A flexible layout configurable with pattern string. The goal of this class is to {@link #format format} a {@link LoggingEvent} and return the results in a {#link StringBuffer}. The format of the result depends on the conversion pattern.
The conversion pattern is closely related to the conversion pattern of the printf function in C. A conversion pattern is composed of literal text and format control expressions called conversion specifiers.
Note that you are free to insert any literal text within the conversion pattern.
Each conversion specifier starts with a percent sign (%) and is followed by optional format modifiers and a conversion character. The conversion character specifies the type of data, e.g. category, priority, date, thread name. The format modifiers control such things as field width, padding, left and right justification. The following is a simple example.
Let the conversion pattern be "%-5p [%t]: %m%n" and assume that the log4j environment was set to use a PatternLayout. Then the statements
Category root = Category.getRoot(); root.debug("Message 1"); root.warn("Message 2"); would yield the output DEBUG [main]: Message 1 WARN [main]: Message 2
Note that there is no explicit separator between text and conversion specifiers. The pattern parser knows when it has reached the end of a conversion specifier when it reads a conversion character. In the example above the conversion specifier %-5p means the priority of the logging event should be left justified to a width of five characters. The recognized conversion characters are
| Conversion Character | Effect |
|---|---|
| c | Used to output the category of the logging event. The category conversion specifier can be optionally followed by precision specifier, that is a decimal constant in brackets. If a precision specifier is given, then only the corresponding number of right most components of the category name will be printed. By default the category name is printed in full. For example, for the category name "a.b.c" the pattern %c{2} will output "b.c". |
| C | Used to output the fully qualified class name of the caller issuing the logging request. This conversion specifier can be optionally followed by precision specifier, that is a decimal constant in brackets. If a precision specifier is given, then only the corresponding number of right most components of the class name will be printed. By default the class name is output in fully qualified form. For example, for the class name "org.apache.xyz.SomeClass", the pattern %C{1} will output "SomeClass". WARNING Generating the caller class information is slow. Thus, it's use should be avoided unless execution speed is not an issue. |
| d | Used to output the date of the logging event. The date conversion specifier may be followed by a set of braces containing a date and time pattern strings {@link java.text.SimpleDateFormat}, ABSOLUTE, DATE or ISO8601. For example, %d{HH:mm:ss,SSS}, %d{dd MMM yyyy HH:mm:ss,SSS} or %d{DATE}. If no date format specifier is given then ISO8601 format is assumed. |
| F | Used to output the file name where the logging request was issued. WARNING Generating caller location information is extremely slow. Its use should be avoided unless execution speed is not an issue. |
| l | Used to output location information of the caller which generated the logging event. The location information depends on the JVM implementation but usually consists of the fully qualified name of the calling method followed by the callers source the file name and line number between parentheses. The location information can be very useful. However, it's generation is extremely slow. It's use should be avoided unless execution speed is not an issue. |
| L | Used to output the line number from where the logging request was issued. WARNING Generating caller location information is extremely slow. It's use should be avoided unless execution speed is not an issue. |
| m | Used to output the application supplied message associated with the logging event. |
| M | Used to output the method name where the logging request was issued. WARNING Generating caller location information is extremely slow. It's use should be avoided unless execution speed is not an issue. |
| n | Outputs the platform dependent line separator character or characters. This conversion character offers practically the same performance as using non-portable line separator strings such as "\n", or "\r\n". Thus, it is the preferred way of specifying a line separator. |
| p | Used to output the priority of the logging event. |
| r | Used to output the number of milliseconds elapsed since the start of the application until the creation of the logging event. |
| t | Used to output the name of the thread that generated the logging event. |
| x | Used to output the NDC (nested diagnostic context) associated with the thread that generated the logging event. |
| X | Used to output the MDC (mapped diagnostic context) associated with the thread that generated the logging event. The X conversion character can be followed by the key for the map placed between braces, as in %X{clientNumber} where See {@link MDC} class for more details. |
| properties | Used to output the Properties associated with the logging event. The properties conversion word can be followed by the key for the map placed between braces, as in %properties{application} where |
| throwable | Used to output the Throwable trace that has been bound to the LoggingEvent, by default this will output the full trace as one would normally find by a call to Throwable.printStackTrace(). The throwable conversion word can be followed by an option in the form %throwable{short} which will only output the first line of the ThrowableInformation. |
| % | The sequence %% outputs a single percent sign. |
By default the relevant information is output as is. However, with the aid of format modifiers it is possible to change the minimum field width, the maximum field width and justification.
The optional format modifier is placed between the percent sign and the conversion character.
The first optional format modifier is the left justification flag which is just the minus (-) character. Then comes the optional minimum field width modifier. This is a decimal constant that represents the minimum number of characters to output. If the data item requires fewer characters, it is padded on either the left or the right until the minimum width is reached. The default is to pad on the left (right justify) but you can specify right padding with the left justification flag. The padding character is space. If the data item is larger than the minimum field width, the field is expanded to accommodate the data. The value is never truncated.
This behavior can be changed using the maximum field width modifier which is designated by a period followed by a decimal constant. If the data item is longer than the maximum field, then the extra characters are removed from the beginning of the data item and not from the end. For example, it the maximum field width is eight and the data item is ten characters long, then the first two characters of the data item are dropped. This behavior deviates from the printf function in C where truncation is done from the end.
Below are various format modifier examples for the category conversion specifier.
| Format modifier | left justify | minimum width | maximum width | comment |
|---|---|---|---|---|
| %20c | false | 20 | none | Left pad with spaces if the category name is less than 20 characters long. |
| %-20c | true | 20 | none | Right pad with spaces if the category name is less than 20 characters long. |
| %.30c | NA | none | 30 | Truncate from the beginning if the category name is longer than 30 characters. |
| %20.30c | false | 20 | 30 | Left pad with spaces if the category name is shorter than 20 characters. However, if category name is longer than 30 characters, then truncate from the beginning. |
| %-20.30c | true | 20 | 30 | Right pad with spaces if the category name is shorter than 20 characters. However, if category name is longer than 30 characters, then truncate from the beginning. |
Below are some examples of conversion patterns.
The above text is largely inspired from Peter A. Darnell and Philip E. Margolis' highly recommended book "C -- a Software Engineering Approach", ISBN 0-387-97389-3. @author James P. Cakalic @author Ceki Gülcü @since 0.8.2
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