How to analyze the file activities due to process? I need to analyze the file i/o operations caused due to a process. 
So I would like to know, which all files are affected by this program. Is there a tool for this particular use case?.

One of our debugging for a slow URL processing got the culprit being the file being read each time a URL was processed by the system. So I would like to get recommendations for a tool which would monitor the file activities per process.
Any suggestions?
You can use lsof with the -p flag, like this : lsof -p $PID .