1、策略模式完善模板方法模式處理DAO中的查詢方法
建議先看模模板方法模式在項目中的應用一節,再與本節對比.
1.1 新建RowMapper接口,定義mapRow(ResultSet rs)方法.
public interface RowMapper {
public Object mapRow(ResultSet rs) throws SQLException;
}
1.2把抽象類AbstractDao改成JdbcDaoTemplate類.
public class JdbcDaoTemplate {
//RowMapper是一個接口類,調用此方法時需傳入對應的實現類
public Object find(String sql, Object[] args, RowMapper rowMapper) {
Connection conn = null;
PreparedStatement ps = null;
ResultSet rs = null;
try {
conn = JdbcUtils.getConnection();
ps = conn.prepareStatement(sql);
for (int i = 0; i < args.length; i++)
ps.setObject(i + 1, args[i]);
rs = ps.executeQuery();
Object obj = null;
if (rs.next()) {
obj = rowMapper.mapRow(rs);
}
return obj;
} catch (SQLException e) {
throw new DaoException(e.getMessage(), e);
} finally {
JdbcUtils.free(rs, ps, conn);
}
}
}
1.3 將UserDaoImpl類改成如下 :
public class UserDaoImpl {
JdbcDaoTemplate template = new JdbcDaoTemplate();
public User findUser(String loginName, String password) {
String sql = "select id, name, money, birthday from user where name=?";
Object[] args = new Object[] { loginName };
RowMapper mapper = new UserRowMapper();
Object user = this.template.find(sql, args, mapper);
return (User) user;
}
//根據用戶id查找對應的用戶名
public String findUserName(int id) {
String sql = "select name from user where id=?";
Object[] args = new Object[] { id };
//通過匿名內部類實現RowMapper接口 策略二:返回的是一個子符串
Object name = this.template.find(sql, args, new RowMapper() {
public Object mapRow(ResultSet rs) throws SQLException {
return rs.getString("name");
}
});
return (String) name;
}
}
//內部類實現的RowMapper接口 策略一:返回的是一個User實體對象
class UserRowMapper implements RowMapper {
public Object mapRow(ResultSet rs) throws SQLException {
User user = new User();
user.setId(rs.getInt("id"));
user.setName(rs.getString("name"));
user.setMoney(rs.getFloat("money"));
user.setBirthday(rs.getDate("birthday"));
return user;
}
}
RowMapper 接口對應的角色是抽象策略,此角色定義了策略類所需的接口.
UserRowMapper 對應的是具體策略角色,該角色給出對抽象策略的具體實現.
JdbcDaoTemplate 對應的是環境角色,持有一個抽象策略接口的引用.
UserDaoImpl
是使者該策略的角色.
2、策略模式結構圖:
代碼表現如下:
2.1 定義接口
/**
* 抽象策略(角色):此角色給出所有的具體策略類所需的接口
* @author Administrator
*
*/
public interface Strategy {
//抽象策略(算法)
public String strategyInterface();
}
2.2 編寫環境角色
/**
* 環境(角色):持有一個Strategy類的引用
* @author Administrator
*/
public class Context {
private Strategy strategy;
//通過將子類對象引用賦值給超類對象引用變量來實現動態方法調用
public Context(Strategy strategy){
//初始化時,傳入具體的策略對象
this.strategy = strategy;
}
public String getResult(){
return strategy.strategyInterface();
}
}
2.3編寫具體策略角色A
/**
* 具體策略角色A
* @author Administrator
*
*/
public class ConcreteStrategyA implements Strategy {
public String strategyInterface() {
return "算法A的實現";
}
}
2.4編寫具體策略角色B
/**
* 具體策略角色B
* @author Administrator
*
*/
public class ConcreteStrategyB implements Strategy {
public String strategyInterface() {
return "算法B的實現";
}
}
2.5編寫使用者:測試類
public class StrateryTest {
public static void main(String[] args) {
Context context = new Context(StrategyFactory.getStrategyA());
System.out.println(context.getResult());
context = new Context(StrategyFactory.getStrategyB());
System.out.println(context.getResult());
context = new Context(StrategyFactory.getStrategyC());
System.out.println(context.getResult());
}
}
打印結果:
算法A的實現
算法B的實現
算法C的實現
3、策略方法模式的優缺點:
優缺: 當業務邏輯存在多個判斷語句時,使用該模式可以轉移if語句.
缺點: 當if語句很多的時候,就會造成編寫很多的具體策略類.