MyBatis从入门到精通—源码剖析之二级缓存细节

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：

开启全局⼆级缓存配置：

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

在需要使⽤⼆级缓存的Mapper配置⽂件中配置标签

<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

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("Mapper's 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。

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