Native Code Throwing Developer-Defined Java Exceptions

[res7cxbi]

How do you throw exceptions i defined myself in java from native code
(C++)?

 

[Gordon Beaton]

How do you throw exceptions i defined myself in java from native
code (C++)?

Have you looked at the JNI specification? There is a section
describing functions for handling exceptions.

Use Throw() or ThrowNew().

/gordon

 

[res7cxbi]

Thanks this is just what i needed

 

[res7cxbi]

Actually, now i have another problem:

Here's my Java Code:
//ExceptionExperiment.java
public class ExceptionExperiment {

public native void testException() throws MyException;

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

public ExceptionExperiment() {
}

public static void main(String[] args) {
ExceptionExperiment app = new ExceptionExperiment();
try {
app.testException();
System.out.print("a");
}catch(MyException e) {
e.printStackTrace();
}
}

public class MyException extends Exception {
public MyException() {
super();
}

public MyException(String s) {
super(s);
}
}
}

And here's the C++ code:
//ExceptionExperimentImpl.cpp
JNIEXPORT void JNICALL Java_ExceptionExperiment_testException
(JNIEnv *env, jobject obj)
{
jclass cls =
env->FindClass("ExceptionExperiment$MyException");
if (cls != NULL) {
env->ThrowNew(cls, NULL);
}
env->DeleteLocalRef(cls);
}

Now, this doesn't throw the MyException, but throws this:
//output
java.lang.NoSuchMethodError: ExceptionExperiment$MyException: method
<init>()V not found
at ExceptionExperiment.testException(Native Method)
at ExceptionExperiment.main(ExceptionExperiment.java:15)

From this output it tells me that it can find the class but can't find

the <init>()V method (which is the constructor, right?).
Am i doing something wrong here?

 

[res7cxbi]

Problem Solved!

I made the MyException class a separate class rather than an inner
class of ExceptionExperiment

The revised java code:
//ExceptionExperiment.java
public class ExceptionExperiment {


public native void testException() throws MyException;


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


public ExceptionExperiment() {
}


public static void main(String[] args) {
ExceptionExperiment app = new ExceptionExperiment();
try {
app.testException();
System.out.print("a");
}catch(MyException e) {
e.printStackTrace();
}
}
}
class MyException extends Exception {
public MyException() {
super();
}

public MyException(String s) {
super(s);
}
}

And here's the revised C++ code:
//ExceptionExperimentImpl.cpp
JNIEXPORT void JNICALL Java_ExceptionExperiment_testException
(JNIEnv *env, jobject obj)
{
jclass cls = env->FindClass("MyException");
if (cls != NULL) {
env->ThrowNew(cls, NULL);
}
env->DeleteLocalRef(cls);
}

 

[Gordon Beaton]

And here's the revised C++ code:
//ExceptionExperimentImpl.cpp
JNIEXPORT void JNICALL Java_ExceptionExperiment_testException
(JNIEnv *env, jobject obj)
{
jclass cls = env->FindClass("MyException");
if (cls != NULL) {
env->ThrowNew(cls, NULL);
}
env->DeleteLocalRef(cls);
}

Note that it is rarely necessary to need DeleteLocalRef(). All local
references will be "deleted" when the native method returns.

/gordon

 

[res7cxbi]

Really? The "A Utility Function" (6.1.2) at the URL
http://java.sun.com/docs/books/jni/html/exceptions.html#11202 does this

 

[Roedy Green]

Actually, now i have another problem:

I would recommend getting a textbook with some examples.

See http://mindprod.com/jgloss/jni.html

 

[Gordon Beaton]

Really? The "A Utility Function" (6.1.2) at the URL
http://java.sun.com/docs/books/jni/html/exceptions.html#11202 does this

Yes it does. Not because it's technically necessary to do so, but
rather to follow a guideline (described in section 5.3 rules for
managing references) that says a utility function should avoid
accumulating local references.

/gordon