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。
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字符串等数据类型没有初始化
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类实例（对象）有用具体的类初始化
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没有判断是否为空
Eg:
源码：
1 public static BookInformation[] ImFromClassification(String a){ 2 Connection conn = null; 3 PreparedStatement ps = null; 4 ResultSet rs = null; 5 try{ 6 int x = 0; 7 conn = LinkMysql.getDBconnection(); 8 if(conn == null){System.out.println("conn");} 9 String sql="select * from bookinformation where classification=?"; 10 ps = conn.prepareStatement(sql); 11 ps.setString(1, a); 12 rs = ps.executeQuery(); 13 rs.beforeFirst(); 14 while(rs.next()){ 15 ++x; 16 } 17 System.out.println(x); 18 BookInformation[] a1 = new BookInformation[x]; 19 rs.first(); 20 for(int i = 0; i < x; i++){ 21 //a1[i] = new BookInformation(); 22 a1[i].setName(rs.getString("name")); 23 a1[i].setAuthor(rs.getString("author")); 24 a1[i].setClassification(rs.getString("classification")); 25 a1[i].setAmount(rs.getInt("amount")); 26 a1[i].setPrice(rs.getInt("price")); 27 a1[i].setSalvesVolum(rs.getInt("sales_volum")); 28 rs.next(); 29 } 30 return a1; 31 32 } catch (SQLException e) { 33 System.out.println("xxx"); 34 return null; 35 } 36 finally{LinkMysql.closeDB(conn, ps, rs);} 37 38 }
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：
1 public static BookInformation[] ImFromClassification(String a){ 2 Connection conn = null; 3 PreparedStatement ps = null; 4 ResultSet rs = null; 5 try{ 6 int x = 0; 7 conn = LinkMysql.getDBconnection(); 8 if(conn == null){System.out.println("conn");} 9 String sql="select * from bookinformation where classification=?"; 10 ps = conn.prepareStatement(sql); 11 ps.setString(1, a); 12 rs = ps.executeQuery(); 13 rs.beforeFirst(); 14 while(rs.next()){ 15 ++x; 16 } 17 System.out.println(x); 18 BookInformation[] a1 = new BookInformation[x]; 19 rs.first(); 20 for(int i = 0; i < x; i++){ 21 a1[i] = new BookInformation(); 22 a1[i].setName(rs.getString("name")); 23 a1[i].setAuthor(rs.getString("author")); 24 a1[i].setClassification(rs.getString("classification")); 25 a1[i].setAmount(rs.getInt("amount")); 26 a1[i].setPrice(rs.getInt("price")); 27 a1[i].setSalvesVolum(rs.getInt("sales_volum")); 28 rs.next(); 29 } 30 return a1; 31 32 } catch (SQLException e) { 33 System.out.println("xxx"); 34 return null; 35 } 36 finally{LinkMysql.closeDB(conn, ps, rs);} 37 38 }