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：
在这里插入图片描述

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());}}