Question

这是我的连接类/方法：

static Connection connection = null;
        Statement stm = null;

public static Connection getConexao() {
            try {  
                // Carregando o JDBC Driver padrão  
                String driverName = "com.mysql.jdbc.Driver";                          
                Class.forName(driverName);  
                // Configurando a nossa conexão com um banco de dados//  
                String serverName = "192.168.0.222";    //caminho do servidor do BD  
                String mydatabase ="bd";        //nome do seu banco de dados  
                String url = "jdbc:mysql://" + serverName + "/" + mydatabase;  
                String username = "user";        //nome de um usuário de seu BD        
                String password = "pass";      //sua senha de acesso  
                connection = DriverManager.getConnection(url, username, password);  
                return connection;  
            }  catch (ClassNotFoundException e) {  //Driver não encontrado  
                System.out.println("O driver expecificado nao foi encontrado.");  
                return null;  
            } catch (SQLException e) {  
                //Não conseguindo se conectar ao banco  
                System.out.println("Nao foi possivel conectar ao Banco de Dados.");  
                return null;  
            }  

    }

TextField失去焦点时调用的方法：

private void puxaemailsiscart(String servico) throws SQLException {
                Connection conn = ConectaBD.getConexao();
                Statement stm = null;
                ResultSet results = null;  
                 stm = conn.createStatement ();


                                    try {
                      System.out.println();
                      results = stm.executeQuery ("SELECT * FROM  " + servico + " WHERE protocolo =" + tfProtocolo.getText());
                      if(results.next()) {

                      tfEmailParte.setText(results.getString("Email").toString());


                      } else { 
                          System.out.println("Protocolo nao encontrado");


                      }
                      results.close();
                      stm.close();
                      conn.close();
....

问题是，第一次TextField失去焦点时，它完美地工作并返回数据库结果，但是当我将焦点放在文本字段中并且它再次失去焦点时，连接返回null，为什么？因为步骤与第一次相同??

栈跟踪

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at telaprincipal$3.puxaemailsiscart(telaprincipal.java:499)
    at telaprincipal$3.focusLost(telaprincipal.java:482)

第499行：= Connection conn = ConectaBD.getConexao();说这是NullPoint，但为什么？

Answer 1

我相信你的classpath上有一个sqlite驱动程序，它在第一个循环之后以某种方式被加载并试图解析你的jdbc url。如果您不使用sqlite，请从运行时类路径中删除它。

此外，如果要在每次调用getConexao（）时打开新连接，请不要将连接存储为静态连接。第二个声明似乎是不必要的（并可能导致意外行为），所以我建议删除：

Statement stm = null;

mysql第一次完美连接，第二次返回nullpointer

1 个答案: