修真院-学习-JDBC-使用java.sql连接数据库遇到的若干问题

今天做的事

1 .学习了JDBC编程

DriverManger 是用于管理JDBC驱动的服务类

//该方法获得url对应的数据库连接
public static synchronized Connection getConnection(String url, String user, String pass) throws SQLException

Connection 代表数据库连接对象，每一个Connection代表一个物理连接会话，该接口的常用方法有三个：

//该方法返回一个Statement对象
Statement createStatement() throws SQLException

//返回预编译的Statment对象，即将SQL语句提交到数据库进行编译
PreparedStatement preparedStatement(String url) throws SQLExcption

之后获得了Statement之后才能执行SQL语句。

Statement是用于执行SQL语句的工具接口，该对象既可用于执行DDL、DCL语句，也可用于执行DML语句，还可用于执行SQL查询（返回结果集）。

PreparedStatement是预编译的Statement对象，是Statement是子接口，它允许数据库预编译SQL语句。

遇到的问题

1.在练习使用getconnection方法时出现错误：

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class TempDemo {

    public static void main(String[] args) {
        //声明Connection对象
        Connection con;
        //驱动程序名
        String driver = "com.mysql.jdbc.Driver";
        //URL指向要访问的数据库名mydata
        String url = "jdbc:mysql://localhost:3306/select_test";
        //MySQL配置时的用户名
        String user = "root";
        //MySQL配置时的密码
        String password = "root123";
        //遍历查询结果集
        try {
            //加载驱动程序
            Class.forName(driver);
            //1.getConnection()方法，连接MySQL数据库！！
            con = DriverManager.getConnection(url,user,password);
            if(!con.isClosed())
                System.out.println("Succeeded connecting to the Database!");
            //2.创建statement类对象，用来执行SQL语句！！
            Statement statement = con.createStatement();
            //要执行的SQL语句
            String sql = "select * from student_table";
            //3.ResultSet类，用来存放获取的结果集！！
            ResultSet rs = statement.executeQuery(sql);
            System.out.println("-----------------");
            System.out.println("执行结果如下所示:");  
            System.out.println("-----------------");  
            System.out.println("姓名" + "\t" + "职称");  
            System.out.println("-----------------");  

            String job = null;
            String id = null;
            while(rs.next()){
                //获取stuname这列数据
                job = rs.getString("student_id");
                //获取stuid这列数据
                id = rs.getString("student_name");

                //输出结果
                System.out.println(id + "\t" + job);
            }
            rs.close();
            con.close();
        } catch(ClassNotFoundException e) {   
            //数据库驱动类异常处理
            System.out.println("Sorry,can`t find the Driver!");   
            e.printStackTrace();   
            } catch(SQLException e) {
            //数据库连接失败异常处理
            e.printStackTrace();  
            }catch (Exception e) {
            // TODO: handle exception
            e.printStackTrace();
        }finally{
            System.out.println("数据库数据成功获取！！");
        }
    }
}

解决办法：导入的包导入错误，Statement应该从java.sql包里面导入而不是从java.beans包里面导入。重新导入之后就解决了。

2.依然是上面的代码，使用editplus运行之后报错：

com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Could not create connection to database server.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:409)
    at com.mysql.jdbc.Util.getInstance(Util.java:384)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1013)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:987)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:973)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:918)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2588)
    at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2321)
    at com.mysql.jdbc.ConnectionImpl.(ConnectionImpl.java:832)
    at com.mysql.jdbc.JDBC4Connection.(JDBC4Connection.java:46)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.jdbc.Util.handleNewInstance(Util.java:409)
    at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:417)
    at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:344)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at cn.sql.test.TempDemo.main(TempDemo.java:29)
Caused by: java.lang.NullPointerException
    at com.mysql.jdbc.ConnectionImpl.getServerCharacterEncoding(ConnectionImpl.java:3307)
    at com.mysql.jdbc.MysqlIO.sendConnectionAttributes(MysqlIO.java:1985)
    at com.mysql.jdbc.MysqlIO.proceedHandshakeWithPluggableAuthentication(MysqlIO.java:1911)
    at com.mysql.jdbc.MysqlIO.doHandshake(MysqlIO.java:1288)
    at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2506)
    at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2539)
    ... 13 more
数据库数据成功获取！！

原因：mysql的链接驱动错误，不再是 com.mysql.jdbc.Driver ，变成了 com.mysql.cj.jdbc.Driver
修改链接驱动之后，错误变成了：

Sorry,can`t find the Driver!
java.lang.ClassNotFoundException: com.mysql.cj.jdbc.Driver
    at java.net.URLClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.Class.forName0(Native Method)
    at java.lang.Class.forName(Unknown Source)
    at cn.sql.test.TempDemo.main(TempDemo.java:27)
数据库数据成功获取！！

进一步怀疑是 mysql-connector-java.jar 版本过旧，目前我的版本是5.1.30，因此我重新下载了一个8.0.12的版本，修改过之后报错：

Wed Aug 08 16:27:44 CST 2018 WARN: Establishing SSL connection without server's identity verification is not recommended. According to MySQL 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn't set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to 'false'. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
java.sql.SQLException: The server time zone value '???ú±ê×??±??' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the serverTimezone configuration property) to use a more specifc time zone value if you want to utilize time zone support.
数据库数据成功获取！！
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:89)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:63)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:73)
    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:76)
    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:832)
    at com.mysql.cj.jdbc.ConnectionImpl.(ConnectionImpl.java:456)
    at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:240)
    at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:207)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at cn.sql.test.TempDemo.main(TempDemo.java:29)
Caused by: com.mysql.cj.exceptions.InvalidConnectionAttributeException: The server time zone value '???ú±ê×??±??' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the serverTimezone configuration property) to use a more specifc time zone value if you want to utilize time zone support.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:85)
    at com.mysql.cj.util.TimeUtil.getCanonicalTimezone(TimeUtil.java:128)
    at com.mysql.cj.protocol.a.NativeProtocol.configureTimezone(NativeProtocol.java:2236)
    at com.mysql.cj.protocol.a.NativeProtocol.initServerSession(NativeProtocol.java:2260)
    at com.mysql.cj.jdbc.ConnectionImpl.initializePropsFromServer(ConnectionImpl.java:1314)
    at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:963)
    at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:822)
    ... 6 more

这表明我计算机的ssl出了问题，经过百度查证，是因为url不正确，修改成：
jdbc:mysql://localhost:3306/select_test?useUnicode=true&characterEncoding=utf-8&useSSL=false

修改之后依然有错，这仅仅解决了ssl的问题，以下报错还没解决：

java.sql.SQLException: The server time zone value '???ú±ê×??±??' is unrecognized or represents more than one time zone. You must configure either the server or JDBC driver (via the serverTimezone configuration property) to use a more specifc time zone value if you want to utilize time zone support.

将url设置为：
jdbc:mysql://localhost:3306/select_testjdbc:mysql://localhost:3306/select_test”
+ “?useUnicode=true&characterEncoding=utf-8&useSSL=falseserverTimezone = GMT％2B8
出现新的报错：

java.sql.SQLNonTransientConnectionException: Cannot load connection class because of underlying exception: com.mysql.cj.exceptions.WrongArgumentException: Malformed database URL, failed to parse the connection string near '= GMT％2B8'.
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:110)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:89)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:63)
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:73)
    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:79)
    at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:131)
    at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:227)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at java.sql.DriverManager.getConnection(Unknown Source)
    at cn.sql.test.TempDemo.main(TempDemo.java:30)
Caused by: com.mysql.cj.exceptions.UnableToConnectException: Cannot load connection class because of underlying exception: com.mysql.cj.exceptions.WrongArgumentException: Malformed database URL, failed to parse the connection string near '= GMT％2B8'.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:105)
    ... 4 more
Caused by: com.mysql.cj.exceptions.WrongArgumentException: Malformed database URL, failed to parse the connection string near '= GMT％2B8'.
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61)
    at com.mysql.cj.conf.ConnectionUrlParser.processKeyValuePattern(ConnectionUrlParser.java:547)
    at com.mysql.cj.conf.ConnectionUrlParser.parseQuerySection(ConnectionUrlParser.java:519)
    at com.mysql.cj.conf.ConnectionUrlParser.getProperties(ConnectionUrlParser.java:644)
    at com.mysql.cj.conf.ConnectionUrl.collectProperties(ConnectionUrl.java:304)
    at com.mysql.cj.conf.ConnectionUrl.(ConnectionUrl.java:289)
    at com.mysql.cj.conf.url.SingleConnectionUrl.(SingleConnectionUrl.java:47)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
    at java.lang.reflect.Constructor.newInstance(Unknown Source)
    at com.mysql.cj.util.Util.handleNewInstance(Util.java:210)
    at com.mysql.cj.util.Util.getInstance(Util.java:185)
    at com.mysql.cj.util.Util.getInstance(Util.java:192)
    at com.mysql.cj.conf.ConnectionUrl.getConnectionUrlInstance(ConnectionUrl.java:201)
    at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:204)
    ... 3 more
数据库数据成功获取！！

上述三个问题参考文章：https://blog.csdn.net/weixin_40916641/article/details/80546040
第三个报错显示 url 不正确，遂检查 url ，发现时区前面没有使用 & 链接。
将数据库url更改为：

"jdbc:mysql://localhost:3306/select_test?useUnicode=true&characterEncoding=utf-8&useSSL=false&serverTimezone = GMT";

问题解决，可以正确输出数据库内容。

Succeeded connecting to the Database!
-----------------
执行结果如下所示:
-----------------
姓名  职称
-----------------
张三  1
张三  2
李四  3
王五  4
_王五 5
null    6
赵六  7
数据库数据成功获取！！

遇到问题引起的思考：使用 java 链接数据库时，url 非常重要，只有正确的 url 格式才能成功。

明日计划

1.继续学习JDBC编程。