创建数据库
ACCOUNT 表
| ACCOUNT_ID |
| 1 |
| 2 |
ORDERS 表
| ORDER_ID | ACCOUNT_ID |
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 2 |
| 5 | 2 |
| 6 | 2 |
创建JavaBean
Account.java
public class Account {
private Integer accountId;
private Set<Orders> ordersSet = new HashSet<Orders>();
// 省略 getter setter
public String toString() {
return "accountId:" + accountId;
}
}
Orders.java
public class Orders {
private Integer orderId;
private Account account;
// 省略getter setter
public String toString() {
return "orderId:"+orderId;
}
}
一、使用Ibatis查询
SqlMapConfig.xml配置
<sqlMapConfig>
<settings cacheModelsEnabled="true" enhancementEnabled="false"
lazyLoadingEnabled="false" maxRequests="32" maxSessions="10"
maxTransactions="5" useStatementNamespaces="true" />
<transactionManager type="JDBC">
<dataSource type="SIMPLE">
<property name="JDBC.Driver"
value="com.microsoft.sqlserver.jdbc.SQLServerDriver" />
<property name="JDBC.ConnectionURL"
value="jdbc:sqlserver://127.0.0.1:1433;DatabaseName=TEST" />
<property name="JDBC.Username" value="xiao" />
<property name="JDBC.Password" value="xiao" />
</dataSource>
</transactionManager>
<sqlMap resource="ibatis/n1.ibatis.xml" />
<sqlMap resource="ibatis/groupby.ibatis.xml" />
</sqlMapConfig>
n1.ibatis.xml 配置
<sqlMap namespace="N1"> <typeAlias alias="Account" type="bean.Account"/> <resultMap id="ResultAccountInfoMap" class="bean.Account"> <result property="accountId" column="ACCOUNT_ID" /> <result property="ordersSet" select="N1.getOrderInfoList" column="ACCOUNT_ID"/> </resultMap> <resultMap id="ResultOrderInfoMap" class="bean.Orders"> <result property="orderId" column="ORDER_ID" /> <result property="account.accountId" column="ACCOUNt_Id" /> </resultMap> <select id="getAccountInfoList" resultMap="ResultAccountInfoMap"> select account_Id from ACCOUNT </select> <select id="getOrderInfoList" resultMap="ResultOrderInfoMap"> select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = #value# </select> </sqlMap>
groupby.ibatis.xml 配置
<sqlMap namespace="GroupBy"> <resultMap id="ResultAccountInfoMap" class="bean.Account" groupBy="accountId"> <result property="accountId" column="account_Id" /> <result property="ordersSet" resultMap="GroupBy.ResultOrderInfoMap"/> </resultMap> <resultMap id="ResultOrderInfoMap" class="bean.Orders" groupBy="orderId"> <result property="orderId" column="ORDER_ID" /> <result property="account.accountId" column="ACCOUNT_ID" /> </resultMap> <select id="getAccountInfoList" resultMap="ResultAccountInfoMap"> select ACCOUNT.ACCOUNT_ID AS ACCOUNT_ID ,ORDERS.ORDER_ID AS ORDER_ID from ACCOUNT join ORDERS on ACCOUNT.account_Id=ORDERS.ACCOUNT_ID group by ACCOUNT.ACCOUNT_ID,ORDERS.ORDER_ID </select> </sqlMap>
测试代码
public static void queryN1() throws SQLException, IOException{
Reader reader=Resources.getResourceAsReader("ibatis/SqlMapConfig.xml");
SqlMapClient sqlMapClient = SqlMapClientBuilder.buildSqlMapClient(reader);
List<Account> accountList=sqlMapClient.queryForList("N1.getAccountInfoList");
for(Account account:accountList){
System.out.println("############"+account+"############");
for(Orders orders:account.getOrdersSet()){
System.out.println("############"+orders+"############");
}
}
}
执行以上代码,log4j打印如下:
[size=xx-small]2012-08-07 10:19:52,000-[HL] DEBUG main java.sql.Connection - {conn-100000} Connection
2012-08-07 10:19:52,000-[HL] DEBUG main java.sql.Connection - {conn-100000} Preparing Statement: select account_Id from ACCOUNT
2012-08-07 10:19:52,031-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Executing Statement: select account_Id from ACCOUNT
2012-08-07 10:19:52,031-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Parameters: []
2012-08-07 10:19:52,031-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Types: []
2012-08-07 10:19:52,078-[HL] DEBUG main java.sql.Connection - {conn-100000} Preparing Statement: select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = ?
2012-08-07 10:19:52,078-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100003} Executing Statement: select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = ?
2012-08-07 10:19:52,078-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100003} Parameters: [1]
2012-08-07 10:19:52,078-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100003} Types: [java.lang.Integer]
2012-08-07 10:19:52,078-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100003} Executing Statement: select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = ?
2012-08-07 10:19:52,078-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100003} Parameters: [2]
2012-08-07 10:19:52,078-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100003} Types: [java.lang.Integer][/size][size=xx-small][/size]
############accountId:1############
############orderId:3############
############orderId:2############
############orderId:1############
############accountId:2############
############orderId:5############
############orderId:6############
############orderId:4############
查看以上log，发现执行了3条查询语句.
将SqlMapConfig.xml中的setting 节点中的lazyLoadingEnabled属性设为true
<settings cacheModelsEnabled="true" enhancementEnabled="false" lazyLoadingEnabled="true" maxRequests="32" maxSessions="10" maxTransactions="5" useStatementNamespaces="true" />
再次执行之前queryN1测试代码,log4j打印如下
2012-08-07 10:26:42,468-[HL] DEBUG main java.sql.Connection - {conn-100000} Connection
2012-08-07 10:26:42,468-[HL] DEBUG main java.sql.Connection - {conn-100000} Preparing Statement: select account_Id from ACCOUNT
2012-08-07 10:26:42,515-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Executing Statement: select account_Id from ACCOUNT
2012-08-07 10:26:42,515-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Parameters: []
2012-08-07 10:26:42,515-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Types: []
############accountId:1############
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.Connection - {conn-100003} Connection
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.Connection - {conn-100003} Preparing Statement: select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = ?
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100004} Executing Statement: select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = ?
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100004} Parameters: [1]
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100004} Types: [java.lang.Integer]
############orderId:3############
############orderId:1############
############orderId:2############
############accountId:2############
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.Connection - {conn-100006} Connection
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.Connection - {conn-100006} Preparing Statement: select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = ?
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100007} Executing Statement: select ORDER_ID,ACCOUNT_ID from ORDERS where ACCOUNT_ID = ?
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100007} Parameters: [2]
2012-08-07 10:26:42,562-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100007} Types: [java.lang.Integer]
############orderId:5############
############orderId:4############
############orderId:6############
查看log，依然有3条查询语句，只是查询的时机不一样了。当设置SqlMapConfig.xml中的setting 节点中的lazyLoadingEnabled属性设为true时(启动ibatis懒加载)，ibatis会生成代理对对象，当集合元素真正使用时才执行查询语句(和hibernate类似)。
目前来看：至少在ibatis中启用或不启用懒加载，都不可以解决n+1问题.而在ibatis中使用groupby是一个解决方案.
测试代码如下:
public static void queryGroupBy() throws SQLException, IOException{
Reader reader=Resources.getResourceAsReader("ibatis/SqlMapConfig.xml");
SqlMapClient sqlMapClient = SqlMapClientBuilder.buildSqlMapClient(reader);
List<Account> accountList=sqlMapClient.queryForList("GroupBy.getAccountInfoList");
for(Account account:accountList){
System.out.println("############"+account+"############");
for(Orders orders:account.getOrdersSet()){
System.out.println("############"+orders+"############");
}
}
}
执行以上测试代码，log打印如下
2012-08-07 10:36:12,328-[HL] DEBUG main java.sql.Connection - {conn-100000} Connection
2012-08-07 10:36:12,328-[HL] DEBUG main java.sql.Connection - {conn-100000} Preparing Statement: select ACCOUNT.ACCOUNT_ID AS ACCOUNT_ID ,ORDERS.ORDER_ID AS ORDER_ID from ACCOUNT join ORDERS on ACCOUNT.account_Id=ORDERS.ACCOUNT_ID group by ACCOUNT.ACCOUNT_ID,ORDERS.ORDER_ID
2012-08-07 10:36:12,359-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Executing Statement: select ACCOUNT.ACCOUNT_ID AS ACCOUNT_ID ,ORDERS.ORDER_ID AS ORDER_ID from ACCOUNT join ORDERS on ACCOUNT.account_Id=ORDERS.ACCOUNT_ID group by ACCOUNT.ACCOUNT_ID,ORDERS.ORDER_ID
2012-08-07 10:36:12,359-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Parameters: []
2012-08-07 10:36:12,359-[HL] DEBUG main java.sql.PreparedStatement - {pstm-100001} Types: []
############accountId:1############
############orderId:3############
############orderId:2############
############orderId:1############
############accountId:2############
############orderId:4############
############orderId:6############
############orderId:5############
查看以上log，发现只有1条查询语句，解决了n+1问题。
目前看来，n+1问题算是解决了，可是配置文件搞得很复杂。
ibatis作为sql mapper并不是orm，非把ibatis当hibernate玩，个人认为这样并没有多大意义。
二 使用hibernate查询
hibernate.cfg.xml配置
<hibernate-configuration>
<session-factory>
<property name="dialect">org.hibernate.dialect.SQLServerDialect</property>
<property name="connection.url">jdbc:sqlserver://127.0.0.1:1433;DatabaseName=TEST</property>
<property name="connection.username">xiao</property>
<property name="connection.password">xiao</property>
<property name="connection.driver_class">com.microsoft.sqlserver.jdbc.SQLServerDriver</property>
<property name="show_sql">true</property>
<mapping resource="hibernate/Account.hibernate.xml"/>
<mapping resource="hibernate/Orders.hibernate.xml"/>
</session-factory>
</hibernate-configuration>
Account.hibernate.xml 配置
<hibernate-mapping> <class name="bean.Account" table="ACCOUNT" > <id name="accountId" column="ACCOUNT_ID" /> <set name="ordersSet" > <key column="ACCOUNT_ID"/> <one-to-many class="bean.Orders"/> </set> </class> </hibernate-mapping>
Orders.hibernate.xml 配置
<hibernate-mapping> <class name="bean.Orders" table="ORDERS"> <id name="orderId" column="ORDER_ID"/> <many-to-one name="account" class="bean.Account" column="ACCOUNT_ID"/> </class> </hibernate-mapping>
a.hibernate 1对多 n+1
测试代码
public static void query1(){
Configuration config = new Configuration().configure("hibernate/hibernate.cfg.xml");
SessionFactory factory = config.buildSessionFactory();
Session session = factory.openSession();
System.out.println("==========start query==========");
Query query=session.createQuery("from Account as account");
List<Account> accountList=query.list();
for(Account account:accountList){
System.out.println("############"+account+"############");
for(Orders orders:account.getOrdersSet()){
System.out.println("############"+orders+"############");
}
}
session.close();
}
执行以上测试代码，打印log如下：
==========start query========== Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_ from ACCOUNT account0_ ############accountId:1############ Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? ############orderId:2############ ############orderId:1############ ############orderId:3############ ############accountId:2############ Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? ############orderId:6############ ############orderId:5############ ############orderId:4############
查看以上log，发现3条查询语句。n+1 问题重现了。因为hibernate对集合查询默认是懒加载的，所以从以上打印log发现，只用在要使用集合时，hibernate才去执行查询。
将Account.hibernate.xml中set节点的lazy属性设置为false，如下
<set name="ordersSet" lazy="false"> <key column="ACCOUNT_ID"/> <one-to-many class="bean.Orders"/> </set>
再次执行query1()测试代码，log打印如下：
==========start query========== Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_ from ACCOUNT account0_ Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? ############accountId:1############ ############orderId:1############ ############orderId:3############ ############orderId:2############ ############accountId:2############ ############orderId:5############ ############orderId:4############ ############orderId:6############
查看以上log，发现依然是3条查询语句，只是查询在一开始就全部执行了。
b.hibernate 多对1 n+1
测试代码
public static void query2(){
Configuration config = new Configuration().configure("hibernate/hibernate.cfg.xml");
SessionFactory factory = config.buildSessionFactory();
Session session = factory.openSession();
System.out.println("==========start query==========");
Query query=session.createQuery("from Orders");
List<Orders> ordersSet=query.list();
for(Orders orders:ordersSet){
System.out.println("############"+orders+"############");
System.out.println("############"+orders.getAccount()+"############");
}
session.close();
}
执行以上测试代码，log如下:
==========start query========== Hibernate: select orders0_.ORDER_ID as ORDER1_1_, orders0_.ACCOUNT_ID as ACCOUNT2_1_ from ORDERS orders0_ ############orderId:1############ Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_0_ from ACCOUNT account0_ where account0_.ACCOUNT_ID=? ############accountId:1############ ############orderId:2############ ############accountId:1############ ############orderId:3############ ############accountId:1############ ############orderId:4############ Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_0_ from ACCOUNT account0_ where account0_.ACCOUNT_ID=? ############accountId:2############ ############orderId:5############ ############accountId:2############ ############orderId:6############ ############accountId:2############
查看以上log，发现3条查询语句。
倘若将Account.hibernate.xml中set节点的lazy属性设置为false，如下
<set name="ordersSet" lazy="false"> <key column="ACCOUNT_ID"/> <one-to-many class="bean.Orders"/> </set>
再次执行query2(),log打印如下:
==========start query========== Hibernate: select orders0_.ORDER_ID as ORDER1_1_, orders0_.ACCOUNT_ID as ACCOUNT2_1_ from ORDERS orders0_ ############orderId:1############ Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_0_ from ACCOUNT account0_ where account0_.ACCOUNT_ID=? Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? ############accountId:1############ ############orderId:2############ ############accountId:1############ ############orderId:3############ ############accountId:1############ ############orderId:4############ Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_0_ from ACCOUNT account0_ where account0_.ACCOUNT_ID=? Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? ############accountId:2############ ############orderId:5############ ############accountId:2############ ############orderId:6############ ############accountId:2############
查看以上log，发现5条查询语句。第一查询语句用来查询Orders信息，而Orders对象中的Account实例变量默认是懒加载的，所以当使用account变量时，hibernate才进行查询，由于Account对象中的ordersSet集合不启用懒加载。所以每次查询Account时会附带查询一次Orders.
c.hibernate 使用iterate n+1
测试代码
public static void query3(){
Configuration config = new Configuration().configure("hibernate/hibernate.cfg.xml");
SessionFactory factory = config.buildSessionFactory();
Session session = factory.openSession();
System.out.println("==========start query==========");
Query query=session.createQuery("from Account");
Iterator<Account> iterator=query.iterate();
while(iterator.hasNext()){
Account account=iterator.next();
System.out.println("############"+account+"############");
System.out.println("############"+account.getOrdersSet()+"############");
}
session.close();
}
执行以上测试代码,log打印语句如下:
==========start query========== Hibernate: select account0_.ACCOUNT_ID as col_0_0_ from ACCOUNT account0_ Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_0_ from ACCOUNT account0_ where account0_.ACCOUNT_ID=? ############accountId:1############ Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? ############[orderId:3, orderId:2, orderId:1]############ Hibernate: select account0_.ACCOUNT_ID as ACCOUNT1_0_0_ from ACCOUNT account0_ where account0_.ACCOUNT_ID=? ############accountId:2############ Hibernate: select ordersset0_.ACCOUNT_ID as ACCOUNT2_1_, ordersset0_.ORDER_ID as ORDER1_1_, ordersset0_.ORDER_ID as ORDER1_1_0_, ordersset0_.ACCOUNT_ID as ACCOUNT2_1_0_ from ORDERS ordersset0_ where ordersset0_.ACCOUNT_ID=? ############[orderId:6, orderId:5, orderId:4]############
查看以上log，发现有5条查询语句。第一条查询Account对象的accountId列表。当迭代使用Account时，hibernate会通过查询到的accountId在内存中找，如果找到则直接使用，如果为未找到则做一次查询。而把iterate查询归纳为n+1查询感觉不妥当,写在这里是想说明iterate没用好会产生n+1次查询或者更多。
如果将query3()中的迭代对象改为Orders，就会有很刺激的事，如果再将Account中的ordersSet懒加载去除(lazy=false),就更有意思了。
在hibernate中要解决n+1可以通过left join fetch。
将query1() 的hql该为from Account as account left join fetch account.ordersSet
重新执行则只有一条查询语句。
总结：不管是hibernate 还是ibatis 如果对象设计中有主从对象(主从对象指:一个对象中包含另一个对象，包含对象为主对象，被包含对象为从对象，从对象可为普通javabean或集合)都有可能碰到n+1问题，其中1指进行一条语句查询到n个主对象，而n指n个主对象要将从对象关联出来要进行n次查询。而解决方案不管怎样就是通过数据库join查询。而hibernate比较复杂点有很多缓存可用，所以具体的时候会有点差异。n+1并不是问题，更是一种策略。