Filter for logging http request parameters
The redirect doesn't seem to be part of the logging. Maybe put it in a separate filter that runs before this one?

The logged string could be built using String.format() or MessageFormat which would be more readable. Most logging frameworks have such formatting built into their logging methods anyway.

Do you really need to convert the post parameters into JSON? Isn't the standard toString representation of the Map good enough for logging? Apart from that most JSON libraries have a built-in mechanism to convert a Map into a JSON object so you don't need to do it yourself.