NullPointerException handling

R

ramu

i,
I have the following code.

package transform;

import oracle.xml.schemavalidator.XSDValidator;
import oracle.xml.parser.schema.XSDException;
import oracle.xml.parser.schema.XMLSchema;
import oracle.xml.parser.schema.XSDBuilder;
import oracle.xml.parser.v2.XMLError;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.helpers.DefaultHandler;
import java.net.URL;

public class SchemaValidator{

public void validateSchema(String SchemaUrl, String
XmlDocumentUrl)
{
try {
XSDValidator xsdValidator=new XSDValidator();
XSDBuilder builder = new XSDBuilder();
URL url = new URL(SchemaUrl);
XMLSchema schemadoc =
(XMLSchema)builder.build(url);
xsdValidator.setSchema(schemadoc);

Validator handler=new Validator();
XMLError xmlError=new XMLError();
xmlError.setErrorHandler(handler);
xsdValidator.setError(xmlError);
xsdValidator.validate(new
URL(XmlDocumentUrl));
if(handler.validationError==true)
System.out.println("This XML Document
has Error: " + handler.saxParseException.getMessage());
else
System.out.println("This XML Document
is valid");

}catch(java.lang.NullPointerException npe)
{
System.out.println("NullPointerException
"+npe.getMessage());
}
catch(java.io.IOException ioe)
{
System.out.println("IOException
"+ioe.getMessage());
}catch (SAXException e) {
System.out.println("SAXException
"+e.getMessage());
}
catch (XSDException e) {
System.out.println("SAXException
"+e.getMessage());
}
}

private class Validator extends DefaultHandler{

public boolean validationError = false;
public SAXParseException saxParseException=null;
alidationError = true;
saxParseException=exception;
}

public void fatalError(SAXParseException exception)
throws SAXException{
validationError = true;
saxParseException=exception;
}
public void warning(SAXParseException exception)
throws SAXException{}

}

public static void main(String[] argv){
try{
String SchemaUrl=argv[0];
String XmlDocumentUrl=argv[1];
SchemaValidator validator=new SchemaValidator();



validator.validateSchema(SchemaUrl, XmlDocumentUrl);

} catch(java.lang.NullPointerException npe)
{

System.out.println("NullPointerException");
}
}


Here XmlDocumentUrl is the xml document. Am trying to validating a xml
document(XmlDocumentUrl) with the schema specified by SchemaUrl. But
when I miss some attribute in the xml document(XmlDocumentUrl) its
throwing NullPointerException. After catching the NullPointerException
I want to handle it. I want to print an error message which specifies
in which file(XmlDocumentUrl) it occured and the line in that
file(XmlDocumentUrl). Can anyone tell me how to do this?
 
M

Manish Pandit

i,
I have the following code.

package transform;

import oracle.xml.schemavalidator.XSDValidator;
import oracle.xml.parser.schema.XSDException;
import oracle.xml.parser.schema.XMLSchema;
import oracle.xml.parser.schema.XSDBuilder;
import oracle.xml.parser.v2.XMLError;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.helpers.DefaultHandler;
import java.net.URL;

public class SchemaValidator{

public void validateSchema(String SchemaUrl, String
XmlDocumentUrl)
{
try {
XSDValidator xsdValidator=new XSDValidator();
XSDBuilder builder = new XSDBuilder();
URL url = new URL(SchemaUrl);
XMLSchema schemadoc =
(XMLSchema)builder.build(url);
xsdValidator.setSchema(schemadoc);

Validator handler=new Validator();
XMLError xmlError=new XMLError();
xmlError.setErrorHandler(handler);
xsdValidator.setError(xmlError);
xsdValidator.validate(new
URL(XmlDocumentUrl));
if(handler.validationError==true)
System.out.println("This XML Document
has Error: " + handler.saxParseException.getMessage());
else
System.out.println("This XML Document
is valid");

}catch(java.lang.NullPointerException npe)
{
System.out.println("NullPointerException
"+npe.getMessage());
}
catch(java.io.IOException ioe)
{
System.out.println("IOException
"+ioe.getMessage());
}catch (SAXException e) {
System.out.println("SAXException
"+e.getMessage());
}
catch (XSDException e) {
System.out.println("SAXException
"+e.getMessage());
}
}

private class Validator extends DefaultHandler{

public boolean validationError = false;
public SAXParseException saxParseException=null;
alidationError = true;
saxParseException=exception;
}

public void fatalError(SAXParseException exception)
throws SAXException{
validationError = true;
saxParseException=exception;
}
public void warning(SAXParseException exception)
throws SAXException{}

}

public static void main(String[] argv){
try{
String SchemaUrl=argv[0];
String XmlDocumentUrl=argv[1];
SchemaValidator validator=new SchemaValidator();

validator.validateSchema(SchemaUrl, XmlDocumentUrl);

} catch(java.lang.NullPointerException npe)
{

System.out.println("NullPointerException");
}
}

Here XmlDocumentUrl is the xml document. Am trying to validating a xml
document(XmlDocumentUrl) with the schema specified by SchemaUrl. But
when I miss some attribute in the xml document(XmlDocumentUrl) its
throwing NullPointerException. After catching the NullPointerException
I want to handle it. I want to print an error message which specifies
in which file(XmlDocumentUrl) it occured and the line in that
file(XmlDocumentUrl). Can anyone tell me how to do this?

Yes. It is called a Stack Trace.

npe.printStackTrace();

-cheers,
Manish
 
M

Manish Pandit

i,
I have the following code.
package transform;
import oracle.xml.schemavalidator.XSDValidator;
import oracle.xml.parser.schema.XSDException;
import oracle.xml.parser.schema.XMLSchema;
import oracle.xml.parser.schema.XSDBuilder;
import oracle.xml.parser.v2.XMLError;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.helpers.DefaultHandler;
import java.net.URL;
public class SchemaValidator{
public void validateSchema(String SchemaUrl, String
XmlDocumentUrl)
{
try {
XSDValidator xsdValidator=new XSDValidator();
XSDBuilder builder = new XSDBuilder();
URL url = new URL(SchemaUrl);
XMLSchema schemadoc =
(XMLSchema)builder.build(url);
xsdValidator.setSchema(schemadoc);
Validator handler=new Validator();
XMLError xmlError=new XMLError();
xmlError.setErrorHandler(handler);
xsdValidator.setError(xmlError);
xsdValidator.validate(new
URL(XmlDocumentUrl));
if(handler.validationError==true)
System.out.println("This XML Document
has Error: " + handler.saxParseException.getMessage());
else
System.out.println("This XML Document
is valid");
}catch(java.lang.NullPointerException npe)
{
System.out.println("NullPointerException
"+npe.getMessage());
}
catch(java.io.IOException ioe)
{
System.out.println("IOException
"+ioe.getMessage());
}catch (SAXException e) {
System.out.println("SAXException
"+e.getMessage());
}
catch (XSDException e) {
System.out.println("SAXException
"+e.getMessage());
}
}
private class Validator extends DefaultHandler{
public boolean validationError = false;
public SAXParseException saxParseException=null;
alidationError = true;
saxParseException=exception;
}
public void fatalError(SAXParseException exception)
throws SAXException{
validationError = true;
saxParseException=exception;
}
public void warning(SAXParseException exception)
throws SAXException{}

public static void main(String[] argv){
try{
String SchemaUrl=argv[0];
String XmlDocumentUrl=argv[1];
SchemaValidator validator=new SchemaValidator();
validator.validateSchema(SchemaUrl, XmlDocumentUrl);
} catch(java.lang.NullPointerException npe)
{

Here XmlDocumentUrl is the xml document. Am trying to validating a xml
document(XmlDocumentUrl) with the schema specified by SchemaUrl. But
when I miss some attribute in the xml document(XmlDocumentUrl) its
throwing NullPointerException. After catching the NullPointerException
I want to handle it. I want to print an error message which specifies
in which file(XmlDocumentUrl) it occured and the line in that
file(XmlDocumentUrl). Can anyone tell me how to do this?

Yes. It is called a Stack Trace.

npe.printStackTrace();

-cheers,
Manish- Hide quoted text -

- Show quoted text -

Oops..sorry! I thought you wanted to show the line number where
exception occured.

You should not catch NPEs - they normally indicated uninitialized
references, or something wrong with the flow. Something in the arglist
is null, or the object you're calling the method on is null. This is
something you debug, not catch.

The exception should spew out a trace on the console (when not
catched) - follow the trace and see what could be the problem.

-cheers,
Manish
 
R

ramu

Hi,
The intention here is to tell the user, that he has not specified
any attribute(NULL value) in the XML document So I have to tell the
user in which line he is missing the attribute in the input XML
document. Any idea how to do this?

Regards
 
O

Oliver Wong

ramu said:
i,
I have the following code.
[snip code]

[...]
Am trying to validating a xml
document(XmlDocumentUrl) with the schema specified by SchemaUrl. But
when I miss some attribute in the xml document(XmlDocumentUrl) its
throwing NullPointerException. After catching the NullPointerException
I want to handle it. I want to print an error message which specifies
in which file(XmlDocumentUrl) it occured and the line in that
file(XmlDocumentUrl). Can anyone tell me how to do this?

Catch the NPE, wrap it in a new exception of your own design (e.g.
MissingAttributeException) and throw that.

Catch the MissingAttributeException within the method which knows the
name of the file. Display the error at that point.

- Oliver
 
M

Manivannan Palanichamy

i,
I have the following code.

package transform;

import oracle.xml.schemavalidator.XSDValidator;
import oracle.xml.parser.schema.XSDException;
import oracle.xml.parser.schema.XMLSchema;
import oracle.xml.parser.schema.XSDBuilder;
import oracle.xml.parser.v2.XMLError;
import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;
import org.xml.sax.helpers.DefaultHandler;
import java.net.URL;

public class SchemaValidator{

public void validateSchema(String SchemaUrl, String
XmlDocumentUrl)
{
try {
XSDValidator xsdValidator=new XSDValidator();
XSDBuilder builder = new XSDBuilder();
URL url = new URL(SchemaUrl);
XMLSchema schemadoc =
(XMLSchema)builder.build(url);
xsdValidator.setSchema(schemadoc);

Validator handler=new Validator();
XMLError xmlError=new XMLError();
xmlError.setErrorHandler(handler);
xsdValidator.setError(xmlError);
xsdValidator.validate(new
URL(XmlDocumentUrl));
if(handler.validationError==true)
System.out.println("This XML Document
has Error: " + handler.saxParseException.getMessage());
else
System.out.println("This XML Document
is valid");

}catch(java.lang.NullPointerException npe)
{
System.out.println("NullPointerException
"+npe.getMessage());
}
catch(java.io.IOException ioe)
{
System.out.println("IOException
"+ioe.getMessage());
}catch (SAXException e) {
System.out.println("SAXException
"+e.getMessage());
}
catch (XSDException e) {
System.out.println("SAXException
"+e.getMessage());
}
}

private class Validator extends DefaultHandler{

public boolean validationError = false;
public SAXParseException saxParseException=null;
alidationError = true;
saxParseException=exception;
}

public void fatalError(SAXParseException exception)
throws SAXException{
validationError = true;
saxParseException=exception;
}
public void warning(SAXParseException exception)
throws SAXException{}

}

public static void main(String[] argv){
try{
String SchemaUrl=argv[0];
String XmlDocumentUrl=argv[1];
SchemaValidator validator=new SchemaValidator();

validator.validateSchema(SchemaUrl, XmlDocumentUrl);

} catch(java.lang.NullPointerException npe)
{

System.out.println("NullPointerException");
}
}

Here XmlDocumentUrl is the xml document. Am trying to validating a xml
document(XmlDocumentUrl) with the schema specified by SchemaUrl. But
when I miss some attribute in the xml document(XmlDocumentUrl) its
throwing NullPointerException. After catching the NullPointerException
I want to handle it. I want to print an error message which specifies
in which file(XmlDocumentUrl) it occured and the line in that
file(XmlDocumentUrl). Can anyone tell me how to do this?

Why dont you add an if condition to check null values?
like,
if (xsdValidator == null)
throw new RuntimeException("xsdValidator is null.");

NullPointerExcetpion is something, an application is not supposed to
throw. Remember, every NullPointerException is considered to be a Bug
by many design principles. Applications should add null handles (like
if condition) and wrap it by a custom exception & then throw.
 
B

Ben Phillips

Manivannan said:
if (xsdValidator == null)
throw new RuntimeException("xsdValidator is null.");

Eh. Wrapping a specific subtype of RuntimeException in a plain vanilla
RuntimeException? Another one for the WTF file...

If it shouldn't ever be null, leave the exception alone. If it could be
null due to invalid user input, the user input needs validating so that
the null never travels deeper than the UI layer. If it could be null for
other reasons, it should be transformed into a checked exception (e.g.
into IOException if nulls sometimes result from a particular species of
network protocol barf) or handled on-site (e.g. as a special-case
allowed parameter value to your method).
 
K

Karl Uppiano

Ben Phillips said:
Eh. Wrapping a specific subtype of RuntimeException in a plain vanilla
RuntimeException? Another one for the WTF file...

I would at least re-throw it wrapped in an IllegalArgumentException. That
would tell the user it was something *they* did wrong.

[snip]
 
G

Greg R. Broderick

when I miss some attribute in the xml document(XmlDocumentUrl) its
throwing NullPointerException. After catching the NullPointerException
I want to handle it. I want to print an error message which specifies
in which file(XmlDocumentUrl) it occured and the line in that
file(XmlDocumentUrl). Can anyone tell me how to do this?


The parser (whether it is your code or the underlying XML processing /
parsing code) should not be throwing a NullPointerException in this case,
but should rather be throwing an exception that is more directly related to
the cause of the exception (e.g. SAXException, DOMException). A
NullPointerException is not suitable for the purposes that you are trying
to put it, as it is far to general of an exception (it could have a very
wide range of causes, only one of which would be a missing XML attribute).


I would track down the actual line of code that is throwing this exception
and:


1. if this is your code, fix it so that the missing attribute results in a
more appropriate thrown exception.

2. if this is in the underlying code that you are calling, work around it
(feed the underlying code different input parameters, or implement the
SAXErrorHandler interface in your code) so that the NullPointerException
isn't thrown.

3. if you cannot do #2 and the NullPointerException is thrown by the
underlying code, then get a different XML parser/validator instead of the
one that you're presently using.

--
---------------------------------------------------------------------
Greg R. Broderick (e-mail address removed)

A. Top posters.
Q. What is the most annoying thing on Usenet?
---------------------------------------------------------------------
 
B

Ben Phillips

Greg said:
3. if you cannot do #2 and the NullPointerException is thrown by the
underlying code, then get a different XML parser/validator instead of the
one that you're presently using.

There's also "transform the exception". Wrap it in SAXException or
whatever. (I'm fairly sure I suggested something similar before, using
an example where IOException was the appropriate checked exception type.)
 
G

Greg R. Broderick

There's also "transform the exception". Wrap it in SAXException or
whatever. (I'm fairly sure I suggested something similar before, using
an example where IOException was the appropriate checked exception type.)

Transforming the exception is fatally flawed -- the NullPointerException can
happen for any number of reasons (one of which might be this missing
attribute). Interpreting every possible NPE as caused by a missing attribute
is erroneous, and is likely to cause more problems than it solves. It would
be analogous to a medical doctor interpreting every instance of a headache as
caused by a brain tumor.

Moreover, other, more specific exception types (like SAXParseException) carry
'interesting' information about the location of the error within the XML
document. NullPointerExceptions don't carry this same level of specific
information.

Cheers!


--
---------------------------------------------------------------------
Greg R. Broderick (e-mail address removed)

A. Top posters.
Q. What is the most annoying thing on Usenet?
---------------------------------------------------------------------
 
B

Ben Phillips

Well if possible you want to catch the null itself before it causes NPE,
and transform it.

If the NPE comes from code you call, but don't control, you might have
to resort to transforming it. In that case you'd want to debug all NPEs
of other causes until there were none first. Or if you can catch the
NPE, evaluate some expression that is true only if the NPE was caused in
a particular way, transform it if so, and rethrow it unaltered otherwise.

Most likely though if you're using a library that throws NPEs when it
should throw some checked exception instead, then you ought to report it
to the library's makers as a bug. Still, until they fix it you may need
to work around it. It's up to you to weigh your various options there,
but often you won't want it to bring your Big Important Server(tm) or
whatever to a screeching halt every time it happens.
 

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