MapViewer supports labels or annotations that contain a newline char (ASCII char code 10). However by default such characters are ignored by MapViewer during the map text labeling process, and the label text appears in a single line (unless there is a text wrap width specified.)
To force MapViewer to honor the newline chars in a theme's label texts, the theme must use a TEXT labeling style with the "Honor Newline" attribute set to true. This can be done from MapBuilder's TEXT style editor, by simply selecting the Honor newline check box. Now any theme that uses this TEXT style will automatically display any label text containing newline characters with multiple lines.
If your application is written using the MapViewer Java API, and you need to add individual geo-features with multi-line text labels to your map, then additional caution must be taken when constructing the label text with the newline char. Namely, the label text for a feature must be constructed like this:
String textLabel = "LINE 1" + "%26#10;" + "LINE 2";
Typically in an XML document (the Java API eventually will convert your current map request into an XML request before sending it to the MapViewer server), one uses to represent the newline char(10). Because we are sending the entire map request string using HTTP POST from map client to the server, the character '&' itself must be escaped as '%26', hence the odd-looking string %26#10; representing an HTTP-safe newline character.
So this is how you can construct a new feature with this text as its label:
mapViewer.addPointFeature(centreX, centreY, 8265, "C.RED", textLabel, "MY_TEXT_STYLE",
null, null, true);
where MY_TEXT_STYLE should be a TEXT type style with "Honor newline" enabled.