架构探险——第二章（为web应用添加业务功能）

第二章不使用框架完成了自己的Web应用。

重点：

服务层的完善优化过程，思路

在看这一段的时候引起了无数次的共鸣。相信大家在开始接触Java Web的时候，都做过类似的封装和优化。

第一版

在Service的静态代码块中获取config.properties配置文件中与JDBC相关的配置项。在service的业务方法中获取数据库的连接，并进行数据库的操作，finally中关闭数据库。

/**
 * 提供客户数据服务
 */
public class CustomerServiceVersion {

    private static final Logger LOGGER = LoggerFactory.getLogger(DatabaseHelper.class);

    private static final String DRIVER;
    private static final String URL;
    private static final String USERNAME;
    private static final String PASSWORD;
    private static final String URL;

    static {
        Properties conf = PropsUtil.loadProps("config.properties");
        DRIVER = conf.getProperty("jdbc.driver");
        URL = conf.getProperty("jdbc.url");
        USERNAME = conf.getProperty("jdbc.username");
        PASSWORD = conf.getProperty("jdbc.password");

        try {
            Class.forName(DRIVER);
        }catch(ClassNotFoundException e){
            LOGGER.error("can not load jdbc driver",e);
        }
    }

    /**
     * 获取客户列表
     * @return
     */
    public List<Customer> getCustomerList(){
        Connection conn = null;
        try {
            List<Customer> customerList = new ArrayList<Customer>();
            String sql = "select * from customer";
            conn = DriverManager.getConnection(URL,USERNAME,PASSWORD);
            PreparedStatement stmt = conn.prepareStatement(sql);
            ResultSet rs = stmt.executeQuery();
            while (rs.next()){
                Customer customer = new Customer();
                customer.setId(rs.getLong("id"));
                customer.setName(rs.getString("name"));
                customer.setContact(rs.getString("contact"));
                customer.setTelephone(rs.getString("telephone"));
                customer.setEmail(rs.getString("email"));
                customer.setRemark(rs.getString("remark"));
                customerList.add(customer);
            }
            return  customerList;
        }catch (SQLException e){
            LOGGER.error("excute sql failure",e);
        }finally {
            if(conn != null){
                try{
                    conn.close();
                }catch (SQLException e){
                    LOGGER.error("close connection failure",e);
                }
            }
        }
    }
}

存在的问题：

1.在service中读取config.properties文件，不合理，其他service使用的时候反复进行io。

2.执行一条SQL语句需要编写一大堆代码，并且使用try...catch...finally结构，开发效率不高）

第二版

开发DatabaseHelper来读取配置文件，获取数据库连接，关闭数据库连接。（解决了第一个问题）

使用Apache Common项目中的DbUtils类库，在DatabaseHelper中写SQL公共类，通过传参的形式，利用反射，执行对应的SQL。（解决了第二个问题）

/**
 * 数据库助手类
 */
public final class DatabaseHelper1 {

    private static final Logger LOGGER = LoggerFactory.getLogger(DatabaseHelperV.class);

    private static final QueryRunner QUERY_RUNNER = new QueryRunner();

    private static final String DRIVER;
    private static final String URL;
    private static final String USERNAME;
    private static final String PASSWORD;
    private static final String URL;

    static {
        Properties conf = PropsUtil.loadProps("config.properties");
        DRIVER = conf.getProperty("jdbc.driver");
        URL = conf.getProperty("jdbc.url");
        USERNAME = conf.getProperty("jdbc.username");
        PASSWORD = conf.getProperty("jdbc.password");

        try {
            Class.forName(DRIVER);
        }catch(ClassNotFoundException e){
            LOGGER.error("can not load jdbc driver",e);
        }
    }

    /**
     * 获取数据库连接
     */
    public static Connection getConnection() {
        Connection conn = null;
        try {
            conn = DriverManager.getConnection(URl,USERNAME,PASSWORD);
        } catch (SQLException e) {
            LOGGER.error("get connection failure", e);
        }
        return conn;
    }

    /**
     * 关闭数据库连接
     */
    public static void closeConnection(Connection conn){
        if(conn != null){
            try{
                conn.close();
            }catch (SQLException e){
                LOGGER.error("close connection failure",e);
            }
        }
    }

    /**
     * 查询实体列表
     */
    public static <T> List<T> queryEntityList(Class<T> entityClass, String sql, Object... params) {
        List<T> entityList;
        try {
            Connection conn = getConnection();
            entityList = QUERY_RUNNER.query(conn, sql, new BeanListHandler<T>(entityClass), params);
        } catch (SQLException e) {
            LOGGER.error("query entity list failure", e);
            throw new RuntimeException(e);
        }
        return entityList;
    }
}

存在的问题：

1.由于Connection是static变量，（静态对象可以节省频繁访问引起的频繁分配内存、释放内存、占用内存的性能开销）但是这样是线程不安全的。

第三版

使用ThreadLocal来存放本地线程变量。在每次获取Connection的时候，在ThreadLocal中查找，不存在，则新创建一个连接。

/**
 * 数据库助手类
 */
public final class DatabaseHelper2 {

    private static final Logger LOGGER = LoggerFactory.getLogger(DatabaseHelperV.class);

    private static final ThreadLocal<Connection> CONNECTION_HOLDER;

    private static final QueryRunner QUERY_RUNNER = new QueryRunner();

    private static final String DRIVER;
    private static final String URL;
    private static final String USERNAME;
    private static final String PASSWORD;
    private static final String URL;

    static {
        CONNECTION_HOLDER = new ThreadLocal<Connection>();

        Properties conf = PropsUtil.loadProps("config.properties");
        DRIVER = conf.getProperty("jdbc.driver");
        URL = conf.getProperty("jdbc.url");
        USERNAME = conf.getProperty("jdbc.username");
        PASSWORD = conf.getProperty("jdbc.password");

        try {
            Class.forName(DRIVER);
        }catch(ClassNotFoundException e){
            LOGGER.error("can not load jdbc driver",e);
        }
    }

    /**
     * 获取数据库连接
     */
    public static Connection getConnection() {
        Connection conn = CONNECTION_HOLDER.get();
        if (conn == null) {
            try {
                conn = DATA_SOURCE.getConnection();
            } catch (SQLException e) {
                LOGGER.error("get connection failure", e);
                throw new RuntimeException(e);
            } finally {
                CONNECTION_HOLDER.set(conn);
            }
        }
        return conn;
    }

    /**
     * 关闭数据库连接
     */
    public static void closeConnection(){
        Connection conn = CONNECTION_HOLDER.get();
        if(conn != null){
            try {
                conn.close();
            }catch (SQLException e){
                LOGGER.error("cose connection failure",e);
                throw new RuntimeException(e);
            }finally {
                CONNECTION_HOLDER.remove();
            }
        }
    }

    /**
     * 查询实体列表
     */
    public static <T> List<T> queryEntityList(Class<T> entityClass, String sql, Object... params) {
        List<T> entityList;
        try {
            Connection conn = getConnection();
            entityList = QUERY_RUNNER.query(conn, sql, new BeanListHandler<T>(entityClass), params);
        } catch (SQLException e) {
            LOGGER.error("query entity list failure", e);
            throw new RuntimeException(e);
        }
        return entityList;
    }
}

存在的问题：每次都需要创建一个Connection，然后进行数据库操作，然后关闭。

第四版

引入了DBCP数据库连接池

/**
 * 数据库助手类
 */
public final class DatabaseHelper {

    private static final Logger LOGGER = LoggerFactory.getLogger(DatabaseHelper.class);

    private static final ThreadLocal<Connection> CONNECTION_HOLDER;

    private static final QueryRunner QUERY_RUNNER;

    private static final BasicDataSource DATA_SOURCE;

    static {
        CONNECTION_HOLDER = new ThreadLocal<Connection>();

        QUERY_RUNNER = new QueryRunner();

        Properties conf = PropsUtil.loadProps("config.properties");
        String driver = conf.getProperty("jdbc.driver");
        String url = conf.getProperty("jdbc.url");
        String username = conf.getProperty("jdbc.username");
        String password = conf.getProperty("jdbc.password");

        DATA_SOURCE = new BasicDataSource();
        DATA_SOURCE.setDriverClassName(driver);
        DATA_SOURCE.setUrl(url);
        DATA_SOURCE.setUsername(username);
        DATA_SOURCE.setPassword(password);
    }

    /**
     * 获取数据库连接
     */
    public static Connection getConnection() {
        Connection conn = CONNECTION_HOLDER.get();
        if (conn == null) {
            try {
                conn = DATA_SOURCE.getConnection();
            } catch (SQLException e) {
                LOGGER.error("get connection failure", e);
                throw new RuntimeException(e);
            } finally {
                CONNECTION_HOLDER.set(conn);
            }
        }
        return conn;
    }

    /**
     * 执行查询语句
     */
    public List<Map<String, Object>> executeQuery(String sql, Object... params) {
        List<Map<String, Object>> result;
        try {
            Connection conn = getConnection();
            result = QUERY_RUNNER.query(conn, sql, new MapListHandler(), params);
        } catch (Exception e) {
            LOGGER.error("execute query failure", e);
            throw new RuntimeException(e);
        }
        return result;
    }
}

在这个过程中，黄老师已经教我们开发了一个轻量级的JDBC框架。

最终代码

干货分享：

读取配置文件——Java读取Properties配置文件

ThreadLocal的用法——传送门-ThreadLocal那点事儿