反射寫一個通用的dao層

• 1. 學了反射，就得學會搞事情
• 2. 反射寫dao具體實現(xiàn)

1. 學了反射，就得學會搞事情

• 我想怎么對實體類操作，就對實體類操作

• 想實現(xiàn)通用的dao，要滿足的條件：

• 實體類名與表名一樣
• 實體字段名與表字段名一樣
• 實體類定義的屬性順序與表名字段名順序一樣
• 使用dao修改，增加時，實體類的屬性都要有賦值，因為根據(jù)反射拿到所有屬性值，和屬性名，來拼接成sql語句滴

• 如何用反射操作操作

• 拼接sql語句
• 拿到表名
• 拿到實體類的字段
• 拿到字段的值

2. 反射寫dao具體實現(xiàn)

• BaseDao 拿到連接和釋放資源

package com.lovely.dao;import java.sql.Connection; import java.sql.DriverManager; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException;public class BaseDao {static {try {Class.forName("oracle.jdbc.OracleDriver");} catch (ClassNotFoundException e) {e.printStackTrace();}}public static Connection getConn() {Connection conn = null;String url = "jdbc:oracle:thin:@127.0.0.1:1521:orcl";try {conn = DriverManager.getConnection(url, "scott", "scott");} catch (SQLException e) {e.printStackTrace();}return conn;}public static void closeAll(Connection conn, PreparedStatement ps, ResultSet rs) {try {if (rs != null)rs.close();if (ps != null)ps.close();if (conn != null)conn.close();} catch (SQLException e) {e.printStackTrace();}} }

• 增刪改查萬能寫法

package com.lovely.dao;import java.lang.reflect.Field; import java.sql.*; import java.util.ArrayList; import java.util.List;public class CommonDao {/*** @param entity 給定一個初始化參數(shù)的實體，來做添加操作* @return 返回數(shù)據(jù)庫受影響行數(shù)*/public int add(Object entity) {int count = -1;Class<?> c = entity.getClass();StringBuffer sql = new StringBuffer();// insert into tab_name values (seq_student.nextval, ?, ?, ? ...);sql.append("insert into " + c.getSimpleName() + " values (seq_" + c.getSimpleName() + ".nextval");// 拿到實體類私有屬性對象的數(shù)組Field[] fs = c.getDeclaredFields();Field.setAccessible(fs, true);// 排除主鍵for (int j = 1; j < fs.length; j++) {sql.append(", ?");}sql.append(")");System.out.println(sql);Connection conn = BaseDao.getConn();PreparedStatement ps = null;try {ps = conn.prepareStatement(sql.toString());// 賦值for (int i = 1; i < fs.length; i++) {ps.setObject(i, fs[i].get(entity));}count = ps.executeUpdate();} catch (SQLException e) {e.printStackTrace();} catch (IllegalArgumentException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} finally {BaseDao.closeAll(conn, ps, null);} return count;}/*** * @param c 描述類的類對象* @return 這張表在數(shù)據(jù)庫里面所有的數(shù)據(jù)*/public List<Object> queryAll(Class<?> c) {ArrayList<Object> list = new ArrayList<Object>();String sql = "SELECT * FROM " + c.getSimpleName();Connection conn = BaseDao.getConn();PreparedStatement ps = null;ResultSet rs = null;conn = BaseDao.getConn();try {ps = conn.prepareStatement(sql);rs = ps.executeQuery();Field[] fs = c.getDeclaredFields();Field.setAccessible(fs, true);while (rs.next()) {// 創(chuàng)建實體類對象Object entity = c.newInstance();// 為實體類的每個屬性賦值for (int i = 0; i < fs.length; i++) {// 取結(jié)果集中的值Object value = rs.getObject(fs[i].getName());// oracle 和 Java 中的類型不一樣 設(shè)置屬性值時 可能報錯if (fs[i].getType() == Integer.class) {value = rs.getInt(fs[i].getName());} else if (fs[i].getType() == Double.class) {value = rs.getDouble(fs[i].getName());} else if (fs[i].getType() == Timestamp.class) {value = rs.getTimestamp(fs[i].getName());}// Can not set java.sql.Timestamp field com.lovely.entity.Product.pdate to java.sql.Date// 屬性是Timstamp類型 Oracle獲取的值是 java.sql.Date類型 /*if (fs[i].getType() == Timestamp.class && value.getClass() == java.sql.Date.class) {// 轉(zhuǎn)型java.sql.Date d = (Date) value; value = new Timestamp(d.getTime());}*/// 為實體類屬性賦值fs[i].set(entity, value);}list.add(entity);}} catch (SQLException e) {e.printStackTrace();} catch (InstantiationException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} finally {BaseDao.closeAll(conn, ps, rs);}return list;}/*** * @param entity 傳入主鍵即可* @return 返回主鍵對應(yīng)的對象*/public Object queryOne(Object entity) {Object obj = null;Class<?> c = entity.getClass();Field[] fs = c.getDeclaredFields();Field.setAccessible(fs, true); // 可訪問私有屬性String sql = "select * from " + c.getSimpleName() + " where " + fs[0].getName() + " = ?";Connection conn = BaseDao.getConn();PreparedStatement ps = null;ResultSet rs = null;try {ps = conn.prepareStatement(sql);ps.setObject(1, fs[0].get(entity));rs = ps.executeQuery();if (rs.next()) {// 創(chuàng)建當前實體類對象obj = c.newInstance();for (int i = 0; i < fs.length; i++) {// 結(jié)果集中每個列的值Object value = rs.getObject(fs[i].getName());// 判斷屬性值的類型if (fs[i].getType() == Integer.class) {value = rs.getInt(fs[i].getName());} else if (fs[i].getType() == Double.class) {value = rs.getDouble(fs[i].getName());} else if (fs[i].getType() == java.sql.Timestamp.class) {value = rs.getTimestamp(fs[i].getName());}// 為每個屬性設(shè)置值fs[i].set(obj, value);}}} catch (Exception e) {e.printStackTrace();} finally {BaseDao.closeAll(conn, ps, rs);}return obj; }/*** * @param entity 要修改的實體對象 實體對象的值都得被初始化* @return 返回受影響行數(shù)*/public int update(Object entity) {int count = -1;Class<?> c = entity.getClass();StringBuffer sql = new StringBuffer();// update tab_name set * = ?, * = ?, * = ? ... where primary_key = ?sql.append("update " + c.getSimpleName() + " set ");Field[] fs = c.getDeclaredFields();Field.setAccessible(fs, true);// 拼接列名try {for (int i = 1; i < fs.length; i++) {if (i < fs.length - 1)sql.append(fs[i].getName() + " = ?, ");elsesql.append(fs[i].getName() + " = ? ");}} catch (IllegalArgumentException e) {e.printStackTrace();}// 拼接條件sql.append("where " + fs[0].getName() + " = ?");System.out.println(sql);Connection conn = BaseDao.getConn();PreparedStatement ps = null;try {ps = conn.prepareStatement(sql.toString());// 設(shè)置 sql 中參數(shù)的值for (int i = 1; i < fs.length; i++) {ps.setObject(i, fs[i].get(entity));}// 設(shè)置主鍵值ps.setObject(fs.length, fs[0].get(entity));count = ps.executeUpdate();} catch (SQLException e) {e.printStackTrace();} catch (IllegalArgumentException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();} finally {BaseDao.closeAll(conn, ps, null);}return count;}/*** * @param entity 要被刪除的對象，傳入主鍵即可* @return 返回受影響行數(shù)*/public int delete(Object entity) {int count = -1;Class<?> c = entity.getClass();Field primaryKey = c.getDeclaredFields()[0];primaryKey.setAccessible(true);String sql = "delete from " + c.getSimpleName() + " where " + primaryKey.getName() + " = ?";Connection conn = BaseDao.getConn();PreparedStatement ps = null;try {ps = conn.prepareStatement(sql);ps.setObject(1, primaryKey.get(entity));count = ps.executeUpdate();} catch (SQLException | IllegalArgumentException | IllegalAccessException e) {e.printStackTrace();} finally {BaseDao.closeAll(conn, ps, null);}return count;}}

• 本代碼適用Oracle，mysql，sqlserver稍微改變下可用。

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