public Transform expression(String expression) {
		Assert.hasText(expression, "expression can not be empty nor null");
		this.expression = expression;
		return this;
	}
Sets the expression to be used by the transform
@param expression The expression to transform the input
@return current instance of the transform fixture.
spring-xd-test-fixtures/src/main/java/org/springframework/xd/test/fixtures/Transform.java