Problem with dynamic dataTable generation

[Kumar Gaurav]

Hi All,

I need to create a table dynamically which means no of columns will vary and will be decided at run time. For that i have written a code but its not working. I am pasting here code and exception stacktrace:
Code:

JSP

<h:commandButton action="#{firstBean.createTable}"></h:commandButton>

<h:dataTable binding="#{firstBean.dataTable}" rendered="#{firstBean.renderFlag}" />

Bean

package pack;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

import javax.el.ValueExpression;
import javax.faces.component.UIComponent;
import javax.faces.component.html.HtmlColumn;
import javax.faces.component.html.HtmlDataTable;
import javax.faces.component.html.HtmlOutputText;
import javax.faces.context.FacesContext;

public class FirstBean {

	private int cols =5;
	private int rows= 3;
	private boolean renderFlag = false;	
	private HtmlDataTable dataTable;
	

	public HtmlDataTable getDataTable() {
		return dataTable;
	}

	public void setDataTable(HtmlDataTable dataTable) {
		if(dataTable == null)
			dataTable = new HtmlDataTable();
		this.dataTable = dataTable;
	}

	public int getCols() {
		return cols;
	}

	public void setCols(int cols) {
		this.cols = cols;
	}

	public int getRows() {
		return rows;
	}

	public void setRows(int rows) {
		this.rows = rows;
	}

	public boolean isRenderFlag() {
		return renderFlag;
	}

	public void setRenderFlag(boolean renderFlag) {
		this.renderFlag = renderFlag;
	}
	private List<List<String>> dataList = new ArrayList<List<String>>();

	public List<List<String>> getDataList() {
		return dataList;
	}

	public void setDataList(List<List<String>> dataList) {
		this.dataList = dataList;
	}

	public void createTable() {

		if (cols > 0 && rows > 0) {

			renderFlag = true;
			String header = "Column";
			String cellValue = "Cell";
			 

			String[] headers = new String[cols];
			for (int i = 0; i < headers.length; i++) {
				headers[i] = header + " " + i;
			}
			for (int j = 0; j < rows; j++) {

				String[] cells = new String[cols];
				for (int k = 0; k < cols; k++) {
					cells[k] = cellValue + j + "" + k;
				}
				dataList.add(Arrays.asList(cells));
			}

			// Create <h:dataTable value="#{myBean.dynamicList}"
			// var="dynamicItem">.
			dataTable = new HtmlDataTable();
			dataTable.setValueExpression("value", createValueExpression(
					"#{myBean.dataList}", List.class));
			dataTable.setVar("dynamicItem");

			// Iterate over columns.
			for (int i = 0; i < dataList.get(0).size(); i++) {
				// Create <h:column>.

				HtmlColumn column = new HtmlColumn();
				dataTable.getChildren().add(column);

				// Create <h:outputText value="dynamicHeaders[i]"> for <f:facet
				// name="header"> of column.
				HtmlOutputText headerT = new HtmlOutputText();
				headerT.setValue(headers[i]);
				column.setHeader(headerT);

				// Create <h:outputText value="#{dynamicItem[" + i + "]}"> for
				// the body of column.
				HtmlOutputText output = new HtmlOutputText();
				output.setValueExpression("value", createValueExpression(
						"#{dynamicItem[" + i + "]}", String.class));
				column.getChildren().add(output);
			}			
	    }

	}



	private ValueExpression createValueExpression(String valueExpression,
			Class<?> valueType) {
		FacesContext facesContext = FacesContext.getCurrentInstance();
		return facesContext.getApplication().getExpressionFactory().
		createValueExpression(facesContext.getELContext(),valueExpression, valueType);
	}

}

Exception :

Exception


org.apache.jasper.el.JspELException: /First.jsp(23,1) '#{firstBean.dataTable}' Cannot convert #{firstBean.dataTable} of type class java.lang.String to class javax.faces.component.UIComponent
at org.apache.jasper.el.JspValueExpression.getValue(JspValueExpression.java:107)
at com.sun.faces.application.ApplicationImpl.createComponent(ApplicationImpl.java:246)
at javax.faces.webapp.UIComponentELTag.createComponent(UIComponentELTag.java:222)
at javax.faces.webapp.UIComponentClassicTagBase.createChild(UIComponentClassicTagBase.java:486)
at javax.faces.webapp.UIComponentClassicTagBase.findComponent(UIComponentClassicTagBase.java:670)
at javax.faces.webapp.UIComponentClassicTagBase.doStartTag(UIComponentClassicTagBase.java:1142)
at com.sun.faces.taglib.html_basic.DataTableTag.doStartTag(DataTableTag.java:382)
at org.apache.jsp.First_jsp._jspx_meth_h_005fdataTable_005f0(First_jsp.java:340)
at org.apache.jsp.First_jsp._jspx_meth_f_005fview_005f0(First_jsp.java:132)
at org.apache.jsp.First_jsp._jspService(First_jsp.java:83)

Please help me out. I am complete stuck.

[Davie Lin]

your class is "FirstBean"

but you can't use "#{firstBean}" that's not what your declare your class as

[Tim Holloway]

There's a "code" button in the editor that you can use to wrap code, XML, and other stuff that you don't want to be reformatted. I applied it to your example. It makes it a lot easier to read!

For the record, I'm not sure what Davie means - JSF doesn't refer to classes, it refers to instances, so "firstBean" is correct. Assuming you defined an instance of FirstBean in your faces-config under the name of "firstBean", at least.

I can't see for certain what makes the system thing your datatable property is a String, but you didn't initialize it in a constructor and the code that creates it would only be executed AFTER the page was displayed and posted back by clicking on the commandButton. So initially it would be null and just possibly, that got turned into a String whose value was "null". And that would definitely NOT be a dataTable.

So create a public no-argument constructor for FirstBean and initialize the dataTable to a basic dataTable object and see if that helps.

Also, be a good chap and put braces around your conditional predicates. It makes them less likely to bite code maintainers.