空指针异常我怎么也找不出问题

Question

java.lang.NullPointerException
com.eshore.dao.ProductDaoImpl.findAll(ProductDaoImpl.java:33)
com.eshore.service.ProductService.findAll(ProductService.java:36)
org.apache.jsp.product_005flist2_jsp._jspService(product_005flist2_jsp.java:101)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:432)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

我是查询语句，JSP JDBC连接MYsql的，用DAO模式，
下面是报错里面这两句有问题的话的出处：

第一句：com.eshore.dao.ProductDaoImpl.findAll(ProductDaoImpl.java:33)
public List findAll(String product_name) throws Exception{
List list = new ArrayList();
String sql = "select product_id,product_name,price,info from product";
if(product_name!=null&&!"".equals(product_name)){
sql = "select product_id,product_name,price,info from product where product_name like? ";
this.pstmt.setString(1,"%" + product_name + "%");

        }
        else {
            this.pstmt = this.conn.prepareStatement(sql);
        }
     ResultSet rs = this.pstmt.executeQuery();
     Product product = null;
        while (rs.next()){
            product = new Product();
            product.setProduct_id(rs.getString(1));
            product.setProduct_name(rs.getString(2));
            product.setPrice(rs.getDouble(3));
            product.setInfo(rs.getString(4));
            list.add(product);
        }
     this.pstmt.close();
     return list;
 }

 标的是：this.pstmt.setString(1,"%" + product_name + "%");这句




 第二句：com.eshore.service.ProductService.findAll(ProductService.java:36)

public List findAll(String product_name) throws Exception{
List list = null;

    try{ 
        list = this.dao.findAll(product_name);
    }
    catch (Exception e){
        throw e;
    }
    finally {
        this.dbconn.close();
    }

    return list;
}

是里面的：list = this.dao.findAll(product_name);


在第二处可疑错中，我开始想是不是product_name是不是没判断非空，
加了if(product_name!=null&&!"".equals(product_name)){


变成：       try{ if(product_name!=null&&!"".equals(product_name)){
        list = this.dao.findAll(product_name);
    }}

什么的还是报这个错：
java.lang.NullPointerException
com.eshore.dao.ProductDaoImpl.findAll(ProductDaoImpl.java:33)
com.eshore.service.ProductService.findAll(ProductService.java:36)

Answer 1

if(product_name!=null&&!"".equals(product_name)){
sql = "select product_id,product_name,price,info from product where product_name like? ";
this.pstmt.setString(1,"%" + product_name + "%");
}
else {
this.pstmt = this.conn.prepareStatement(sql);
}
product_name不是空的时候 this.pstmt 没设置值？？是空的吗，你else里设置了