将byte []从Java传递给C ++

Question

I'm trying to pass a byte[] param with binary data and a string param from Java to C++ code using SWIG .

Here are my .i and .cpp files:

my_module.i

%module mymodule
%inline %{
   extern void compress_buffer_to_file(unsigned char *buffer, char *ofname);
%}

my_module.cpp

void compress_buffer_to_file(unsigned char *buffer, char *ofname){
   .........
}

The generated method in the Java wrapper:

public static void compress_buffer_to_file(SWIGTYPE_p_unsigned_char buffer, String ofname) {
   my_moduleJNI.compress_buffer_to_file(SWIGTYPE_p_unsigned_char.getCPtr(buffer), ofname);
}

How can I define a .i file that will generate a Java wrapper that will let me pass byte[] instead of SWIGTYPE_p_unsigned_char to the compress_buffer_to_file method or alternatively? How do I associate an existing byte[] on the Java side with a SWIGTYPE_p_unsigned_char?

I've tried to use typemap with no success.

Answer 1

You want SWIG to perform a conversion between Java and C++ types, namely

1. Java byte[] to unsigned char * and
2. String to char * .

The general tools for such conversions are SWIG typemaps . Conveniently, many are already provided by SWIG and simply need to be applied . Have a look at the Java typemap documentation .

The latter conversion ( String to char * ) is done automatically by SWIG (using predefined typemaps that match char * arguments).

Using predefined typemaps

A standard conversion very similar to what you want ( byte[] to char * ) is handled by the " char * BYTE " typemaps defined in the file various.i . To use it, all you need to do is add

%include various.i
%apply char *BYTE { char *buffer_variable_name };

at the top of the swig interface file, where buffer_variable_name is the name of the variable in the function argument (typemaps can be matched by name). For details, see the general typemap documentation and the Java typemap documentation .

However, this is not exactly what you want, since your function takes an unsigned char * . If you can use java.nio.Buffer instead of byte[] on the Java side (which needs to be allocated via allocateDirect ), there is a different set of typemaps predefined in various.i that can be used via

%apply unsigned char *NIOBUFFER { unsigned char *buffer_variable_name };

Writing your "own" typemaps

In your case you want essentially the same as the provided " char * BYTE " typemaps, but for functions taking unsigned char * . So you can just copy these from various.i to a new file ubyte.i and tweak them slightly:

ubyte.i

%typemap(jni) unsigned char *UBYTE "jbyteArray"
%typemap(jtype) unsigned char *UBYTE "byte[]"
%typemap(jstype) unsigned char *UBYTE "byte[]"
%typemap(in) unsigned char *UBYTE {
  $1 = (unsigned char *) JCALL2(GetByteArrayElements, jenv, $input, 0); 
}

%typemap(argout) unsigned char *UBYTE {
  JCALL3(ReleaseByteArrayElements, jenv, $input, (jbyte *) $1, 0); 
}

%typemap(javain) unsigned char *UBYTE "$javainput"

/* Prevent default freearg typemap from being used */
%typemap(freearg) unsigned char *UBYTE "" 

Use these in the same way, by adding at the top of your swig interface file:

%include ubyte.i
%apply unsigned char *UBYTE { unsigned char *buffer_variable_name };

Using these predefined typemaps as a basic example, and reading the docs, you can then start writing your own custom typemaps if you need to.

Hope this helps.