SQL开发知识：SQL语句解析执行的过程及原理

/**
* 通过sqlSessionFactory.openSession进行获取sqlSession对象
* 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSession()
*/
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}

/**
* 通过数据源去获取SqlSession
* 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(ExecutorType, TransactionIsolationLevel, boolean)
*/
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level,
boolean autoCommit) {
Transaction tx = null;
try {
// 获取环境变量
final Environment environment = configuration.getEnvironment();
// 获取事务工厂
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
// 获取一个事务
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
// 获取执行器，这边获得的执行器已经代理拦截器的功能
final Executor executor = configuration.newExecutor(tx, execType);
// 根据获取的执行器创建SqlSession
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();
}
}

/**
* 获取执行器
* 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSessionFactory.openSessionFromDataSource(ExecutorType, TransactionIsolationLevel, boolean)
*/
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
// 默认使用SIMPLE的执行器
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;

if (ExecutorType.BATCH == executorType) {
// 批量的执行器
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
// 可重复使用的执行器
executor = new ReuseExecutor(this, transaction);
} else {
// 简单的sql执行器
executor = new SimpleExecutor(this, transaction);
}

// 判断Mybatis的全局配置文件是否开启二级缓存
if (cacheEnabled) {
// 开启缓存，吧executor包装为CachingExecutor
executor = new CachingExecutor(executor);
}

// 插件的调用：责任链模式
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}

/**
* 查询的入口方法
* 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(String, Object)
*/
public <T> T selectOne(String statement, Object parameter) {
// Popular vote was to return null on 0 results and throw exception on too many.
// 查询数据
List<T> list = this.<T>selectList(statement, parameter);

// 长度为1，拿第一个
if (list.size() == 1) {
return list.get(0);
} else if (list.size() > 1) {
// 长度大于一，抛异常
throw new TooManyResultsException(
“Expected one result (or null) to be returned by selectOne(), but found: ” + list.size());
} else {
// 没有拿到返回null
return null;
}
}

/**
* 查询数据
* 源码位置：org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(String, Object, RowBounds)
*/
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
// 通过statement去全局配置文件中获取MappedStatement（得到mapper中增删改查的节点）
MappedStatement ms = configuration.getMappedStatement(statement);

// 通过执行器去执行SQL
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();
}
}

/**
* 执行查询操作的准备工作
* 源码位置：org.apache.ibatis.executor.CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler)
*/
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds,
ResultHandler resultHandler) throws SQLException {

// 通过参数进行sql解析
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

/**
* 执行查询操作的准备工作
* 源码位置：org.apache.ibatis.executor.CachingExecutor.query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql)
*/
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds,
ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
// 判断sql是否开启了缓存 <cache></cache>
Cache cache = ms.getCache();

// 有缓存
if (cache != null) {
// 判断是否需要刷新缓存
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, boundSql);
@SuppressWarnings(“unchecked”)

// 去二级缓存中获取（装饰者模式）
List<E> list = (List<E>) tcm.getObject(cache, key);

// 二级缓存没有找到
if (list == null) {

// 查询数据，并放入缓存
list = delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
// 查询数据
return delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

/**
* 一级缓存查询的调用
* 源码位置：org.apache.ibatis.executor.BaseExecutor.query(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql)
*/
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,
CacheKey key, BoundSql boundSql) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity(“executing a query”).object(ms.getId());

// 已经关闭了，抛异常
if (closed) {
throw new ExecutorException(“Executor was closed.”);
}

// 清空本地缓存
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
// 从一级缓存中获取数据
queryStack++;
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
// 缓存里面有，进行处理
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
// 缓存没有，进行查询
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack–;
}
if (queryStack == 0) {
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
// issue #601
deferredLoads.clear();
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
// issue #482
clearLocalCache();
}
}
return list;
}

/**
* 在数据库中查询
* 源码位置：org.apache.ibatis.executor.BaseExecutor.queryFromDatabase(MappedStatement, Object, RowBounds, ResultHandler, CacheKey, BoundSql)
*/
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds,
ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
// 去数据库查询
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
localCache.removeObject(key);
}

// 一级缓存进行缓存
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}

/**
* 查询逻辑
* 源码位置：org.apache.ibatis.executor.SimpleExecutor.doQuery(MappedStatement, Object, RowBounds, ResultHandler, BoundSql)
*/
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler,
BoundSql boundSql) throws SQLException {
Statement stmt = null;
try {
// 得到整体的配置对象
Configuration configuration = ms.getConfiguration();

// 内部封装了ParameterHandler和ResultSetHandler
StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds,
resultHandler, boundSql);
stmt = prepareStatement(handler, ms.getStatementLog());

// 执行查询
return handler.<E>query(stmt, resultHandler);
} finally {
closeStatement(stmt);
}
}

/**
* 执行查询语句
* 源码位置：org.apache.ibatis.executor.statement.SimpleStatementHandler.query(Statement, ResultHandler)
*/
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
// 得到要执行的sql
String sql = boundSql.getSql();

// 执行sql
statement.execute(sql);

// 处理结果集
return resultSetHandler.<E>handleResultSets(statement);
}