首先来看下mybatis对缓存的规范,规范嘛就是定义的接口啦。
Cache接口 定义了缓存的方法
public interface Cache {
/**获取缓存的id
* @return The identifier of this cache
*/
String getId();
/**添加缓存
* @param key Can be any object but usually it is a {@link CacheKey}
* @param value The result of a select.
*/
void putObject(Object key, Object value);
/**根据缓存键获取缓存
* @param key The key
* @return The object stored in the cache.
*/
Object getObject(Object key);
/**移除缓存
* As of 3.3.0 this method is only called during a rollback
* for any previous value that was missing in the cache.
* This lets any blocking cache to release the lock that
* may have previously put on the key.
* A blocking cache puts a lock when a value is null
* and releases it when the value is back again.
* This way other threads will wait for the value to be
* available instead of hitting the database.
*
*
* @param key The key
* @return Not used
*/
Object removeObject(Object key);
/**
* Clears this cache instance
*/
void clear();
/**
* Optional. This method is not called by the core.
*
* @return The number of elements stored in the cache (not its capacity).
*/
int getSize();
/**
* Optional. As of 3.2.6 this method is no longer called by the core.
*
* Any locking needed by the cache must be provided internally by the cache provider.
*
* @return A ReadWriteLock
*/
ReadWriteLock getReadWriteLock();
}
mybatis实现了多种缓存,比如perpetualCache 是Cache接口的默认实现,通过hashMap来操作缓存,logginCache,在具有缓存的功能下,添加了打印日志的功能。
BlockingCache:阻塞版本的缓存装饰器,能够保证同一时间只有一个线程到缓存中查找指定的Key对应的数据。
FifoCache:先入先出缓存装饰器,FifoCache内部有一个维护具有长度限制的Key键值链表(LinkedList实例)和一个被装饰的缓存对象,Key值链表主要是维护Key的FIFO顺序,而缓存存储和获取则交给被装饰的缓存对象来完成。
LoggingCache:为缓存增加日志输出功能,记录缓存的请求次数和命中次数,通过日志输出缓存命中率。LruCache:最近最少使用的缓存装饰器,当缓存容量满了之后,使用LRU算法淘汰最近最少使用的Key和Value。
LruCache中通过重写LinkedHashMap类的removeEldestEntry()方法获取最近最少使用的Key值,将Key值保存在LruCache类的eldestKey属性中,然后在缓存中添加对象时,淘汰eldestKey对应的Value值。具体实现细节读者可参考LruCache类的源码。
ScheduledCache:自动刷新缓存装饰器,当操作缓存对象时,如果当前时间与上次清空缓存的时间间隔大于指定的时间间隔,则清空缓存。清空缓存的动作由getObject()、putObject()、removeObject()等方法触发。
SerializedCache:序列化缓存装饰器,向缓存中添加对象时,对添加的对象进行序列化处理,从缓存中取出对象时,进行反序列化处理。
SoftCache:软引用缓存装饰器,SoftCache内部维护了一个缓存对象的强引用队列和软引用队列,缓存以软引用的方式添加到缓存中,并将软引用添加到队列中,获取缓存对象时,如果对象已经被回收,则移除Key,如果未被回收,则将对象添加到强引用队列中,避免被回收,如果强引用队列已经满了,则移除最早入队列的对象的引用。
SynchronizedCache:线程安全缓存装饰器,SynchronizedCache的实现比较简单,为了保证线程安全,对操作缓存的方法使用synchronized关键字修饰。
TransactionalCache:事务缓存装饰器,该缓存与其他缓存的不同之处在于,TransactionalCache增加了两个方法,即commit()和rollback()。当写入缓存时,只有调用commit()方法后,缓存对象才会真正添加到TransactionalCache对象中,如果调用了rollback()方法,写入操作将被回滚。WeakCache:弱引用缓存装饰器,功能和SoftCache类似,只是使用不同的引用类型。
会话(session)级别的缓存称为一级缓存,默认开启的。
mybatis毕竟是查询数据库的一个半orm框架,查询数据库势必要消耗服务器的性能,为了减少服务器的性能,使用了缓存。将每次查询到的结果结果缓存起来,当下次查询的时候,如果判断先前有个完全一样的查询,会直接从缓存中直接将结果取出,返回给用户,不需要再进行一次数据库查询了。
当程序与数据库建立了一次会话,中间开始查询数据,每次查询会根据mapper的id、命名空间、sql等等创建缓存key,先去查询本地缓存是否有值,如果有值,则获取解析值,返回,如果没有值,则去查询数据库,再把结果缓存到本地缓存。
首先来看下缓存实例是存在哪里的,在BaseExecutor中有本地缓存localCache,所以继承BaseExecutor的执行器都有localCache,包括但不限于
SimpleExecutor、BatchExecutor,ReuseExecutor。
接下来大概介绍查询流程,具体介绍用到一级缓存的地方
@Test
public void testMybatisCache () throws IOException {
// 获取配置文件输入流
InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml");
// 通过SqlSessionFactoryBuilder的build()方法创建SqlSessionFactory实例
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
// 调用openSession()方法创建SqlSession实例
SqlSession sqlSession = sqlSessionFactory.openSession();
// 获取UserMapper代理对象
UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
// 执行Mapper方法,获取执行结果
List<UserEntity> userList = userMapper.listAllUser();
UserMapper userMapper1 = sqlSession.getMapper(UserMapper.class);
// 执行Mapper方法,获取执行结果
List<UserEntity> userList1 = userMapper.listAllUser();
System.out.println(JSON.toJSONString(userList));
}
@Override
public SqlSession openSession() {
return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
// 获取Mybatis主配置文件配置的环境信息
final Environment environment = configuration.getEnvironment();
// 创建事务管理器工厂
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
// 创建事务管理器
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
// 根据Mybatis主配置文件中指定的Executor类型创建对应的Executor实例
final Executor executor = configuration.newExecutor(tx, execType);
// 创建DefaultSqlSession实例
return new DefaultSqlSession(configuration, executor, autoCommit);
} 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();
}
}
根据上面的流程可以知道,SqlSession,Executor,localcache之间的关系
接下来主要看SqlSession的select方法
@Override
public void select(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
executor.query(ms, wrapCollection(parameter), rowBounds, handler);
} catch (Exception e) {
throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);
} finally {
ErrorContext.instance().reset();
}
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
// 获取BoundSql对象,BoundSql是对动态SQL解析生成的SQL语句和参数映射信息的封装
BoundSql boundSql = ms.getBoundSql(parameter);
// 创建CacheKey,用于缓存Key
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
// 调用重载的query()方法
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
@Override
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++;
// 从缓存中获取结果
list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
if (list != null) {
handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
} else {
// 缓存中获取不到,则调用queryFromDatabase()方法从数据库中查询
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;
}
@Override
public CacheKey createCacheKey(MappedStatement ms, Object parameterObject, RowBounds rowBounds, BoundSql boundSql) {
if (closed) {
throw new ExecutorException("Executor was closed.");
}
CacheKey cacheKey = new CacheKey();
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List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
TypeHandlerRegistry typeHandlerRegistry = ms.getConfiguration().getTypeHandlerRegistry();
// 所有参数值
for (ParameterMapping parameterMapping : parameterMappings) {
if (parameterMapping.getMode() != ParameterMode.OUT) {
Object value;
String propertyName = parameterMapping.getProperty();
if (boundSql.hasAdditionalParameter(propertyName)) {
value = boundSql.getAdditionalParameter(propertyName);
} else if (parameterObject == null) {
value = null;
} else if (typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
value = parameterObject;
} else {
MetaObject metaObject = configuration.newMetaObject(parameterObject);
value = metaObject.getValue(propertyName);
}
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}
}
// Environment Id
if (configuration.getEnvironment() != null) {
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}
return cacheKey;
}
注意:在分布式环境下,务必将MyBatis的localCacheScope属性设置为STATEMENT,避免其他应用节点执行SQL更新语句后,本节点缓存得不到刷新而导致的数据一致性问题。
在MyBatis中,关于缓存设置的参数一共有2个:localCacheScope,cacheEnabled。
二级缓存是全局的缓存,即使不同会话之间也能共享二级缓存,默认是不开启的;
首先说下如何开启他,在mybatis配置文件添加
和在对应的mapper.xml添加cache实例
其次看下二级缓存是如何生效的
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
// 根据executor类型创建对象的Executor对象
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);
}
// 如果cacheEnabled属性为ture,这使用CachingExecutor对上面创建的Executor进行装饰
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
// 执行拦截器链的拦截逻辑
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}
public class CachingExecutor implements Executor {
private final Executor delegate;
private final TransactionalCacheManager tcm = new TransactionalCacheManager();
public CachingExecutor(Executor delegate) {
this.delegate = delegate;
delegate.setExecutorWrapper(this);
}
@Override
public Transaction getTransaction() {
return delegate.getTransaction();
}
@Override
public void close(boolean forceRollback) {
try {
//issues #499, #524 and #573
if (forceRollback) {
tcm.rollback();
} else {
tcm.commit();
}
} finally {
delegate.close(forceRollback);
}
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
// 调用createCacheKey()方法创建缓存Key
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
@Override
public <E> Cursor<E> queryCursor(MappedStatement ms, Object parameter, RowBounds rowBounds) throws SQLException {
flushCacheIfRequired(ms);
return delegate.queryCursor(ms, parameter, rowBounds);
}
@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
// 获取MappedStatement对象中维护的二级缓存对象
Cache cache = ms.getCache();
if (cache != null) {
// 判断是否需要刷新二级缓存
flushCacheIfRequired(ms);
if (ms.isUseCache() && resultHandler == null) {
ensureNoOutParams(ms, boundSql);
// 从MappedStatement对象对应的二级缓存中获取数据
@SuppressWarnings("unchecked")
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
// 如果缓存数据不存在,则从数据库中查询数据
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
// 將数据存放到MappedStatement对象对应的二级缓存中
tcm.putObject(cache, key, list); // issue #578 and #116
}
return list;
}
}
return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
}
public class TransactionalCacheManager {
// 通过HashMap对象维护二级缓存对应的TransactionalCache实例
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) {
// 获取二级缓存对应的TransactionalCache对象,然后根据缓存Key获取缓存对象
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) {
// 获取二级缓存对应的TransactionalCache对象
TransactionalCache txCache = transactionalCaches.get(cache);
if (txCache == null) {
// 如果获取不到则创建,然后添加到Map中
txCache = new TransactionalCache(cache);
transactionalCaches.put(cache, txCache);
}
return txCache;
}
}
还是在sqlSessionFactory.openSession();
时,会创建执行器,当cacheEnabled属性为ture,会创建CachingExecutor缓存执行器。
看下CachingExecutor的结构,它包含了一个委托执行器(使用了委托模式),用来真正执行的sql,而自己主要的作用是放在了建立和使用二级缓存
当执行sql,最后会进入到执行器,如果执行器是CachingExecutor时,会调用他的query方法,进入方法后,首先会从MappedStatement
拿出二级缓存实例(你以为这就是二级缓存?不,你错了),然后判断是否要刷新缓存,再根据二级缓存实例从缓存管理器
(CacheExecutor维护了TransactionalCacheManager缓存管理器,缓存管理器里面维护了二级缓存实例和TransactionalCache的关系)中得到TransactionalCache,再利用cacheKey获取TransactionalCache中对应的二级缓存,如果缓存不存在,则使用委托执行器去数据库查询数据,再缓存结果,如果存在,则直接返回。
再回过头看,当时的MappedStatement
是如何get二级缓存实例的;
private void configurationElement(XNode context) {
try {
// 获取命名空间
String namespace = context.getStringAttribute("namespace");
if (namespace == null || namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
// 设置当前正在解析的Mapper配置的命名空间
builderAssistant.setCurrentNamespace(namespace);
// 解析标签
cacheRefElement(context.evalNode("cache-ref"));
// 解析标签
cacheElement(context.evalNode("cache"));
// 解析所有的标签
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
// 解析所有的标签
resultMapElements(context.evalNodes("/mapper/resultMap"));
// 解析所有的标签
sqlElement(context.evalNodes("/mapper/sql"));
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} 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) {
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();
builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
}
}
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))
.cache(currentCache);
如上面的代码所示,在获取标签的所有属性信息后,调用MapperBuilderAssistant对象的userNewCache()方法创建二级缓存实例,然后通过MapperBuilderAssistant的currentCache属性保存二级缓存对象的引用。在调用MapperBuilderAssistant对象的addMappedStatement()方法创建MappedStatement对象时会将当前命名空间对应的二级缓存对象的引用添加到MappedStatement对象中,所以这就是需要配置的原因。
流程
![image-20211116235954488](https://img-blog.csdnimg.cn/img_convert/695760986f3bbfb9701b238c790e5892.png)
### mybatis二级缓存解决了什么问题
解决了一级缓存在不同session存在脏读的问题,但是分布式二级缓存也存在脏读。
![image-20211117000030143](https://img-blog.csdnimg.cn/img_convert/ae605add0674bdcb75bfb19684740c78.png)
### 总结
MyBatis一级缓存是SqlSession级别的缓存,默认就是开启的,而且无法关闭;二级缓存需要在MyBatis主配置文件中通过设置cacheEnabled参数值来开启。