ibatis源码学习(一)整体设计和核心流程

转自：http://www.iteye.com/topic/1121467

本文主要从ibatis框架的基本代码骨架进行切入，理解ibatis框架的整体设计思路，各组件的实现细节将在后文进行分析。

背景

介绍ibatis实现之前，先来看一段jdbc代码:

Java代码

Class.forName("com.mysql.jdbc.Driver");
String url = "jdbc:mysql://localhost:3306/learnworld";
Connection con = DriverManager.getConnection(url, "root","learnworld");
String sql = "select * from test";
PreparedStatement ps = con.prepareStatement(sql);
ResultSet rs = ps.executeQuery();
while(rs.next()){
System.out.println("id=" + rs.getInt(1)+". score=" + rs.getInt(2));

}

Class.forName("com.mysql.jdbc.Driver");
String url = "jdbc:mysql://localhost:3306/learnworld";
Connection con = DriverManager.getConnection(url, "root","learnworld");
String sql = "select * from test";
PreparedStatement ps = con.prepareStatement(sql);
ResultSet rs = ps.executeQuery();
while(rs.next()){
System.out.println("id=" + rs.getInt(1)+". score=" + rs.getInt(2));
}

上面这段代码大家比较熟悉，这是一个典型的jdbc方式处理流程: 建立连接->传递参数->sql执行->处理结果->关闭连接。

问题

上面的代码中包含了很多不稳定的因素，可能会经常发生修改:

1. 数据源和事务管理等

2. sql语句

3. 入参和结果处理

如果这些发生变化，我们需要直接修改代码，重新编译、打包、发布等。

DIY

下面从我们自己DIY的视角来考虑如何设计框架，应对这些问题。框架的核心思想是抽取共性，封装可能出现的变化。

如何处理数据源变化？

将数据源连接等过程固定，将数据源中易变的信息封装在一起(如driverClassName, url等)，放在配置文件中。

如何处理sql语句的变化？

将sql语句统一放在配置文件中，为每条sql语句设置标识，在代码中使用标识进行调用。

如何应对入参和结果处理的变化？

将参数传递，结果映射到java bean等统一封装在配置文件中。

结论: 将不变的流程固化到代码中，将变化的信息封装在配置文件中。

核心接口

ibatis抽取了以下几个重要接口:

1. SqlMapExecutor

该接口是对SQL操作行为的抽象，提供了SQL单条执行和批处理涉及的所有操作方法。



2. SqlMapTransactionManager

该接口是对事务行为的抽象，提供了事务执行过程中的涉及的所有方法。



3. SqlMapClient

该接口定位是SQL执行客户端，是线程安全的，用于处理多个线程的sql执行。它继承了上面两个接口，这意味着该接口具有SQL执行、批处理和事务处理的能力，如果你拥有该接口的实现类，意味着执行任何SQL语句对你来说是小菜一碟。该接口的核心实现类是SqlMapClientImpl。

4. SqlMapSession

该接口在继承关系上和SqlMapClient一致，但它的定位是保存单线程sql执行过程的session信息。该接口的核心实现类是SqlMapSessionImpl。

5. MappedStatement

该接口定位是单条SQL执行时的上下文环境信息，如SQL标识、SQL、参数信息、返回结果、操作行为等。

6. ParameterMap/ResultMap

该接口用于在SQL执行的前后提供参数准备和执行结果集的处理。

以上接口的类图关系如下(部分):



这里必须要强调SqlMapExecutorDelegate这个类，他是一个执行代理类，在ibatis框架中地位非常重要，因为他耦合了用户端的操作行为和执行环境，他持有执行操作的所需要的所有数据，同时管理着执行操作依赖的环境。

初始化过程

1. 读取和解析sqlmap配置文件。

2. 注册Statement对象。

3. 创建SqlMapClientImpl对象。

下面是一个Spring中sqlMapClient的bean配置:

Java代码

<bean id="sqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">

<property name="dataSource" ref="dataSource" />
<property name="configLocation" value="classpath/sqlmap/sqlmap-ibatis.xml" />
</bean>

<bean id="sqlMapClient" class="org.springframework.orm.ibatis.SqlMapClientFactoryBean">
<property name="dataSource" ref="dataSource" />
<property name="configLocation" value="classpath/sqlmap/sqlmap-ibatis.xml" />
</bean>

下面看一下SqlMapClientFactoryBean的初始化方法afterPropertiesSet()，用于构建sqlMapClient对象:

Java代码

public void afterPropertiesSet() throws Exception {

...

this.sqlMapClient = buildSqlMapClient(this.configLocations, this.mappingLocations, this.sqlMapClientProperties); //初始化核心方法，构建sqlMapClient对象

...
}

public void afterPropertiesSet() throws Exception {
...

this.sqlMapClient = buildSqlMapClient(this.configLocations, this.mappingLocations, this.sqlMapClientProperties);   //初始化核心方法，构建sqlMapClient对象

...
}

看一下buildSqlMapClient()的实现:

Java代码

protected SqlMapClient buildSqlMapClient(
Resource[] configLocations, Resource[] mappingLocations, Properties properties)
throws IOException {
...
SqlMapClient client = null;
SqlMapConfigParser configParser = new SqlMapConfigParser();
for (int i = 0; i < configLocations.length; i++) {

InputStream is = configLocations[i].getInputStream();
try {
client = configParser.parse(is, properties); //通过SqlMapConfigParser解析配置文件，生成SQLMapClientImpl对象

}
catch (RuntimeException ex) {
throw new NestedIOException("Failed to parse config resource: " + configLocations[i], ex.getCause());

}
}
...
return client;
}

protected SqlMapClient buildSqlMapClient(
Resource[] configLocations, Resource[] mappingLocations, Properties properties)
throws IOException {
...
SqlMapClient client = null;
SqlMapConfigParser configParser = new SqlMapConfigParser();
for (int i = 0; i < configLocations.length; i++) {
InputStream is = configLocations[i].getInputStream();
try {
client = configParser.parse(is, properties); //通过SqlMapConfigParser解析配置文件，生成SQLMapClientImpl对象
}
catch (RuntimeException ex) {
throw new NestedIOException("Failed to parse config resource: " + configLocations[i], ex.getCause());
}
}
...
return client;
}

初始化的核心是通过配置文件构建SQLMapClientImpl对象和其内部的各个属性，这里就不详述了，详细构建过程将另文分析。

SQL执行过程

下面以一个select语句的执行过程，分析一下以上各ibatis核心接口相互如何配合。

1. dao调用SqlMapClientTemplate的query()方法：

Java代码

public Object queryForObject(final String statementName, final Object parameterObject)

throws DataAccessException {

return execute(new SqlMapClientCallback() {
public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {

return executor.queryForObject(statementName, parameterObject);
}
});
}

public Object queryForObject(final String statementName, final Object parameterObject)
throws DataAccessException {

return execute(new SqlMapClientCallback() {
public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {
return executor.queryForObject(statementName, parameterObject);
}
});
}

2. execute()方法中展示了执行过程中的核心逻辑：获取session -> 获取connection -> 执行sql ->释放connection -> 关闭session。

部分源码如下：

Java代码

public Object execute(SqlMapClientCallback action) throws DataAccessException {

...
SqlMapSession session = this.sqlMapClient.openSession(); //获取session信息

Connection ibatisCon = null; //获取Connection

try {
Connection springCon = null;
DataSource dataSource = getDataSource();
boolean transactionAware = (dataSource instanceof TransactionAwareDataSourceProxy);

try {
ibatisCon = session.getCurrentConnection();
if (ibatisCon == null) {
springCon = (transactionAware ?
dataSource.getConnection() : DataSourceUtils.doGetConnection(dataSource));
session.setUserConnection(springCon);
...
}
}
...

// Execute given callback...

try {
return action.doInSqlMapClient(session); //执行SQL

}
...
finally {
// 关闭Connection

try {
if (springCon != null) {
if (transactionAware) {
springCon.close();
}
else {
DataSourceUtils.doReleaseConnection(springCon, dataSource);
}
}
}

if (ibatisCon == null) {
session.close(); //关闭session

}
}

public Object execute(SqlMapClientCallback action) throws DataAccessException {
...
SqlMapSession session = this.sqlMapClient.openSession(); //获取session信息

Connection ibatisCon = null;     //获取Connection

try {
Connection springCon = null;
DataSource dataSource = getDataSource();
boolean transactionAware = (dataSource instanceof TransactionAwareDataSourceProxy);

try {
ibatisCon = session.getCurrentConnection();
if (ibatisCon == null) {
springCon = (transactionAware ?
dataSource.getConnection() : DataSourceUtils.doGetConnection(dataSource));
session.setUserConnection(springCon);
...
}
}
...

// Execute given callback...
try {
return action.doInSqlMapClient(session); //执行SQL
}
...
finally {
// 关闭Connection
try {
if (springCon != null) {
if (transactionAware) {
springCon.close();
}
else {
DataSourceUtils.doReleaseConnection(springCon, dataSource);
}
}
}

if (ibatisCon == null) {
session.close(); //关闭session
}
}

3. action.doInSqlMapClient(session)调用SqlMapSessionImpl().queryForObject()方法。

注意: 这里调用对象主体为SqlMapSessionImpl，表示在线程session中执行sql(session是ThreadLocal的)，而不在负责整体协调的SqlMapClientImpl中执行sql语句。

源码如下：

Java代码

public Object queryForObject(final String statementName, final Object parameterObject)

throws DataAccessException {

return execute(new SqlMapClientCallback() {
//这里的SqlMapExecutor对象类型为SqlMapSessionImpl

public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {

return executor.queryForObject(statementName, parameterObject);
}
});
}

public Object queryForObject(final String statementName, final Object parameterObject)
throws DataAccessException {

return execute(new SqlMapClientCallback() {
//这里的SqlMapExecutor对象类型为SqlMapSessionImpl
public Object doInSqlMapClient(SqlMapExecutor executor) throws SQLException {
return executor.queryForObject(statementName, parameterObject);
}
});
}

4. SqlMapSessionImpl().queryForObject()的方法很简单，直接交给代理对象SqlMapExecutorDelegate处理:

Java代码

public Object queryForObject(String id, Object paramObject) throws SQLException {

return delegate.queryForObject(session, id, paramObject);
}

public Object queryForObject(String id, Object paramObject) throws SQLException {
return delegate.queryForObject(session, id, paramObject);
}

5. SqlMapExecutorDelegate的queryForObject()方法代码如下:

Java代码

public Object queryForObject(SessionScope session, String id, Object paramObject, Object resultObject) throws SQLException {

Object object = null;

MappedStatement ms = getMappedStatement(id); //MappedStatement对象集是上文中提及的初始化方法SqlMapClientFactoryBean.afterPropertiesSet()中，由配置文件构建而成

Transaction trans = getTransaction(session); // 用于事务执行

boolean autoStart = trans == null;

try {
trans = autoStartTransaction(session, autoStart, trans);

RequestScope request = popRequest(session, ms); // 从RequestScope池中获取该次sql执行中的上下文环境RequestScope

try {
object = ms.executeQueryForObject(request, trans, paramObject, resultObject); // 执行sql

} finally {
pushRequest(request); //归还RequestScope

}

autoCommitTransaction(session, autoStart);
} finally {
autoEndTransaction(session, autoStart);
}

return object;
}

public Object queryForObject(SessionScope session, String id, Object paramObject, Object resultObject) throws SQLException {
Object object = null;

MappedStatement ms = getMappedStatement(id); //MappedStatement对象集是上文中提及的初始化方法SqlMapClientFactoryBean.afterPropertiesSet()中，由配置文件构建而成
Transaction trans = getTransaction(session); // 用于事务执行
boolean autoStart = trans == null;

try {
trans = autoStartTransaction(session, autoStart, trans);

RequestScope request = popRequest(session, ms);  // 从RequestScope池中获取该次sql执行中的上下文环境RequestScope
try {
object = ms.executeQueryForObject(request, trans, paramObject, resultObject);   // 执行sql
} finally {
pushRequest(request);  //归还RequestScope
}

autoCommitTransaction(session, autoStart);
} finally {
autoEndTransaction(session, autoStart);
}

return object;
}

6. MappedStatement携带了SQL语句和执行过程中的相关信息，MappedStatement.executeQueryForObject()方法部分源码如下:

Java代码

public Object executeQueryForObject(RequestScope request, Transaction trans, Object parameterObject, Object resultObject)
throws SQLException {
try {
Object object = null;

DefaultRowHandler rowHandler = new DefaultRowHandler();
executeQueryWithCallback(request, trans.getConnection(), parameterObject, resultObject, rowHandler, SqlExecutor.NO_SKIPPED_RESULTS, SqlExecutor.NO_MAXIMUM_RESULTS); //执行sql语句

//结果处理，返回结果

List list = rowHandler.getList();
if (list.size() > 1) {
throw new SQLException("Error: executeQueryForObject returned too many results.");

} else if (list.size() > 0) {

object = list.get(0);
}

return object;
}
....
}

public Object executeQueryForObject(RequestScope request, Transaction trans, Object parameterObject, Object resultObject)
throws SQLException {
try {
Object object = null;

DefaultRowHandler rowHandler = new DefaultRowHandler();
executeQueryWithCallback(request, trans.getConnection(), parameterObject, resultObject, rowHandler, SqlExecutor.NO_SKIPPED_RESULTS, SqlExecutor.NO_MAXIMUM_RESULTS); //执行sql语句

//结果处理，返回结果
List list = rowHandler.getList();
if (list.size() > 1) {
throw new SQLException("Error: executeQueryForObject returned too many results.");
} else if (list.size() > 0) {
object = list.get(0);
}

return object;
}
....
}

7. MappedStatement.executeQueryWithCallback()方法包含了参数值映射、sql准备和sql执行等关键过程，部分源码如下:

Java代码

protected void executeQueryWithCallback(RequestScope request, Connection conn, Object parameterObject, Object resultObject, RowHandler rowHandler, int skipResults, int maxResults)

throws SQLException {
...
try {
parameterObject = validateParameter(parameterObject); //验证入参

Sql sql = getSql(); //获取SQL对象

errorContext.setMoreInfo("Check the parameter map.");
ParameterMap parameterMap = sql.getParameterMap(request, parameterObject);// 入参映射

errorContext.setMoreInfo("Check the result map.");
ResultMap resultMap = sql.getResultMap(request, parameterObject); //结果映射

request.setResultMap(resultMap);
request.setParameterMap(parameterMap);

errorContext.setMoreInfo("Check the parameter map.");
Object[] parameters = parameterMap.getParameterObjectValues(request, parameterObject); //获取参数值

errorContext.setMoreInfo("Check the SQL statement.");
String sqlString = sql.getSql(request, parameterObject); //获取拼装后的sql语句

errorContext.setActivity("executing mapped statement");
errorContext.setMoreInfo("Check the SQL statement or the result map.");
RowHandlerCallback callback = new RowHandlerCallback(resultMap, resultObject, rowHandler);
sqlExecuteQuery(request, conn, sqlString, parameters, skipResults, maxResults, callback); //sql执行

errorContext.setMoreInfo("Check the output parameters.");
if (parameterObject != null) {
postProcessParameterObject(request, parameterObject, parameters);
}

errorContext.reset();
sql.cleanup(request);
notifyListeners();
....
}

protected void executeQueryWithCallback(RequestScope request, Connection conn, Object parameterObject, Object resultObject, RowHandler rowHandler, int skipResults, int maxResults)
throws SQLException {
...
try {
parameterObject = validateParameter(parameterObject);  //验证入参

Sql sql = getSql();  //获取SQL对象

errorContext.setMoreInfo("Check the parameter map.");
ParameterMap parameterMap = sql.getParameterMap(request, parameterObject);// 入参映射

errorContext.setMoreInfo("Check the result map.");
ResultMap resultMap = sql.getResultMap(request, parameterObject); //结果映射

request.setResultMap(resultMap);
request.setParameterMap(parameterMap);

errorContext.setMoreInfo("Check the parameter map.");
Object[] parameters = parameterMap.getParameterObjectValues(request, parameterObject);  //获取参数值

errorContext.setMoreInfo("Check the SQL statement.");
String sqlString = sql.getSql(request, parameterObject);  //获取拼装后的sql语句

errorContext.setActivity("executing mapped statement");
errorContext.setMoreInfo("Check the SQL statement or the result map.");
RowHandlerCallback callback = new RowHandlerCallback(resultMap, resultObject, rowHandler);
sqlExecuteQuery(request, conn, sqlString, parameters, skipResults, maxResults, callback);  //sql执行

errorContext.setMoreInfo("Check the output parameters.");
if (parameterObject != null) {
postProcessParameterObject(request, parameterObject, parameters);
}

errorContext.reset();
sql.cleanup(request);
notifyListeners();
....
}

8. 到了执行中最核心的一步，也是最后一步: MappedStatement.sqlExecuteQuery()方法，它负责sql的最后执行，内部调用了SqlExecutor.executeQuery()方法，部分源码如下:

Java代码

public void executeQuery(RequestScope request, Connection conn, String sql, Object[] parameters, int skipResults, int maxResults, RowHandlerCallback callback) throws SQLException {

...
PreparedStatement ps = null;
ResultSet rs = null;
setupResultObjectFactory(request);
try {
errorContext.setMoreInfo("Check the SQL Statement (preparation failed).");
Integer rsType = request.getStatement().getResultSetType();
//初始化PreparedStatement，设置sql、参数值等

if (rsType != null) {
ps = prepareStatement(request.getSession(), conn, sql, rsType);
} else {
ps = prepareStatement(request.getSession(), conn, sql);
}
setStatementTimeout(request.getStatement(), ps);
Integer fetchSize = request.getStatement().getFetchSize();
if (fetchSize != null) {
ps.setFetchSize(fetchSize.intValue());
}
errorContext.setMoreInfo("Check the parameters (set parameters failed).");
request.getParameterMap().setParameters(request, ps, parameters);
errorContext.setMoreInfo("Check the statement (query failed).");
ps.execute(); //执行

errorContext.setMoreInfo("Check the results (failed to retrieve results).");

// ResultSet处理

rs = handleMultipleResults(ps, request, skipResults, maxResults, callback);
} finally {
try {
closeResultSet(rs);
} finally {
closeStatement(request.getSession(), ps);
}
}

}

public void executeQuery(RequestScope request, Connection conn, String sql, Object[] parameters, int skipResults, int maxResults, RowHandlerCallback callback) throws SQLException {
...
PreparedStatement ps = null;
ResultSet rs = null;
setupResultObjectFactory(request);
try {
errorContext.setMoreInfo("Check the SQL Statement (preparation failed).");
Integer rsType = request.getStatement().getResultSetType();
//初始化PreparedStatement，设置sql、参数值等
if (rsType != null) {
ps = prepareStatement(request.getSession(), conn, sql, rsType);
} else {
ps = prepareStatement(request.getSession(), conn, sql);
}
setStatementTimeout(request.getStatement(), ps);
Integer fetchSize = request.getStatement().getFetchSize();
if (fetchSize != null) {
ps.setFetchSize(fetchSize.intValue());
}
errorContext.setMoreInfo("Check the parameters (set parameters failed).");
request.getParameterMap().setParameters(request, ps, parameters);
errorContext.setMoreInfo("Check the statement (query failed).");
ps.execute(); //执行
errorContext.setMoreInfo("Check the results (failed to retrieve results).");

// ResultSet处理
rs = handleMultipleResults(ps, request, skipResults, maxResults, callback);
} finally {
try {
closeResultSet(rs);
} finally {
closeStatement(request.getSession(), ps);
}
}

}

上面这段代码大家会非常熟悉，和本文开始处的代码很相似，ibatis归根到底，是对JDBC操作一定程度上的封装而已。

下面在总体上概括sql的一般执行过程:

SqlMapClientImpl接到请求后，创建SqlMapSessionImpl对象(ThreadLocal，保证线程安全)，SqlMapSessionImpl交由内部的代理类SqlMapExecutorDelegate执行，代理类获取相应的MappedStatement，交由MappedStatement对象执行，MappedStatement交由SqlExecutor执行，最终使用JDBC方式执行sql。

小结

ibatis源码规模较小，整体设计思路清晰，阅读ibatis源码可以按以下思路进行:

1. 了解ibatis框架的整体目标，用于解决哪些问题。

2. ibatis如何解决这些问题，带着问题去学习。

3. 了解ibatis框架的核心接口和整体设计思路。

4. 抓住ibatis核心流程: 初始化和请求处理流程。

5. 详细ibatis框架的关键细节实现，如ibatis中的配置文件解析，参数和结果映射等。