How do you use JNI to call native code from Java?

Table of Contents

• Introduction
• Steps to Call Native Code Using JNI
  • Step 1: Declare Native Methods in Java
  • Step 2: Generate JNI Header File
  • Step 3: Implement the Native Method in C/C++
  • Step 4: Compile the Native Code into a Shared Library
  • Step 5: Load the Native Library in Java
  • Step 6: Call the Native Method from Java
  • Step 7: Run the Java Application
• Conclusion

Introduction

The Java Native Interface (JNI) allows Java applications to call native code written in languages like C or C++. This capability is useful for accessing platform-specific features, optimizing performance, or integrating existing libraries. This guide outlines the steps to use JNI for calling native code from Java.

Steps to Call Native Code Using JNI

Step 1: Declare Native Methods in Java

Begin by declaring the native methods in your Java class using the native keyword. This informs the Java compiler that the method's implementation will be provided externally.

Example:

Step 2: Generate JNI Header File

Compile the Java class to create a .class file, then use the javah tool (or javac -h for newer Java versions) to generate the JNI header file, which contains the function signatures for the native methods.

Command:

Step 3: Implement the Native Method in C/C++

Create a C or C++ file to implement the native methods defined in the header file. Use JNI types and functions to interface with Java.

Example (MyNativeClass.c):

Step 4: Compile the Native Code into a Shared Library

Compile the C/C++ code into a shared library. The output format depends on your operating system (e.g., .dll for Windows, .so for Linux, or .dylib for macOS).

Example Command for Linux:

Step 5: Load the Native Library in Java

Ensure that the native library is in your library path. The System.loadLibrary() method in the static block of your Java class loads the library at runtime.

Step 6: Call the Native Method from Java

You can now create an instance of your Java class and call the native method, which will execute the code defined in your C/C++ implementation.

Example:

Step 7: Run the Java Application

Compile and run your Java application. If everything is set up correctly, you should see the output from your native code.

Command:

Conclusion

Using the Java Native Interface (JNI) to call native code from Java involves several steps, including declaring native methods, generating JNI headers, implementing the native methods in C/C++, and loading the shared library. By following this process, developers can effectively leverage native code to enhance Java applications, optimize performance, and access platform-specific functionalities. Understanding these steps is essential for integrating Java with native resources in a seamless manner.