native C wrapper/loader for the IBM AS/400 Java 1.1 JVM.
They use the Java Native Interface (JNI).
I've never called the JVM from C, so I don't know about that.
There is an example here about how to start a JVM from inside your C program:
http://www.inonit.com/cygwin/jni/invocationApi/c.html
"Java Native Access (JNA) provides Java programs easy access to native shared libraries (DLLs on Windows) without writing anything but Java code" (quotation from their homepage).
Personally never tried it so far.
Is there an open source application that could integrate applications using multiple programming languages into a Java application?
Some options:
Languages that can be compiled and run into the JVM, like python and javascript. But you might have a difficult time if the programs where not built from the beggining to run inside the JVM.
JNI, java's native interface. This allows interfacing Java with native (i.e. C) languages. If your other language is not C or C++ then probably you will need to write a native interface for them too.
API. Using web services or socket communication have the two languages communicate.
Sharing data. Having both programs share files or databases in a common format.
Not quite sure what you mean, but there are several languages that could be compiled to Java byte code and run under JVM.
http://en.wikipedia.org/wiki/Java_Native_Interface
The Java Native Interface is a programming framework that allows Java code running in a Java Virtual Machine to call and to be called by native applications (programs specific to a hardware and operating system platform) and libraries written in other languages, such as C, C++ and assembly.
Whenever JVM fires up, i.e when java command is run; it looks for other libraries in /java/jre/lib folder. These libraries along with the third party libraries like X11 libs, are loaded into the memory by the system's dynamic loader (like dld.so in HP Unix).
So is it possible to load third party libraries from code in java? If yes, what could be the side effects?
What you are looking for is the Java Native Interface (JNI). Using external native libraries will make your code less portable. It will make your code less stable since none of the Java language guarantees will hold. There can be security manager implications since you application will need permissions to load the library. In my experience there are many difficulties in writing good JNI code; specifically deallocation and debugging.
SWIG may be of use automatically generate the necessary JNI code.
Native libraries are loaded with System/Runtime.loadLibrary. You will then need some code to make translate between the libraries native calls and JNI (Java Native Interface). Then you will need some Java code with native methods defined for calling into the translation code.