MyBatis从入门到精通—源码剖析之二级缓存细节 _源码剖析

仓廪实则知礼节，衣食足则知荣辱。这篇文章主要讲述MyBatis从入门到精通—源码剖析之二级缓存细节相关的知识，希望能为你提供帮助。
?级缓存构建在?级缓存之上，在收到查询请求时，MyBatis ?先会查询?级缓存，若?级缓存未命中，再去查询?级缓存，?级缓存没有，再查询数据库。
?级缓存------》 ?级缓存------》数据库
与?级缓存不同，?级缓存和具体的命名空间绑定，?个Mapper中有?个Cache，相同Mapper中的MappedStatement共??个Cache，?级缓存则是和 SqlSession 绑定。
启用二级缓存分为三步?：

开启全局?级缓存配置：

< settings> < setting name="cacheEnabled" value="https://www.songbingjia.com/android/true"/> < /settings>
在需要使??级缓存的Mapper配置?件中配置标签

< cache> < /cache>
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【MyBatis从入门到精通—源码剖析之二级缓存细节】根据之前的mybatis源码剖析，xml的解析?作主要交给XMLConfigBuilder.parse()?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?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?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?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?会将?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?我们可以?定义cache的实现类，?如redisCache，如果没有?定义，这?使?和?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.?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?如果我们定义了< cache/> 中的type，就使??定义的Cache,否则使?和?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?个Mapper.xml只会解析?次标签，也就是只创建?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?条执?语句转换成?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?将之前?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?到了每个MappedStatement对象中，也就是同?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?的 Cache 是从MappedStatement中获取的 // 也就是我们上?解析Mapper中< cache/> 标签中创建的，它保存在Configration中 // 我们在上?解析mapper.xml时分析过每?个MappedStatement都有?个Cache对象，就是这? Cache cache = ms.getCache(); // 如果配置?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?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?查询，这个查询实际也是先??级缓存查询，?级缓存也没有的话，则进?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 #116return 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?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?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??个缓存实例，会导致脏读问题。?于脏读问题，需要借助其他类来处理，也就是上?代码中tcm 变量对应的类型。下?分析?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?该对象的 clear ?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?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?种装饰类，为 Cache 增加事务功能 // 创建?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?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?处理的。下?分析?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?的Map< Cache, TransactionalCache> 中的Cache是同?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?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?到 entriesToAddOnCommit 这个Map中中，??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?空值 delegate.putObject(entry, null); private void unlockMissedEntries() for (Object entry : entriesMissedInCache) try // 调? removeObject 进?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?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?级缓存查询数据库后，设置缓存值是没有?刻?效的，主要是因为直接存到 delegate 会导致脏数据问题。
SqlSession.commit()?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?句 reset(); // TransactionalCache.flushPendingEntries() private void flushPendingEntries() for (Map.Entry< Object, Object> entry : entriesToAddOnCommit.entrySet()) // 在这?真正的将entriesToAddOnCommit的对象逐个添加到delegate中，只有这时，?级缓存才真正的?效 delegate.putObject(entry.getKey(), entry.getValue()); for (Object entry : entriesMissedInCache) if (!entriesToAddOnCommit.containsKey(entry)) delegate.putObject(entry, null);

?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?清空 Cache cache = ms.getCache(); //SQL需设置flushCache="true" 才会执?清空 if (cache != null & & ms.isFlushCacheRequired()) this.tcm.clear(cache);

MyBatis?级缓存只适?于不常进?增、删、改的数据，?如国家?政区省市区街道数据。?但数据变更，MyBatis会清空缓存。因此?级缓存不适?于经常进?更新的数据。
总结：