java.text.MessageFormat 介紹

 

MessageFormat provides a means to produce concatenated messages in language-neutral way. Use this to construct messages displayed for end users.

MessageFormat takes a set of objects, formats them, then inserts the formatted strings into the pattern at the appropriate places.

Note: MessageFormat differs from the other Format classes in that you create a MessageFormat object with one of its constructors (not with a getInstance style factory method). The factory methods aren't necessary because MessageFormat itself doesn't implement locale specific behavior. Any locale specific behavior is defined by the pattern that you provide as well as the subformats used for inserted arguments.

Patterns and Their Interpretation

MessageFormat uses patterns of the following form:

 MessageFormatPattern:         String         MessageFormatPattern FormatElement String FormatElement:         { ArgumentIndex }         { ArgumentIndex , FormatType }         { ArgumentIndex , FormatType , FormatStyle } FormatType: one of          number date time choice FormatStyle:         short         medium         long         full         integer         currency         percent         SubformatPattern String:         StringPartopt         String StringPart StringPart:         ''         ' QuotedString '         UnquotedString SubformatPattern:         SubformatPatternPartopt         SubformatPattern SubformatPatternPart SubFormatPatternPart:         ' QuotedPattern '         UnquotedPattern 

Within a String, "''" represents a single quote. A QuotedString can contain arbitrary characters except single quotes; the surrounding single quotes are removed. An UnquotedString can contain arbitrary characters except single quotes and left curly brackets. Thus, a string that should result in the formatted message "'{0}'" can be written as "'''{'0}''" or "'''{0}'''".

Within a SubformatPattern, different rules apply. A QuotedPattern can contain arbitrary characters except single quotes; but the surrounding single quotes are not removed, so they may be interpreted by the subformat. For example, "{1,number,$'#',##}" will produce a number format with the pound-sign quoted, with a result such as: "$#31,45". An UnquotedPattern can contain arbitrary characters except single quotes, but curly braces within it must be balanced. For example, "ab {0} de" and "ab '}' de" are valid subformat patterns, but "ab {0'}' de" and "ab } de" are not.

 

Warning:

The rules for using quotes within message format patterns unfortunately have shown to be somewhat confusing. In particular, it isn't always obvious to localizers whether single quotes need to be doubled or not. Make sure to inform localizers about the rules, and tell them (for example, by using comments in resource bundle source files) which strings will be processed by MessageFormat. Note that localizers may need to use single quotes in translated strings where the original version doesn't have them.

The ArgumentIndex value is a non-negative integer written using the digits '0' through '9', and represents an index into the arguments array passed to the format methods or the result array returned by the parse methods.

The FormatType and FormatStyle values are used to create a Format instance for the format element. The following table shows how the values map to Format instances. Combinations not shown in the table are illegal. A SubformatPattern must be a valid pattern string for the Format subclass used.

Format Type	Format Style	Subformat Created
(none)	(none)	null
number	(none)	NumberFormat.getInstance(getLocale())
	integer	NumberFormat.getIntegerInstance(getLocale())
	currency	NumberFormat.getCurrencyInstance(getLocale())
	percent	NumberFormat.getPercentInstance(getLocale())
	SubformatPattern	new DecimalFormat(subformatPattern, new DecimalFormatSymbols(getLocale()))
date	(none)	DateFormat.getDateInstance(DateFormat.DEFAULT, getLocale())
	short	DateFormat.getDateInstance(DateFormat.SHORT, getLocale())
	medium	DateFormat.getDateInstance(DateFormat.DEFAULT, getLocale())
	long	DateFormat.getDateInstance(DateFormat.LONG, getLocale())
	full	DateFormat.getDateInstance(DateFormat.FULL, getLocale())
	SubformatPattern	new SimpleDateFormat(subformatPattern, getLocale())
time	(none)	DateFormat.getTimeInstance(DateFormat.DEFAULT, getLocale())
	short	DateFormat.getTimeInstance(DateFormat.SHORT, getLocale())
	medium	DateFormat.getTimeInstance(DateFormat.DEFAULT, getLocale())
	long	DateFormat.getTimeInstance(DateFormat.LONG, getLocale())
	full	DateFormat.getTimeInstance(DateFormat.FULL, getLocale())
	SubformatPattern	new SimpleDateFormat(subformatPattern, getLocale())
choice	SubformatPattern	new ChoiceFormat(subformatPattern)

 

Usage Information

Here are some examples of usage:

 Object[] arguments = {     new Integer(7),     new Date(System.currentTimeMillis()),     "a disturbance in the Force" }; String result = MessageFormat.format(     "At {1,time} on {1,date}, there was {2} on planet {0,number,integer}.",     arguments); output: At 12:30 PM on Jul 3, 2053, there was a disturbance           in the Force on planet 7. 

Typically, the message format will come from resources, and the arguments will be dynamically set at runtime.

Example 2:

 Object[] testArgs = {new Long(3), "MyDisk"}; MessageFormat form = new MessageFormat(     "The disk /"{1}/" contains {0} file(s)."); System.out.println(form.format(testArgs)); // output, with different testArgs output: The disk "MyDisk" contains 0 file(s). output: The disk "MyDisk" contains 1 file(s). output: The disk "MyDisk" contains 1,273 file(s). 

For more sophisticated patterns, you can use a ChoiceFormat to get output such as:

 MessageFormat form = new MessageFormat("The disk /"{1}/" contains {0}."); double[] filelimits = {0,1,2}; String[] filepart = {"no files","one file","{0,number} files"}; ChoiceFormat fileform = new ChoiceFormat(filelimits, filepart); form.setFormatByArgumentIndex(0, fileform); Object[] testArgs = {new Long(12373), "MyDisk"}; System.out.println(form.format(testArgs)); // output, with different testArgs output: The disk "MyDisk" contains no files. output: The disk "MyDisk" contains one file. output: The disk "MyDisk" contains 1,273 files. 

You can either do this programmatically, as in the above example, or by using a pattern (see ChoiceFormat for more information) as in:

 form.applyPattern(    "There {0,choice,0#are no files|1#is one file|1<are {0,number,integer} files}."); 

Note: As we see above, the string produced by a ChoiceFormat in MessageFormat is treated specially; occurances of '{' are used to indicated subformats, and cause recursion. If you create both a MessageFormat and ChoiceFormat programmatically (instead of using the string patterns), then be careful not to produce a format that recurses on itself, which will cause an infinite loop.

When a single argument is parsed more than once in the string, the last match will be the final result of the parsing. For example,

 MessageFormat mf = new MessageFormat("{0,number,#.##}, {0,number,#.#}"); Object[] objs = {new Double(3.1415)}; String result = mf.format( objs ); // result now equals "3.14, 3.1" objs = null; objs = mf.parse(result, new ParsePosition(0)); // objs now equals {new Double(3.1)} 

Likewise, parsing with a MessageFormat object using patterns containing multiple occurances of the same argument would return the last match. For example,

 MessageFormat mf = new MessageFormat("{0}, {0}, {0}"); String forParsing = "x, y, z"; Object[] objs = mf.parse(forParsing, new ParsePosition(0)); // result now equals {new String("z")} 

Synchronization

Message formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

See Also:

java.util.Locale

Format

NumberFormat

DecimalFormat

ChoiceFormat

Author:

Mark Davis

@version

1.51, 01/23/03