Passing buffer between Java and C++ using SWIG

It's not entirely clear what you want to achieve exactly.

If you want to map your Buffer to a Java byte[], you can do it with a custom typemap:

%typemap(jni) Buffer "jbyteArray"
%typemap(jtype) Buffer "byte[]"
%typemap(jstype) Buffer "byte[]"
%typemap(in) Buffer {
    $1.addr = (unsigned char *) JCALL2(GetByteArrayElements, jenv, $input, 0);
    $1.size = JCALL1(GetArrayLength, jenv, $input);
}
%typemap(argout) Buffer {
    JCALL3(ReleaseByteArrayElements, jenv, $input, (jbyte *) $1.addr, 0);
}
%typemap(out) Buffer {
    $result = JCALL1(NewByteArray, jenv, $1.size);
    JCALL4(SetByteArrayRegion, jenv, $result, 0, $1.size, (jbyte *) $1.addr);
    delete[] $1.addr;
}
%typemap(javain) Buffer "$javainput"
%typemap(javaout) Buffer { return $jnicall; }

Then C++ code like

  Buffer getData();
  void sendData(Buffer arg);

Will be mapped to Java:

  public static byte[] getData() { ... }
  public static void sendData(byte[] arg) { ... }

The difficulty with passing data to Java is to get it into the JVM heap and/or to manage the lifetime of the data. It's easy to achieve with some copying, but a truly 0-copy solution will often require a change in the C++ interface.

Complete example:

example.h

#include <stddef.h>
struct Buffer
{
    unsigned char *addr;
    size_t        size;
};
Buffer getData();
void sendData(Buffer);

example.cxx

#include "example.h"

Buffer getData() {
    Buffer rc { new unsigned char[64], 64 };
    for (int i = 0; i < rc.size; ++i)
        rc.addr[i] = 0x40 + i;
    return rc;
}
void sendData(Buffer buf) {
    // use buf.addr
}

example.i

%module example
%{ 
#include "example.h"
%}
%typemap(jni) Buffer "jbyteArray"
%typemap(jtype) Buffer "byte[]"
%typemap(jstype) Buffer "byte[]"
%typemap(in) Buffer {
    $1.addr = (unsigned char *) JCALL2(GetByteArrayElements, jenv, $input, 0);
    $1.size = JCALL1(GetArrayLength, jenv, $input);
}
%typemap(argout) Buffer {
    JCALL3(ReleaseByteArrayElements, jenv, $input, (jbyte *) $1.addr, 0);
}
%typemap(out) Buffer {
    $result = JCALL1(NewByteArray, jenv, $1.size);
    JCALL4(SetByteArrayRegion, jenv, $result, 0, $1.size, (jbyte *) $1.addr);
    delete[] $1.addr;
}
%typemap(javain) Buffer "$javainput"
%typemap(javaout) Buffer { return $jnicall; }
%ignore Buffer;
%include "example.h"

test.java

class test {
    public static void main(String[] args) throws Exception {
       System.loadLibrary("_example");
       byte[] data = example.getData();
       System.out.println(new String(data));
    }
}

Output:

@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~⌂