CODEWITHSUNDEEP
androidjava-native-interfacenative
Buda Gavril
Buda Gavril

Reputation: 21647

JNI export string from c++ to java

I have a class written in c++. I want to write a method to return a string so I can read it from my java class. I've tried to do something like:

readstring.cpp:

#include "common.h"
#include <string.h>

#ifdef __cplusplus
extern "C" {
#endif  /* __cplusplus */

/************
 * ReadString *
 ************/

 jstring Java_com_googlecode_leptonica_android_ReadFile__getString(JNIEnv *env, jclass clazz)
{
  jstring result = env->NewStringUTF("My String");
 return result;
}

#ifdef __cplusplus
}
#endif  /* __cplusplus */

Readstring.java:

package com.testjni;

    public class ReadString {

        static {
            System.loadLibrary("lept");
        }

        public String getText(){
            return getString();
        }
        private static native String getString();
    }

Reading string from an activity:

public class TestJNI extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        ReadString readString = new ReadString();
        Toast.makeText(this, readString.getText(), Toast.LENGTH_SHORT).show();
    }
}

and stack trace is:

10-16 12:35:09.974: ERROR/AndroidRuntime(12303): FATAL EXCEPTION: main java.lang.UnsatisfiedLinkError: getString at com.testjni.ReadString.getString(Native Method) at com.testjni.ReadString.getText(ReadString.java:10) at com.testjni.TestJNI.onCreate(TestJNI.java:13) at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1780) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1837) at android.app.ActivityThread.access$1500(ActivityThread.java:132) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1033) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:143) at android.app.ActivityThread.main(ActivityThread.java:4196) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:507) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597) at dalvik.system.NativeStart.main(Native Method)

What am I doing wrong?

Upvotes: 1

Views: 2593

Answers (3)

KP_
KP_

Reputation: 1866

"Java_com_googlecode_leptonica_android_ReadFile__getString"

here your package is " com.googlecode.leptonica.android" and class is "ReadFile" and the function is "getString".

Actually you didnt mention this function in your code .Please check that.

use like this

"Java_com_testjni_ReadString__getString()" in your c code instead of

"Java_com_googlecode_leptonica_android_ReadFile__getString"

Upvotes: 1

Anup Cowkur
Anup Cowkur

Reputation: 20563

This tutorial might help you

http://marakana.com/bookshelf/java_fundamentals_tutorial/_java_native_interface_jni.html

Upvotes: 1

user207421
user207421

Reputation: 310911

You haven't included the generated .h file in the .c file, and you either didn't generate a .c file at all or you have mangled the function declaration from what was generated.

Upvotes: 0

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