JDBC Detailed Explanation (2): Get Database Connection (Super Detailed Explanation)

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foreword

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1. Element 1: Driver interface implementation class

1. Driver interface introduction

⭕Interface java.sql.Driveris the interface that all JDBCdrivers need to implement . This interface is provided for use by database vendors, and different database vendors provide different implementations.

⭕ There is no need to directly access the class that implements Driverthe interface , but the driver manager class java.sql.DriverManagercalls these Driverimplementations.

1. OracleDriver:oracle.jdbc.driver.OracleDriver
2. mySqlDriver:com.mysql.jdbc.Driver

2. Load and register JDBC driver

⭕ Load the driver: To load JDBCthe driver, you need to call Classthe static method of the class forName(), and pass it the class name of JDBCthe driver
Class.forName("com.mysql.jdbc.Driver");

⭕ Register driver: DriverManagerThe class is the driver manager class, which is responsible for managing the driver

• Register DriverManager.registerDriver(com.mysql.jdbc.Driver)the driver with

• Normally, you don't need to DriverManagerexplicitly registerDriver()call the method of the class to register an instance of the driver class, because the Driverdriver class of the interface contains a static code block, and in this static code block, DriverManager.registerDriver()the method is called to register an instance of itself.

The following figure is the source code MySQLof Driverthe implementation class:

2. Element 2: URL

⭕ JDBC URLis used to identify a registered driver, and the driver manager URLselects to establish a connection to the database.

The criteria for ⭕ JDBC URLconsists of three parts separated by colons.

protocol:subprotocol:subname

• protocol : JDBC URLthe protocol in alwaysjdbc
• Subprotocol : The subprotocol is used to identify a database driver
• Subname : A way to identify a database. The subname can vary according to different subprotocols, and the purpose of using the subname is to provide enough information for locating the database . Contains the host name (corresponding to the ip address of the server), port number, database name

⭕ Example:

⭕ Several commonly used databases JDBC URL:

• How to write MySQL connection URL:
  jdbc:mysql://主机名称:mysql服务端口号/数据库名称?参数=值&参数=值

  • Example 1:jdbc:mysql://localhost:3306/xiaolaoshiir
  • Example 2: jdbc:mysql://localhost:3306/xiaolaoshiir?useUnicode=true&characterEncoding=utf8(If the JDBC program is inconsistent with the server-side character set, which will cause garbled characters, you can specify the server-side character set through parameters)
  • Example 3:jdbc:mysql://localhost:3306/atguigu?user=root&password=123456

• The way to write the connection URL of Oracle 9i:
  jdbc:oracle:thin:@主机名称:oracle服务端口号:数据库名称

  • example:jdbc:oracle:thin:@localhost:1521:xiaolaoshiir

• How to write the connection URL of SQL Server:
  jdbc:sqlserver://主机名称:sqlserver服务端口号:DatabaseName=数据库名称

  • example:jdbc:sqlserver://localhost:1433:DatabaseName=atguigu

3. Element 3: Username and Password

⭕ user, passwordyou can use " 属性名=属性值" key-value pair to tell the database

⭕ You can DriverManagercall getConnection()the method of the class to establish a connection to the database

4. Examples of database connection methods

4.1 Connection method 1

@Test
	public void testConnection1() throws SQLException {
     
     
		// 获取Driver实现类对象
		Driver driver = new com.mysql.jdbc.Driver();

		// url:http://localhost:8080/gmall/keyboard.jpg
		// jdbc:mysql:协议
		// localhost:ip地址
		// 3306：默认mysql的端口号
		// test:test数据库
		String url = "jdbc:mysql://localhost:3306/test";
		// 将用户名和密码封装在Properties中
		Properties info = new Properties();
		info.setProperty("user", "root");
		info.setProperty("password", "abc123");

		Connection conn = driver.connect(url, info);

		System.out.println(conn);
	}

Disadvantages: The third-party database explicitly appears in the above codeAPI

4.2 Connection method 2

// 方式二：对方式一的迭代:在如下的程序中不出现第三方的api,使得程序具有更好的可移植性
	@Test
	public void testConnection2() throws Exception {
     
     
		// 1.获取Driver实现类对象：使用反射
		Class clazz = Class.forName("com.mysql.jdbc.Driver");
		Driver driver = (Driver) clazz.newInstance();

		// 2.提供要连接的数据库
		String url = "jdbc:mysql://localhost:3306/test";

		// 3.提供连接需要的用户名和密码
		Properties info = new Properties();
		info.setProperty("user", "root");
		info.setProperty("password", "abc123");

		// 4.获取连接
		Connection conn = driver.connect(url, info);
		System.out.println(conn);

	}

Note: Compared with method 1, reflection instantiation is used here Driver, and the third-party database is not reflected in the code API. It embodies the idea of ​​interface-oriented programming.

4.3 Connection method 3

// 方式三：使用DriverManager替换Driver
	@Test
	public void testConnection3() throws Exception {
     
     
		// 1.获取Driver实现类的对象
		Class clazz = Class.forName("com.mysql.jdbc.Driver");
		Driver driver = (Driver) clazz.newInstance();

		// 2.提供另外三个连接的基本信息：
		String url = "jdbc:mysql://localhost:3306/test";
		String user = "root";
		String password = "abc123";

		// 注册驱动
		DriverManager.registerDriver(driver);

		// 获取连接
		Connection conn = DriverManager.getConnection(url, user, password);
		System.out.println(conn);
	}

Description: Use DriverManagerto realize the connection to the database. Discover the 4 essential elements necessary to get connected.

4.4 Connection method 4

// 方式四：可以只是加载驱动，不用显示的注册驱动过了。
	@Test
	public void testConnection4() throws Exception {
     
     
		// 1.提供三个连接的基本信息：
		String url = "jdbc:mysql://localhost:3306/test";
		String user = "root";
		String password = "abc123";
		
		// 2.加载Driver
		Class.forName("com.mysql.jdbc.Driver");
		//相较于方式三，可以省略如下的操作：
//		Driver driver = (Driver) clazz.newInstance();
//		// 注册驱动
//		DriverManager.registerDriver(driver);
		//为什么可以省略上述操作呢？
		/*
		 * 在mysql的Driver实现类中，声明了如下的操作：
		 * static {
				try {
					java.sql.DriverManager.registerDriver(new Driver());
				} catch (SQLException E) {
					throw new RuntimeException("Can't register driver!");
				}
			}
		 */

		// 3.获取连接
		Connection conn = DriverManager.getConnection(url, user, password);
		System.out.println(conn);
	}

Note: It is not necessary to explicitly register the driver. Because DriverManagerthere is already a static code block in the source code, the registration of the driver is realized.

4.5 Connection method five (final version)

//方式五(final版)：将数据库连接需要的4个基本信息声明在配置文件中，通过读取配置文件的方式，获取连接
	/*
	 * 此种方式的好处？
	 * 1.实现了数据与代码的分离。实现了解耦
	 * 2.如果需要修改配置文件信息，可以避免程序重新打包。
	 */
	@Test
	public void getConnection5() throws Exception{
     
     
		
		//1.读取配置文件中的4个基本信息
		InputStream is = ConnectionTest.class.getClassLoader().getResourceAsStream("jdbc.properties");
		
		Properties pros = new Properties();
		pros.load(is);
		
		String user = pros.getProperty("user");
		String password = pros.getProperty("password");
		String url = pros.getProperty("url");
		String driverClass = pros.getProperty("driverClass");
		
		//2.加载驱动
		Class.forName(driverClass);
		
		//3.获取连接
		Connection conn = DriverManager.getConnection(url, user, password);
		System.out.println(conn);			
	}

Among them, the configuration file is declared in the src directory of the project: [ jdbc.properties]

user=root
password=abc123
url=jdbc:mysql://localhost:3306/test
driverClass=com.mysql.jdbc.Driver

Description: Use the configuration file to save the configuration information, and load the configuration file in the code

Benefits of using configuration files:

⭕ Realized the separation of code and data. If you need to modify the configuration information, you can modify it directly in the configuration file without going deep into the code.
⭕ If you modify the configuration information, the process of recompilation is omitted.