Native Methods in Java

In this post, we will see a detailed explanation of the native methods in Java. We will see the basic syntax of it along with their working. There will be Java code examples showing the use of native methods.

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Here is a basic syntax of how native methods are used in Java:

[ public / protected / private] native [return-type] methodName ();

The above syntax shows how a native method is declared in Java. Similar to a regular method, it necessitates the definition of an access modifier, followed by the native keyword, then the return type of the method, and lastly, the method name with any necessary input parameters specified.

How Does Native Method Works In Java?

We can define it as methods implemented in languages other than Java, such as C/C++. The general idea behind using such methods may be to take advantage of high performance or memory management available in C/C++.

To support methods written in other languages, Java provides an interface called Java Native Interface, which acts as a mediator between Java and other languages. The motivation for using JNI comes from its ability to provide code reusability and high performance. Note that code written in other languages like C/C++ is not portable.

The following are the main components of the Java native interface:

• javah: This tool is available in the java development kit that creates header files compatible with C from an existing java file java class containing native methods.
• h: This is a C/C++-based header file available in the Java development kit that provides a mapping between Java data types and native data types. Javah file described above generates this file automatically.

Examples of Native Methods in Java

Now we will see an example that clearly understands how native methods work in Java. The following are the steps involved in using native methods:

• Program java code.
• Compile java code.
• Create a C header file.
• Implement native method logic in C/C++.
• Create a shared library.
• Run and Test java applications.

The below example will be divided into two parts: having java code and having native code.

Part #1: Java

Here is our java class containing the native method.

Code:

package com.edubca.nativedemo;
class NativeDemo
{
public native String encryptData (String inputdata);
static
{
System.loadLibrary ("nativedemo");   /* lowercase of classname! */
}
public static void main (String[] args)
{
NativeDemo demo = new NativeDemo ();
System.out.println("Encrypted data is " + demo.encryptData ("This is Edubca"));
}
}

The above example contains a native method declared in the NativeDemo class. We have written the implementation of the encryptData method in C. As you can see, we have used a static block whose purpose is to load the native C library in which the implementation of the encryptData method is available. One important note is that you should supply the lowercase name of the enclosing Java class as the string parameter in the System.loadLibrary method. As per the steps declared above, it is time to compile our java code.

The following command compiles the above java code.

In the next step, we will create a header file using javah utility as described below.

The above command will generate a javah file with the same name as the name of the class. While writing the C implementation of the native method, you should include this file.

Part #2: C Code

Here is the C implementation of the native function encryptData.

Code:

#include <jni.h>
#include <stdio.h>
#include "NativeDemo.h"
JNIEXPORT void JNICALL
Java_NativeDemo_encryptData(JNIEnv *env, jobject obj, jstring inputstr)
{
const char *str= (*env)->GetStringUTFChars(env,inputstr,0) // create string from jstring
char Newch = '@';
for(i = 0; i <= strlen(str); i++)
{
if(str[i] == 'a' || str[i]== 'e' || str[i]== 'i' || str[i]== 'o' || str[i]== 'u' || str[i] == 'A' || str[i]== 'E' || str[i]== 'I' || str[i]== 'O' || str[i]== 'U')
{
str[i] = Newch;
}
}
return env->NewStringUTF(str); // convert string to jstring
}

• The above files are saved with NativeDemo.c.
• From the above code, we can see the logic of encryption is written in C language. The logic is based on replacing all vowels with @ and returning the string.
• In the next step, will compile the above C code using the below command:

• The above command creates a shared library used by the java programming layer to call code written in C. We can use different compilation strategies based on our compiler and operating system.
• After completing the above steps, we can call run our java code like the below:

If everything goes well, you will see the below output:

Conclusion

From the above article, we have a clear understanding of it. Most real-time applications written in Java make use of native methods to take performance as well as memory management advantages available in native programming languages like C/C++.

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