Creating ArrayList through Form submitting ( Thymeleaf + Spring MVC )

Question

i wanted to know, how Lists are getting binded in Spring...and how to access them in Thymeleaf. With and without WrapperClass How could achieve adding single / multiple objects through an form to an ArrayList or normal Array..

I would be very thankful for an explanation.

Thats what i have tried: Form

<form action="#" th:action="@{/tests}" th:object="${features}" method="post">



        <input type="text" th:field="*{featureArrayList[0].name}"/>
    <input type="text" th:field="*{featureArrayList[1].name}"/>




    <p><input type="submit" value="Submit" /> <input type="reset" value="Reset" /></p>
</form>

Wrapper Class

  ArrayList<Feature> featureArrayList = new ArrayList<Feature>();

    public FeatureWrapper() {

    }

    public ArrayList<Feature> getFeatureArrayList() {
        return featureArrayList;
    }

    public void setFeatureArrayList(ArrayList<Feature> featureArrayList) {
        this.featureArrayList = featureArrayList;
    }

Model:

   private String name;

    public Feature(String name) {
        this.name = name;
    }

    public Feature() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

Controller


      @GetMapping("/tests")
    public String showForm(Model model)
    {
        FeatureWrapper featureWrapper = new FeatureWrapper();

        model.addAttribute("features",  featureWrapper);
        return "Feature";
    }
    @PostMapping("/tests")
    public String proceedForm(@ModelAttribute("features") FeatureWrapper features,Model model)
    {
       for(Feature feature: features.getFeatureArrayList())
       {
           System.out.println(feature.getName());
           System.out.println(features.getFeatureArrayList().size());
       }

        return "Feature";
    }

Edit1: Now its working with an Wrapper Class and using static Indexes... How would i achieve this without a wrapper class , i read much about the EL expression ${var.index} , but how is this working ? And how should i use this to dynamically add my Objects. Edit2: Now im able to add single objects , and multiple objects to the list, but im not using any EL Expression, so i wanted to know when would i need to use something like this:

 <div th:each="feature,stat:*{featureArrayList}">
        <input  id="myInput" type="text" th:field="*{featureArrayList[__${stat.index}__].name}"/>
    </div>

Possible duplicate of EL1008E: Property or field 'content' cannot be found on object of type 'java.util.ArrayList' - maybe not public or not valid? — Tom
– Tom, Commented Apr 11, 2019 at 12:37
As far as I know, you can't bind to a list without a wrapper class. It just doesn't work that way. — Metroids
– Metroids, Commented Apr 11, 2019 at 16:55
@Metroids Thanks for you answer, do you maybe know when to use the el expression i mentioned — T. Lekar
– T. Lekar, Commented Apr 11, 2019 at 18:29

Metroids · Accepted Answer · 2019-04-11 19:32:44Z

1

As far as I know, you can't bind to a list without a wrapper class (as stated in comments).
In normal Thymeleaf expressions, you can directly use variables as indexes. Expressions like this for example:
```
<th:block th:with="i=0">
    <span th:text="${array[i]}" />
</th:block>
```
However, th:field expressions are different. If you read the section on dynamic fields:

The problem is that Spring EL does not evaluate variables inside array index brackets, so when executing the above expression we would obtain an error telling us that rows[rowStat.index] (instead of rows[0], rows[1], etc) is not a valid position in the rows collection. That’s why preprocessing is needed here.

So, when you are using th:field to bind inputs to arrays, you must use preprocessing which is why you see expressions like this:
```
<input type="text" th:field="*{featureArrayList[__${stat.index}__].name}"/>
```

answered Apr 11, 2019 at 19:32

Metroids

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1 Comment

T. Lekar Over a year ago

Thanks for your answer, finally i understood the usecase.

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