How to pass C structs back and forth to Java code in JNI?

Question

I've got some C functions which I am calling through JNI which take a pointer to a structure, and some other functions which will allocate/free a pointer to the same type of structure so that it is a bit easier to deal with my wrapper. Surprisingly, the JNI documentation says very little about how to deal with C structures.

My C header file looks like so:

typedef struct _MyStruct {   float member; } MyStruct;  MyStruct* createNewMyStruct(); void processData(int *data, int numObjects, MyStruct *arguments); 

The corresponding JNI C wrapper file contains:

JNIEXPORT jobject JNICALL Java_com_myorg_MyJavaClass_createNewMyStruct(JNIEnv *env, jobject this) {   return createNewMyStruct(); }  JNIEXPORT void JNICALL Java_com_myorg_MyJavaClass_processData(JNIEnv *env, jobject this, jintArray data,                                        jint numObjects, jobject arguments) {   int *actualData = (*env)->GetIntArrayElements(env, data, NULL);   processData(actualData, numObjects, arguments);   (*env)->ReleaseIntArrayElements(env, data, actualData, NULL); } 

...and finally, the corresponding Java class:

public class MyJavaClass {   static { System.loadLibrary("MyJniLibrary"); }    private native MyStruct createNewMyStruct();   private native void processData(int[] data, int numObjects, MyStruct arguments);    private class MyStruct {     float member;   }    public void test() {     MyStruct foo = createNewMyStruct();     foo.member = 3.14159f;     int[] testData = new int[10];     processData(testData, 10, foo);   } } 

Unfortunately, this code crashes the JVM right after hitting createNewMyStruct(). I'm a bit new to JNI and have no idea what the problem could be.

Edit: I should note that the C code is very vanilla C, is well-tested and was ported from a working iPhone project. Also, this project is using the Android NDK framework, which lets you run native C code from an Android project from within JNI. However, I don't think that this is strictly an NDK issue... it seems like a JNI setup/initialization error on my part.

Answer 1

You need to create a Java class with the same members as C struct, and 'map' them in the C code via methods env->GetIntField, env->SetIntField, env->GetFloatField, env->SetFloatField, and so on - in short, lots of manual labor, hopefully there already exist programs that do it automatically: JNAerator (http://code.google.com/p/jnaerator) and SWIG (http://www.swig.org/). Both have their pros and cons, the choice is up to you.

Answer 2

It's crashing because Java_com_myorg_MyJavaClass_createNewMyStruct is declared to return jobject, but is actually returning struct MyStruct. If you ran this with CheckJNI enabled, the VM would complain loudly and abort. Your processData() function is also going to be fairly upset about what it gets handed in arguments.

A jobject is an object on the managed heap. It can have extra stuff before or after the declared fields, and the fields don't have to be laid out in memory in any particular order. So you can't map a C struct on top of a Java class.

The most straightforward way to deal with this was identified in an earlier answer: manipulate the jobject with JNI functions. Allocate the objects from Java or with NewObject, Get/Set the object fields with appropriate calls.

There are various ways to "cheat" here. For example, you could include a byte[] in your Java object that holds sizeof(struct MyStruct) bytes and then use GetByteArrayElements to get a pointer to it. A bit ugly, especially if you want to access the fields from the Java side as well.