Prior to Spring-Boot 2.3.x

You should either specify logging.file or logging.path, but not both (
when both are specified, logging.path is ignored and only logging.file is considered).

1. Using logging.file

You may use logging.file one of the following way:

logging.file = logfile.log                     //in current folder
logging.file = relativepath/to/logfile.log     //relative path with filename
logging.file = /fullpath/to/logfile.log        //full path with filename

In Spring Boot Documentation:

By default, Spring Boot will only log to the console and will not
write log files. If you want to write log files in addition to the
console output you need to set a logging.file or logging.path property
(for example in your application.properties).

In Spring Boot’s how to logging doc:

If the only change you need to make to logging is to set the levels of
various loggers then you can do that in application.properties using
the “logging.level” prefix, e.g.
You can also set the location of a file to log to (in addition to the
console) using “logging.file”.

2. Using logging.path

You could also use logging.path to set the path, in which case the logfile will automatically be named spring.log:

logging.path = ./                         // -> spring.log in current folder
logging.path = relativepath/to/logs       // -> relativepath/to/logs/spring.log
logging.path = /fullpath/to/logs          // -> /fullpath/to/logs/spring.log

In Spring Boot doc:

[Using logging.path] Writes spring.log to the specified directory.
Names can be an exact location or relative to the current directory.

springframework.guru on Spring Boot logging:

There is also a logging.path property to specify a path for a logging
file. If you use it, Spring Boot creates a spring.log file in the
specified path. However, you cannot specify both the logging.file and
logging.path properties together. If done, Spring Boot will ignore
both.

Since Spring-Boot 2.3.x

The previously used properties logging.file or logging.path became deprecated in Spring-Boot 2.2.x and were replaced in version 2.3.x with logging.file.name and logging.file.path.

See Spring Boot Documentation.