The FileHandler can either write to a specified file, or it can write to a rotating set of files.

For a rotating set of files, as each file reaches a given size limit, it is closed, rotated out, and a new file opened. Successively older files are named by adding "0", "1", "2", etc into the base filename.

By default buffering is enabled in the IO libraries but each log record is flushed out when it is complete.

By default the XMLFormatter class is used for formatting.

Configuration: By default each FileHandler is initialized using the following LogManager configuration properties. If properties are not defined (or have invalid values) then the specified default values are used.

• java.util.logging.FileHandler.level specifies the default level for the Handler (defaults to Level.ALL).
• java.util.logging.FileHandler.filter specifies the name of a Filter class to use (defaults to no Filter).
• java.util.logging.FileHandler.formatter specifies the name of a Formatter class to use (defaults to java.util.logging.XMLFormatter)
• java.util.logging.FileHandler.encoding the name of the character set encoding to use (defaults to the default platform encoding).
• java.util.logging.FileHandler.limit specifies an approximate maximum amount to write (in bytes) to any one file. If this is zero, then there is no limit. (Defaults to no limit).
• java.util.logging.FileHandler.count specifies how many output files to cycle through (defaults to 1).
• java.util.logging.FileHandler.pattern specifies a pattern for generating the output file name. See below for details. (Defaults to "%h/java%u.log").
• java.util.logging.FileHandler.append specifies whether the FileHandler should append onto any existing files (defaults to false).

A pattern consists of a string that includes the following special components that will be replaced at runtime:

• "/" the local pathname separator
• "%t" the system temporary directory
• "%h" the value of the "user.home" system property
• "%g" the generation number to distinguish rotated logs
• "%u" a unique number to resolve conflicts
• "%%" translates to a single percent sign "%"

Thus for example a pattern of "%t/java%g.log" with a count of 2 would typically cause log files to be written on Solaris to /var/tmp/java0.log and /var/tmp/java1.log whereas on Windows 95 they would be typically written to C:\TEMP\java0.log and C:\TEMP\java1.log

Generation numbers follow the sequence 0, 1, 2, etc.

Normally the "%u" unique field is set to 0. However, if the FileHandler tries to open the filename and finds the file is currently in use by another process it will increment the unique number field and try again. This will be repeated until FileHandler finds a file name that is not currently in use. If there is a conflict and no "%u" field has been specified, it will be added at the end of the filename after a dot. (This will be after any automatically added generation number.)

Thus if three processes were all trying to log to fred%u.%g.txt then they might end up using fred0.0.txt, fred1.0.txt, fred2.0.txt as the first file in their rotating sequences.

Note that the use of unique ids to avoid conflicts is only guaranteed to work reliably when using a local disk file system.