Fill a list from JSP in Spring

Question

Hello,

I have something like this in my Spring Application:

public class Book{ 
    public Book(){
       sheets = new LinkedList<Sheet>();
    }
    protected List<Sheet> sheets;
    //getter and setter 
}

I add several Sheets to the sheet list and I print a form in a JSP like this:

<form:form modelAttribute="book"  action="${dest_url}" method="POST">   
    <c:forEach items="${mybook.sheets}" var="sheet" varStatus="status">
        <form:hidden path="sheet[${status.count -1}].header"/>
        <form:hidden path="sheet[${status.count -1}].footer"/>
        <form:hidden path="sheet[${status.count -1}].operador"/>
        <form:hidden path="sheet[${status.count -1}].number"/>
        <form:hidden path="sheet[${status.count -1}].lines"/>
    </c:forEach>
    ...
</form:form>

I need to get back this list in the controller when the form is submitted. So in my controller I have a method with a parameter like this:

public String myMethod (@ModelAttribute("book") Book book, Model model){
    ...
}

The problem is that it doesn't fill the sheets list unless in the constructor of Book I add as much Sheet's as I want to get. The problem is that I don't know in advance the number of Sheets the book is going to have.

I think the problem is that in my method it instantiates Book which has a list of sheets with 0 elements. When it tries to access to sheets[0] the list is empty and it doen't add a Sheet. I've tried to create a getter method for the list with an index parameter (so it can create the element if it doesn't exists in the list like in Struts framework) like this one:

public Sheet getSheets(int index){
    if(sheets.size() <= index){
        Sheet sheet = new Sheet();
        sheets.add(index, sheet);
    }
    Sheet sheetToReturn = sheets.get(index);
    if(sheetToReturn == null){
        sheetToReturn = new Sheet();
        sheets.add(index, sheetToReturn);
    }
    return sheetToReturn;
}

but with this method the JSP doesn't work because sheets has an invalid getter.

What's the proper way of filling a list when you don't know the number of items in advanced?

Thanks

Answer 1

This getter is indeed not going to work. With collections in EL, ${bean.list[0]} would roughly resolve to bean.getList().get(0), not to bean.getList(0) as you seem to expect.

You need to pass the size of the list as a request parameter as well and prepare the list accordingly. You can use JSTL fn:length() to get the size:

<input type="hidden" name="size" value="${fn:length(mybook.sheets)}">

(sorry, no Struts targeted example with form:hidden, I don't use it, but it should give you an idea)

When the size is set, then you can prepare the list as follows:

for (int i = 0; i < this.size; i++) {
    sheets.add(new Sheet());
}

That said, you can better use ${status.index} instead of ${status.count - 1}. See the LoopTagStatus API for all available methods.

Answer 2

If you make your list a lazy list (Apache Commons Collections LazyList or Spring's AutoPopulatingList) it will instantiate a new Sheet as a given list element at the moment you bind to it. Then you don't have to worry about how many items you've prepopulated the list with -- you can call list[17] on an empty list and it will be created on the fly and bound.