Format Modifiers
Format modifiers allow you to specify the exact number of characters used to represent a field. Format modifiers come after the percent sign and before the conversion specifier.
The following table lists format modifiers and their affects:
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Format Modifier
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Justified
|
Min Width
|
Max Width
|
Comments
|
|
%20
|
Right
|
20
|
None
|
Field is left padded with spaces to be a minimum of 20 characters long.
|
|
%-10
|
Left
|
10
|
None
|
Field is right padded with spaces to be a minimum of 10 characters long.
|
|
%.15
|
NA
|
None
|
15
|
Field is truncated if it is longer than 15 characters.
|
|
%-20.30
|
Left
|
20
|
30
|
Combines both justification and truncation. In this case, right padding up to 20 spaces and truncating if necessary after 30.
|
|
%5.5
|
Right
|
5
|
5
|
In this example, we are specifying a field with a fixed width of 5. Fixed width fields can be very useful with continuous string data.
|
As an example, consider the following log entry.
2012-05-09 10:50:14 A Test14Start - Initializing...There are two things to note about this log entry. First, there are 5 spaces after the 'A' that follows the date. Second, the string 'Test14Start' actually contains three pieces of information an ID, a Position, and an Action.
The above log entry can be parsed with the conversion pattern:
%d %-5c %4.4s{ID}%2.2s{POS}%5.5s{ACTION} - %m%nUsing this conversion pattern, the log entry will be parsed as:
|
Format Modifier
|
Value
|
Comments
|
|
%d
|
2012-05-09 10:50:14
|
The date.
|
|
%-5c
|
A
|
The extra 4 spaces are ignored. If our conversion pattern had contained two spaces between this specifier and the next, we would read into the next field and parsing would fail.
|
|
%4.4s{ID}
|
Test
|
The next field is a fixed 11 characters. The first four will be the ID.
|
|
%2.2s{POS}
|
14
|
...the next two will be the Position.
|
|
%5.5s{ACTION}
|
Start
|
...and the final 5 will be the Action. Taken together, these three fields must always be exactly 11 characters long.
|
|
%m
|
Initializing...
|
The message. This could be any length and may span multiple lines.
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You can find out more about format modifiers online. The Apache documentation is a particularly good place to start.