我正在尝试下面的代码来获取spa对象字段中的数据。因此，我正在使用以下代码，但它显示了null值并给出了以下错误

Error setting expression 'spa.amount' with value '[Ljava.lang.String;@10dd65e'
ognl.OgnlException: target is null for setProperty(null, "amount", [Ljava.lang.String;@10dd65e)

Jsp代码：

<s:form action="UpdatedPaid" method="post">  
        <s:if test="paidList.size >  0"> 
            <s:iterator value="paidList" var="pdlst">
                <div class="box">
                    <span class="label">Amount</span>
                    <span class="ib"> 
                        <s:hidden name="spa.id" id="paidId">
                            <s:param name="value">
                                <s:property value="paidId"/>
                            </s:param>
                        </s:hidden>
                        <s:textfield name="spa.amount" id="amount">
                            <s:param name="value">
                                <s:property value="amount"/>
                            </s:param>
                        </s:textfield>
                    </span>
                </div>
                <div class="box">
                    <span class="label">Payment Date</span>
                    <span class="ib"> <s:textfield name="spa.paymentDate" id="paymentDate">
                            <s:param name="value">
                                <s:property value="paymentDate"/>
                            </s:param>
                        </s:textfield>
                    </span>
                </div>
                <div class="box">
                    <span class="label">Payment Mode</span>
                    <span class="ib">
                        <s:textfield name="spa.paymentMode" id="payment_mode" readonly="true">
                            <s:param name="value">
                                <s:property value="payment_mode"/>
                            </s:param>
                        </s:textfield>
                    </span>
                </div>
            </s:iterator>
        </s:if>
    </s:form>

我的动作课

package iland.payment;

import static com.opensymphony.xwork2.Action.SUCCESS;
import com.opensymphony.xwork2.ActionSupport;
import com.opensymphony.xwork2.ModelDriven;
import iland.hbm.SupplierPaidDetails;

public class hbmCashAction extends ActionSupport implements ModelDriven<SupplierPaidDetails> {

    SupplierPaidDetails spa = new SupplierPaidDetails();

    public SupplierPaidDetails getSpa() {
        return spa;
    }

    public void setSpa(SupplierPaidDetails spa) {
        this.spa = spa;
    }

    public String update() {
        System.out.println("--------");
        System.out.println(spa.getId() + " " + spa.getPaymentMode() + " " + spa.getAmount());
        System.out.println("--------");
        return SUCCESS;
    }

    @Override
    public SupplierPaidDetails getModel() {
        return spa;
    }

}

SupplierPayDetails

public class SupplierPaidDetails  implements java.io.Serializable {


     private Long id;
     private SupplierPaymentDetails supplierPaymentDetails;
     private Float amount;
     private String paymentMode;
     private Date paymentDate;
     private Date addDate;
     private String status;
     private Set supplierPaidOnlines = new HashSet(0);
     private Set supplierPaidCashes = new HashSet(0);
     private Set supplierPaidChecks = new HashSet(0);

    public SupplierPaidDetails() {
    }


    public SupplierPaidDetails(SupplierPaymentDetails supplierPaymentDetails, Date addDate) {
        this.supplierPaymentDetails = supplierPaymentDetails;
        this.addDate = addDate;
    }
    public SupplierPaidDetails(SupplierPaymentDetails supplierPaymentDetails, Float amount, String paymentMode, Date paymentDate, Date addDate, String status, Set supplierPaidOnlines, Set supplierPaidCashes, Set supplierPaidChecks) {
       this.supplierPaymentDetails = supplierPaymentDetails;
       this.amount = amount;
       this.paymentMode = paymentMode;
       this.paymentDate = paymentDate;
       this.addDate = addDate;
       this.status = status;
       this.supplierPaidOnlines = supplierPaidOnlines;
       this.supplierPaidCashes = supplierPaidCashes;
       this.supplierPaidChecks = supplierPaidChecks;
    }

    public Long getId() {
        return this.id;
    }

    public void setId(Long id) {
        this.id = id;
    }
    public SupplierPaymentDetails getSupplierPaymentDetails() {
        return this.supplierPaymentDetails;
    }

    public void setSupplierPaymentDetails(SupplierPaymentDetails supplierPaymentDetails) {
        this.supplierPaymentDetails = supplierPaymentDetails;
    }
    public Float getAmount() {
        return this.amount;
    }

    public void setAmount(Float amount) {
        this.amount = amount;
    }
    public String getPaymentMode() {
        return this.paymentMode;
    }

    public void setPaymentMode(String paymentMode) {
        this.paymentMode = paymentMode;
    }
    public Date getPaymentDate() {
        return this.paymentDate;
    }

    public void setPaymentDate(Date paymentDate) {
        this.paymentDate = paymentDate;
    }
    public Date getAddDate() {
        return this.addDate;
    }

    public void setAddDate(Date addDate) {
        this.addDate = addDate;
    }
    public String getStatus() {
        return this.status;
    }

    public void setStatus(String status) {
        this.status = status;
    }
    public Set getSupplierPaidOnlines() {
        return this.supplierPaidOnlines;
    }

    public void setSupplierPaidOnlines(Set supplierPaidOnlines) {
        this.supplierPaidOnlines = supplierPaidOnlines;
    }
    public Set getSupplierPaidCashes() {
        return this.supplierPaidCashes;
    }

    public void setSupplierPaidCashes(Set supplierPaidCashes) {
        this.supplierPaidCashes = supplierPaidCashes;
    }
    public Set getSupplierPaidChecks() {
        return this.supplierPaidChecks;
    }

    public void setSupplierPaidChecks(Set supplierPaidChecks) {
        this.supplierPaidChecks = supplierPaidChecks;
    }
}

编辑
我想设置的各个领域SupplierPaidDetails，包括集（supplierPaidOnlines，supplierPaidCashes，supplierPaidChecks这些可以包含许多情况下），并supplierPaymentDetails从JSP

如何解决以上错误。