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Pass and return String value to JNI method
  • If you didn't know how to write sample JNI please follow previous blog.
  • Java and C represent String as diffrent way.
  • In Java language String are sequence of UTF-16 code and in C it is bytes.
  • Thus in JNI it has two two functions to convert String to UTF-8 and one to convert it back to UTF-16
  • Let's implement simple Greeting Class which is get person's name and return String greeting
  • Define the Greeting.java class as below.
    class Greeting {
    	public static native String sayHello(String name);
  • Compile and generate corresponding header definition
    javac Greeting.java
    javah Greeting
  • Then implement the Greeting.c as below
  • unlike before we use GetStringUTFChars to convert utf-8 chars and NewStringUTF to get UTF-16 Sequence
    #include "Greeting.h"
    JNIEXPORT jstring JNICALL Java_Greeting_sayHello (JNIEnv *env, jclass cl, jstring name)
    	jstring jstr;
    	char greeting[] = "Hello, ";
    	char* cname;
    	cname = (*env)->GetStringUTFChars(env, name, NULL);
    	jstr = (*env)->NewStringUTF(env, greeting);
    	return jstr;
  • Generate the dynamic library according to the environment
    //on Cygwin programming environment in windows
    gcc -mno-cygwin -D __int64="long long" -I JAVA_HOME/include/ -I JAVA_HOME/include/win32 -shared -Wl,--add-stdcall-alias -o Greeting.dll Greeting.c
    // Gnu C compiler on Linux
    gcc -fPIC -I JAVA_HOME/include -I JAVA_HOME/include/linux -shared -o libGreeting.so Greeting.c
    //With tSun compiler under the Solaris Operating System
    cc -G -I JAVA_HOME/include -I JAVA_HOME/include/solaris -o libGreeting.so Greeting.c
    //in Microsoft C++ compiler on Windows
    cl -I JAVA_HOME\include -I JAVA_HOME\include\win32 -LD Greeting.c -FeGreeting.dll
  • Create GreetingTest class
    class GreetingTest
        public static void main(String[] args)
  • Run the GreetingTest class
    javac GreetingTest.java
    java GreetingTest
  • download the sample code here.

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Topic: Pass and return String value to JNI method
Philip Jones
Apr 2023
First Poster
Philip Jones says...

Very useful, thanks.

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