MyBatis从入门到精通—源码剖析之二级缓存细节

Posted 2022-07-19 共饮一杯无

⼆级缓存构建在⼀级缓存之上，在收到查询请求时，MyBatis ⾸先会查询⼆级缓存，若⼆级缓存未命中，再去查询⼀级缓存，⼀级缓存没有，再查询数据库。
⼆级缓存------》 ⼀级缓存------》数据库
与⼀级缓存不同，⼆级缓存和具体的命名空间绑定，⼀个Mapper中有⼀个Cache，相同Mapper中的MappedStatement共⽤⼀个Cache，⼀级缓存则是和 SqlSession 绑定。

启用二级缓存

分为三步⾛：

1. 开启全局⼆级缓存配置：

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

2. 在需要使⽤⼆级缓存的Mapper配置⽂件中配置标签

   <cache></cache>

3. 在具体CURD标签上配置 useCache=true

   <select id="findById" resultType="com.zjq.pojo.User" useCache="true">
   select * from user where id = #id
   </select>

   标签 < cache/> 的解析

   根据之前的mybatis源码剖析，xml的解析⼯作主要交给XMLConfigBuilder.parse()⽅法来实现

   public class XMLConfigBuilder extends BaseBuilder      
   public Configuration parse() 
   if (parsed) 
     throw new BuilderException("Each XMLConfigBuilder can only be used once.");
   
   parsed = true;
   parseConfiguration(parser.evalNode("/configuration"));
   return configuration;
   
   
   // 既然是在xml中添加的，那么我们就直接看关于mappers标签的解析
   private void parseConfiguration(XNode root) 
   try 
     //issue #117 read properties first
     propertiesElement(root.evalNode("properties"));
     Properties settings = settingsAsProperties(root.evalNode("settings"));
     loadCustomVfs(settings);
     typeAliasesElement(root.evalNode("typeAliases"));
     pluginElement(root.evalNode("plugins"));
     objectFactoryElement(root.evalNode("objectFactory"));
     objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
     reflectorFactoryElement(root.evalNode("reflectorFactory"));
     settingsElement(settings);
     // read it after objectFactory and objectWrapperFactory issue #631
     environmentsElement(root.evalNode("environments"));
     databaseIdProviderElement(root.evalNode("databaseIdProvider"));
     typeHandlerElement(root.evalNode("typeHandlers"));
     // 这⾥这里，继续进入这个方法跟进去看看
     mapperElement(root.evalNode("mappers"));
    catch (Exception e) 
     throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
   
   
   
   private void mapperElement(XNode parent) throws Exception 
   if (parent != null) 
     for (XNode child : parent.getChildren()) 
       if ("package".equals(child.getName())) 
         String mapperPackage = child.getStringAttribute("name");
         configuration.addMappers(mapperPackage);
        else 
         String resource = child.getStringAttribute("resource");
         String url = child.getStringAttribute("url");
         String mapperClass = child.getStringAttribute("class");
         // 按照我们本例的配置，则直接⾛该if判断
         if (resource != null && url == null && mapperClass == null) 
           ErrorContext.instance().resource(resource);
           InputStream inputStream = Resources.getResourceAsStream(resource);
           XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
           //进入XMLMapperBuilder实例，并执行parse解析方法
           mapperParser.parse();
          else if (resource == null && url != null && mapperClass == null) 
           ErrorContext.instance().resource(url);
           InputStream inputStream = Resources.getUrlAsStream(url);
           XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
           mapperParser.parse();
          else if (resource == null && url == null && mapperClass != null) 
           Class<?> mapperInterface = Resources.classForName(mapperClass);
           configuration.addMapper(mapperInterface);
          else 
           throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
         
       
     
   
   
   

   follow me，继续看看解析Mapper.xml

   public class XMLMapperBuilder extends BaseBuilder 
   public void parse() 
   if (!configuration.isResourceLoaded(resource)) 
     // 进入解析mapper属性
     configurationElement(parser.evalNode("/mapper"));
     configuration.addLoadedResource(resource);
     bindMapperForNamespace();
   
   
   parsePendingResultMaps();
   parsePendingCacheRefs();
   parsePendingStatements();
   
   
   private void configurationElement(XNode context) 
   try 
     String namespace = context.getStringAttribute("namespace");
     if (namespace == null || namespace.equals("")) 
       throw new BuilderException("Mappers namespace cannot be empty");
     
     builderAssistant.setCurrentNamespace(namespace);
     cacheRefElement(context.evalNode("cache-ref"));
     // 最终在这⾥看到了关于cache属性的处理,跟进去看看
     cacheElement(context.evalNode("cache"));
     parameterMapElement(context.evalNodes("/mapper/parameterMap"));
     resultMapElements(context.evalNodes("/mapper/resultMap"));
     sqlElement(context.evalNodes("/mapper/sql"));
     // 这⾥会将⽣成的Cache包装到对应的MappedStatement
     buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    catch (Exception e) 
     throw new BuilderException("Error parsing Mapper XML. The XML location is " + resource + ". Cause: " + e, e);
   
   
   
   private void cacheElement(XNode context) throws Exception 
   if (context != null) 
     //解析<cache/>标签的type属性，这⾥我们可以⾃定义cache的实现类，⽐如redisCache，如果没有⾃定义，这⾥使⽤和⼀级缓存相同的PERPETUAL
     String type = context.getStringAttribute("type", "PERPETUAL");
     Class<? extends Cache> typeClass = typeAliasRegistry.resolveAlias(type);
     String eviction = context.getStringAttribute("eviction", "LRU");
     Class<? extends Cache> evictionClass = typeAliasRegistry.resolveAlias(eviction);
     Long flushInterval = context.getLongAttribute("flushInterval");
     Integer size = context.getIntAttribute("size");
     boolean readWrite = !context.getBooleanAttribute("readOnly", false);
     boolean blocking = context.getBooleanAttribute("blocking", false);
     Properties props = context.getChildrenAsProperties();
     // 构建Cache对象
     builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
   
   
   

   让我们继续来看看是如何构建Cache对象的

   MapperBuilderAssistant.useNewCache()

   public class MapperBuilderAssistant extends BaseBuilder   
   // 1.⽣成Cache对象
   public Cache useNewCache(Class<? extends Cache> typeClass,
     Class<? extends Cache> evictionClass,
     Long flushInterval,
     Integer size,
     boolean readWrite,
     boolean blocking,
     Properties props) 
   Cache cache = new CacheBuilder(currentNamespace)
       //这⾥如果我们定义了<cache/>中的type，就使⽤⾃定义的Cache,否则使⽤和⼀级缓存相同的PerpetualCache
       .implementation(valueOrDefault(typeClass, PerpetualCache.class))
       .addDecorator(valueOrDefault(evictionClass, LruCache.class))
       .clearInterval(flushInterval)
       .size(size)
       .readWrite(readWrite)
       .blocking(blocking)
       .properties(props)
       .build();
   // 2.添加到Configuration中
   configuration.addCache(cache);
   // 3.并将cache赋值给MapperBuilderAssistant.currentCache
   currentCache = cache;
   return cache;
   
   

   我们看到⼀个Mapper.xml只会解析⼀次标签，也就是只创建⼀次Cache对象，放进configuration中，并将cache赋值给MapperBuilderAssistant.currentCache。
   XMLMapperBuilder的configurationElement(XNode context) 方法中的buildStatementFromContext(context.evalNodes("select|insert|update|delete"));将Cache包装到MappedStatement。

   public class XMLMapperBuilder extends BaseBuilder 
   private void buildStatementFromContext(List<XNode> list) 
   if (configuration.getDatabaseId() != null) 
     buildStatementFromContext(list, configuration.getDatabaseId());
   
   //调用重载方法
   buildStatementFromContext(list, null);
   
   
   private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) 
   for (XNode context : list) 
     final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
     try 
       // 每⼀条执⾏语句转换成⼀个MappedStatement
       statementParser.parseStatementNode();
      catch (IncompleteElementException e) 
       configuration.addIncompleteStatement(statementParser);
     
   
   
   

   XMLStatementBuilder.parseStatementNode()

   public class XMLStatementBuilder extends BaseBuilder 
   public void parseStatementNode() 
   String id = context.getStringAttribute("id");
   String databaseId = context.getStringAttribute("databaseId");
   
   if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) 
     return;
   
   
   Integer fetchSize = context.getIntAttribute("fetchSize");
   Integer timeout = context.getIntAttribute("timeout");
   String parameterMap = context.getStringAttribute("parameterMap");
   String parameterType = context.getStringAttribute("parameterType");
   Class<?> parameterTypeClass = resolveClass(parameterType);
   String resultMap = context.getStringAttribute("resultMap");
   String resultType = context.getStringAttribute("resultType");
   String lang = context.getStringAttribute("lang");
   LanguageDriver langDriver = getLanguageDriver(lang);
   
   Class<?> resultTypeClass = resolveClass(resultType);
   String resultSetType = context.getStringAttribute("resultSetType");
   StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
   ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
   
   String nodeName = context.getNode().getNodeName();
   SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
   boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
   boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
   boolean useCache = context.getBooleanAttribute("useCache", isSelect);
   boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
   
   // Include Fragments before parsing
   XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
   includeParser.applyIncludes(context.getNode());
   
   // Parse selectKey after includes and remove them.
   processSelectKeyNodes(id, parameterTypeClass, langDriver);
   
   // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
   SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
   String resultSets = context.getStringAttribute("resultSets");
   String keyProperty = context.getStringAttribute("keyProperty");
   String keyColumn = context.getStringAttribute("keyColumn");
   KeyGenerator keyGenerator;
   String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
   keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
   if (configuration.hasKeyGenerator(keyStatementId)) 
     keyGenerator = configuration.getKeyGenerator(keyStatementId);
    else 
     keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
         configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
         ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
   
   //创建MappedStatement对象
   builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
       fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
       resultSetTypeEnum, flushCache, useCache, resultOrdered, 
       keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
   
   

   BuilderAssistant.addMappedStatement()

   public MappedStatement addMappedStatement(
     String id,
     SqlSource sqlSource,
     StatementType statementType,
     SqlCommandType sqlCommandType,
     Integer fetchSize,
     Integer timeout,
     String parameterMap,
     Class<?> parameterType,
     String resultMap,
     Class<?> resultType,
     ResultSetType resultSetType,
     boolean flushCache,
     boolean useCache,
     boolean resultOrdered,
     KeyGenerator keyGenerator,
     String keyProperty,
     String keyColumn,
     String databaseId,
     LanguageDriver lang,
     String resultSets) 
   
   if (unresolvedCacheRef) 
     throw new IncompleteElementException("Cache-ref not yet resolved");
   
   
   id = applyCurrentNamespace(id, false);
   boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
   //创建MappedStatement对象
   MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
       .resource(resource)
       .fetchSize(fetchSize)
       .timeout(timeout)
       .statementType(statementType)
       .keyGenerator(keyGenerator)
       .keyProperty(keyProperty)
       .keyColumn(keyColumn)
       .databaseId(databaseId)
       .lang(lang)
       .resultOrdered(resultOrdered)
       .resultSets(resultSets)
       .resultMaps(getStatementResultMaps(resultMap, resultType, id))
       .resultSetType(resultSetType)
       .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
       .useCache(valueOrDefault(useCache, isSelect))
       //在这⾥将之前⽣成的Cache封装到MappedStatement
       .cache(currentCache);
   
   ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
   if (statementParameterMap != null) 
     statementBuilder.parameterMap(statementParameterMap);
   
   
   MappedStatement statement = statementBuilder.build();
   configuration.addMappedStatement(statement);
   return statement;
   

   我们看到将Mapper中创建的Cache对象，加⼊到了每个MappedStatement对象中，也就是同⼀个
   Mapper中所有的MappedStatement实例，有关于<cache>标签的解析就到这。

   查询源码分析

   CachingExecutor

   @Override
   public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException 
   BoundSql boundSql = ms.getBoundSql(parameterObject);
   //创建 CacheKey
   CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
   //调用重载方法
   return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
   
   
   @Override
   public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
     throws SQLException 
   // 从 MappedStatement 中获取 Cache，注意这⾥的 Cache 是从MappedStatement中获取的
   // 也就是我们上⾯解析Mapper中<cache/>标签中创建的，它保存在Configration中
   // 我们在上⾯解析mapper.xml时分析过每⼀个MappedStatement都有⼀个Cache对象，就是这⾥
   Cache cache = ms.getCache();
   // 如果配置⽂件中没有配置 <cache>，则 cache 为空
   if (cache != null) 
     //如果需要刷新缓存的话就刷新：flushCache="true"
     flushCacheIfRequired(ms);
     if (ms.isUseCache() && resultHandler == null) 
       ensureNoOutParams(ms, boundSql);
       // 访问⼆级缓存
       @SuppressWarnings("unchecked")
       List<E> list = (List<E>) tcm.getObject(cache, key);
       // 缓存未命中
       if (list == null) 
         // 如果没有值，则执⾏查询，这个查询实际也是先⾛⼀级缓存查询，⼀级缓存也没有的话，则进⾏DB查询
         list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
         // 缓存查询结果
         tcm.putObject(cache, key, list); // issue #578 and #116
       
       return list;
     
   
   return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
   

   如果设置了flushCache="true"，则每次查询都会刷新缓存

   <!-- 执⾏此语句清空缓存 -->
   <select id="findbyId" resultType="com.zjq.pojo.user" useCache="true" flushCache="true" >
   select * from user
   </select>

   如上，注意⼆级缓存是从 MappedStatement 中获取的。由于 MappedStatement 存在于全局配置中，可以多个 CachingExecutor 获取到，这样就会出现线程安全问题。除此之外，若不加以控制，多个事务共⽤⼀个缓存实例，会导致脏读问题。⾄于脏读问题，需要借助其他类来处理，也就是上⾯代码中tcm 变量对应的类型。下⾯分析⼀下。

   TransactionalCacheManager

   
   /** 事务缓存管理器 */
   public class TransactionalCacheManager 
   // Cache 与 TransactionalCache 的映射关系表
   private final Map<Cache, TransactionalCache> transactionalCaches = new HashMap<Cache, TransactionalCache>();
   
   public void clear(Cache cache) 
   // 获取 TransactionalCache 对象，并调⽤该对象的 clear ⽅法，下同
   getTransactionalCache(cache).clear();
   
   
   public Object getObject(Cache cache, CacheKey key) 
   // 直接从TransactionalCache中获取缓存
   return getTransactionalCache(cache).getObject(key);
   
   
   public void putObject(Cache cache, CacheKey key, Object value) 
   // 直接存⼊TransactionalCache的缓存中
   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) 
   // 从映射表中获取 TransactionalCache
   TransactionalCache txCache = transactionalCaches.get(cache);
   if (txCache == null) 
     // TransactionalCache 也是⼀种装饰类，为 Cache 增加事务功能
     // 创建⼀个新的TransactionalCache，并将真正的Cache对象存进去
     txCache = new TransactionalCache(cache);
     transactionalCaches.put(cache, txCache);
   
   return txCache;
   

TransactionalCacheManager 内部维护了 Cache 实例与 TransactionalCache 实例间的映射关系，该类也仅负责维护两者的映射关系，真正做事的还是 TransactionalCache。TransactionalCache 是⼀种缓存装饰器，可以为Cache 实例增加事务功能。我在之前提到的脏读问题正是由该类进⾏处理的。下⾯分析⼀下该类的逻辑。
### TransactionalCache
```java
public class TransactionalCache implements Cache 

  private static final Log log = LogFactory.getLog(TransactionalCache.class);
  //真正的缓存对象，和上⾯的Map<Cache, TransactionalCache>中的Cache是同⼀个
  private final Cache delegate;
  private boolean clearOnCommit;
  // 在事务被提交前，所有从数据库中查询的结果将缓存在此集合中
  private final Map<Object, Object> entriesToAddOnCommit;
  // 在事务被提交前，当缓存未命中时，CacheKey 将会被存储在此集合中
  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
    // 查询的时候是直接从delegate中去查询的，也就是从真正的缓存对象中查询
    Object object = delegate.getObject(key);
    if (object == null) 
      // 缓存未命中，则将 key 存⼊到 entriesMissedInCache 中
      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) 
    // 将键值对存⼊到 entriesToAddOnCommit 这个Map中中，⽽⾮真实的缓存对象delegate 中
    entriesToAddOnCommit.put(key, object);
  

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

  @Override
  public void clear() 
    clearOnCommit = true;
    // 清空 entriesToAddOnCommit，但不清空 delegate 缓存
    entriesToAddOnCommit.clear();
  

  public void commit() 
    // 根据 clearOnCommit 的值决定是否清空 delegate
    if (clearOnCommit) 
      delegate.clear();
    
    // 刷新未缓存的结果到 delegate 缓存中
    flushPendingEntries();
    // 重置 entriesToAddOnCommit 和 entriesMissedInCache
    reset();
  

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

  private void reset() 
    clearOnCommit = false;
    // 清空集合
    entriesToAddOnCommit.clear();
    entriesMissedInCache.clear();
  

  private void flushPendingEntries() 
    for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) 
      // 将 entriesToAddOnCommit 中的内容转存到 delegate 中
      delegate.putObject(entry.getKey(), entry.getValue());
    
    for (Object entry : entriesMissedInCache) 
      if (!entriesToAddOnCommit.containsKey(entry)) 
        // 存⼊空值
        delegate.putObject(entry, null);
      
    
  

  private void unlockMissedEntries() 
    for (Object entry : entriesMissedInCache) 
      try 
        // 调⽤ removeObject 进⾏解锁
        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);
      
    
  

存储⼆级缓存对象的时候是放到了TransactionalCache.entriesToAddOnCommit这个map中，但是每次查询的时候是直接从TransactionalCache.delegate中去查询的，所以这个⼆级缓存查询数据库后，设置缓存值是没有⽴刻⽣效的，主要是因为直接存到 delegate 会导致脏数据问题。
SqlSession.commit()⽅法做了什么

SqlSession.commit()

public class DefaultSqlSession implements SqlSession  
  @Override
  public void commit(boolean force) 
    try 
      // 主要是这句
      executor.commit(isCommitOrRollbackRequired(force));
      dirty = false;
     catch (Exception e) 
      throw ExceptionFactory.wrapException("Error committing transaction.  Cause: " + e, e);
     finally 
      ErrorContext.instance().reset();
    
  


// CachingExecutor.commit()
@Override
public void commit(boolean required) throws SQLException 
    delegate.commit(required);
    //调用TransactionalCacheManager.commit()
    tcm.commit();


// TransactionalCacheManager.commit()
public void commit() 
    for (TransactionalCache txCache : transactionalCaches.values()) 
        //调用TransactionalCache.commit()
        txCache.commit();
    

// TransactionalCache.commit()
public void commit() 
    if (clearOnCommit) 
        delegate.clear();
    
    flushPendingEntries();//这⼀句
    reset();

// TransactionalCache.flushPendingEntries()
private void flushPendingEntries() 
    for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) 
        // 在这⾥真正的将entriesToAddOnCommit的对象逐个添加到delegate中，只有这时，⼆级缓存才真正的⽣效
        delegate.putObject(entry.getKey(), entry.getValue());
        
        for (Object entry : entriesMissedInCache) 
            if (!entriesToAddOnCommit.containsKey(entry)) 
                delegate.putObject(entry, null);
            
        

⼆级缓存的刷新

我们来看看SqlSession的更新操作

//DefaultSqlSession.update
public int update(String statement, Object parameter) 
    int var4;
    try 
        this.dirty = true;
        MappedStatement ms = this.configuration.getMappedStatement(statement);
        var4 = this.executor.update(ms, this.wrapCollection(parameter));
     catch (Exception var8) 
        throw ExceptionFactory.wrapException("Error updating database. Cause:" + var8, var8);
     finally 
        ErrorContext.instance().reset();
    
    return var4;
    


//CachingExecutor.update
public int update(MappedStatement ms, Object parameterObject) throws SQLException 
    this.flushCacheIfRequired(ms);
    return this.delegate.update(ms, parameterObject);


private void flushCacheIfRequired(MappedStatement ms) 
    //获取MappedStatement对应的Cache，进⾏清空
    Cache cache = ms.getCache();
    //SQL需设置flushCache="true" 才会执⾏清空
    if (cache != null && ms.isFlushCacheRequired()) 
        this.tcm.clear(cache);
    

MyBatis⼆级缓存只适⽤于不常进⾏增、删、改的数据，⽐如国家⾏政区省市区街道数据。⼀但数据变更，MyBatis会清空缓存。因此⼆级缓存不适⽤于经常进⾏更新的数据。
总结：

• 在⼆级缓存的设计上，MyBatis⼤量地运⽤了装饰者模式，如CachingExecutor, 以及各种Cache接⼝的装饰器。
• ⼆级缓存实现了Sqlsession之间的缓存数据共享，属于namespace级别
• ⼆级缓存具有丰富的缓存策略。
• ⼆级缓存可由多个装饰器，与基础缓存组合⽽成
• ⼆级缓存⼯作由 ⼀个缓存装饰执⾏器CachingExecutor和 ⼀个事务型预缓存TransactionalCache完成。