How do I populate a drop down with a list using thymeleaf and spring

For generating dropdown for the list of String returned in model you just need to use these lines. You don't need to create any model class with extra getter and setter methods. Your code is correct, you just missed the variable name for storing the value returned in countryName list in th:each.

<select th:field="*{countryName}">
<option value="">Select Country</option>
<option th:each="country : ${countryName}" th:value="${country}" th:text="${country}"></option>
</select>

First thank to your question and answer! I've done with this solution.

My Model

@Entity
@Table(name = "test")
public class Test {

    @Id
    private String testCode;
    private String testName;
    private int price;

    public Test() {}

    public Test(String testCode, String testName, int price) {
        this.testCode = testCode;
        this.testName = testName;
        this.price = price;
    }

    public String getTestCode() {
        return testCode;
    }

    public String getTestName() {
        return testName;
    }

    public int getPrice() {
        return price;
    }
}

My View

    List<Test> test = new ArrayList<>();
    model.addAttribute("test", test);
    List<Test> tests = testRepository.findAll();
    model.addAttribute("tests", tests);

My HTML

<div class="col-lg-3" th:object="${test}">
    <select class="form-control" id="testOrder" name="testOrder">
        <option value="">Select Test Order</option>
        <option th:each="test : ${tests}"
                th:value="${test.testCode}"
                th:text="${test.testCode}+' : '+${test.testName}"></option>
    </select>
</div>

My Result

Image - tymeleaf dropdown from model

This is how I populate the drop down list.I think it may help to you get some idea about it.

Controller

List<Operator> operators =  operatorService.getAllOperaors()
model.addAttribute("operators", operators);

Model

  @Entity
  @Table(name = "operator")
  public class Operator {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    @JsonIgnore
    private Long id;

    @NotBlank(message="operator Name cannot be empty")
    @Column(name = "operator_name", nullable = false)
    private String operatorName;

    getters and setters ...

}   

View

<div class="form-group blu-margin">
    <select class="form-control" th:field="${operator.opeId}"  id="dropOperator">
    <option value="0">select operator</option>
    <option th:each="operator : ${operators}" th:value="${operator.id}" th:text="${operator.operatorName}"></option>
    </select>
</div>