JDBC---概述

 

/**
     * 【JDBC】
     *      what？
     *          java database connectivity
     *
     *      java.sql.DriverManager
     *          The basic service for managing a set of JDBC drivers.
     *          DriverManager 是 一个 管理 JDBC驱动 的服务；
     *
     *          The {@link javax.sql.DataSource} interface, new in the JDBC 2.0 API, provides another way to connect to a data source.
     *          The use of a DataSource object is the preferred means of connecting to a data source.
     *          javax.sql.DataSource 接口 是 JDBC 2.0 新增的API，提供了 另一种方式 连接一个 数据源；
     *          推荐 使用 DataSource 连接 一个数据源；
     *
     *          As part of its initialization, the DriverManager class will attempt to load the driver classes referenced in the "jdbc.drivers" system property.
     *          This allows a user to customize the JDBC Drivers used by their applications.
     *          DriverManager 的初始化块中，尝试加载 系统属性jdbc.drivers [System.getProperty("jdbc.drivers")] 引用的驱动；
     *          DriverManager 允许 用户自定义 JDBC驱动 ；
     *
     *          When the method getConnection is called,the DriverManager will attempt to locate a suitable driver from amongst those loaded at initialization and those loaded explicitly using the same classloader as the current applet or application.
     *          当 调用 DriverManager 的getConnection()， 将会 从初始化块 或 使用类加载器 找到合适的 驱动；
     *
     *
     *     java.sql.Connection
     *          A connection (session) with a specific database.
     *          SQL statements are executed and results are returned within the context of a connection.
     *          指定数据库的 一个 connection；
     *          在 一个connection上下文 中，SQL语句 将被执行 且 结果将被返回；
     *
     *          A Connection object's database is able to provide information describing its tables, its supported SQL grammar, its stored procedures, the capabilities of this connection, and so on.
     *          This information is obtained with the getMetaData method.
     *          一个Connection的数据库 应该提供表信息、SQL语法、存储过程...
     *          通过 Connection的 getMetaData() 可以获取 数据库的元数据；
     *
     *          When configuring a Connection, JDBC applications should use the appropriate Connection method such as setAutoCommit or setTransactionIsolation.
     *          Applications should not invoke SQL commands directly to change the connection's configuration when there is a JDBC method available.
     *          By default a Connection object is in auto-commit mode, which means that it automatically commits changes after executing each statement.
     *          If auto-commit mode has been disabled, the method commit must be called explicitly in order to commit changes; otherwise, database changes will not be saved.
     *          配置一个Connection时，JDBC应用程序 应该使用合适的Connection方法 （比如 setAutoCommit 、setTransactionIsolation）；
     *          当 有一个可用的JDBC可用时，应用程序 不能 直接 调用SQL 直接改变 Connection的配置；
     *          默认情况下，一个 Connection 是自动提交事务；
     *          如果 自动提交 被禁用，必须显式 提交，否则数据库将不会保存；
     *
     *          A new Connection object created using the JDBC 2.1 core API has an initially empty type map associated with it.
     *          使用 JDBC2.1 API 创建一个新的Connection 会关联一个 初始化的map类型；
     *
     *          public class ConnectionImpl implements JdbcConnection, SessionEventListener, Serializable {
     *              private NativeSession session = null;
     *          }
     *
     *
     *    java.lang.AutoCloseable
     *          An object that may hold resources (such as file or socket handles) until it is closed.
     *          The {@link #close()} method of an {@code AutoCloseable} object is called automatically when exiting a {@code try}-with-resources block for which the object has been declared in the resource specification header.
     *          This construction ensures prompt release, avoiding resource exhaustion exceptions and errors that may otherwise occur.
     *          AutoCloseable 是一个 持有资源（比如 文件、Socket处理） 直到 资源被关闭；
     *          当 存在try{}  AutoCloseable 的 close方法 将被调用 ；
     *          这种 结构 确保资源释放；
     *
     *    java.sql.Statement
     *          The object used for executing a static SQL statement and returning the results it produces.
     *          Statement 被用来 执行静态SQL 且 返回执行结果；
     *
     *          By default, only one <code>ResultSet</code> object per <code>Statement</code>object can be open at the same time.
     *          All execution methods in the <code>Statement</code> interface implicitly close a current <code>ResultSet</code> object of the statement if an open one exists.
     *          默认情况，一个 Statement 同一时间 只能有一个 ResultSet；
     *          Statement 执行的方法 必须显式 关闭ResultSet；
     *
     *
     *    java.sql.PreparedStatement
     *          An object that represents a precompiled SQL statement.
     *          A SQL statement is precompiled and stored in a <code>PreparedStatement</code> object.
     *          This object can then be used to efficiently execute this statement multiple times.
     *          PreparedStatement 代表一个预编译的SQL statement；
     *          一个 SQL statement 被 预编译 且 存储在 PreparedStatement中；
     *          PreparedStatement 能被高效率的执行；
     *
     *
     *          【PreparedStatement 与 Statement比较】
     *              1、PreparedStatement 预编译SQL，性能更好；
     *                  只需要处理参数，不需要每次重新传入SQL；
     *              2、PreparedStatement 不需要 拼接处理参数，易编写易读；
     *              3、PreparedStatement 防止SQL注入；
     *                  将 传入的参数 整个字符串 作为 某个字段的值 处理，而不是作为 SQL的一部分；
     *
     *
     *    java.sql.ResultSet
     *          A table of data representing a database result set, which is usually generated by executing a statement that queries the database.
     *          ResultSet 代表 数据库的结果集；
     *
     *          A <code>ResultSet</code> object maintains a cursor pointing to its current row of data.
     *          Initially the cursor is positioned before the first row.
     *          The <code>next</code> method moves the cursor to the next row, and because it returns <code>false</code> when there are no more rows in the <code>ResultSet</code> object,it can be used in a <code>while</code> loop to iterate through the result set.
     *          ResultSet 包含一个 游标 对象（指向 当前的数据行）；
     *          游标 初始 指向 第一行；
     *          ResultSet的 next方法 移动游标到下一行，当没有数据返回时 return false，因此 可以使用 while 迭代处理数据；
     *
     */