public BooleanExpression isNotNull() {
        if (isnotnull == null) {
            isnotnull = Expressions.booleanOperation(Ops.IS_NOT_NULL, mixin);
        }
        return isnotnull;
    }
Create a {@code this is not null} expression

@return this is not null
querydsl-core/src/main/java/com/querydsl/core/types/dsl/SimpleExpression.java