bytedeco/javacpp

Passing data between languages with no leaks

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#351 opened on Oct 16, 2019

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 (12 comments) (0 reactions) (0 assignees)Java (4,279 stars) (620 forks)batch import
enhancementhelp wanted

Description

Hi everyone. The issue is the following. I have to transfer ownership of an allocated data (an array) from C++ to Java. How would I do that?

As a side note: I've tried to work something out with @ArrayAllocator annotation but haven't quite understood the idea of how it works. I got a couple of NullPointerExceptions during my experiments that look like this:

javac -cp javacpp.jar:/home/vasniktel/javacpp-test test/Lib.java 
Generating /home/vasniktel/javacpp-test/jnijavacpp.cpp
Generating /home/vasniktel/javacpp-test/test/jniLib.cpp
Warning: Method "public static native void test.Lib.allocateArray(int)" cannot behave like a "ArrayAllocator". No code will be generated.
Exception in thread "main" java.lang.NullPointerException
        at org.bytedeco.javacpp.tools.Generator.methods(Generator.java:1748)
        at org.bytedeco.javacpp.tools.Generator.classes(Generator.java:1645)
        at org.bytedeco.javacpp.tools.Generator.generate(Generator.java:206)
        at org.bytedeco.javacpp.tools.Builder.generateAndCompile(Builder.java:604)
        at org.bytedeco.javacpp.tools.Builder.build(Builder.java:1107)
        at org.bytedeco.javacpp.tools.Builder.main(Builder.java:1341)

The question is how those annotations work and whether this exception is an intended behaviour?

Contributor guide

Tech stack
javacpp
Domain
backend
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
needs investigation
Prerequisites
JavaC++JavaCPPJNI
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20
Research direction
The issue involves a NullPointerException while using the @ArrayAllocator annotation in JavaCPP. To resolve, first reproduce the error by following the user's example. Then examine the Generator.java file at line 1748 to understand the cause of the null pointer. Review the JavaCPP documentation and examples for proper usage of @ArrayAllocator for transferring ownership of arrays between C++ and Java. The user's warning suggests the method signature is incorrect, so investigate correct signature patterns.