所謂模板板式，就是在父類中定義算法的主要流程，而把一些個(gè)性化的步驟延遲到子類中去實(shí)現(xiàn)，父類始終控制著整個(gè)流程的主動(dòng)權(quán)，子類只是輔助父類實(shí)現(xiàn)某些可定制的步驟。
先用一小段代碼回顧模板模式
//模板父類 控制execute的執(zhí)行流程 而doExecute的具體實(shí)現(xiàn)交于之類完成
public abstract class TemplateClass {
private void before() {
System.out.println("before execute");
}
private void after() {
System.out.println("after execute");
}
protected abstract void doExecute();
public void execute() {
before();
doExecute();
after();
}
}
//模板子類 完成doExecute方法的具體實(shí)現(xiàn)
public class TemplateSubClass extends TemplateClass{
@Override
protected void doExecute() {
System.out.println("執(zhí)行 doExecute");
}
}
//測(cè)試結(jié)果
public class TestCase {
public static void main(String[] args) {
TemplateSubClass subClass = new TemplateSubClass();
subClass.execute();
}
}
輸出結(jié)果
before execute
執(zhí)行 doExecute
after execute
簡(jiǎn)單的模板模式回顧完畢，下面通過JDBC的數(shù)據(jù)庫查詢講述Spring的回調(diào)模式
//傳統(tǒng)的JDBC查詢應(yīng)該是這樣
public List<AppUser> query(String sql) {
Connection con = DbUtil.openConnection();
PreparedStatement pst = null;
ResultSet rs = null;
List<AppUser> list = new ArrayList<AppUser>();
try {
pst = con.prepareStatement(sql);
rs = pst.executeQuery();
while (rs.next()) {
AppUser appUser = new AppUser();
appUser.setName(rs.getString("name"));
appUser.setAge(rs.getInt("age"));
list.add(appUser);
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
if (rs != null)
rs.close();
if (pst != null)
pst.close();
if (con != null)
con.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
return list;
}
傳統(tǒng)的JDBC查詢最明顯的弊端就是代碼重復(fù)?？梢酝ㄟ^模板模式解決重復(fù)代碼的問題。
//模板父類
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
public abstract class JdbcTemplate {
public Object query(String sql) {
Connection con = DbUtil.openConnection();
PreparedStatement pst = null;
ResultSet rs = null;
Object object = null;
try {
pst = con.prepareStatement(sql);
rs = pst.executeQuery();
object = doConvert(rs);
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
if (rs != null)
rs.close();
if (pst != null)
pst.close();
if (con != null)
con.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
return object;
}
protected abstract Object doConvert(ResultSet resultSet);
}
//數(shù)據(jù)庫查詢的所有DAO都繼承模板父類，自己實(shí)現(xiàn)doConvert方法
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
public class AppUserDao extends JdbcTemplate {
@Override
protected Object doConvert(ResultSet rs) {
List<AppUser> list = new ArrayList<AppUser>();
try {
while (rs.next()) {
AppUser appUser = new AppUser();
appUser.setName(rs.getString("name"));
appUser.setAge(rs.getInt("age"));
list.add(appUser);
}
} catch (SQLException e) {
e.printStackTrace();
}
return list;
}
}
上面的模板模式解決了代碼重復(fù)的問題。在面向?qū)ο笊系穆飞嫌侄嘧吡艘徊?。在設(shè)計(jì)模式里一個(gè)很重要的準(zhǔn)則聚合優(yōu)于繼承。每一個(gè)Dao都必須繼承模板，這是不可取的。回調(diào)模式就是為了解決這樣的問題。
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public abstract class JdbcTemplate {
private <T> T execute(StatementCallback<T> callback) throws SQLException {
Connection con = DbUtil.openConnection();
Statement pst = null;
ResultSet rs = null;
T object = null;
try {
pst = con.createStatement();
object = callback.doStatement(pst);
} catch (SQLException e) {
e.printStackTrace();
} finally {
try {
if (rs != null)
rs.close();
if (pst != null)
pst.close();
if (con != null)
con.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
return object;
}
public <T> T query(StatementCallback<T> callback) throws SQLException {
return execute(callback);
}
public void delete(final String sql) throws SQLException {
execute(new StatementCallback<Object>() {
@Override
public Object doStatement(Statement statement) {
try {
statement.executeUpdate(sql);
} catch (SQLException e) {
e.printStackTrace();
}
return null;
}
});
}
}
import java.sql.Statement;
public interface StatementCallback<T> {
T doStatement(Statement statement);
}
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
public class AppUserDao {
private JdbcTemplate jdbcTemplate;
public List<AppUser> query(final String sql) throws SQLException {
return jdbcTemplate.query(new StatementCallback<List<AppUser>>() {
@Override
public List<AppUser> doStatement(Statement statement) {
List<AppUser> list = new ArrayList<AppUser>();
try {
ResultSet rs = statement.executeQuery(sql);
while (rs.next()) {
AppUser appUser = new AppUser();
appUser.setName(rs.getString("name"));
appUser.setAge(rs.getInt("age"));
list.add(appUser);
}
} catch (SQLException e) {
e.printStackTrace();
}
return list;
}
});
}
}
public void delete(String sql) throws SQLException{
jdbcTemplate.delete(sql);
}
