BaseExecutor是Executor的一个子类，是一个抽象类，其实现了接口Executor的部分方法，并提供了三个抽象方法doUpdate、doFlushStatements和doQuery在他的子类SimpleExecutor、ReuseExecutor和BatchExecutor中实现。
BaseExecutor也算是一个模板类，几个抽象方法在子类中实现，具体选用哪个子类SimpleExecutor、ReuseExecutor和BatchExecutor实现，可以在Mybatis的配置文件中进行配，配置如下：
<settings>
<setting name="defaultExecutorType" value="REUSE"/> <!--SIMPLE、REUSE、BATCH-->
</settings>
配置之后在Configuration类中的newExecutor（）函数会选择具体使用的子类，实现如下：
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
//根据executorType来选择实现子类
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
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);
}
//cacheEnabled默认是true，所以还是会使用CachingExecutor，不知何意
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
BaseExecutor的实现源码及解析如下如下：
/**
* @author Clinton Begin
*/
public abstract class BaseExecutor implements Executor {
private static final Log log = LogFactory.getLog(BaseExecutor.class);
//事务
protected Transaction transaction;
//executor
protected Executor wrapper;
//线程安全队列
protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
//缓存 永久缓存 本地缓存
protected PerpetualCache localCache;
//
protected PerpetualCache localOutputParameterCache;
//mybatis的配置信息
protected Configuration configuration;
//查询堆栈
protected int queryStack = 0;
private boolean closed;
//构造函数
protected BaseExecutor(Configuration configuration, Transaction transaction) {
this.transaction = transaction;
this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>();
this.localCache = new PerpetualCache("LocalCache");
this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
this.closed = false;
this.configuration = configuration;
this.wrapper = this;
}
public Transaction getTransaction() {
if (closed) throw new ExecutorException("Executor was closed.");
return transaction;
}
public void close(boolean forceRollback) {
try {
try {
rollback(forceRollback);
} finally {
if (transaction != null) transaction.close();
}
} catch (SQLException e) {
// Ignore. There's nothing that can be done at this point.
log.warn("Unexpected exception on closing transaction. Cause: " + e);
} finally {
transaction = null;
deferredLoads = null;
localCache = null;
localOutputParameterCache = null;
closed = true;
}
}
public boolean isClosed() {
return closed;
}
//SqlSession的update/insert/delete会调用此方法
public int update(MappedStatement ms, Object parameter) throws SQLException {
ErrorContext.instance().resource(ms.getResource()).activity("executing an update").object(ms.getId());
if (closed) throw new ExecutorException("Executor was closed.");
//先清局部缓存，再更新，如何更新由子类实现，模板方法模式
clearLocalCache();
return doUpdate(ms, parameter);
}
public List<BatchResult> flushStatements() throws SQLException {
return flushStatements(false);
}
public List<BatchResult> flushStatements(boolean isRollBack) throws SQLException {
if (closed) throw new ExecutorException("Executor was closed.");
return doFlushStatements(isRollBack);
}
//SqlSession.selectList会调用此方法
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//得到绑定sql
BoundSql boundSql = ms.getBoundSql(parameter);
//创建缓存key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//查询
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
@SuppressWarnings("unchecked")
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());
//如果已经关闭，报错
if (closed) throw new ExecutorException("Executor was closed.");
//先清局部缓存，再查询，但仅仅查询堆栈为0才清，为了处理递归调用
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
//加一，这样递归调用到上面的时候就不会再清局部缓存了
queryStack++;
//根据cachekey从localCache去查
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
//如果查到localCache缓存，处理localOutputParameterCache
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//从数据库查
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
//清空堆栈
queryStack--;
}
if (queryStack == 0) {
//延迟加载队列中所有元素
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
//清空延迟加载队列
deferredLoads.clear(); // issue #601
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
//如果是statement，清本地缓存
clearLocalCache(); // issue #482
}
}
return list;
}
//延迟加载
public void deferLoad(MappedStatement ms, MetaObject resultObject, String property, CacheKey key, Class<?> targetType) {
if (closed) throw new ExecutorException("Executor was closed.");
DeferredLoad deferredLoad = new DeferredLoad(resultObject, property, key, localCache, configuration, targetType);
//如果能加载则立即加载，否则加入到延迟加载队列中
if (deferredLoad.canLoad()) {
deferredLoad.load();
} else {
//这里怎么又new了一个新的，性能有点问题
deferredLoads.add(new DeferredLoad(resultObject, property, key, localCache, configuration, targetType));
}
}
//创建缓存key
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
if (closed) throw new ExecutorException("Executor was closed.");
CacheKey cacheKey = new CacheKey();
//MyBatis 对于其 Key 的生成采取规则为：[mappedStementId + offset + limit + SQL + queryParams + environment]生成一个哈希码
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 (int i = 0; i < parameterMappings.size(); i++) { // mimic DefaultParameterHandler logic
ParameterMapping parameterMapping = parameterMappings.get(i);
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);
}
}
return cacheKey;
}
public boolean isCached(MappedStatement ms, CacheKey key) {
return localCache.getObject(key) != null;
}
public void commit(boolean required) throws SQLException {
if (closed) throw new ExecutorException("Cannot commit, transaction is already closed");
clearLocalCache();
flushStatements();
if (required) {
transaction.commit();
}
}
public void rollback(boolean required) throws SQLException {
if (!closed) {
try {
clearLocalCache();
flushStatements(true);
} finally {
if (required) {
transaction.rollback();
}
}
}
}
//清空本地缓存，一个map结构
public void clearLocalCache() {
if (!closed) {
localCache.clear();
localOutputParameterCache.clear();
}
}
//子类中实现
protected abstract int doUpdate(MappedStatement ms, Object parameter)
throws SQLException;
//子类中实现
protected abstract List<BatchResult> doFlushStatements(boolean isRollback)
throws SQLException;
//子类中实现
protected abstract <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql)
throws SQLException;
protected void closeStatement(Statement statement) {
if (statement != null) {
try {
statement.close();
} catch (SQLException e) {
// ignore
}
}
}
//处理存储过程的out参数
private void handleLocallyCachedOutputParameters(MappedStatement ms, CacheKey key, Object parameter, BoundSql boundSql) {
if (ms.getStatementType() == StatementType.CALLABLE) {
final Object cachedParameter = localOutputParameterCache.getObject(key);
if (cachedParameter != null && parameter != null) {
final MetaObject metaCachedParameter = configuration.newMetaObject(cachedParameter);
final MetaObject metaParameter = configuration.newMetaObject(parameter);
for (ParameterMapping parameterMapping : boundSql.getParameterMappings()) {
if (parameterMapping.getMode() != ParameterMode.IN) {
final String parameterName = parameterMapping.getProperty();
final Object cachedValue = metaCachedParameter.getValue(parameterName);
metaParameter.setValue(parameterName, cachedValue);
}
}
}
}
}
//从数据库中查
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
//向缓存中放入占位符
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
//清除占位符
localCache.removeObject(key);
}
//加入缓存
localCache.putObject(key, list);
//如果是存储过程，OUT参数也加入缓存
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
protected Connection getConnection(Log statementLog) throws SQLException {
Connection connection = transaction.getConnection();
if (statementLog.isDebugEnabled()) {
return ConnectionLogger.newInstance(connection, statementLog, queryStack);
} else {
return connection;
}
}
public void setExecutorWrapper(Executor wrapper) {
this.wrapper = wrapper;
}
//延迟加载
private static class DeferredLoad {
private final MetaObject resultObject;
private final String property;
private final Class<?> targetType;
private final CacheKey key;
private final PerpetualCache localCache;
private final ObjectFactory objectFactory;
private final ResultExtractor resultExtractor;
public DeferredLoad(MetaObject resultObject,
String property,
CacheKey key,
PerpetualCache localCache,
Configuration configuration,
Class<?> targetType) { // issue #781
this.resultObject = resultObject;
this.property = property;
this.key = key;
this.localCache = localCache;
this.objectFactory = configuration.getObjectFactory();
this.resultExtractor = new ResultExtractor(configuration, objectFactory);
this.targetType = targetType;
}
//缓存中找到，且不为占位符，代表可以加载
public boolean canLoad() {
return localCache.getObject(key) != null && localCache.getObject(key) != EXECUTION_PLACEHOLDER;
}
//加载
public void load() {
@SuppressWarnings( "unchecked" ) // we suppose we get back a List
List<Object> list = (List<Object>) localCache.getObject(key);
Object value = resultExtractor.extractObjectFromList(list, targetType);
resultObject.setValue(property, value);
}
}
}
本文原文地址：https://blog.csdn.net/qq924862077/article/details/52634178