JNI problem when populating jobject in C

[Martin Stephenson]

Hi all,

I'm trying to populate an object with data from a C DLL, called via JNI from
Java.

What I have is this:

public class returnValue
{
private String username;
private String cert;

public void setUsername(String username)
{
this.username = username;
}

public String getUsername()
{
return this.username;
}

public void setCert(String cert)
{
this.cert = cert;
}

public String getCert()
{
return this.cert;
}
}

here's the function Declaration in the Java (for the API in the DLL I want
to call):

public native void getBiometric(String username,
String nonce,
String cert,
long sessionId,
long keyId,
short enroll,
short typeQualifier,
short usageQualifier,
short deviceType,
returnValue r);

Here's where I call it from the Java:

r = new returnValue();
r.setCert("C");
r.setUsername("U");
System.out.println("Username being sent is : " + r.getUsername());
System.out.println("Cert being sent is : " + r.getCert());
getBiometric(username, nonce, "ABC", (long)1000, keyId, (short)0,
(short)2, (short)22, (short)1, r);
System.out.println("Username returned is : " + r.getUsername());
System.out.println("Cert returned is : " + r.getCert());

Basically I want to populate the returnValue object in the C.

Anyhow here's the C Code:

JNIEXPORT void JNICALL Java_JavaCapture_getBiometric(JNIEnv *env,
jobject jo,
jstring u, // The username
jstring n, // The nonce
jstring c, // The cert
jlong sid, // The Session ID
jlong kid, // The Key ID
jshort e, // The Enroll data
jshort tq, // The Type Qualifier
jshort uq, // The Usage Qualifier
jshort dt, // The Device Type
jobject retVal) // The return value
{
//do stuff

// Populate the return object
jclass cls = env->GetObjectClass(retVal);

jmethodID setUsername = env->GetMethodID(cls, "setUsername",
"(Ljava/lang/String;)V");
if (setUsername == 0)
return;
env->CallVoidMethod(retVal, setUsername, "Martin");

jmethodID setCert = env->GetMethodID(cls, "setCert",
"(Ljava/lang/String;)V");
if (setUsername == 0)
return;
env->CallVoidMethod(retVal, setCert, "Cert!!");
}

This works fine in the C, but as soon as the DLL returns to java it bombs -
it never gets to the second set of println's

The error is:

Exception in thread "main" java.lang.NullPointerException
at java.lang.StringBuffer.append(StringBuffer.java:398)
at JavaCapture.getBio(JavaCapture.java:54)
at JavaCapture.main(JavaCapture.java:131)

Any help would be great !!

Thanks !!

Martin.

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[Blair Wyman]

Martin Stephenson wrote:
Signature mismatch:

> jmethodID setUsername = env->GetMethodID(cls, "setUsername",
> "(Ljava/lang/String;)V");
> if (setUsername == 0)
> return;
> env->CallVoidMethod(retVal, setUsername, "Martin");

setUserName is prototyped to take a java.lang.String, but you are
passing "Martin" which is NOT a java.lang.String...

Try:

//...
env->CallVoidMethod(retVal, setUserName,
env->NewStringUTF("Martin));

//...
env->CallVoidMethod(retVal, setCert,
env->NewStringUTF("Cert!!"));

HTH.

--
___ _ Blair Wyman IBM Rochester
( /_) / _ ' _ (507)253-2891 bla...@us.ibm.com
__/__)_/_<_/_/_/_' Opinions expressed may not be those of IBM