mybatis是一個查詢數據庫的封裝框架,主要是封裝提供靈活的增刪改sql,開發中,service層可以經過mybatis組件查詢和修改數據庫中表的數據;做爲查詢工具,mybatis有使用緩存,這裏講一下mybatis的緩存相關源碼。java
緩存sql
在計算機裏面,任何信息都有源頭,緩存通常指源頭信息讀取後,放在內存或者其餘讀取較快的地方,下次讀取相同信息不去源頭查詢而是直接從內存(或者能快速存取的硬件)讀取。這樣能夠減小硬件使用,提升讀取速度。數據庫
mybatis也是這樣,查詢數據庫的數據以後,mybatis能夠把查詢結果緩存到內存,下次查詢若是查詢語句相同,而且查詢相關的表的數據沒被修改過,就能夠直接返回緩存中的結果,而不用去查詢數據庫的語句,有效節省了時間。apache
簡單看一下mybatis一級緩存和二級緩存相關源碼,學習使用緩存
經過查看源碼可知,一級緩存是綁定sqSsession中的,因此每次查詢sqlSession不一樣就失效,相同的sqlSession可使用一級緩存。session
mybatis默認sqlsession:org.apache.ibatis.session.defaults.DefaultSqlSession
mybatis
構造方法中傳入executor(查詢執行對象)app
public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) { this.configuration = configuration; this.executor = executor; this.dirty = false; this.autoCommit = autoCommit; }
executor中攜帶一級緩存成員:框架
protected BaseExecutor(Configuration configuration, Transaction transaction) { this.transaction = transaction; this.deferredLoads = new ConcurrentLinkedQueue<>(); this.localCache = new PerpetualCache("LocalCache"); //默認一級緩存 this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache"); this.closed = false; this.configuration = configuration; this.wrapper = this; }
org.apache.ibatis.executor.BaseExecutor.query()
ide
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()); List<E> list; try { queryStack++; //localCache 一級緩存 list = resultHandler == null ? (List<E>) localCache.getObject(key) : null; //先從一級緩存中獲取,key是經過sql語句生成 if (list != null) { handleLocallyCachedOutputParameters(ms, key, parameter, boundSql); } else { // 若是緩存中沒有 才從數據庫查詢 list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql); } } finally { queryStack--; } return list; } //從數據庫讀取數據 private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException { List<E> list; localCache.putObject(key, EXECUTION_PLACEHOLDER); try { list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql); } finally { localCache.removeObject(key);//將一級緩存清除 } localCache.putObject(key, list);//返回查詢結果以前,先放入一級緩存 刷新 if (ms.getStatementType() == StatementType.CALLABLE) { localOutputParameterCache.putObject(key, parameter); } return list; }
二級緩存mapper中的,默認是開啓的,但須要在映射文件mapper.xml中添加<cache/>
標籤
<mapper namespace="userMapper"> <cache/><!-- 添加cache標籤表示此mapper使用二級緩存 --> </mapper>
配置false能夠關閉二級緩存
mybatis: configuration: cache-enabled: false #默認值爲true,表示開啓
org.apache.ibatis.builder.xml.XMLMapperBuilder
private void configurationElement(XNode context) { try { //... cacheElement(context.evalNode("cache")); //解析cache標籤 } catch (Exception e) { throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e); } } private void cacheElement(XNode context) { if (context != null) { // if hava cache tag 若是有cache標籤才執行下面的邏輯 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);//創建二級緩存 } }
org.apache.ibatis.builder.MapperBuilderAssistant.useNewCache()
:
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) .implementation(valueOrDefault(typeClass, PerpetualCache.class)) .addDecorator(valueOrDefault(evictionClass, LruCache.class)) .clearInterval(flushInterval) .size(size) .readWrite(readWrite) .blocking(blocking) .properties(props) .build(); configuration.addCache(cache);//二級緩存賦值,若是cache標籤爲空,不會執行此方法,currentCache爲空 currentCache = cache; return cache; }
在映射文件mapper中若是沒有cache標籤,不會執行上面的useNewCache方法,cache爲null,就不會使用二級緩存(至關於失效)。
org.apache.ibatis.executor.CachingExecutor
:
@Override 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) {//若是二級緩存對象不爲空 嘗試在二級緩存中獲取(沒有cache標籤此對象就是空) flushCacheIfRequired(ms); if (ms.isUseCache() && resultHandler == null) { ensureNoOutParams(ms, boundSql); @SuppressWarnings("unchecked") List<E> list = (List<E>) tcm.getObject(cache, key); //從二級緩存中獲取數據 if (list == null) { list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); //若是爲空,使用delegate查詢(BaseExecutor) tcm.putObject(cache, key, list); // 查詢結果保存到二級緩存 } return list; } } return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql); }
二級緩存和一級緩存不用想,數據庫的數據被修改是要清空緩存的,否則數據有誤,至於怎麼清空,是另外一套邏輯了,mapper中的cache標籤能夠配置一些參數,好比緩存按期清空。
mybatis默認是先查詢二級緩存,沒有,再查看一級緩存,都爲空,最後查詢數據庫