java.lang.NullPointerException的可能原因及處理

java.lang.NullPointerException的可能原因及處理

java.lang.NullPointerException具體意思是空指針異常，最常見的問題就是沒有初始化。

1. 字符串等數據類型沒有初始化
2. 類實例（對象）有用具體的類初始化
3. 沒有判斷是否為空

 

Eg:

源碼：

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);}
        
    }

報錯：

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：

數組定義(BookInformation[] a1 = new BookInformation[5])之后，沒有對每個數組元素初始化。

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);}
        
    }