Mybatis#BaseExecutor原始碼解析
摘要：
BaseExecutor是Executor的一個子類，是一個抽象類，實現介面Executor的部分方法，並提供了三個抽象方法
doUpdate
doFlushStatements
doQuery
在他的子類SimpleExecutor、ReuseExecuto...
BaseExecutor是Executor的一個子類，是一個抽象類，實現介面Executor的部分方法，並提供了三個抽象方法
- doUpdate
- doFlushStatements
- doQuery
在他的子類SimpleExecutor、ReuseExecutor和BatchExecutor中實現。
BaseExecutor也算是一個模板類，幾個抽象方法在子類中實現
具體選用哪個子類實現，可以在Mybatis的配置檔案中進行配，配置如下：
<settings> <setting name="defaultExecutorType" value="REUSE"/> <!--SIMPLE、REUSE、BATCH--> </settings>
配置之後在Configuration類中的newExecutor（）函式會選擇具體使用的子類，實現如下：
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
//根據executorType來選擇實現子類
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預設是true，所以還是會使用CachingExecutor，不知何意
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
BaseExecutor原始碼解析
/**
* @author Clinton Begin
*/
public abstract class BaseExecutor implements Executor {
protected Transaction transaction;
protected Executor wrapper;
protected ConcurrentLinkedQueue<DeferredLoad> deferredLoads;
//快取 永久快取本地快取
protected PerpetualCache localCache;
protected PerpetualCache localOutputParameterCache;
//mybatis配置資訊
protected Configuration configuration;
//查詢堆疊
protected int queryStack = 0;
private boolean closed;
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 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;
}
}
//SqlSession的update/insert/delete會呼叫該方法
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.");
//先清區域性快取，再更新，如何更新由子類實現，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會呼叫此方法
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
//得到繫結sql
BoundSql boundSql = ms.getBoundSql(parameter);
//建立快取key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
//查詢
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());
//如果已經關閉，報錯
if (closed) throw new ExecutorException("Executor was closed.");
//先清區域性快取，再查詢，但僅當查詢堆疊為0時才清，為了處理遞迴呼叫
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
//加一，這樣遞迴呼叫到上面的時候就不會再清區域性快取了
queryStack++;
//根據cachekey從localCache去查
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
//如果查到localCache快取，處理localOutputParameterCache
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
//從資料庫查
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
} finally {
//清空堆疊
queryStack--;
}
if (queryStack == 0) {
//延遲載入佇列中所有元素
for (DeferredLoad deferredLoad : deferredLoads) {
deferredLoad.load();
}
//清空延遲載入佇列
deferredLoads.clear(); // issue #601
if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
//如果是statement，清本地快取
clearLocalCache(); // issue #482
}
}
return list;
}
//延遲載入
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);
//如果能載入則立即載入，否則加入到延遲載入佇列中
if (deferredLoad.canLoad()) {
deferredLoad.load();
} else {
//這裡怎麼又new了一個新的，效能有點問題
deferredLoads.add(new DeferredLoad(resultObject, property, key, localCache, configuration, targetType));
}
}
//建立快取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 對於其 Key 的生成採取規則為：[mappedStementId + offset + limit + SQL + queryParams + environment]生成一個雜湊碼
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 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();
}
}
}
}
//清空本地快取，一個map結構
public void clearLocalCache() {
if (!closed) {
localCache.clear();
localOutputParameterCache.clear();
}
}
//子類中實現
protected abstract int doUpdate(MappedStatement ms, Object parameter)
throws SQLException;
//子類中實現
protected abstract List<BatchResult> doFlushStatements(boolean isRollback)
throws SQLException;
//子類中實現
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
}
}
}
//處理儲存過程的out引數
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);
}
}
}
}
}
//從資料庫中查
private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> list;
//向快取中放入佔位符
localCache.putObject(key, EXECUTION_PLACEHOLDER);
try {
list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
} finally {
//清除佔位符
localCache.removeObject(key);
}
//加入快取
localCache.putObject(key, list);
//如果是儲存過程，OUT引數也加入快取
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;
}
//延遲載入
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;
}
//快取中找到，且不為佔位符，代表可以載入
public boolean canLoad() {
return localCache.getObject(key) != null && localCache.getObject(key) != EXECUTION_PLACEHOLDER;
}
//載入
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);
}
}
}