java.lang.NullPointerException的可能原因及处理

java.lang.NullPointerException的可能原因及处理

java.lang.NullPointerException具体意思是空指针异常，最常见的问题就是没有初始化。

1. 字符串等数据类型没有初始化
2. 类实例（对象）有用具体的类初始化
3. 没有判断是否为空

Eg:

源码：

 public static BookInformation[] ImFromClassification(String a){
         Connection conn = null;
         PreparedStatement ps = null;
         ResultSet rs = null;
         try{
                 int x = 0;
                 conn = LinkMysql.getDBconnection();
                 if(conn == null){System.out.println("conn");}
                 String sql="select * from bookinformation where classification=?";
                 ps = conn.prepareStatement(sql);
                 ps.setString(1, a);
                 rs = ps.executeQuery();
                 rs.beforeFirst();
                 while(rs.next()){
                     ++x;
                 }
                 System.out.println(x);
                 BookInformation[] a1 = new BookInformation[x];
                 rs.first();
                 for(int i = 0; i < x; i++){
                     //a1[i] = new BookInformation();
                     a1[i].setName(rs.getString("name"));
                     a1[i].setAuthor(rs.getString("author"));
                     a1[i].setClassification(rs.getString("classification"));
                     a1[i].setAmount(rs.getInt("amount"));
                     a1[i].setPrice(rs.getInt("price"));
                     a1[i].setSalvesVolum(rs.getInt("sales_volum"));
                     rs.next();
                 }
                 return a1;

         } catch (SQLException e) {
             System.out.println("xxx");
             return null;
         }
         finally{LinkMysql.closeDB(conn, ps, rs);}

     }

报错：

root cause

java.lang.NullPointerException
	Dao.BookInfor.ImFromClassification(BookInfor.java:31)
	org.apache.jsp.front.home_jsp._jspService(home_jsp.java:120)
	org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
	org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:439)
	org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:395)
	org.apache.jasper.servlet.JspServlet.service(JspServlet.java:339)
	javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
	org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

分析：

数组定义(BookInformation[] a1 = new BookInformation[5])之后，没有对每个数组元素初始化。

更改：

 public static BookInformation[] ImFromClassification(String a){
         Connection conn = null;
         PreparedStatement ps = null;
         ResultSet rs = null;
         try{
                 int x = 0;
                 conn = LinkMysql.getDBconnection();
                 if(conn == null){System.out.println("conn");}
                 String sql="select * from bookinformation where classification=?";
                 ps = conn.prepareStatement(sql);
                 ps.setString(1, a);
                 rs = ps.executeQuery();
                 rs.beforeFirst();
                 while(rs.next()){
                     ++x;
                 }
                 System.out.println(x);
                 BookInformation[] a1 = new BookInformation[x];
                 rs.first();
                 for(int i = 0; i < x; i++){
                     a1[i] = new BookInformation();
                     a1[i].setName(rs.getString("name"));
                     a1[i].setAuthor(rs.getString("author"));
                     a1[i].setClassification(rs.getString("classification"));
                     a1[i].setAmount(rs.getInt("amount"));
                     a1[i].setPrice(rs.getInt("price"));
                     a1[i].setSalvesVolum(rs.getInt("sales_volum"));
                     rs.next();
                 }
                 return a1;

         } catch (SQLException e) {
             System.out.println("xxx");
             return null;
         }
         finally{LinkMysql.closeDB(conn, ps, rs);}

     }