最近接触的老项目，总是爱用hibernate，维护起来觉得很麻烦，这里随便总结一下hibernate的特点。

首先：

1.hibernate可以直接对java实例操作，达到数据库的增删改查作用，但是这个实例，只对应一个表。

2.hibernate可以通过编写hql语句，达成自定义SQL查询，但是返回结果，可能是一个map，而无法直接对应成JAVA实例，要自己组装查询结果。

3.hibernate可以通过创建配置文件，配置文件里，定义一个复杂的JAVA实例Class，对应数据库中一个复杂的视图table，并指定在这个复杂视图中，多个表之间的one-to-one, one-to-many, many-to-many关系，和指定哪个字段作为关联条件。而在复杂的java实例中，根据一对一还是一对多的关系，创建其他表的实例作为成员变量，接收其他表的查询结果。既查询一次这个复杂的java实例，就等于关联查询了好多张表。

该文件的命名，一般以XXXX.hbm.xml结尾。

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping auto-import="false"><class name="com.nstc.cpm.biz.acm.Account" table="V_CPM_BKUNIT_ACCOUNT" lazy="true"><id name="id" type="java.lang.Integer"><column name="ACCOUNT_ID" /><generator class="sequence"><param name="sequence">S_CPM_BOOK_UNIT</param></generator></id><discriminator column="ACC_CLASS" /><version name="version" type="java.lang.Integer"><column name="VERSION_COLUMN" /></version><many-to-one name="customer" class="com.nstc.cpm.biz.acm.Customer" fetch="join" outer-join="true"><column name="CUST_ID" /></many-to-one><many-to-one name="subject" class="com.nstc.cpm.biz.acm.Subject" fetch="join"><column name="SUBJECT_ID" length="8" not-null="true" /></many-to-one><many-to-one name="currency" class="com.nstc.cpm.biz.model.BCurrency"fetch="select" update="false" insert="false"><column name="CUR_CODE" not-null="true" /></many-to-one><property name="number" type="java.lang.String" unique="true"><column name="ACCOUNT_NO" length="32" not-null="true" /></property><one-to-one name="accIntr"  class="com.nstc.cpm.biz.acm.DefaultAccIntr" property-ref="accountId"/><many-to-one name="type" class="com.nstc.cpm.biz.acm.AccountType" fetch="select" not-null="true"><column name="TYPE_ID" length="8" /></many-to-one><many-to-one name="bizType" class="com.nstc.cpm.biz.model.BizType" fetch="select"><column name="BTY_ID" length="6" /></many-to-one><many-to-one name="smartBizType" class="com.nstc.cpm.biz.model.SmartBizType" fetch="select"><column name="SMART_BTY_ID" /></many-to-one><property name="name" type="java.lang.String"><column name="ACCOUNT_NAME" length="128" /></property><property name="openDate" type="java.util.Date"><column name="OPEN_DATE" length="7" /></property><property name="endDate" type="java.util.Date"><column name="END_DATE" length="7" /></property><property name="interBranchFlag" type="java.lang.Integer"><column name="INTER_BRANCH_FLAG"  /></property><property name="currencyNo" type="java.lang.String"><column name="CUR_CODE" length="32" not-null="true" /></property><!-- MODIFY BY ZHUQW FOR ZMJT-388 START 20170815 --><many-to-one name="branch" class="com.nstc.cpm.biz.model.Branch" fetch="join"><column name="BRANCH_NO" length="8" not-null="true" /></many-to-one><!-- MODIFY BY ZHUQW FOR ZMJT-388 END 20170815 --><property name="balanceDir" type="java.lang.Integer"><column name="BALANCE_DIR"  /></property><property name="state" type="java.lang.Integer"><column name="ACCOUNT_STATE"  /></property><property name="clearDate" type="java.util.Date"><column name="CLEAR_DATE"/></property><property name="floorAmount" type="java.lang.Double"><column name="FLOOR_AMOUNT" /></property><property name="sourceId" type="java.lang.String"><column name="SOURCE_MARK" length="32" /></property><property name="thirdCustomerId" type="java.lang.Integer"><column name="THIRD_CUSTOM_ID"  /></property><property name="initBalance" type="java.lang.Double"><column name="INIT_BALANCE"/></property><property name="sleepState" type="java.lang.Integer"><column name="SLEEP_STATE"/></property><property name="accTxDate" type="java.util.Date"><column name="ACCOUNT_TX_DATE"/></property><property name="sleepSDate" type="java.util.Date"><column name="SLEEP_SDATE"/></property><property name="sleepEDate" type="java.util.Date"><column name="SLEEP_EDATE"/></property><set name="intrSbUnits" inverse="true"><key><column name="CPM_ACCOUNT_ID" not-null="true" /></key><one-to-many class="com.nstc.cpm.biz.acm.StandingBookUnit" /></set><subclass name="com.nstc.cpm.biz.acm.CustomerAccount" discriminator-value="CustomerAccount"><subclass name="com.nstc.cpm.biz.acm.EntitySettlementAccount" discriminator-value="EntitySettlementAccount"><subclass name="com.nstc.cpm.biz.acm.DemandDepositAccount" discriminator-value="DemandDepositAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.VirtualSettlementAccount" discriminator-value="VirtualSettlementAccount"><subclass name="com.nstc.cpm.biz.acm.CustLinkageAccount" discriminator-value="CustLinkageAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.DepositAccount" discriminator-value="DepositAccount"><subclass name="com.nstc.cpm.biz.acm.FixedDepositAccount" discriminator-value="FixedDepositAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.CallDepositAccount" discriminator-value="CallDepositAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.MarginDepositAccount" discriminator-value="MarginDepositAccount"><subclass name="com.nstc.cpm.biz.acm.MarginDemandDepositAccount" discriminator-value="MarginDemandDepositAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.MarginFixedDepositAccount" discriminator-value="MarginFixedDepositAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.LoanAccount" discriminator-value="LoanAccount"><subclass name="com.nstc.cpm.biz.acm.CommonLoanAccount" discriminator-value="CommonLoanAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.SyndicatedLoanAccount" discriminator-value="SyndicatedLoanAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.BankAccount" discriminator-value="BankAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.InsideAccountingAccount" discriminator-value="InsideAccountingAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><subclass name="com.nstc.cpm.biz.acm.OffSheetAccountingAccount" discriminator-value="OffSheetAccountingAccount"><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></subclass><sql-delete>delete from CPM_BOOK_UNIT a where a.ACCOUNT_ID=? AND A.VERSION_COLUMN = ?</sql-delete></class>
</hibernate-mapping>

4.hibernate基于JPA，通过注解的方式，来完成数据库的增删改查。使用这个方式，类的成员属性，都需要加上注解，标识这个成员属性，对应表的哪个字段，是否是主键，是否是关联条件。同样的，实体类新增了成员变量，而改变量不对应表的字段时，也要加上注解，标识该字段与表无关。

在多表关联查询的时候，父类查询结果，持有子类查询结果时，还要在父类中还需要持有子查询类的实体类对象，并添加注解，标识改子查询是one-to-one还是one-to-many类型，如果是one-to-many，就需要持有子表实体类的集合作为成员变量。

以上，是我在项目中，遇到的hibernate几种用法。

下面，我们来说一下mybatis，实际上mybatis主要有两种用法：

1.基于注解的，在dao文件中，定义SQL方法，而在每个自定义的SQL方法上，编写mybatis的注解，来对数据库进行操作。但是缺点是多表关联查询，需要编写的SQL比较复杂，使用注解的方式，会更复杂。所以我们一般不用。

2.基于配置文件，基于配置文件的mybatis用法比较常见，有点是，适合编写复杂，超长的sql语句，而且配套的动态sql子句，用起来很好用。