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数据库和实体类设计好以后，有几个实体类就需要几个Dao吗？显然不是。。。
利用java的反射机制，一个Dao就可以实现对所有实体类的操作
思路就是，传过来一个实体类，利用实体类得到表，把实体类的每一个字段取出，放到表对应的字段。。。。
但是，如果实体类字段发生变化，这个时候往数据库插入数据就会出错，原因是表里面没有与实体类属性对应的字段，
这个时候就要用到java的反射机制了
我们首先需要得到表的所有字段，然后得到实体类的所有属性，两个一一比较，实体类中有的属性而数据库没有与之对应的字段，就舍去，这就是基本的思路
代码如下：
package com.lp.DAO;
import java.lang.reflect.Field;
import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import java.util.List;
import com.lp.bean.goods;
/**
* 2015.7.20 利用java反射实现万能DAO 即使bean层的实体类字段改变 也能实现增删查改
*
* @author lp
*/
public class DaoPro {
// 由数据库名称得到数据库中的所有列
public List<String> getColumns(String tableName) {
Connection connection = DbHelper.getCon();
Statement statement = DbHelper.getStatement();
ResultSet resultSet = null;
List<String> list = new ArrayList<String>();
try {
String sql = "select * from " + tableName + " where 1=2";
resultSet = DbHelper.getStatement().executeQuery(sql);
ResultSetMetaData data = resultSet.getMetaData();
int columnsCount = data.getColumnCount();
for (int i = 1; i <= columnsCount; i++) {
String s = data.getColumnName(i);
list.add(s);
}
} catch (SQLException e) {
e.printStackTrace();
} finally {
DbHelper.cloRes(connection, statement, resultSet);
}
return list;
}
// 填充实体类，返回一个实体类集合
public List getList(Class cl) {
int i = 0;
List objs = new ArrayList();
Connection connection = DbHelper.getCon();
Statement statement = DbHelper.getStatement();
ResultSet resultSet = null;
Field[] fields = cl.getDeclaredFields();
List<String> columnsName = getColumns(cl.getSimpleName());
String sql = "select * from " + cl.getSimpleName();
try {
System.out.println(sql);
resultSet = statement.executeQuery(sql);
System.out.println(i++);
while (resultSet.next()) {
System.out.println(i++);
Object obj = cl.newInstance();
for (String str : columnsName) {
for (Field ff : fields) {
if (str.equals(ff.getName())) {
ff.setAccessible(true);
ff.set(obj, resultSet.getObject(ff.getName()));
break;
}
}
}
objs.add(obj);
}
} catch (SQLException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
}
return objs;
}
public void addData(Object object) {
Connection connection = DbHelper.getCon();
ResultSet resultSet = null;
PreparedStatement preparedStatement = null;
String tableName = object.getClass().getSimpleName();
Field[] fields = object.getClass().getDeclaredFields();
List<String> allColumns = getColumns(tableName);
StringBuffer buffer = new StringBuffer();
StringBuffer buffer1 = new StringBuffer();
buffer.append("insert into ");
buffer.append(tableName);
buffer.append(" (");
for (int i = 1; i < allColumns.size(); i++) {
buffer.append(allColumns.get(i));
buffer1.append("?");
if (i != allColumns.size() - 1) {
buffer.append(",");
buffer1.append(",");
}
}
buffer.append(") values (");
buffer.append(buffer1);
buffer.append(")");
// System.out.println(buffer.toString());
try {
preparedStatement = connection.prepareStatement(buffer.toString());
for (int i = 1; i < allColumns.size(); i++) {
for (int j = 0; j < fields.length; j++) {
if ((fields[j].getName()).equals(allColumns.get(i))) {
fields[j].setAccessible(true);
if (fields[j].get(object) == null) {
preparedStatement.setObject(i, "此字段无填充");
} else {
preparedStatement.setObject(i, fields[j].get(object));
}
break;
}
}
}
int a = preparedStatement.executeUpdate();
System.out.println(a);
} catch (SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalArgumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IllegalAccessException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} finally {
DbHelper.cloRes(connection, preparedStatement, resultSet);
}
}
public static void main(String[] args) {
goods c = new goods();
new DaoPro().addData(c);
}
}
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原文地址：http://www.cnblogs.com/nongmei/p/4663402.html
