什么会导致java.lang.NoClassDefFoundError错误？

    Exception in thread "main" java.lang.NoClassDefFoundError: org/apache/xmlbeans/XmlObject
 at ExcelReader.main(ExcelReader.java:32)
Caused by: java.lang.ClassNotFoundException: org.apache.xmlbeans.XmlObject
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 1 more

请帮助我。 首先打开.xlsx文件，然后给出第一个表格。 最后在控制台上打印excel文件的数据。 提示：我已经将poi-ooxml-3.9-20121203.jar添加到我的项目中。

    import java.io.File;
    import java.io.FileInputStream;
    import javax.swing.text.html.HTMLDocument.Iterator;
    import org.apache.poi.hssf.usermodel.HSSFSheet;
    import org.apache.poi.hssf.usermodel.HSSFWorkbook;
    import org.apache.poi.ss.usermodel.Cell;
    import org.apache.poi.ss.usermodel.Row;
    import org.apache.poi.xssf.usermodel.XSSFSheet;
    import java.util.*;
    import org.apache.poi.xssf.usermodel.XSSFWorkbook;
    /**
     * @author mohammad hosein
    *
    */
    public class ExcelReader {

/**
 * @param args
 */
public static void main(String[] args) {
    try
    {
    FileInputStream file = new FileInputStream(new File("E:\\test.xlsx"));

    //Get the workbook instance for XLS file 
    XSSFWorkbook workbook = new XSSFWorkbook (file);

    //Get first sheet from the workbook
    XSSFSheet sheet = workbook.getSheetAt(0);

    //Get iterator to all the rows in current sheet
    java.util.Iterator<Row> rowIterator = sheet.iterator();

    while(rowIterator.hasNext())
    {
        Row row = rowIterator.next();
        java.util.Iterator<Cell> cellIterator = row.cellIterator();

        while(cellIterator.hasNext())
        {
            Cell cell = cellIterator.next();
            System.out.print(cell.getStringCellValue() + "\t");
        }
        System.out.println("");
    }
    }
    catch(Exception e)
    {
        System.out.println("EROR!");
    }

    //Get iterator to all cells of current row

}

}