Скачать или смотреть Resolving JNI Errors When Invoking Java Methods From a C+ + Wrapper

A step-by-step guide on how to successfully call Java methods from a C+ + wrapper using JNI, addressing common pitfalls and including a working code example.
---
This video is based on the question https://stackoverflow.com/q/63016784/ asked by the user 'Valeriy K.' ( https://stackoverflow.com/u/7804595/ ) and on the answer https://stackoverflow.com/a/63032634/ provided by the user 'Valeriy K.' ( https://stackoverflow.com/u/7804595/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Can't call java methods from c+ + wrapper in JNI

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Resolving JNI Errors When Invoking Java Methods From a C+ + Wrapper

The Java Native Interface (JNI) allows Java code to interact with native applications and libraries written in other programming languages like C and C+ + . While powerful, JNI can sometimes be tricky to navigate, especially for those new to it. One common issue that developers face is the inability to call Java methods from a C+ + wrapper, leading to Java Virtual Machine (JVM) errors and application crashes. In this guide, we will explore this issue and walk through a detailed solution to effectively make Java method calls from C+ + .

The Problem: JNI Method Invocation Failure

Consider the case where you have a Java method that you want to invoke from a C+ + wrapper. The method typically follows a structure like this:

[[See Video to Reveal this Text or Code Snippet]]

The Handler class in Java contains a method called call, which you want to invoke after subscribing to some event or messages. However, when attempting to do this from a C+ + implementation, you may encounter a fatal error that results in a crash. For example:

[[See Video to Reveal this Text or Code Snippet]]

This error often indicates that the method was not found due to improper handling of the Java environment (JNIEnv) across threads. So, how can we resolve this?

The Solution: Handle JNI Properly

To resolve JNI invocation issues, follow the steps and guidelines below to set up your JNI environment correctly:

Step 1: Preserve the Java Virtual Machine (JVM) Instance

You need a reference to the Java Virtual Machine (JVM) instance to manipulate the Java environment properly. Here’s how you can do this:

[[See Video to Reveal this Text or Code Snippet]]

Step 2: Create a Wrapper Class for Java Handler

The wrapper class helps to manage JNI calls and ensure correct thread attachment and environment handling.

[[See Video to Reveal this Text or Code Snippet]]

Step 3: Attach and Detach Threads When Necessary

Ensure that threads executing JNI calls are attached and detached properly to avoid memory mismanagement:

Before a call, retrieve the Java environment using GetEnv.

If the current thread is detached, use AttachCurrentThread to attach it temporarily.

Conclusion

By properly managing your JNI environment and JVM instances, you can successfully invoke Java methods from C+ + . Always ensure that you handle thread attachment and memory management to avoid common pitfalls, such as segmentation faults and application crashes. With these solutions in place, you can now effectively leverage the power of JNI in your applications.

With patience and practice, you'll find that using JNI can greatly enhance the capabilities of your Java and C+ + integrations. If you encounter further issues or have questions about specific scenarios, feel free to reach out in the comments below!