一、mybatis接口层
-
在讲创建SqlSession之前，先来介绍下myabtis中的接口层API
SqlSession相关类结构
1、SqlSession是mybatis的核心接口之一，是myabtis接口层的主要组成部分，对外提供了mybatis常用的api。myabtis提供了两个SqlSesion接口的实现，常用的实现类是DefaultSqlSession。
2、SqlSessionFactory负责创建SqlSession，其中包含多个openSession方法的重载，具体创建在DefaultSqlSessionFactory实现类中。
1、DefaultSqlSession
- SqlSession接口的主要实现类，在DefaultSqlSession中使用了策略模式，把数据库相关的操作全部封装到了Executor接口实现中，并通过executor字段选择不同的Executor实现类，最终操作由Executor接口实现类完成。
public class DefaultSqlSession implements SqlSession {
//configuration配置对象
private final Configuration configuration;
//数据库操作相关的executor接口（策略模式）
private final Executor executor;
//是否自动提交
private final boolean autoCommit;
//当前缓存中是否有脏数据
private boolean dirty;
//为防止用户忘记关闭打开的游标对象，会通过cursorList字段记录由该sqlsession对象生成的游标对象
private List> cursorList;
2、DefaultSqlSessionFactory
- 是SqlSessionFactory接口的具体实现类，该类主要提供了两种方式创建相应的DefaultSqlSession对象。一种是通过数据源获取数据库连接，并创建Executor对象以及DefaultSqlSession对象
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
//获取配置的environment对象
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
//根据配置创建Executor 对象
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
- 另一种是用户提供数据库连接对象，DefaultSqlSessionFactory会使用该数据库连接对象创建相应的Executor对象以及DefaultSqlSession对象
private SqlSession openSessionFromConnection(ExecutorType execType, Connection connection) {
try {
boolean autoCommit;
try {
autoCommit = connection.getAutoCommit();
} catch (SQLException e) {
// Failover to true, as most poor drivers
// or databases won't support transactions
autoCommit = true;
}
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
final Transaction tx = transactionFactory.newTransaction(connection);
final Executor executor = configuration.newExecutor(tx, execType);
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
二、Executor接口
- mybatis的核心接口之一，其中定义了数据库操作的基本方法。实际应用中经常涉及的sqlSession接口的功能，都是基于Executor接口实现的。
-
mybatis中提供了Executor的接口实现，这些接口实现中涉及两种涉及模式，分别是模板方法模式和装饰器模式。其中CachingEecutor扮演了装饰器的角色，为Executor添加了二级缓存的功能。
Executor接口实现类结构
1、BaseExecutor
- Executor接口的抽象实现类，实现了Executor接口的发部分方法，其中就是使用了模板方法模式。在BaseExecutor中主要提供了缓存管理（比如一级固定模式不变的一级缓存）和事物管理的基本功能，继承BaseExecutor的子类只要实现四个基本方法来完成数据库的相关操作既可以，四个方法是doUpdate、doQuery、doQueryCursor、doFlushStatement方法。
public abstract class BaseExecutor implements Executor {
private static final Log log = LogFactory.getLog(BaseExecutor.class);
//实现事务的提交和回滚和关闭操作
protected Transaction transaction;
//封装executor对象
protected Executor wrapper;
//延迟队列
protected ConcurrentLinkedQueue deferredLoads;
//一级缓存用于该Executor对象查询结果集映射得到的结果对象
protected PerpetualCache localCache;
//一级缓存用于缓存输出类型的参数
protected PerpetualCache localOutputParameterCache;
protected Configuration configuration;
protected int queryStack;
private boolean closed;
其他方法
2、SimpleExecutor
- 继承自BaseExecutor抽象类，主要专注的实现上面介绍的四个类。例如doQuery查询方法：
@Override
public List doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
//获取配置文件
Configuration configuration = ms.getConfiguration();
//根据相应参数创建具体额StatementHandler
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());
//执行StatementHandler.query方法执行sql语句并通过ResultSetHandler完成结果集映射。
return handler.query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
Connection connection = getConnection(statementLog);
//创建Statement对象
stmt = handler.prepare(connection, transaction.getTimeout());
//处理占位符号
handler.parameterize(stmt);
return stmt;
}
2、ReuseExecutor
- ReuseExecutor提供了Statement重用的功能，ReuseExecutor中通过statementMap字段缓存使用过的Statement对象，key是sql语句，value是对应的Statement对象。
- 和SimpleExecutor的区别是在prepareStatement方法，会尝试重用StatementMap中的缓存的Statement对象。
private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
Statement stmt;
BoundSql boundSql = handler.getBoundSql();
//获取sql语句
String sql = boundSql.getSql();
if (hasStatementFor(sql)) {
stmt = getStatement(sql);
//修改超时时间
applyTransactionTimeout(stmt);
} else {
//获取数据库连接并创建Statement并缓存发哦StatementMap中
Connection connection = getConnection(statementLog);
stmt = handler.prepare(connection, transaction.getTimeout());
putStatement(sql, stmt);
}
handler.parameterize(stmt);
return stmt;
}
private Statement getStatement(String s) {
return statementMap.get(s);
}
private boolean hasStatementFor(String sql) {
try {
return statementMap.keySet().contains(sql) && !statementMap.get(sql).getConnection().isClosed();
} catch (SQLException e) {
return false;
}
}
private void putStatement(String sql, Statement stmt) {
statementMap.put(sql, stmt);
}
4、BatchExecutor
- BatchExexutor提供了批量操作sql语句的功能，也就是在可以在客户端缓存多条sql语句，并在何时的时机将多条sql语句打包发送给数据库执行，从而减少网络方面的开销。
public class BatchExecutor extends BaseExecutor {
//缓存多个statement对象，其中每一个statement对象中都缓存可许多条sql语句
private final List statementList = new ArrayList();
//记录批处理的结果
private final List batchResultList = new ArrayList();
//当前的sql语句
private String currentSql;
//当前的MappedStatement对象
private MappedStatement currentStatement;
public BatchExecutor(Configuration configuration, Transaction transaction) {
super(configuration, transaction);
}
@Override
public int doUpdate(MappedStatement ms, Object parameterObject) throws SQLException {
final Configuration configuration = ms.getConfiguration();
final StatementHandler handler = configuration.newStatementHandler(this, ms, parameterObject, RowBounds.DEFAULT, null, null);
final BoundSql boundSql = handler.getBoundSql();
final String sql = boundSql.getSql();
final Statement stmt;
//如果当前执行的sql模式与上次执行的sql模式相同且对应的MappedStatement对象相同
if (sql.equals(currentSql) && ms.equals(currentStatement)) {
int last = statementList.size() - 1;
stmt = statementList.get(last);
//参数绑定处理？占位符号
applyTransactionTimeout(stmt);
handler.parameterize(stmt);//fix Issues 322
BatchResult batchResult = batchResultList.get(last);
batchResult.addParameterObject(parameterObject);
} else {
Connection connection = getConnection(ms.getStatementLog());
stmt = handler.prepare(connection, transaction.getTimeout());
handler.parameterize(stmt); //fix Issues 322
currentSql = sql;
currentStatement = ms;
statementList.add(stmt);
batchResultList.add(new BatchResult(ms, sql, parameterObject));
}
// 底层使用statement.addBatch方法添加sql语句
handler.batch(stmt);
return BATCH_UPDATE_RETURN_VALUE;
}
5、CacheExecutor
- 是Executor接口的装饰器，为Executor对象增加二级缓存的相关功能。
- 具体实现讲缓存部分在说
三、创建SqlSession流程
- 前面介绍了关于myabtis接口层的东西，下面我们步入正题，研究创建SqlSession的流程。
- 入口：SqlSession sqlSession = sqlSessionFactory.openSession();
1、DefaultSqlSessionFactory的openSession()方法
- 最终调用openSessionFromDataSource方法，前面已经讲过
public SqlSession openSession() {
/*
configuration.getDefaultExecutorType()是获取ExecutorType类型，在Config配置文件中可以配置
有三种形式SIMPLE（普通的执行器;）, REUSE（执行器会重用预处理语句）, BATCH（执行器将重用语句并执行批量更新），
默认的是SIMPLE
*/
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
2、Configuration的newExecutor方法创建Executor对象
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
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
//如果要求缓存，生成另一种CachingExecutor(默认就是有缓存),装饰者模式,所以默认都是返回CachingExecutor
/**
* 二级缓存开关配置示例
*
*
*/
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
//此处调用插件,通过插件可以改变Executor行为
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
3、new DefaultSqlSession创建SqlSession对象
public DefaultSqlSession(Configuration configuration, Executor
executor, boolean autoCommit) {
this.configuration = configuration;
this.executor = executor;
this.dirty = false;
this.autoCommit = autoCommit;
}
四、创建SqlSession流程图
创建SqlSession流程图