refactor UserDao: extract JdbcTemplate

src/main/java/core/jdbc/JdbcTemplate.java

@@ -0,0 +1,71 @@
+package core.jdbc;
+
+import java.sql.Connection;
+import java.sql.PreparedStatement;
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.Optional;
+
+public class JdbcTemplate {
+    private <R> R execute(String query,
+                          ParameterSetter parameterSetter,
+                          StatementExecutor<R> executor) {
+        try (Connection conn = ConnectionManager.getConnection();
+             PreparedStatement stmt = conn.prepareStatement(query)){
+            parameterSetter.setParams(stmt);
+            return executor.execute(stmt);
+        } catch(SQLException e) {
+            throw new DataAccessException(e);
+        }
+    }
+
+    public void insert(String query, Object... params) {
+        execute(query, stmt -> parameterSetter(stmt, params), PreparedStatement::execute);
+    }
+
+    public <T> Optional<T> queryForObject(String query, RowMapper<T> mapper, Object... params)  {
+        List<T> rows = findAll(query, stmt -> parameterSetter(stmt, params), mapper);
+        if (rows.size() > 0) {
+            return Optional.of(rows.get(0));
+        }
+        return Optional.empty();
+    }
+
+    public <T> List<T> findAll(String query, RowMapper<T> mapper, Object... params) {
+        return execute(query, stmt -> parameterSetter(stmt, params), stmt -> fetchAllRows(stmt, mapper));
+    }
+
+    public <T> List<T> findAll(String query, ParameterSetter parameterSetter, RowMapper<T> mapper) {
+        return execute(query, parameterSetter, stmt -> fetchAllRows(stmt, mapper));
+    }
+
+    private void parameterSetter(PreparedStatement stmt, Object[] params) throws SQLException {
+        int i = 1;
+        for(Object param : params) {
+            stmt.setObject(i++, param);
+        }
+    }
+
+    private <T> List<T> fetchAllRows(PreparedStatement stmt, RowMapper<T> mapper) throws SQLException {
+        List<T> rows = new ArrayList<>();
+        try (ResultSet rs = stmt.executeQuery()) {
+            while (rs.next()) {
+                rows.add(mapper.map(rs));
+            }
+        }
+        return rows;
+    }
+}
+
+class DataAccessException extends RuntimeException {
+    public DataAccessException(Throwable cause) {
+        super(cause);
+    }
+}
+
+@FunctionalInterface
+interface StatementExecutor<R> {
+    R execute(PreparedStatement stmt) throws SQLException;
+}

src/main/java/core/jdbc/ParameterSetter.java

@@ -0,0 +1,9 @@
+package core.jdbc;
+
+import java.sql.PreparedStatement;
+import java.sql.SQLException;
+
+@FunctionalInterface
+public interface ParameterSetter {
+    void setParams(PreparedStatement rs) throws SQLException;
+}

src/main/java/core/jdbc/RowMapper.java

@@ -0,0 +1,9 @@
+package core.jdbc;
+
+import java.sql.ResultSet;
+import java.sql.SQLException;
+
+@FunctionalInterface
+public interface RowMapper<T> {
+    T map(ResultSet rs) throws SQLException;
+}

src/main/java/next/dao/UserDao.java

@@ -1,78 +1,40 @@
 package next.dao;
 
-import java.sql.Connection;
-import java.sql.PreparedStatement;
 import java.sql.ResultSet;
 import java.sql.SQLException;
-import java.util.ArrayList;
 import java.util.List;
 
-import core.jdbc.ConnectionManager;
+import core.jdbc.JdbcTemplate;
 import next.model.User;
 
 public class UserDao {
-    public void insert(User user) throws SQLException {
-        Connection con = null;
-        PreparedStatement pstmt = null;
-        try {
-            con = ConnectionManager.getConnection();
-            String sql = "INSERT INTO USERS VALUES (?, ?, ?, ?)";
-            pstmt = con.prepareStatement(sql);
-            pstmt.setString(1, user.getUserId());
-            pstmt.setString(2, user.getPassword());
-            pstmt.setString(3, user.getName());
-            pstmt.setString(4, user.getEmail());
+    private final JdbcTemplate template;
 
-            pstmt.executeUpdate();
-        } finally {
-            if (pstmt != null) {
-                pstmt.close();
-            }
-
-            if (con != null) {
-                con.close();
-            }
-        }
+    public UserDao() {
+        template = new JdbcTemplate();
     }
-
-    public void update(User user) throws SQLException {
-        // TODO 구현 필요함.
+    public void insert(User user) {
+        String sql = "INSERT INTO USERS VALUES (?, ?, ?, ?)";
+        template.insert(sql, user.getUserId(), user.getPassword(), user.getName(), user.getEmail());
     }
 
-    public List<User> findAll() throws SQLException {
-        // TODO 구현 필요함.
-        return new ArrayList<User>();
+    public void update(User user) {
+        String sql = "UPDATE \"USERS\" SET password =?, name = ?, email = ? WHERE userId = ?";
+        template.insert(sql, user.getPassword(), user.getName(), user.getEmail(), user.getUserId());
     }
 
-    public User findByUserId(String userId) throws SQLException {
-        Connection con = null;
-        PreparedStatement pstmt = null;
-        ResultSet rs = null;
-        try {
-            con = ConnectionManager.getConnection();
-            String sql = "SELECT userId, password, name, email FROM USERS WHERE userid=?";
-            pstmt = con.prepareStatement(sql);
-            pstmt.setString(1, userId);
-
-            rs = pstmt.executeQuery();
+    public List<User> findAll() {
+        String sql = "SELECT * FROM USERS";
+        return template.findAll(sql, UserDao::mapRow);
+    }
 
-            User user = null;
-            if (rs.next()) {
-                user = new User(rs.getString("userId"), rs.getString("password"), rs.getString("name"),
-                        rs.getString("email"));
-            }
+    public User findByUserId(String userId)  {
+        String sql = "SELECT userId, password, name, email FROM USERS WHERE userid=?";
+        return template.queryForObject(sql, UserDao::mapRow, userId).get();
+    }
 
-            return user;
-        } finally {
-            if (rs != null) {
-                rs.close();
-            }
-            if (pstmt != null) {
-                pstmt.close();
-            }
-            if (con != null) {
-                con.close();
-            }
-        }
+    private static User mapRow(ResultSet rs) throws SQLException {
+        return new User(rs.getString("userId"), rs.getString("password"), rs.getString("name"),
+                rs.getString("email"));
     }
 }