javah Commnad in Linux

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In classic Linux operating systems, a command line utility named javah is used to generate C header and source files. These files and headers are required to implement native methods. These files allow C programs to access an object's instance variables from the native source code.

Table of Contents

Here is a comprehensive guide to the options available with the javah command −

• What is javah?
• Syntax of javah Command
• Options Javah Command
• How to Check if javah is Installed or Not?
• Examples of javah Command in Linux

What is javah?

javah stands for Java Header. As the name itself suggests, this command is used to generate C header and source files from Java class files. The generated file has an extension ".h". This file contains a struct definition that matches the structure of the related class. The fields in the struct are the same as the instance variables in the class.

Syntax of javah Command

To use this command in Linux, type javah followed by the class name for which you want to generate a header −

javah [options] className

Options Javah Command

The javah command can accept the following optional flags to manage the output format −

For better understanding, you can access the man page of the javah command, as follows −

man javah 

You can also use the javah command with the -help option to achieve the similar functionality −

javah -help

This command returns the basic usage and options description −

How to Check if javah is Installed or Not?

To check the existence of javah on your system, run the following command −

javah -version

The output shows that we are using "1.8.0_422" version of the javah command −

You can use the following command to install javah in Linux, in case it is not installed on your system −

#for Ubuntu or Debian
sudo apt install default-jdk
#for RHEL/CentOS/Fedora
sudo yum install java-11-openjdk
#For Arch Linux
sudo pacman -S jdk-openjdk

Examples javah Command in Linux?

Lets go through the following examples to learn how the javah command works in Linux −

• Creating a Header File in the Current Directory
• Creating a Header File in a Specific Directory
• Redirecting Output to a Specific File

Creating a Header File in the Current Directory

We have already created a Java file named javaExample.java, lets compile it using the following command −

javac javaExample.java

Now run the javah command to generate a header file −

javah javaExample

This command generates a header file the in the current working directory as shown below −

We use the ls command to verify the creation of the header file in the current directory.

Creating a Header File in a Specific Directory

You can create a header file in a specific directory. For this purpose, you must use the -d option followed by the directory name, as shown below −

javah -d Documents javaExample

After running this command, we use the cd command to access the Documents directory and then the ls command to confirm the header file generation −

Redirecting Output to a Specific File

The -o option in javah lets you choose the name of the output file. It redirected the generated header to that file instead of following the default naming pattern −

javah -o javaOutput.h javaExample

This command generates a header file named javaOutput.h for the Java class javaExample −

This sums up the working of the javah command in Linux.

Conclusion

The javah command is a valuable command line tool in classic Linux systems that generates C header and source files from Java class files. This utility is essential for implementing native methods, allowing C programs to access Java object's instance variables.

In this tutorial, we explored the syntax, options, and practical usage of the javah command through examples. We demonstrated how to generate header files in the current directory, in specific directories, and how to redirect output to a specified file.