Executor接口基于以下方法可以完成增，删，改查以及事务处理等操作。事实上，mybatis中的所有数据库操作是通过调用这些方法实现的。
public interface Executor {
ResultHandler NO_RESULT_HANDLER = null;
// 数据更新操作，其中数据的增加、删除、更新均可由该方法实现
int update(MappedStatement ms, Object parameter) throws SQLException;
// 数据查询操作，返回结果为列表形式
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey cacheKey, BoundSql boundSql) throws SQLException;
// 数据查询操作，返回结果为列表形式
<E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException;
// 数据查询操作，返回结果为游标形式
<E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException;
// 清理缓存
List<BatchResult> flushStatements() throws SQLException;
// 提交事务
void commit(boolean required) throws SQLException;
// 回滚事务
void rollback(boolean required) throws SQLException;
// 创建当前查询的缓存键值
CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql);
// 本地缓存是否有指定值
boolean isCached(MappedStatement ms, CacheKey key);
// 清理本地缓存
void clearLocalCache();
// 懒加载
void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType);
// 获取事务
Transaction getTransaction();
// 关闭执行器
void close(boolean forceRollback);
// 判断执行器是否关闭
boolean isClosed();
// 设置执行器包装
void setExecutorWrapper(Executor executor);
}BaseExecutor是一个抽象类，并用到了模板模式，实现了其子类的一些共有的基础功能，而将与子类直接相关的操作交给子类处理。
public abstract class BaseExecutor implements Executor {
private static final Log log = LogFactory.getLog(BaseExecutor.class);
protected Transaction transaction;
protected Executor wrapper;
protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
// 查询操作的结果缓存
protected PerpetualCache localCache;
// Callable查询的输出参数缓存
protected PerpetualCache localOutputParameterCache;
protected Configuration configuration;
protected int queryStack;
private boolean closed;
@Override
public int update(MappedStatement ms, Object parameter) throws SQLException {
ErrorContext.instance().resource(ms.getResource())
.activity("executing an update").object(ms.getId());
if (closed) {
// 执行器已经关闭
throw new ExecutorException("Executor was closed.");
}
// 清理本地缓存
clearLocalCache();
// 返回调用子类进行操作
return doUpdate(ms, parameter);
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameter);
// 生成缓存的键
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
@SuppressWarnings("unchecked")
@Override
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());
if (closed) {
// 执行器已经关闭
throw new ExecutorException("Executor was closed.");
}
if (queryStack == 0 && ms.isFlushCacheRequired()) { // 新的查询栈且要求清除缓存
// 清除一级缓存
clearLocalCache();
}
List<E> list;
try {
queryStack++;
// 尝试从本地缓存获取结果
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
// 本地缓存中有结果，则对于CALLABLE语句还需要绑定到IN/INOUT参数上
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
// 本地缓存没有结果，故需要查询数据库
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
queryStack--;
}
if (queryStack == 0) {
// 懒加载操作的处理
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
deferredLoads.clear();
// 如果本地缓存的作用域为STATEMENT，则立刻清除本地缓存
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
clearLocalCache();
}
}
return list;
}
@Override
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
// 创建CacheKey，并将所有查询参数依次更新写入
CacheKey cacheKey = new CacheKey();
cacheKey.update(ms.getId());
cacheKey.update(rowBounds.getOffset());
cacheKey.update(rowBounds.getLimit());
cacheKey.update(boundSql.getSql());
List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
for (ParameterMapping parameterMapping : parameterMappings) {
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) {
value = boundSql.getAdditionalParameter(propertyName);
} else if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
cacheKey.update(value);
}
}
if (configuration.getEnvironment() != null) {
cacheKey.update(configuration.getEnvironment().getId());
}
return cacheKey;
}
}BaseExecutor有四个实现类：
CloseExecutor：仅表明自身已经关闭的执行器，没有其他实际功能SimpleExecutor: 一个最为简单的执行器BatchExecutor：支持批量执行功能的执行器ReuseExecutor: 支持Statement对象复用的执行器。
SimpleExecutor，BatchExecutor，ReuseExecutor 这三个执行器的选择是在mybatis的配置文件中进行的，可选的值由session包中的ExecutorType定义，这三个执行器主要基于StatementHandler完成创建Statement对象，绑定参数等工作。
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