创建SqlSession
前面的两篇文章我们已经得到了SqlSessionFactory，那么SqlSession将由SqlSessionFactory进行创建。
SqlSession sqlSession=sqlSessionFactory.openSession();
我们就来
看看这个SqlSessionFactory的 openSession方法是如何创建SqlSession对象的。根据上面的分析，这里的SqlSessionFactory类型对象其实是一个DefaultSqlSessionFactory对象，因此，需要到DefaultSqlSessionFactory类中去看openSession方法。
@Override
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
调用了openSessionFromDataSource方法，并且第一个参数获取了默认的执行器类型，第二个参数为null,第三个参数为false,看看这个默认的执行器类型是啥
默认的执行器类型SIMPLE，我们跟进openSessionFromDataSource方法
/**
* ExecutorType 指定Executor的类型，分为三种：SIMPLE, REUSE, BATCH，默认使用的是SIMPLE
* TransactionIsolationLevel 指定事务隔离级别，使用null,则表示使用数据库默认的事务隔离界别
* autoCommit 是否自动提交，传过来的参数为false，表示不自动提交
*/
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
// 获取配置中的环境信息，包括了数据源信息、事务等
final Environment environment = configuration.getEnvironment();
// 创建事务工厂
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
// 创建事务，配置事务属性
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
// 创建Executor，即执行器
// 它是真正用来Java和数据库交互操作的类，后面会展开说。
final Executor executor = configuration.newExecutor(tx, execType);
// 创建DefaultSqlSession对象返回，其实现了SqlSession接口
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx);
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
主要包含以下几个步骤：
- 首先从configuration获取Environment对象，里面主要包含了DataSource和TransactionFactory对象
- 创建TransactionFactory
- 创建Transaction
- 从configuration获取Executor
- 构造DefaultSqlSession对象
我们先来看看常规的environment配置
//配置environment环境
<environments default="development">
<environment id="development">
/** 事务配置 type= JDBC、MANAGED
* 1.JDBC:这个配置直接简单使用了JDBC的提交和回滚设置。它依赖于从数据源得到的连接来管理事务范围。
* 2.MANAGED:这个配置几乎没做什么。它从来不提交或回滚一个连接。
*/
<transactionManager type="JDBC" />
/** 数据源类型：type = UNPOOLED、POOLED、JNDI
* 1.UNPOOLED：这个数据源的实现是每次被请求时简单打开和关闭连接。
* 2.POOLED：这是JDBC连接对象的数据源连接池的实现。
* 3.JNDI：这个数据源的实现是为了使用如Spring或应用服务器这类的容器
*/
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost:3306/xhm" />
<property name="username" value="root" />
<property name="password" value="root" />
//默认连接事务隔离级别
<property name="defaultTransactionIsolationLevel" value=""/>
</dataSource>
</environment>
</environments>
还记得前面文章是怎么解析environments的吗，Mybaits 源码解析 （二）----- 根据配置文件创建SqlSessionFactory（Configuration的创建过程），我们简单的回顾一下
private void environmentsElement(XNode context) throws Exception {
if (context != null) {
if (environment == null) {
// 获取 default 属性
environment = context.getStringAttribute("default");
}
for (XNode child : context.getChildren()) {
// 获取 id 属性
String id = child.getStringAttribute("id");
/*
* 检测当前 environment 节点的 id 与其父节点 environments 的属性 default
* 内容是否一致，一致则返回 true，否则返回 false
* 将其default属性值与子元素environment的id属性值相等的子元素设置为当前使用的Environment对象
*/
if (isSpecifiedEnvironment(id)) {
// 将environment中的transactionManager标签转换为TransactionFactory对象
TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
// 将environment中的dataSource标签转换为DataSourceFactory对象
DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
// 创建 DataSource 对象
DataSource dataSource = dsFactory.getDataSource();
Environment.Builder environmentBuilder = new Environment.Builder(id)
.transactionFactory(txFactory)
.dataSource(dataSource);
// 构建 Environment 对象，并设置到 configuration 中
configuration.setEnvironment(environmentBuilder.build());
}
}
}
}
private TransactionFactory transactionManagerElement(XNode context) throws Exception {
if (context != null) {
String type = context.getStringAttribute("type");
Properties props = context.getChildrenAsProperties();
//通过别名获取Class,并实例化
TransactionFactory factory = (TransactionFactory)this.resolveClass(type).newInstance();
factory.setProperties(props);
return factory;
} else {
throw new BuilderException("Environment declaration requires a TransactionFactory.");
}
}
private DataSourceFactory dataSourceElement(XNode context) throws Exception {
if (context != null) {
String type = context.getStringAttribute("type");
//通过别名获取Class,并实例化
Properties props = context.getChildrenAsProperties();
DataSourceFactory factory = (DataSourceFactory)this.resolveClass(type).newInstance();
factory.setProperties(props);
return factory;
} else {
throw new BuilderException("Environment declaration requires a DataSourceFactory.");
}
}
获取TransactionFactory
我们的environment配置中transactionManager type="JDBC"和dataSource type="POOLED"，则生成的transactionManager为JdbcTransactionFactory，DataSourceFactory为PooledDataSourceFactory
我们回到openSessionFromDataSource，接着看看getTransactionFactoryFromEnvironment方法
private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
return (TransactionFactory)(environment != null && environment.getTransactionFactory() != null ? environment.getTransactionFactory() : new ManagedTransactionFactory());
}
创建Transaction
很明显 environment.getTransactionFactory() 就是JdbcTransactionFactory，看看这个工厂是如何创建Transaction的
public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
return new JdbcTransaction(ds, level, autoCommit);
}
直接通过工厂方法创建了一个JdbcTransaction对象，并传参DataSource ，事务隔离级别null，自动提交false三个参数，我们来看看JdbcTransaction
public class JdbcTransaction implements Transaction {
//数据库连接对象
protected Connection connection;
//数据库DataSource
protected DataSource dataSource;
//数据库隔离级别
protected TransactionIsolationLevel level;
//是否自动提交
protected boolean autoCommmit;
public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
//设置dataSource和隔离级别，是否自动提交属性
//这里隔离级别传过来的是null,表示使用数据库默认隔离级别，自动提交为false，表示不自动提交
this.dataSource = ds;
this.level = desiredLevel;
this.autoCommmit = desiredAutoCommit;
}
public Connection getConnection() throws SQLException {
if (this.connection == null) {
this.openConnection();
}
return this.connection;
}
//提交功能是通过Connection去完成的
public void commit() throws SQLException {
if (this.connection != null && !this.connection.getAutoCommit()) {
if (log.isDebugEnabled()) {
log.debug("Committing JDBC Connection [" + this.connection + "]");
}
this.connection.commit();
}
}
//回滚功能是通过Connection去完成的
public void rollback() throws SQLException {
if (this.connection != null && !this.connection.getAutoCommit()) {
if (log.isDebugEnabled()) {
log.debug("Rolling back JDBC Connection [" + this.connection + "]");
}
this.connection.rollback();
}
}
//关闭功能是通过Connection去完成的
public void close() throws SQLException {
if (this.connection != null) {
this.resetAutoCommit();
if (log.isDebugEnabled()) {
log.debug("Closing JDBC Connection [" + this.connection + "]");
}
this.connection.close();
}
}
//获取连接是通过dataSource来完成的
protected void openConnection() throws SQLException {
if (log.isDebugEnabled()) {
log.debug("Opening JDBC Connection");
}
this.connection = this.dataSource.getConnection();
if (this.level != null) {
this.connection.setTransactionIsolation(this.level.getLevel());
}
this.setDesiredAutoCommit(this.autoCommmit);
}
}
JdbcTransaction主要维护了一个默认autoCommit为false的Connection对象，对事物的提交，回滚，关闭等都是接见通过Connection完成的。
创建Executor
//创建一个执行器，默认是SIMPLE
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
//根据executorType来创建相应的执行器,Configuration默认是SIMPLE
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
//创建SimpleExecutor实例，并且包含Configuration和transaction属性
executor = new SimpleExecutor(this, transaction);
}
//如果要求缓存，生成另一种CachingExecutor,装饰者模式,默认都是返回CachingExecutor
/**
* 二级缓存开关配置示例
* <settings>
* <setting name="cacheEnabled" value="true"/>
* </settings>
*/
if (cacheEnabled) {
//CachingExecutor使用装饰器模式，将executor的功能添加上了二级缓存的功能，二级缓存会单独文章来讲
executor = new CachingExecutor(executor);
}
//此处调用插件,通过插件可以改变Executor行为，此处我们后面单独文章讲
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
executor包含了Configuration和刚刚创建的Transaction，默认的执行器为SimpleExecutor，如果开启了二级缓存(默认开启)，则CachingExecutor会包装SimpleExecutor，然后依次调用拦截器的plugin方法返回一个被代理过的Executor对象。
CachingExecutor 对象里面包含了刚创建的SimpleExecutor，后面文章我们会及具体讲这个类
public class CachingExecutor implements Executor {
private Executor delegate;
private TransactionalCacheManager tcm = new TransactionalCacheManager();
public CachingExecutor(Executor delegate) {
this.delegate = delegate;
delegate.setExecutorWrapper(this);
}
//略
}
构造DefaultSqlSession对象
new DefaultSqlSession(this.configuration, executor, autoCommit);
传参configuration和刚生成的executor，我们来简单看看
public class DefaultSqlSession implements SqlSession {
/**
* mybatis全局配置新
*/
private final Configuration configuration;
/**
* SQL执行器
*/
private final Executor executor;
/**
* 是否自动提交
*/
private final boolean autoCommit;
private List<Cursor<?>> cursorList;
public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
this.configuration = configuration;
this.executor = executor;
this.dirty = false;
this.autoCommit = autoCommit;
}
@Override
public <T> T selectOne(String statement) {
return this.<T>selectOne(statement, null);
}
@Override
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
List<T> list = this.<T>selectList(statement, parameter);
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
} else {
return null;
}
}
@Override
public <E> List<E> selectList(String statement) {
return this.selectList(statement, null);
}
@Override
public <E> List<E> selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
@Override
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
//略....update等方法
}
SqlSession的所有查询接口最后都归结位Exector的方法调用。后面文章我们来分析其调用流程