Textview show nothing with jni

I want to show log file by Textview and Textview log file content is called by jni.

But Textview nothing show(blank black screen),When give just "hello /n How low",was shown by Textview correct.

return (*env)->NewStringUTF(env, "hello /n How low"); was shown.

return (*env)->NewStringUTF(env, str); was not shown.

--application.java--

package com.showlog;

import android.app.Activity;
import android.widget.TextView;
import android.os.Bundle;

public class showlog extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        TextView  tv = new TextView(this);
        tv.setText( stringFromJNI(开发者_如何学Go) );
        setContentView(tv);
    }

    public native String  stringFromJNI();

    public native String  unimplementedStringFromJNI();

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

--showlog.c--

#include <string.h>
#include <stdio.h>
#include <jni.h>

#define MAX 119 // MAX of one line length of log file
#define Log_file "./Log_file.log" // log file path

jstring
Java_com_showlog_stringFromJNI(JNIEnv* env, jobject thiz)
{
    char line[120]; // one line length of Log_file
    char str[2000]; // Log_file length
    FILE *fin;

    fin=fopen(Log_file, "r");
    while( ! feof(fin)){
        fgets(line, MAX, fin);
        strcat(str, line);
    }
    fclose(fin);

    return (*env)->NewStringUTF(env, str);
}

Then I try just c code(not be jni lib),It works.

--just show log file--

#include <string.h>
#include <stdio.h>

#define MAX 119 // MAX of one line length of log file
#define Log_file "./Log_file.log" // log file path

main()
{
    char line[120]; // one line length of Log_file
    char str[2000]; // Log_file length
    FILE *fin;

    fin=fopen(Log_file, "r");
    while( ! feof(fin)){
        fgets(line, MAX, fin);
        strcat(str, line);
    }
    fclose(fin);

        printf("%s", str);

    return 0;
}

How does Textview show it ?

Thanks in advance.

Return value is not proper....just check it.....and return proper value.....