架构探险（二）：DBUtils简单使用

继续我们的冒险体验，之前综述了一些构建一个项目的大致流程，相信大家对于系统的框架已经了然于胸。 今天我们就来谈一谈，如何构建数据的桥梁。

这里我们使用commons-dbutils 来简单的封装一个jdbc template, 帮助我们实现快速的CRUD. 废话不多讲直接上代码。

package org.foothold.learn2.helper;

import org.apache.commons.dbcp2.BasicDataSource;
import org.apache.commons.dbutils.QueryRunner;
import org.apache.commons.dbutils.handlers.BeanHandler;
import org.apache.commons.dbutils.handlers.BeanListHandler;
import org.apache.commons.dbutils.handlers.MapListHandler;
import org.foothold.learn2.util.PropsUtil;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Properties;

/**
 * Created by Jaden.Gu on 2018/11/16.
 */
public class DatabaseHelper {
    private static final Logger logger = LoggerFactory.getLogger(DatabaseHelper.class);
    private static final QueryRunner QUERY_RUNNER = new QueryRunner();

    private static final ThreadLocal<Connection> CONNECTION_HOLDER = new ThreadLocal<>();

    private static final BasicDataSource basicDataSource;

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

    static {
        Properties properties = PropsUtil.loadProps("db.properties");
        basicDataSource = new BasicDataSource();

        DRIVER = properties.getProperty("jdbc.driver");
        URL = properties.getProperty("jdbc.url");
        USERNAME = properties.getProperty("jdbc.username");
        PASSWORD = properties.getProperty("jdbc.password");

        basicDataSource.setDriverClassName(DRIVER);
        basicDataSource.setUrl(URL);
        basicDataSource.setUsername(USERNAME);
        basicDataSource.setPassword(PASSWORD);

//        try {
//            Class.forName(DRIVER);
//        } catch (ClassNotFoundException ex) {
//            logger.error("DatabaseHelper ClassNotFoundException", ex);
//        }
    }

    public static Connection getConnection() {
        Connection conn = null;

        try {
            if(CONNECTION_HOLDER.get() == null) {
//                conn = DriverManager.getConnection(URL, USERNAME, PASSWORD);
                conn = basicDataSource.getConnection();
                CONNECTION_HOLDER.set(conn);
            }

        } catch (SQLException ex) {
            logger.error("getConnection", ex);
        }

        return  CONNECTION_HOLDER.get();
    }

    public static void closeConnection() {
        if(CONNECTION_HOLDER.get() != null) {
            try {
                CONNECTION_HOLDER.get().close();
            } catch (SQLException ex) {
                logger.error("closeConnection", ex);
            }
        }
    }

    public static <T> List<T> queryEntityList(Class<T> entityClass, String sql, Object... params) {
        List<T> entityList = null;

        try {
            entityList = QUERY_RUNNER.query(getConnection(), sql, new BeanListHandler<T>(entityClass), params);
        }catch (SQLException ex) {
            logger.error("query entity list failure", ex);
            throw new RuntimeException(ex);
        } finally {
            closeConnection();
        }

        return entityList;
    }

    public static <T> T queryEntity(Class<T> entityClass, String sql, Object... params) {
        T entity = null;

        try {
            Connection connection = getConnection();

            entity = QUERY_RUNNER.query(connection, sql, new BeanHandler<T>(entityClass), params);
        } catch (SQLException ex) {
            logger.error("queryEntity sql exception", ex);
            throw new RuntimeException(ex);
        } finally {
            closeConnection();
        }

        return  entity;
    }

    public static List<Map<String, Object>> executeQuery(String sql, Object... params) {
        List<Map<String, Object>> result = null;

        try{
            Connection conn = getConnection();

            result = QUERY_RUNNER.query(conn, sql, new MapListHandler(), params);
        } catch (SQLException ex) {
            logger.error("executeQuery sql exception", ex);
            throw new RuntimeException(ex);
        } finally {
            closeConnection();
        }

        return result;
    }

    public static int executeUpdate(String sql, Object... params) {
        int rows = 0;

        try{
            rows = QUERY_RUNNER.update(getConnection(), sql, params);

        } catch (SQLException ex) {
            logger.error("executeUpdate sql exception", ex);
            throw new RuntimeException(ex);
        } finally {
            closeConnection();
        }

        return rows;
    }

    public static <T> boolean insertEntity(Class<T> entityClass, Map<String, Object> fieldMap) {
        if(fieldMap.isEmpty()) {
            logger.error("Can't insert entity : field is empty");
            return false;
        }

        String sql = String.format("INSERT INTO %s", getTableName(entityClass));

        StringBuilder columns = new StringBuilder("(");
        StringBuilder values = new StringBuilder("(");

        for(String column : fieldMap.keySet()) {
            columns.append(column).append(", ");
            values.append("?, ");
        }

        columns.replace(columns.lastIndexOf(", "), columns.length(), ")");
        values.replace(values.lastIndexOf(", "), values.length(), ")");
        sql += columns + " VALUES " + values;

        Object[] parmas = fieldMap.values().toArray();

        return executeUpdate(sql, parmas) == 1;
    }

    public static <T> boolean updateEntity(Class<T> entityClass, Map<String, Object> fieldMap, long id) {

        if(fieldMap.isEmpty()) {
            logger.error("Can't update entity : field is empty");
            return false;
        }

        String sql = "UPDATE " + getTableName(entityClass) + " SET ";

        StringBuilder columns = new StringBuilder();

        for(String column : fieldMap.keySet()) {
            columns.append(String.format(" %s = ? ", column));
        }

        sql += columns.substring(0, columns.length() -1) + " WHERE id = ? ";

        List<Object> paramList = new ArrayList<>();

        paramList.addAll(fieldMap.values());

        paramList.add(id);

        Object[] params = paramList.toArray();

        return  executeUpdate(sql, params) == 1;
    }

    public static <T> boolean deleteEntity(Class<T> entityClass, long id) {
        String sql = "DELETE FROM " + getTableName(entityClass) + " WHERE id = ?";
        return executeUpdate(sql, id) == 1;
    }

    private static String getTableName(Class<?> entityClass) {
        return entityClass.getSimpleName().toLowerCase();
    }

}

在static 代码快里，我们读取了配置文件，然后通过配置的数据库驱动类，数据库用户名、密码。初始化一个dpcp2 的数据源，然后通过dpcp2数据库连接池管理数据库连接，这样有助于减少频繁创建数据库所带来的昂贵开销。另外我们把从连接池中取到的连接放到了ThreadLocal中，实现了统一请求的连接复用。

若要使用dpcp2我们需要在pom.xml里添加以下依赖：

   <dependency>
          <groupId>org.apache.commons</groupId>
          <artifactId>commons-dbcp2</artifactId>
          <version>2.1.1</version>
   </dependency>

若想了解数据库连接池可以点击下方链接：
https://www.cnblogs.com/aspirant/p/6747238.html