java.lang.NullPointerException

Question

接口：public 《T》 List《T》 query(String sql, Class《T》 clazz, Object[] parameters);

实现方法：
public static 《T》 List《T》 query(String sql, Class《T》 clazz,
Object[] parameters) {
Connection conn = null;
PreparedStatement pstmt = null;
ResultSet rs = null;
ResultSetMetaData metaData = null;
List《T》 list = new ArrayList《T》();
int index = 1;
try {
pstmt = conn.prepareStatement(sql); ///问题行
if (parameters != null) {
for (int i = 0; i < parameters.length; i++) {
// pstmt.setObject(第几个参数, 替换成什么)；
pstmt.setObject(index, parameters[i]);
}
}
rs = pstmt.executeQuery(sql);
// 封装resultset
metaData = rs.getMetaData();
// 取出列的信息
int columnLength = metaData.getColumnCount();
// 获取列数
while (rs.next()) {
// 通过反射机制创建一个对象
T result = clazz.newInstance();
for (int i = 0; i < columnLength; i++) {
String metaDataKey = metaData.getColumnName(i + 1);
Object resultsetValue = rs.getObject(metaDataKey);
if (resultsetValue == null) {
resultsetValue = "";
}
Field field = clazz.getDeclaredField(metaDataKey);
field.setAccessible(true);
field.set(result, resultsetValue);
}
list.add(result);
}
} catch (Exception e) {
e.printStackTrace();
} finally {
DBCloseUtils.closeCSR(conn, pstmt, rs);
}
return list;
}

测试：

  String sql = null;
List《User》 list = new ArrayList《User》();
    sql = "select * from user_info where id > ? order by id asc";
    list = databaseFactory.query(sql, User.class, new Object[] { 2 }); //问题行
    if (list != null) {
        for (User user : list) {
            System.out.println("username =" + user.username);
            System.out.println("password =" + user.password);
        }
        System.out.println(sql);
        System.out.println("inquiry4");
        System.out.println("-------------------------------");
    }

问题：

java.lang.NullPointerException
at study.java.DBOperateTUtilts.query(DBOperateTUtilts.java:74)
at study.java.DatabaseFactoryImp.query(DatabaseFactoryImp.java:55)
select * from user_info where id > ? order by id asc

inquiry4

at study.java.test.Demo.main(Demo.java:106)

Answer 1

if(conn==null){
System.out.println("conn is null");
}

connection是需要获取的，
Class.forName("com.mysql.jdbc.Driver");
String url="jdbc:mysql://localhost:3306/briup?user=root&password=root";

//定义连接数据库的url

conn=DriverManager.getConnection(url); //获取数据库连接