ThreadLocal

	ThreadLocal是线程的本地变量，可以防止线程安全问题，
			因为它隔离了相关的资源（它只是一种手段，
			我们可以采用同步机制解决线程安全问题），并在同一个线程中可以共享这个资源


			public class ConnectionManager {

				private static ThreadLocal connectionHolder = new ThreadLocal();
				
				/**
				 * 取得Connection
				 * @return
				 */
				public static Connection getConnection() {
					Connection conn = (Connection)connectionHolder.get();
					if (conn == null) {
						try {
							Class.forName("oracle.jdbc.driver.OracleDriver");
							conn = DriverManager.getConnection("jdbc:oracle:thin:@127.0.0.1:1521:bt", "xx", "xx");
							connectionHolder.set(conn);
						} catch (ClassNotFoundException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
							throw new java.lang.RuntimeException(e);
						} catch (SQLException e) {
							// TODO Auto-generated catch block
							e.printStackTrace();
							throw new java.lang.RuntimeException(e);
						}
					}
					return conn;
				}
			}
				
				/**
				 * 关闭Connection
				 */
				public static void close() {
					Connection conn = (Connection)connectionHolder.get();
					if (conn != null) {
						try {
							conn.close();
							connectionHolder.remove();
							//connectionHolder.set(null);
						} catch (SQLException e) {
							e.printStackTrace();
						}
					}
				}