mybatis源码(十)mybatis二级缓存的使用
mybatis源码(十)mybatis二级缓存的使用
mybatis的二级缓存是mapper级别的缓存
1.mybatis中如何使用二级缓存
a.mybatis的主配置文件的settings中设置cacheEnabled=true
b.mybatis的mapper的配置文件中，配置缓存策略、缓存刷新时间、缓存容量等属性.这个有专门的cache标签。
c.在配置mapper时，通过usecache属性指定是否使用缓存、通过flushCache指定mapper执行后是否刷新缓存。
2.mybatis二级缓存的实现原理
2.1 mybatis二级缓存开启的地方
当mybatis的主配置文件的settings中设置cacheEnabled=true时，创建Executor的时候，默认是创建简单的执行器，这里替换成CacheExecutor，是执行器接口的一个实现类
newExecutor 方法是在sqlSessionFactory.openSession()是调用的
public Executor newExecutor(Transaction transaction) {
return newExecutor(transaction, defaultExecutorType);
}
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;
}
代码执行到这里，我们的执行器已经不是默认的simple执行器了，而是CachingExecutor. 后续执行代码的时候，就是通过该执行器执行代码
2.2 mybatis二级缓存的创建过程
二级缓存的创建是在解析mapper配置文件的时候创建的。
流程如下：
XMLConfigBuilder：解析mybatis主配置文件--->解析mybatis的mapper配置文件---->解析cache标签---->创建缓存对象
public class XMLConfigBuilder extends BaseBuilder {
// 解析主配置文件
public Configuration parse() {
// 防止parse（）方法被同一个实例多次调用
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
// 调用XPathParser.evalNode（）方法，创建表示configuration节点的XNode对象。
// 调用parseConfiguration（）方法对XNode进行处理
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
// 解析mybatis主配置文件下configuration对象
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"));
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
// 解析mapper标签
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
// 通过<package>标签指定包名
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");
// 通过resource属性指定XML文件路径
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());
mapperParser.parse();
} else if (resource == null && url != null && mapperClass == null) {
// 通过url属性指定XML文件路径
ErrorContext.instance().resource(url);
InputStream inputStream = Resources.getUrlAsStream(url);
// 解析具体的mapper文件
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url == null && mapperClass != null) {
// 通过class属性指定接口的完全限定名
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.");
}
}
}
}
}
// 解析具体的mapper文件 XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments()); mapperParser.parse();
从这里进去XMLMapperBuilder，解析具体的mapper文件,代码如下：
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();
// 通过MapperBuilderAssiatant创建Cache
builderAssistant.useNewCache(typeClass, evictionClass, flushInterval, size, readWrite, blocking, props);
}
}
MapperBuilderAssistant
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);
currentCache = cache; // 这里添加进来了
return cache;
}
如上面的代码所示，在获取<cache>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。
下面是创建MappedStatement对象的关键代码:
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;
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); // 在这里添加进来了
ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id); if (statementParameterMap != null) { statementBuilder.parameterMap(statementParameterMap); } MappedStatement statement = statementBuilder.build(); configuration.addMappedStatement(statement); return statement; }
2.3 mybaits的二级缓存的原理
CacheExcutor的query操作源码如下：
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 <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> 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);
}
}
如上面的代码所示，在CachingExecutor的query()方法中， 首先调用createCacheKey()方法创建缓存Key对象，然后调用MappedStatement对象的getCache()方法获取MappedStatement对象
中维护的二级缓存对象。然后尝试从二级缓存对象中获取结果，如果获取不到，则调用目标Executor对象的query()方法从数据库获取数据,再将数据添加到二级缓存中。
由上代码可知，二级缓存是从TransactionalCacheManager中获取的，该类的源码如下：
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;
}
}
2.4 mybaits的二级缓存删除地方
当执行update更新操作的时候，同一命名空间下的二级缓存会被清空，下面是CachingExecutor的update方法
@Override
public int update(MappedStatement ms, Object parameterObject) throws SQLException {
// 如果需要刷新，则更新缓存
flushCacheIfRequired(ms);
return delegate.update(ms, parameterObject);
}
public boolean isFlushCacheRequired() {
return flushCacheRequired;
}
private void flushCacheIfRequired(MappedStatement ms) {
Cache cache = ms.getCache();
if (cache != null && ms.isFlushCacheRequired()) {
tcm.clear(cache);
}
}
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