Return java.lang.Object using SWIG %native function

Question

I'm declaring an inline native JNI function, getId. I'm unsure of how to tell SWIG, in the %native snippet below, how to just return a java.lang.Object. I have TBD there for now. I believe a typemap is the proper approach, but I'm unsure of how to declare it for a native JNI method using the %native function. Any ideas?

%module Sample
%{
JNIEXPORT jobject JNICALL Java_com_test_SampleJNI_getID(JNIEnv *env, jclass cls) 
{
    jmethodID cnstrctr;
    jvalue args[2];
    jobject obj;
    jbyteArray bArray;

    bArray = (*env)->NewByteArray(env, 32);

   (*env)->SetByteArrayRegion(env, bArray, 0, 32, (jbyte *)foo);

   cls = (*env)->FindClass(env, "com/test/jni/DeviceId");
   cnstrctr = (*env)->GetMethodID(env, cls, "<init>", "(ILjava/lang/String;[B)V");

   args[0].i = (*env)->NewStringUTF(env, "id");
   args[1].i = bArray;

   obj = (*env)->NewObjectA(env, cls, cnstrctr, args);

   return obj;
}
%}
%native(getID) TBD getID();

DeviceId.java:

package com.test.jni;

public class DeviceId {
    private String id;
    private byte[] cache;

    public DeviceId(String id, byte[] cache){
        this.id=id;
        this.cache=cache;
    }

    public byte[] getCache() {
        return cache;
    }

    public void setCache(byte[] cache) {
        this.cache = cache;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }   
}

What do you mean "find a class and populate it?". Do you want to return a new Java Object? Or a SWIG wrapped C++ object? — Flexo - Save the data dump
– Flexo - Save the data dump ♦, Commented Apr 12, 2012 at 10:06
@awoodland - A new Java object (instace of DeviceId). I added some code to show what the intent is. The main issue is knowing how to handle the jobject return type of the getID inline function within the %native snippet. — c12
– c12, Commented Apr 12, 2012 at 16:08

Flexo - Save the data dump · Accepted Answer · 2012-04-12 16:28:24Z

1

You can use typemaps (e.g. jtype and jstype most likely) with %native, but you can also just say simply jobject if you have an Object being returned and have it handled for you, e.g.:

%module Sample
%{
JNIEXPORT jobject JNICALL Java_com_test_SampleJNI_getID(JNIEnv *env, jclass cls) 
{
  jclass c = (*jenv)->FindClass(env, "java/lang/Object");
  jmethodID m = (*jenv)->GetMethodID(env, c, "<init>", "()V");
  return (*env)->NewObject(env, c, m);
}
%}
%native(getID) jobject getID();

If you wanted to return something else (say java.lang.Integer) you would do something like:

%module Sample
%{
JNIEXPORT jobject JNICALL Java_com_test_SampleJNI_getID(JNIEnv *env, jclass cls) 
{
  jclass c = (*jenv)->FindClass(env, "java/lang/Integer");
  jmethodID m = (*jenv)->GetMethodID(env, c, "<init>", "()V");
  return (*env)->NewObject(env, c, m);
}
%}

%typemap(jstype) int getID "Integer"
%typemap(jtype) int getID "Integer"
%native(getID) int getID();

I specified getID explicitly in that typemap to avoid it being applied to all ints.

(I tested the SWIG part of this is generating sensible code, but I didn't verify the JNI and it clearly needs checking of return values)

For your specific example you could do:

%typemap(jstype) DeviceID getID "com.test.jni.DeviceId"
%typemap(jtype) DeviceID getID "com.test.jni.DeviceId"
%typemap(javaout) DeviceID getID { return $jnicall; }
%native(getID) DeviceID getID();

i.e. you can write literally anything name that isn't already being used for something else and just apply a typemap.

But it's probably best to go with jobject as the return type as then you can just write:

%typemap(jstype) jobject getID "com.test.jni.DeviceId"
%typemap(jtype) jobject getID "com.test.jni.DeviceId"
%native(getID) jobject getID();

edited Apr 12, 2012 at 16:28

answered Apr 12, 2012 at 16:23

Flexo - Save the data dump♦

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9 Comments

c12 Over a year ago

It wont let me edit your post but I think there is a small typo, *jenv should be *env, was getting compile issues.

c12 Over a year ago

are you able to compile with the JNI function in a header file and not inlining it? I'm only able to get it to work if I place the full JNI function/code in the swig.i file. I keep getting a complie error of Sample.h:797: Error: Syntax error in input(1) when I declare JNIEXPORT jobject JNICALL Java_com_test_Sample_getID(JNIEnv *env, jclass cls); in my Sample.h file.

c12 Over a year ago

one more question...shouldn't you be able to do jclass c = (*env)->FindClass(env, "com.test.jni.DeviceId"); if I wanted to return a particular instance of a class? The reason for this is that I want to populate the com.test.jni.DeviceId class in the JNI method and return a fully populated version. Thanks for all the help!

Flexo - Save the data dump Over a year ago

@c12 - You're right about the extra j creeping in - that's what SWIG calls it in typemaps normally. If it's in a different file I think you will need an extra #include. I don't follow the FindClass question. FindClass just looks up the information about the class, not about specific instances of it.

Flexo - Save the data dump Over a year ago

@c12 yes. There will be a module.java and moduleJNI.java generated. jtype is the JNI result (often just a long for a pointer) and jstype is for the proxy. The jstype typemap has to convert between the two as required.

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