JNI C calling java with String gets null pointer in Java

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xingshan_he@hotmail.com - 19 Oct 2005 00:47 GMT

I have a c program that calls a java method. When trying to access the
string parms passed into the jave pointer, I get null pointer
exception.

Here is part of the code:

char *urlChar = "http://this_host/service";
char *userChar = "tester";
char *passwordChar = "tester";
char *phoneNumberChar = "1234567890";

jmethodID mid = (*env)->GetStaticMethodID(env,
cls, "getPhoneMessage",
"(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;");
if (mid == NULL) {
    printf("unable to get method getPhoneMessage()...");
    return;
}

jobject result = (*env)->CallStaticObjectMethod(
    env, cls, mid, phoneNumberChar, urlChar, userChar, passwordChar);

--- java class ---

public class IVRPhoneUtil {
public static String getPhoneMessage(String phoneNumber, String
url, String userID, String password) throws Exception {
System.out.println("java getPhoneMessage()");
if (url == null) {
System.out.println("url is null");
}
else {
System.out.println("length " + url.length());
System.out.println("url is " + url);
}
return "test message";
}
}

-- error / stack trace ---
Exception in thread "main" java.lang.NullPointerException
at java.io.Writer.write(Writer.java:126)
at java.io.PrintStream.write(PrintStream.java:303)
at java.io.PrintStream.print(PrintStream.java:448)
at java.io.PrintStream.println(PrintStream.java:585)
at
com.interop.ivr.IVRPhoneUtil.getPhoneMessage(IVRPhoneUtil.java:21)
Segmentation fault (core dumped)

The line that's causing the null pointer is where I tried to print out
the length of the string.

Any idea what's going wrong here?

Thanks.

XS

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Roedy Green - 19 Oct 2005 04:04 GMT

On 18 Oct 2005 16:47:15 -0700, xingshan_he@hotmail.com wrote or quoted

>jmethodID mid = (*env)->GetStaticMethodID(env,

You are a glutton for punishment with code like this. Do the
Javaesque stuff like reflection in Java and the Cesque stuff in C.

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http://mindprod.com Again taking new Java programming contracts.

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Chris Uppal - 19 Oct 2005 08:42 GMT

> char *urlChar = "http://this_host/service";
> char *userChar = "tester";
> char *passwordChar = "tester";
> char *phoneNumberChar = "1234567890";

[...]

> jobject result = (*env)->CallStaticObjectMethod(
> env, cls, mid, phoneNumberChar, urlChar, userChar, passwordChar);

You are passing four pointers to a nul-terminated array of 8-byte characters
when the function is expecting 4 references to objects of type
java.lang.String. These things are not even remotely similar.

Create four java.lang.Strings from your char*s and pass those.

-- chris

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xingshan_he@hotmail.com - 19 Oct 2005 20:06 GMT

This is a great group with great programmers! Using advice given here,
I got my stuff to work.
Here is the change to get it working:

Instead of doing this:
jobject result = (*env)->CallStaticObjectMethod(
env, cls, mid, phoneNumberChar, urlChar, userChar, passwordChar);

I need to do this:

jobject result = (*env)->CallStaticObjectMethod(
    env, cls, mid,
    (*env)->NewStringUTF(env, phoneNumberChar),
    (*env)->NewStringUTF(env, urlChar),
    (*env)->NewStringUTF(env, userChar),
    (*env)->NewStringUTF(env, passwordChar));

It works great now. My c program calls a java program that does SOAP
to a Tomcat server.

Thanks alot folks!

XS

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xingshan_he@hotmail.com - 19 Oct 2005 23:46 GMT

I tried a different way of creating jstring from char* and the result
is very different:

This works as it successfully returns from Java method, but then it get
segmentation fault when I tried to get char* from the jstring:

jstring result = (jstring)(*env)->CallStaticObjectMethod(
    env, cls, mid, //I have seen people calling this method without the
'env' here???
    (*env)->NewStringUTF(env, phoneNumberChar),
    (*env)->NewStringUTF(env, urlChar),
    (*env)->NewStringUTF(env, userChar),
    (*env)->NewStringUTF(env, passwordChar));
if (result != NULL) {
    const char *str = (*env)->GetStringUTFChars(env, result, 0);
//here I get segmentation fault
    printf("message returned is %s\n", str);

}

While this causes segmentation fault when calling the java method:

jstring phoneNumber = (jstring)(*env)->NewStringUTF(env,
phoneNumberChar);
jstring url = (jstring)(*env)->NewStringUTF(env, urlChar);
jstring user = (jstring)(*env)->NewStringUTF(env, userChar);
jstring password = (jstring)(*env)->NewStringUTF(env, passwordChar);

jstring result = (jstring)(*env)->CallStaticObjectMethod(
    env, cls, mid, phoneNumber, url, user, password);

Can anyone explain why this is happening? What's the correct way of
doing this?

Thanks,
XS

Reply to this Message

Gordon Beaton - 20 Oct 2005 09:24 GMT

> I tried a different way of creating jstring from char* and the result
> is very different:
[quoted text clipped - 5 lines]
> env, cls, mid, //I have seen people calling this method without the
> 'env' here???

In C++ env is automatically passed to the function. In C you need to
pass it explicitly.

>     (*env)->NewStringUTF(env, phoneNumberChar),
>     (*env)->NewStringUTF(env, urlChar),
>     (*env)->NewStringUTF(env, userChar),
>     (*env)->NewStringUTF(env, passwordChar));
> if (result != NULL) {
>     const char *str = (*env)->GetStringUTFChars(env, result, 0);

> //here I get segmentation fault
> printf("message returned is %s\n", str);

If str is NULL after calling GetStringUTFChars(), you can't pass it to
printf(). You should check first, and determine whether
GetStringUTFChars() raised an exception before proceeding.

Note that every call to GetStringUTFChars() should be matched with a
call to ReleaseStringUTFChars(). If you are calling this method
repeatedly it will ultimately fail otherwise.

> While this causes segmentation fault when calling the java method:
>
[quoted text clipped - 9 lines]
> Can anyone explain why this is happening? What's the correct way of
> doing this?

None of the casts to (jstring) should be necessary, and the use of
explicit typecasts should be considered a warning flag. Otherwise I
can't see anything obviously wrong with the code you've posted,

Your results seem a little erratic, so I wonder if you haven't done
something wrong elsewhere in the native code. Check for array bounds
errors, stray pointers, unitialized variables, unchecked return values
and things like that.

To help determine the location of the segmentation fault more
accurately, use fprintf(stderr) rather than printf().

/gordon

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Gordon Beaton - 19 Oct 2005 08:56 GMT

> I have a c program that calls a java method. When trying to access the
> string parms passed into the jave pointer, I get null pointer
> exception.

[...]

> char *urlChar = "http://this_host/service";
> char *userChar = "tester";
> char *passwordChar = "tester";
> char *phoneNumberChar = "1234567890";

[...]

> jobject result = (*env)->CallStaticObjectMethod(
> env, cls, mid, phoneNumberChar, urlChar, userChar, passwordChar);

You can't pass a char* where a java String is required. Use
NewString() or NewStringUTF to create the necessary objects.

/gordon

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[ do not email me copies of your followups ]
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JNI C calling java with String gets null pointer in Java

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