doc: update docs/java.md (#909)

docs/java.md

@@ -1558,6 +1558,118 @@ int frequency = Collections
       .frequency(list, 2); // frequency = 2
 ```
 
+操纵数据库
+----
+
+### JDBC
+
+```java
+String url = "jdbc:mysql://localhost:3306/test?useUnicode=true&characterEncoding=utf8&useSSL=false&serverTimezone=UTC";
+String user = "root";
+String password = "123456";
+String sql = "SELECT 1 as a, '2' as b";
+String preparedSql = "SELECT * FROM t_user WHERE id = ?";
+
+Connection conn = null;
+Statement sm = null;
+ResultSet rs = null;
+try {
+  // 1.注册驱动
+  Class.forName("com.mysql.cj.jdbc.Driver");
+} catch (ClassNotFoundException e) {
+  // 驱动找不到
+  throw new RuntimeException(e);
+}
+
+// 2.建立连接
+try (Connection connection = DriverManager.getConnection(url, user, password)) {
+
+  conn = connection;
+
+  // 3.创建Statement对象
+  try (Statement statement = connection.createStatement()) {
+
+    sm = statement;
+
+    // 4.执行SQL语句
+    try (ResultSet resultSet = statement.executeQuery(sql)) {
+
+      rs = resultSet;
+
+      // 5.处理结果集
+      while (resultSet.next()) {
+        // 按照列名取值
+        System.out.println(resultSet.getLong("a"));
+        // 按照索引取值
+        System.out.println(resultSet.getString(2));
+      }
+    }
+  }
+
+  // 3.创建PreparedStatement对象
+  try (PreparedStatement preparedStatement = connection.prepareStatement(preparedSql)) {
+
+    sm = preparedStatement;
+
+    preparedStatement.setLong(1, 1_000L);
+    // 4.执行SQL语句
+    try (ResultSet resultSet = preparedStatement.executeQuery()) {
+
+      rs = resultSet;
+
+      // 5.处理结果集
+      while (resultSet.next()) {
+        System.out.println(resultSet.getLong("id"));
+        System.out.println(resultSet.getString(2));
+      }
+    }
+  }
+} catch (SQLException e) {
+  // 数据库异常
+  throw new RuntimeException(e);
+} finally {
+  // 6.关闭资源
+  // 上面的try块里已经自动关闭，否则（JDK 7以前）按照以下顺序关闭
+  // 先打开的后关闭，后打开的先关闭
+  if (null != rs) {
+    try {
+      rs.close();
+    } catch (SQLException ignored) {
+    }
+  }
+
+  if (null != sm) {
+    try {
+      sm.close();
+    } catch (SQLException ignored) {
+    }
+  }
+
+  if (null != conn) {
+    try {
+      conn.close();
+    } catch (SQLException ignored) {
+    }
+  }
+
+  // 也可以直接工具类， 注意顺序
+  IOUtils.close(rs);
+  IOUtils.close(sm);
+  IOUtils.close(conn);
+}
+```
+
+### JDBC注册驱动
+
+```java
+Class.forName("com.mysql.cj.jdbc.Driver");
+
+DriverManager.registerDriver(new org.postgresql.Driver());
+
+// 支持多个同时注册
+System.setProperty("jdbc.drivers", "com.mysql.cj.jdbc.Driver:org.postgresql.Driver");
+```
+
 另见
 ---