How To Use Java Strings In C?

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Handling strings in Java differs from handling strings in C. A string in Java is encoded in Unicode. Conversely, C uses the Universal Character Set (UCS) Transformation Format-8 (UTF-8) to represent strings. The native equivalent of a string in Java is jstring, which cannot be used as a string in C.

Accessing a Java String in C

Code:

class StringDemo 
{
	String str;
	//Declaring a native method
	private native void strDemo();
	public static void main(String args[])
   	{
    	StringDemo dem  = new StringDemo();
	    dem.str = "This is a string declared in Java";
	    //invoking the native method
	    dem.strDemo();
	    System.out.println("String in Java:\n");
    	System.out.println("  dem.str = \"" + dem.str + "\"");
  	}
  	static 
	{
    	    //loading library
	    System.loadLibrary("StringDemo_lib");
	}
}//end class

This code shows the Java code that declares a native method to access a Java string from C. The implementation of the native code is achieved by loading the corresponding library.

Header File for the StringDemo File

Code:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class StringDemo */
#ifndef _Included_StringDemo
#define _Included_StringDemo
#ifdef __cplusplus
extern "C" 
{
	#endif
	/*
	 * Class:     StringDemo
	 * Method:    strDemo
	 * Signature: ()V
	 */
	JNIEXPORT void JNICALL Java_StringDemo_strDemo (JNIEnv *, jobject);
	#ifdef __cplusplus
}
#endif
#endif

This code shows how to generate the header file from the StringDemo class file with the help of the javah tool.

Implementation of the strDemo Method

Code:

#include <stdio.h>
#include <jni.h>
#include "StringDemo.h"
JNIEXPORT void JNICALL 
Java_StringDemo_strDemo(JNIEnv *exeenv, jobject javaobj)
{
	jclass cls = (*exeenv)->GetObjectClass(exeenv, javaobj);
	jfieldID fid;
	jstring jstring;
	const char *string;
	printf("String accessed in C\n");
	fid = (*exeenv)->GetFieldID(exeenv, cls, "str", "Ljava/lang/String;");
	if (fid == 0) 
	{
    		return;
  	}
  	jstring = (*exeenv)->GetObjectField(exeenv, javaobj, fid);
  	string = (*exeenv)->GetStringUTFChars(exeenv, jstring, 0);
	printf("dem.s = \"%s\"\n", string);
  	(*exeenv)->ReleaseStringUTFChars(exeenv, jstring, string);
}

The above code accesses a string declared in Java code, converts it into an equivalent C string with the help of the GetStringUTFChars method, and then prints it.