Expression templating

Expression templates allow mixing literal text with one or more evaluation blocks. Each evaluation block is delimited with prefix and suffix characters that you can define. A common choice is to use #{ } as the delimiters, as the following example shows:

  • Java

  • Kotlin

String randomPhrase = parser.parseExpression(
		"random number is #{T(java.lang.Math).random()}",
		new TemplateParserContext()).getValue(String.class);

// evaluates to "random number is 0.7038186818312008"
val randomPhrase = parser.parseExpression(
		"random number is #{T(java.lang.Math).random()}",
		TemplateParserContext()).getValue(String::class.java)

// evaluates to "random number is 0.7038186818312008"

The string is evaluated by concatenating the literal text 'random number is ' with the result of evaluating the expression inside the #{ } delimiter (in this case, the result of calling that random() method). The second argument to the parseExpression() method is of the type ParserContext. The ParserContext interface is used to influence how the expression is parsed in order to support the expression templating functionality. The definition of TemplateParserContext follows:

  • Java

  • Kotlin

public class TemplateParserContext implements ParserContext {

	public String getExpressionPrefix() {
		return "#{";
	}

	public String getExpressionSuffix() {
		return "}";
	}

	public boolean isTemplate() {
		return true;
	}
}
class TemplateParserContext : ParserContext {

	override fun getExpressionPrefix(): String {
		return "#{"
	}

	override fun getExpressionSuffix(): String {
		return "}"
	}

	override fun isTemplate(): Boolean {
		return true
	}
}