Wednesday, January 16, 2008

Functions with a Variable Argument List II

After using functions with a variable argument list to implement a simple logging sollution I found out that those functions do a great job also in the exception handling.
When you want to throw an exception you usually implement a utility method like this:

void throwByName(JNIEnv *env, const char *name, const char *msg)

jclass cls = env->FindClass(name);
/* if cls is NULL, an exception has already been thrown */
if (cls != NULL)
env->ThrowNew(cls, msg);
/* free the local ref */

This method can be invoked in such a way:
char buffer[128];
throwByName(env, "com/exceptions/MyException", buffer);

The problem is that if you have a lot of methods and exception condition you have to copy/paste every time the exception class that is inconvenient. Also after the first refactoring you'll probably have to change the exception class on so much places.
To avoid those problems you can implement utility methods for all the exceptions that are used (or maybe the most often used) to store the exception class:
void throwMyException(JNIEnv * env, const char * errorMessage, ...)
char buffer[STACK_TRACE_SIZE];
va_list args;
va_start(args, errorMessage);
vsnprintf(buffer, sizeof(buffer), errorMessage, args);

throwByName(env, "com/exceptions/MyException", buffer);

The benefit is that you can throw that particular exception very easy and without much copy/paste:
throwMyException(env, "Error finishing encryption!");

You can also take a look at this post for more tips&trick for JNI.

Functions with a Variable Argument List

My current project is a JNI application - invokes C++ code from Java. I began implementation of a Logger component for the C++ part. As usually I wanted to integrate it very easy with just replacing the previous printing to stdout (printf) with a utility method (log) eg:
replace: printf("Error = %d (%s)", error, errorMessage);
with : log("Error = %d (%s)", error, errorMessage);
The signature of the log method is: log(const char * message, ...) that should print the log message to a file with: fprintf (file, messageString).

The big problem turned out to be - how to pass all the arguments from log to fprintf - both methods with variable argument list ...

Some ideas:
  1. All the tutorials learn you how to iterate those variable arguments and manipulate them separately - time consuming
  2. Overloade operator << - a lot of refactoring required.
  3. A cool idea was to redirect the System.out from Java to a file with System.setOut(printStream) - stdout is not the same as System.out
A possible sollution is to redirect the stdout from the C++ code to a file and skip the whole Logger component - the printf will append to the log file:
std::freopen(LOG_FILE, "a", stdout);
The big side effect is that the Java System.out stream is also redirected and writes to the file.

And the winner is:

#include "stdio.h"
#include "stdarg.h"
log(const char * message, ...) {
char buffer[512]
va_list args;
va_start(args, message);
// Returns the size of the created message
vsnprintf(buffer, sizeof(buffer), message, args);
fprintf (file, buffer);

vsnprintf formats the message with the argument list and writes it to the buffer that is easily logged.

Clean sollution but far from my Java-stuffed brain.

A solution for exception handling in JNI is in: Functions with a Variable Argument List II
You can also take a look at this post for more tips&trick for JNI.