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 o

Answer 1

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.