模板设计模式

模板方法模式（Template Method Pattern），又叫模板模式(Template Pattern)，在一个抽象类公开定义了执行它的方法的模板。它的子类可以按需要重写方法实现，但调用将以抽象类中定义的方式进行。

该模式的主要优点如下。
1） 它封装了不变部分，扩展可变部分。它把认为是不变部分的算法封装到父类中实现，而把可变部分算法由子类继承实现，便于子类继续扩展。
2） 它在父类中提取了公共的部分代码，便于代码复用。
3） 部分方法是由子类实现的，因此子类可以通过扩展方式增加相应的功能，符合开闭原则。

案例

JdbcTemplate uml 图：

1 pom

 <dependencies>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>
        <!-- https://mvnrepository.com/artifact/mysql/mysql-connector-java -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.47</version>
        </dependency>
        <dependency>
            <groupId>com.alibaba</groupId>
            <artifactId>druid</artifactId>
            <version>1.1.16</version>
        </dependency>

    </dependencies>

2 JdbcFactory

import com.alibaba.druid.pool.DruidDataSourceFactory;

import javax.sql.DataSource;
import java.util.HashMap;
import java.util.Map;

public class JdbcFactory {

    /**
     * ###########################DB配置########################
     */
    private static final String URL = "jdbc:mysql://localhost:3306/test?charset=utf8mb4&serverTimezone=UTC&useUnicode=true&useSSL=false";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "123456";
    private static final String DRIVER = "com.mysql.jdbc.Driver";


    private static DataSource db;

    static {

        Map<String, String> map = new HashMap<>();
        map.put(DruidDataSourceFactory.PROP_DRIVERCLASSNAME, DRIVER);
        map.put(DruidDataSourceFactory.PROP_URL, URL);
        map.put(DruidDataSourceFactory.PROP_USERNAME, USERNAME);
        map.put(DruidDataSourceFactory.PROP_PASSWORD, PASSWORD);
        try {
            db = DruidDataSourceFactory.createDataSource(map);
        } catch (Exception e) {
            db = null;
            e.printStackTrace();
        }

    }

    private JdbcFactory() {

    }


    public static DataSource getInstance() {
        return db;
    }


}

3 RowMapper

/**
 * ORM 定制化接口
 */
public interface RowMapper<T> {

    T mapRow(ResultSet rs, int rowNum) throws SQLException;

}

4 JdbcTemplate

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;

public abstract class JdbcTemplate {


    private DataSource dataSource;

    public JdbcTemplate(DataSource dateSource) {

        this.dataSource = dateSource;
    }


    public <T> List<T> executeQuery(String sql, RowMapper<T> rowMapper, Object[] values) {

        try {
            //1 获取连接
            Connection connection = this.getConnection();
            //2 创建语句集
            PreparedStatement pstm = this.createPrepareStatement(connection, sql);
            //3 执行语句集
            ResultSet rs = execute(pstm, values);
            //4 处理结果集
            List<T> list = paresResultSet(rs, rowMapper);
            //5 关闭结果集
            this.closeResultSet(rs);
            //6 关闭语句集
            this.closeStatement(pstm);
            //7 关闭连接
            this.closeConnection(connection);
            //8 返回
            return list;
        } catch (Exception e) {
            e.printStackTrace();
        }


        return new ArrayList<>();
    }

    protected void closeConnection(Connection connection) throws SQLException {

        if (connection != null) {
            connection.close();
        }

    }

    protected void closeStatement(PreparedStatement pstm) throws SQLException {
        pstm.close();
    }


    protected void closeResultSet(ResultSet rs) throws SQLException {
        rs.close();
    }

    protected <T> List<T> paresResultSet(ResultSet rs, RowMapper<T> rowMapper) throws SQLException {
        List<T> result = new ArrayList<>();
        int rowNum = 1;
        while (rs.next()) {
            result.add(rowMapper.mapRow(rs, rowNum++));
        }
        return result;

    }

    private ResultSet execute(PreparedStatement pstm, Object[] values) throws SQLException {

        if (values != null) {
            for (int i = 0; i < values.length; i++) {
                pstm.setObject(i, values[i]);
            }
        }
        return pstm.executeQuery();
    }

    protected PreparedStatement createPrepareStatement(Connection connection, String sql) throws SQLException {


        return connection.prepareStatement(sql);

    }

    private Connection getConnection() throws SQLException {

        return this.dataSource.getConnection();
    }


}

5 业务类

@Data
@AllArgsConstructor
@NoArgsConstructor
@ToString
public class Student {

    private Integer id;

    private String name;

    private Integer age;


}



public class StudentDao extends JdbcTemplate {

    public StudentDao(DataSource dateSource) {
        super(dateSource);
    }


    public List<Student> selectAll() {
        String sql = "select * from t_student";
        return super.executeQuery(sql, (rs, rowNum) -> {
            Student student = new Student();
            student.setId(rs.getInt("id"));
            student.setAge(rs.getInt("age"));
            student.setName(rs.getString("name"));
            return student;
        }, null);

    }


}

6 测试

public class JdbcTest {


    public static void main(String[] args) throws Exception {

        DataSource instance = JdbcFactory.getInstance();
        StudentDao studentDao=new StudentDao(instance);
        List<Student> students = studentDao.selectAll();
        System.out.println(students.toString());


    }

}