Can we have multiline comments in a Java properties file?
If you use Eclipse, you can select multiple lines and comment all with a shortcut (Ctrl+/ by default). Same shortcut uncomments the lines, but you have to pay attention no to select any empty line, which will cause the non-empty ones to get commented more than once.
These apply to Eclipse, but I guess many IDE:s and some editors offer similar functionality.
.properties files do not have multiline comments.
Since JDK 1.5 Properties have XML support, and XML format supports multiline comments. See javadocs.