@Override
	@AssertTrue(message = "the 'script' and 'expression' options are mutually exclusive")
	public boolean isValid() {
		// default value for expression is always set; this is safe as profilesToActivate() will activate 'use-script'
		// if script is non-null
		return getScript() == null || expression.equals(DEFAULT_EXPRESSION);
	}
User can't explicitly set both script and expression.
spring-xd-module-spi/src/main/java/org/springframework/xd/module/options/mixins/ExpressionOrScriptMixin.java