ibatis中selectKey的用法与源码

最近用到插入oracle数据库时，主键id是sequnce自动生成，具体写法如下
对于oracle：
<insert id="insertUser" parameterClass="ibatis.User">
<selectKey resultClass="long" keyProperty="id">
select SEQ_USER_ID.nextval as id from dual
</selectKey>
insert into user
(id,name,password)
values
(#id#,#name#,#password#)
</insert>
对于mysql：
<insert id="insertUser" parameterClass="ibatis.User">
insert into user
(name,password)
values
(#name#,#password#)
<selectKey resultClass="long" keyProperty="id">
SELECT LAST_INSERT_ID() AS ID
</selectKey>
</insert>

上面是两种数据库的实现。

下面我们看看源码是如何做的
//-- Basic Methods
/**
* Call an insert statement by ID
*
* @param sessionScope - the session
* @param id - the statement ID
* @param param - the parameter object
* @return - the generated key (or null)
* @throws SQLException - if the insert fails
*/
public Object insert(SessionScope sessionScope, String id, Object param) throws SQLException {
Object generatedKey = null;

MappedStatement ms = getMappedStatement(id);
Transaction trans = getTransaction(sessionScope);
boolean autoStart = trans == null;

try {
//开始事务
trans = autoStartTransaction(sessionScope, autoStart, trans);

SelectKeyStatement selectKeyStatement = null;
if (ms instanceof InsertStatement) {
selectKeyStatement = ((InsertStatement) ms).getSelectKeyStatement();
}

// Here we get the old value for the key property. We'll want it later if for some reason the
// insert fails.
Object oldKeyValue = null;
String keyProperty = null;
boolean resetKeyValueOnFailure = false;
//对于oracle适用于执行insert之前执行selectKey获取id值
if (selectKeyStatement != null && !selectKeyStatement.isRunAfterSQL()) {
keyProperty = selectKeyStatement.getKeyProperty();
oldKeyValue = PROBE.getObject(param, keyProperty);
//sequnce获取id的值
generatedKey = executeSelectKey(sessionScope, trans, ms, param);
resetKeyValueOnFailure = true;
}

StatementScope statementScope = beginStatementScope(sessionScope, ms);
try {
//执行sql语句
ms.executeUpdate(statementScope, trans, param);
}catch (SQLException e){
// uh-oh, the insert failed, so if we set the reset flag earlier, we'll put the old value
// back...
if(resetKeyValueOnFailure) PROBE.setObject(param, keyProperty, oldKeyValue);
// ...and still throw the exception.
throw e;
} finally {
endStatementScope(statementScope);
}
//对于mysql适用于执行完insert后，获取id值
if (selectKeyStatement != null && selectKeyStatement.isRunAfterSQL()) {
generatedKey = executeSelectKey(sessionScope, trans, ms, param);
}
//提交事务
autoCommitTransaction(sessionScope, autoStart);
} finally {
//结束事务
autoEndTransaction(sessionScope, autoStart);
}

return generatedKey;
}