Java RestService writing and reading data from log files

I have a Java Rest Servcie which Consumes data Posted by java Rest Client. When Client Post the Data into my Rest Service,i am updating data in some tables. If my database is down.I want to store Client posted data into some log files.

when client post new data again,that time we need to read the log file data again.

Could any one i want to know how to write and read data from log files in java Rest service application.where log files get stored?