ArrayList的添加方法抛出org.apache.jasper.JasperException：java.lang.NullPointerException

Question

The add method of ArrayList throws org.apache.jasper.JasperException: java.lang.NullPointerException when its called in cart.jsp . Please Please see my code. Thanks.

@Remote
public interface BookSessionBeanRemote {
    void persist(Book b);
    List<Book> getBookContent();
    Book getBook(int id);
}

@Stateful
public class CartBean implements CartBeanRemote {

Book b = new Book();
private ArrayList<Book> bk;

@PostConstruct
public void initialize()
{
    bk = new ArrayList<Book>();
}
@Override
public void addBook(Book b)throws Exception
{
    if(b==null)throw new Exception("Book is null");
    bk.add(b);
}
@Override
public void removeBook(int id)throws Exception
{
    BigDecimal bg = new BigDecimal(id);
    b.setBookid(bg);
    boolean result = bk.remove(b);
    if (result == false) 
    {
        throw new Exception("Book not in cart.");
    }
}
@Override
public ArrayList<Book> getContent()
{
    return bk;
}

}

cart.jsp

 <%! CartBeanRemote ct;  %>
        <%
                String path = (String)session.getAttribute("path");
                String bkid = (String)session.getAttribute("bookid");
                String bookname = (String)session.getAttribute("bookname");
                String author = (String)session.getAttribute("author");
                String price= (String)session.getAttribute("price");
               // out.println(path);
              //  out.println(bkid);
              //  out.println(bookname);
              //  out.println(author);
               // out.println(price);

                 BigDecimal bid = new BigDecimal(bkid);
                 BigInteger pr = new BigInteger(price);

                   if(session.isNew())
                    {
                        Context ctx = new InitialContext();
                        ct = (CartBeanRemote)ctx.lookup(CartBeanRemote.class.getName());
                        session.setAttribute("ct", ct);
                    }
                    ct = (CartBeanRemote)session.getAttribute("ct");
                    Book book = new Book();
                    book.setAuthor(author);                        
                    book.setImagepath(path);
                    book.setName(bookname);
                    book.setBookid(bid);
                    book.setPrice(pr);

                    ct.addBook(book);
                   // out.println(book.getAuthor());
                   // out.println(book.getBookid());
                   // out.println(book.getPrice());

                  //  response.sendRedirect("index.jsp");

    %>

Answer 1

Somewhere your code is causing a NullPointerException. From your description, I'd guess that your ArrayList is null when you try to add something to it.

When your code causes the NullPointerException, the JSP will catch it, and then wrap it in a JasperException before rethrowing that.

Have a look at Throwable.getCause(). Also see the constructor for Exception, which looks like Exception(String message, Throwable cause). It is common practice in Java to catch one type of Exception, and wrap it in a new Exception with additional information. This may happen multiple times before the Exception is propogated to a point where the code can do something meaningful to deal with it.