Data exchange between Java and C/C++

Java has its own rationale Java programs are proper in source code (software technological) and run optimal after byte code translation on several platforms.

C and C++ are a settled world, also rational. Both worlds seems to be not related, except both can run on the same processors. A bridge between Java and C/++ is the Java Native Interface https://en.wikipedia.org/wiki/Java_Native_Interface or also the Java Native Access https://en.wikipedia.org/wiki/Java_Native_Access. Both concepts enables combination between Java and C/++ programming on the same processor / in the same process.

But often Java and C/++ can be well separated. This has some advantages, on the one hand an independence between both platforms, on the other hand an independence in terms of dependencies and developers. If it is separated, both runs in independent processes (on an Operation System, on PC) or maybe in different compilation units or software blocks.

The question is: Data exchange. With the focus on data exchange it is very more simple:

But how data exchange is organized?

There are some very simple approaches:

In C usual data structures can be defined via a struct. To make it compatible some rules should be regarded. But that rules should be known and are the same topics as data exchange between different C/++ parts. See for example ../../../emc/html/Base/int_pack_endian.html. In other world: A proper data image is clarified for C/++.

In Java a compatible data image can be offered using the byte[] type. The only one problem is: How to read and write data from a byte[] field without primitive (low level) access and conversion.

An answer is the class org.vishia.byteData/ByteDataAccessBase.java.

Another answer may be usage of the features of java.nio.ByteBuffer

This class was one of the first Java programs which I have written on exploration good features in Java, coming from width knowledge of C/++ programming, in the first years of the 2000^th, written with knowledge of deal with data (from C/++ experience). Studying the Java functions later I found that there is another approach to access float etc. in a byte[] buffer, for example the java.nio.ByteBuffer#getFloat(). It seems to be more better and especially a standard. But a calculation time comparison has shown, that my own functions, which are written at low level byte access with conversion routines, were not slower, partially there were faster than the java.nio…​ routines. Why is it so? The Java Bytecode Compiler (JIT) generates access code to the bytes which is so fast as C access. Simple shift and or operations are optimized as also in C compiling. The standard routines in java.nio should regard some system settings too, in the end they are not faster. Because the org.vishia.bytedata.ByteDataAccessBase have some interesting special features, I have improved this class by using and used furthermore.

The principle is: org.vishia.bytedata.ByteDataAccessBase is a base class which contains access and build of primitive Java data on any position in the buffer. Which position is used, it is determined by the implementing class, which should be present user data structures. The positions can be constant, mapping a struct definition from C language. But they can be also dynamic. Especially for evaluating payloads of communication telegrams starting from the beginning there was a tree structure: The data mapping can depend from given data before. The idea in the beginning was: Data can be also present in an XML tree. Thats why older docs talk about 'XmlBinCoding'. But 'presentation in XML' is an independent thing. It is possible, of course, but not necessary to do so.

The derived class should only contain final routines for access, for example:

whereas k_nrofBytes is a const definition in an extra part to have overview. The called getInt16(…​) routine is also protected final, hence the JIT compiler can optimize the machine code.

A derived class for a given data C-struct or fix mapping can/should be generated, not written manually. It reduce effort if many similar structs are necessary.

You need know two things:

This is offered in ../../../Jc/html/ByteDataAccess.html.

The documentation in org.vishia.byteData/ByteDataAccessBase.java should explain the usage. Last but not least there are some test examples in