經過源碼分析MyBatis的緩存

MyBatis緩存介紹

MyBatis支持聲明式數據緩存(declarative data caching)。當一條SQL語句被標記爲「可緩存」後,首次執行它時從數據庫獲取的全部數據會被存儲在一段高速緩存中,從此執行這條語句時就會從高速緩存中讀取結果,而不是再次命中數據庫。MyBatis提供了默認下基於Java HashMap的緩存實現,以及用於與OSCache、Ehcache、Hazelcast和Memcached鏈接的默認鏈接器。MyBatis還提供API供其餘緩存實現使用。html

重點的那句話就是:MyBatis執行SQL語句以後,這條語句就是被緩存,之後再執行這條語句的時候,會直接從緩存中拿結果,而不是再次執行SQLjava

這也就是你們常說的MyBatis一級緩存,一級緩存的做用域scope是SqlSession。node

MyBatis同時還提供了一種全局做用域global scope的緩存,這也叫作二級緩存,也稱做全局緩存。sql

一級緩存

測試

同個session進行兩次相同查詢:數據庫

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@Test
public void test() {
SqlSession sqlSession = sqlSessionFactory.openSession();
try {
User user = (User)sqlSession.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 1 );
log.debug(user);
User user2 = (User)sqlSession.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 1 );
log.debug(user2);
} finally {
sqlSession.close();
}
}

MyBatis只進行1次數據庫查詢:apache

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==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }

同個session進行兩次不一樣的查詢:設計模式

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@Test
public void test() {
SqlSession sqlSession = sqlSessionFactory.openSession();
try {
User user = (User)sqlSession.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 1 );
log.debug(user);
User user2 = (User)sqlSession.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 2 );
log.debug(user2);
} finally {
sqlSession.close();
}
}

MyBatis進行兩次數據庫查詢:緩存

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==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }
==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 2 (Integer)
<== Total: 1
User{id= 2 , name= 'FFF' , age= 50 , birthday=Sat Dec 06 17 : 12 : 01 CST 2014 }

不一樣session,進行相同查詢:session

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@Test
public void test() {
SqlSession sqlSession = sqlSessionFactory.openSession();
SqlSession sqlSession2 = sqlSessionFactory.openSession();
try {
User user = (User)sqlSession.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 1 );
log.debug(user);
User user2 = (User)sqlSession2.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 1 );
log.debug(user2);
} finally {
sqlSession.close();
sqlSession2.close();
}
}

MyBatis進行了兩次數據庫查詢:mybatis

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==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }
==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }

同個session,查詢以後更新數據,再次查詢相同的語句:

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@Test
public void test() {
SqlSession sqlSession = sqlSessionFactory.openSession();
try {
User user = (User)sqlSession.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 1 );
log.debug(user);
user.setAge( 100 );
sqlSession.update( "org.format.mybatis.cache.UserMapper.update" , user);
User user2 = (User)sqlSession.selectOne( "org.format.mybatis.cache.UserMapper.getById" , 1 );
log.debug(user2);
sqlSession.commit();
} finally {
sqlSession.close();
}
}

更新操做以後緩存會被清除:

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==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }
==> Preparing: update USERS SET NAME = ? , AGE = ? , BIRTHDAY = ? where ID = ?
==> Parameters: format(String), 23 (Integer), 2014 - 10 - 12 23 : 20 : 13.0 (Timestamp), 1 (Integer)
<== Updates: 1
==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }

很明顯,結果驗證了一級緩存的概念,在同個SqlSession中,查詢語句相同的sql會被緩存,可是一旦執行新增或更新或刪除操做,緩存就會被清除

源碼分析

在分析MyBatis的一級緩存以前,咱們先簡單看下MyBatis中幾個重要的類和接口:

org.apache.ibatis.session.Configuration類:MyBatis全局配置信息類

org.apache.ibatis.session.SqlSessionFactory接口:操做SqlSession的工廠接口,具體的實現類是DefaultSqlSessionFactory

org.apache.ibatis.session.SqlSession接口:執行sql,管理事務的接口,具體的實現類是DefaultSqlSession

org.apache.ibatis.executor.Executor接口:sql執行器,SqlSession執行sql最終是經過該接口實現的,經常使用的實現類有SimpleExecutor和CachingExecutor,這些實現類都使用了裝飾者設計模式

一級緩存的做用域是SqlSession,那麼咱們就先看一下SqlSession的select過程:

這是DefaultSqlSession(SqlSession接口實現類,MyBatis默認使用這個類)的selectList源碼(咱們例子上使用的是selectOne方法,調用selectOne方法最終會執行selectList方法):

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public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
List<E> result = executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
return result;
} catch (Exception e) {
throw ExceptionFactory.wrapException( "Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}

咱們看到SqlSession最終會調用Executor接口的方法。

接下來咱們看下DefaultSqlSession中的executor接口屬性具體是哪一個實現類。

DefaultSqlSession的構造過程(DefaultSqlSessionFactory內部):

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private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null ;
try {
final Environment environment = configuration.getEnvironment();
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
final Executor executor = configuration.newExecutor(tx, execType, autoCommit);
return new DefaultSqlSession(configuration, executor);
} catch (Exception e) {
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException( "Error opening session. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}

咱們看到DefaultSqlSessionFactory構造DefaultSqlSession的時候,Executor接口的實現類是由Configuration構造的:

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public Executor newExecutor(Transaction transaction, ExecutorType executorType, boolean autoCommit) {
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);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor, autoCommit);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}

Executor根據ExecutorType的不一樣而建立,最經常使用的是SimpleExecutor,本文的例子也是建立這個實現類。 最後咱們發現若是cacheEnabled這個屬性爲true的話,那麼executor會被包一層裝飾器,這個裝飾器是CachingExecutor。其中cacheEnabled這個屬性是mybatis總配置文件中settings節點中cacheEnabled子節點的值,默認就是true,也就是說咱們在mybatis總配置文件中不配cacheEnabled的話,它也是默認爲打開的。

如今,問題就剩下一個了,CachingExecutor執行sql的時候到底作了什麼?

帶着這個問題,咱們繼續走下去(CachingExecutor的query方法):

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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);
if (!dirty) {
cache.getReadWriteLock().readLock().lock();
try {
@SuppressWarnings ( "unchecked" )
List<E> cachedList = (List<E>) cache.getObject(key);
if (cachedList != null ) return cachedList;
} finally {
cache.getReadWriteLock().readLock().unlock();
}
}
List<E> list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list); // issue #578. Query must be not synchronized to prevent deadlocks
return list;
}
}
return delegate.<E>query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

其中Cache cache = ms.getCache();這句代碼中,這個cache實際上就是個二級緩存,因爲咱們沒有開啓二級緩存(二級緩存的內容下面會分析),所以這裏執行了最後一句話。這裏的delegate也就是SimpleExecutor,SimpleExecutor沒有Override父類的query方法,所以最終執行了SimpleExecutor的父類BaseExecutor的query方法。

因此一級緩存最重要的代碼就是BaseExecutor的query方法!

BaseExecutor的屬性localCache是個PerpetualCache類型的實例,PerpetualCache類是實現了MyBatis的Cache緩存接口的實現類之一,內部有個Map類型的屬性用來存儲緩存數據。 這個localCache的類型在BaseExecutor內部是寫死的。 這個localCache就是一級緩存!

接下來咱們看下爲什麼執行新增或更新或刪除操做,一級緩存就會被清除這個問題。

首先MyBatis處理新增或刪除的時候,最終都是調用update方法,也就是說新增或者刪除操做在MyBatis眼裏都是一個更新操做。

咱們看下DefaultSqlSession的update方法:

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

很明顯,這裏調用了CachingExecutor的update方法:

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public int update(MappedStatement ms, Object parameterObject) throws SQLException {
flushCacheIfRequired(ms);
return delegate.update(ms, parameterObject);
}

這裏的flushCacheIfRequired方法清除的是二級緩存,咱們以後會分析。 CachingExecutor委託給了(以前已經分析過)SimpleExecutor的update方法,SimpleExecutor沒有Override父類BaseExecutor的update方法,所以咱們看BaseExecutor的update方法:

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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." );
clearLocalCache();
return doUpdate(ms, parameter);
}

咱們看到了關鍵的一句代碼: clearLocalCache(); 進去看看:

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public void clearLocalCache() {
if (!closed) {
localCache.clear();
localOutputParameterCache.clear();
}
}

沒錯,就是這條,sqlsession沒有關閉的話,進行新增、刪除、修改操做的話就是清除一級緩存,也就是SqlSession的緩存。

二級緩存

二級緩存的做用域是全局,換句話說,二級緩存已經脫離SqlSession的控制了。

在測試二級緩存以前,我先把結論說一下:

二級緩存的做用域是全局的,二級緩存在SqlSession關閉或提交以後纔會生效。

在分析MyBatis的二級緩存以前,咱們先簡單看下MyBatis中一個關於二級緩存的類(其餘相關的類和接口以前已經分析過):

org.apache.ibatis.mapping.MappedStatement:

MappedStatement類在Mybatis框架中用於表示XML文件中一個sql語句節點,即一個<select />、<update />或者<insert />標籤。Mybatis框架在初始化階段會對XML配置文件進行讀取,將其中的sql語句節點對象化爲一個個MappedStatement對象。

配置

二級緩存跟一級緩存不一樣,一級緩存不須要配置任何東西,且默認打開。 二級緩存就須要配置一些東西。

本文就說下最簡單的配置,在mapper文件上加上這句配置便可:

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< cache />

其實二級緩存跟3個配置有關:

  1. mybatis全局配置文件中的setting中的cacheEnabled須要爲true(默認爲true,不設置也行)
  2. mapper配置文件中須要加入<cache>節點
  3. mapper配置文件中的select節點須要加上屬性useCache須要爲true(默認爲true,不設置也行)

測試

不一樣SqlSession,查詢相同語句,第一次查詢以後commit SqlSession:

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@Test
public void testCache2() {
SqlSession sqlSession = sqlSessionFactory.openSession();
SqlSession sqlSession2 = sqlSessionFactory.openSession();
try {
String sql = "org.format.mybatis.cache.UserMapper.getById" ;
User user = (User)sqlSession.selectOne(sql, 1 );
log.debug(user);
// 注意,這裏必定要提交。 不提交仍是會查詢兩次數據庫
sqlSession.commit();
User user2 = (User)sqlSession2.selectOne(sql, 1 );
log.debug(user2);
} finally {
sqlSession.close();
sqlSession2.close();
}
}

MyBatis僅進行了一次數據庫查詢:

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==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }

不一樣SqlSession,查詢相同語句,第一次查詢以後close SqlSession:

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@Test
public void testCache2() {
SqlSession sqlSession = sqlSessionFactory.openSession();
SqlSession sqlSession2 = sqlSessionFactory.openSession();
try {
String sql = "org.format.mybatis.cache.UserMapper.getById" ;
User user = (User)sqlSession.selectOne(sql, 1 );
log.debug(user);
sqlSession.close();
User user2 = (User)sqlSession2.selectOne(sql, 1 );
log.debug(user2);
} finally {
sqlSession2.close();
}
}

MyBatis僅進行了一次數據庫查詢:

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==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }

不一樣SqlSesson,查詢相同語句。 第一次查詢以後SqlSession不提交:

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@Test
public void testCache2() {
SqlSession sqlSession = sqlSessionFactory.openSession();
SqlSession sqlSession2 = sqlSessionFactory.openSession();
try {
String sql = "org.format.mybatis.cache.UserMapper.getById" ;
User user = (User)sqlSession.selectOne(sql, 1 );
log.debug(user);
User user2 = (User)sqlSession2.selectOne(sql, 1 );
log.debug(user2);
} finally {
sqlSession.close();
sqlSession2.close();
}
}

MyBatis執行了兩次數據庫查詢:

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==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }
==> Preparing: select * from USERS WHERE ID = ?
==> Parameters: 1 (Integer)
<== Total: 1
User{id= 1 , name= 'format' , age= 23 , birthday=Sun Oct 12 23 : 20 : 13 CST 2014 }

源碼分析

咱們從在mapper文件中加入的<cache/>中開始分析源碼,關於MyBatis的SQL解析請參考另一篇博客Mybatis解析動態sql原理分析。接下來咱們看下這個cache的解析:

XMLMappedBuilder(解析每一個mapper配置文件的解析類,每個mapper配置都會實例化一個XMLMapperBuilder類)的解析方法:

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private void configurationElement(XNode context) {
try {
String namespace = context.getStringAttribute( "namespace" );
if (namespace.equals( "" )) {
throw new BuilderException( "Mapper's namespace cannot be empty" );
}
builderAssistant.setCurrentNamespace(namespace);
cacheRefElement(context.evalNode( "cache-ref" ));
cacheElement(context.evalNode( "cache" ));
parameterMapElement(context.evalNodes( "/mapper/parameterMap" ));
resultMapElements(context.evalNodes( "/mapper/resultMap" ));
sqlElement(context.evalNodes( "/mapper/sql" ));
buildStatementFromContext(context.evalNodes( "select|insert|update|delete" ));
} catch (Exception e) {
throw new BuilderException( "Error parsing Mapper XML. Cause: " + e, e);
}
}

咱們看到了解析cache的那段代碼:

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private void cacheElement(XNode context) throws Exception {
if (context != null ) {
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 );
Properties props = context.getChildrenAsProperties();
builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, props);
}
}

解析完cache標籤以後會使用builderAssistant的userNewCache方法,這裏的builderAssistant是一個MapperBuilderAssistant類型的幫助類,每一個XMLMappedBuilder構造的時候都會實例化這個屬性,MapperBuilderAssistant類內部有個Cache類型的currentCache屬性,這個屬性也就是mapper配置文件中cache節點所表明的值:

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public Cache useNewCache(Class<? extends Cache> typeClass,
Class<? extends Cache> evictionClass,
Long flushInterval,
Integer size,
boolean readWrite,
Properties props) {
typeClass = valueOrDefault(typeClass, PerpetualCache. class );
evictionClass = valueOrDefault(evictionClass, LruCache. class );
Cache cache = new CacheBuilder(currentNamespace)
.implementation(typeClass)
.addDecorator(evictionClass)
.clearInterval(flushInterval)
.size(size)
.readWrite(readWrite)
.properties(props)
.build();
configuration.addCache(cache);
currentCache = cache;
return cache;
}

ok,如今mapper配置文件中的cache節點被解析到了XMLMapperBuilder實例中的builderAssistant屬性中的currentCache值裏。

接下來XMLMapperBuilder會解析select節點,解析select節點的時候使用XMLStatementBuilder進行解析(也包括其餘insert,update,delete節點):

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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))
? new Jdbc3KeyGenerator() : new NoKeyGenerator();
}
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}

這段代碼前面都是解析一些標籤的屬性,咱們看到了最後一行使用builderAssistant添加MappedStatement,其中builderAssistant屬性是構造XMLStatementBuilder的時候經過XMLMappedBuilder傳入的,咱們繼續看builderAssistant的addMappedStatement方法:

進入setStatementCache:

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private void setStatementCache(
boolean isSelect,
boolean flushCache,
boolean useCache,
Cache cache,
MappedStatement.Builder statementBuilder) {
flushCache = valueOrDefault(flushCache, !isSelect);
useCache = valueOrDefault(useCache, isSelect);
statementBuilder.flushCacheRequired(flushCache);
statementBuilder.useCache(useCache);
statementBuilder.cache(cache);
}

最終mapper配置文件中的<cache/>被設置到了XMLMapperBuilder的builderAssistant屬性中,XMLMapperBuilder中使用XMLStatementBuilder遍歷CRUD節點,遍歷CRUD節點的時候將這個cache節點設置到這些CRUD節點中,這個cache就是所謂的二級緩存!

接下來咱們回過頭來看查詢的源碼,CachingExecutor的query方法:

進入TransactionalCacheManager的putObject方法:

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public void putObject(Cache cache, CacheKey key, Object value) {
getTransactionalCache(cache).putObject(key, value);
}
private TransactionalCache getTransactionalCache(Cache cache) {
TransactionalCache txCache = transactionalCaches.get(cache);
if (txCache == null ) {
txCache = new TransactionalCache(cache);
transactionalCaches.put(cache, txCache);
}
return txCache;
}

TransactionalCache的putObject方法:

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public void putObject(Object key, Object object) {
entriesToRemoveOnCommit.remove(key);
entriesToAddOnCommit.put(key, new AddEntry(delegate, key, object));
}

咱們看到,數據被加入到了entriesToAddOnCommit中,這個entriesToAddOnCommit是什麼東西呢,它是TransactionalCache的一個Map屬性:

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private Map<Object, AddEntry> entriesToAddOnCommit;

AddEntry是TransactionalCache內部的一個類:

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private static class AddEntry {
private Cache cache;
private Object key;
private Object value;
public AddEntry(Cache cache, Object key, Object value) {
this .cache = cache;
this .key = key;
this .value = value;
}
public void commit() {
cache.putObject(key, value);
}
}

好了,如今咱們發現使用二級緩存以後:查詢數據的話,先從二級緩存中拿數據,若是沒有的話,去一級緩存中拿,一級緩存也沒有的話再查詢數據庫。有了數據以後在丟到TransactionalCache這個對象的entriesToAddOnCommit屬性中。

接下來咱們來驗證爲何SqlSession commit或close以後,二級緩存纔會生效這個問題。

DefaultSqlSession的commit方法:

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public void commit( boolean force) {
try {
executor.commit(isCommitOrRollbackRequired(force));
dirty = false ;
} catch (Exception e) {
throw ExceptionFactory.wrapException( "Error committing transaction. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}

CachingExecutor的commit方法:

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public void commit( boolean required) throws SQLException {
delegate.commit(required);
tcm.commit();
dirty = false ;
}

tcm.commit即 TransactionalCacheManager的commit方法:

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

TransactionalCache的commit方法:

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public void commit() {
delegate.getReadWriteLock().writeLock().lock();
try {
if (clearOnCommit) {
delegate.clear();
} else {
for (RemoveEntry entry : entriesToRemoveOnCommit.values()) {
entry.commit();
}
}
for (AddEntry entry : entriesToAddOnCommit.values()) {
entry.commit();
}
reset();
} finally {
delegate.getReadWriteLock().writeLock().unlock();
}
}

發現調用了AddEntry的commit方法:

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public void commit() {
cache.putObject(key, value);
}

發現了! AddEntry的commit方法會把數據丟到cache中,也就是丟到二級緩存中!

關於爲什麼調用close方法後,二級緩存纔會生效,由於close方法內部會調用commit方法。本文就不具體說了。 讀者有興趣的話看一看源碼就知道爲何了。

其餘

Cache接口簡介

org.apache.ibatis.cache.Cache是MyBatis的緩存接口,想要實現自定義的緩存須要實現這個接口。

MyBatis中關於Cache接口的實現類也使用了裝飾者設計模式

咱們看下它的一些實現類

簡單說明:

LRU – 最近最少使用的:移除最長時間不被使用的對象。

FIFO – 先進先出:按對象進入緩存的順序來移除它們。

SOFT – 軟引用:移除基於垃圾回收器狀態和軟引用規則的對象。

WEAK – 弱引用:更積極地移除基於垃圾收集器狀態和弱引用規則的對象。

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< cache
eviction = "FIFO"
flushInterval = "60000"
size = "512"
readOnly = "true" />

能夠經過cache節點的eviction屬性設置,也能夠設置其餘的屬性。

cache-ref節點

mapper配置文件中還能夠加入cache-ref節點,它有個屬性namespace。

若是每一個mapper文件都是用cache-ref,且namespace都同樣,那麼就表明着真正意義上的全局緩存。

若是隻用了cache節點,那僅表明這個這個mapper內部的查詢被緩存了,其餘mapper文件的不起做用,這並非所謂的全局緩存。

總結

整體來講,MyBatis的源碼看起來仍是比較輕鬆的,本文從實踐和源碼方面深刻分析了MyBatis的緩存原理,但願對讀者有幫助

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