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) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      //從localCache緩存里查數(shù)據(jù)，沒(méi)有就去查數(shù)據(jù)庫(kù)
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        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;
  }

public abstract class BaseExecutor implements Executor {


  protected PerpetualCache localCache;

public class PerpetualCache implements Cache {

  private final String id;

  private Map<Object, Object> cache = new HashMap<Object, Object>();

  public PerpetualCache(String id) {
    this.id = id;
  }

  @Override
  public String getId() {
    return id;
  }

  @Override
  public int getSize() {
    return cache.size();
  }

  @Override
  public void putObject(Object key, Object value) {
    cache.put(key, value);
  }

  @Override
  public Object getObject(Object key) {
    return cache.get(key);
  }

<settings>
<setting name="cacheEnabled" value="true"/>
</settings>

<cache></cache>

 public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, parameterObject, boundSql);
        @SuppressWarnings("unchecked")
        List<E> list = (List<E>) tcm.getObject(cache, key);
        if (list == null) {
          list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
          tcm.putObject(cache, key, list); // issue #578 and #116
        }
        return list;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

public class TransactionalCacheManager {

  private final Map<Cache, TransactionalCache> transactionalCaches = new HashMap<Cache, TransactionalCache>();

  public void clear(Cache cache) {
    getTransactionalCache(cache).clear();
  }

  public Object getObject(Cache cache, CacheKey key) {
    return getTransactionalCache(cache).getObject(key);
  }
  
  public void putObject(Cache cache, CacheKey key, Object value) {
    getTransactionalCache(cache).putObject(key, value);
  }

  public void commit() {
    for (TransactionalCache txCache : transactionalCaches.values()) {
      txCache.commit();
    }
  }

  public void rollback() {
    for (TransactionalCache txCache : transactionalCaches.values()) {
      txCache.rollback();
    }
  }

  private TransactionalCache getTransactionalCache(Cache cache) {
    TransactionalCache txCache = transactionalCaches.get(cache);
    if (txCache == null) {
      txCache = new TransactionalCache(cache);
      transactionalCaches.put(cache, txCache);
    }
    return txCache;
  }

}

public class TransactionalCache implements Cache {

  private static final Log log = LogFactory.getLog(TransactionalCache.class);

  // 真正的緩存對(duì)象
  private final Cache delegate;
  private boolean clearOnCommit;
  //在事務(wù)被提交前，所有從數(shù)據(jù)庫(kù)中查詢的結(jié)果將緩存在此集合中
  private final Map<Object, Object> entriesToAddOnCommit;
  //在事務(wù)被提交前，當(dāng)緩存未命中時(shí)，CacheKey 將會(huì)被存儲(chǔ)在此集合中
  private final Set<Object> entriesMissedInCache;

  public TransactionalCache(Cache delegate) {
    this.delegate = delegate;
    this.clearOnCommit = false;
    this.entriesToAddOnCommit = new HashMap<Object, Object>();
    this.entriesMissedInCache = new HashSet<Object>();
  }

  @Override
  public String getId() {
    return delegate.getId();
  }

  @Override
  public int getSize() {
    return delegate.getSize();
  }

  @Override
  public Object getObject(Object key) {
    // issue #116
    //獲取緩存的時(shí)候從delegate里獲取的
    Object object = delegate.getObject(key);
    if (object == null) {
      //緩存未命中，將key存入entriesMissedInCache.
      entriesMissedInCache.add(key);
    }
    // issue #146
    if (clearOnCommit) {
      return null;
    } else {
      return object;
    }
  }

  @Override
  public ReadWriteLock getReadWriteLock() {
    return null;
  }

  @Override
  public void putObject(Object key, Object object) {
    //put的時(shí)候只是將數(shù)據(jù)庫(kù)的數(shù)據(jù)放入到了entriesToAddOnCommit
    entriesToAddOnCommit.put(key, object);
  }

  @Override
  public Object removeObject(Object key) {
    return null;
  }

  @Override
  public void clear() {
    clearOnCommit = true;
    entriesToAddOnCommit.clear();
  }

  public void commit() {
    if (clearOnCommit) {
      delegate.clear();
    }
    //刷新未緩存的結(jié)果到delegate中去
    flushPendingEntries();
    reset();
  }

  public void rollback() {
    unlockMissedEntries();
    reset();
  }

  private void reset() {
    clearOnCommit = false;
    entriesToAddOnCommit.clear();
    entriesMissedInCache.clear();
  }

  private void flushPendingEntries() {
    for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) {
      delegate.putObject(entry.getKey(), entry.getValue());
    }
    for (Object entry : entriesMissedInCache) {
      if (!entriesToAddOnCommit.containsKey(entry)) {
        delegate.putObject(entry, null);
      }
    }
  }

  private void unlockMissedEntries() {
    for (Object entry : entriesMissedInCache) {
      try {
        delegate.removeObject(entry);
      } catch (Exception e) {
        log.warn("Unexpected exception while notifiying a rollback to the cache adapter."
            + "Consider upgrading your cache adapter to the latest version.  Cause: " + e);
      }
    }
  }

}