众所周知mybatis缓存体系分为一级缓存和二级缓存，因此今天就分别聊聊这两级缓存。spring
一级缓存的使用是不须要任何配置的，直接使用session就可使用一级缓存。代码以下:sql
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
}...
}
}
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameter);
CacheKey key = createCacheKey(ms, parameter, rowBounds, boundSql);
return query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}
public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
...
list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
...
return list;
}
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);
}
localCache.putObject(key, list);
if (ms.getStatementType() == StatementType.CALLABLE) {
localOutputParameterCache.putObject(key, parameter);
}
return list;
}
复制代码
localCache就是所谓的一级缓存。数据库
this.localCache = new PerpetualCache("LocalCache");
public class PerpetualCache implements Cache {
private final String id;
private Map<Object, Object> cache = new HashMap<Object, Object>();
public PerpetualCache(String id) {
this.id = id;
}
}
复制代码
从代码中能够看出localCache是一个PerpetualCache的实现类。在该类里面，存了一个hashmap,这即是一级缓存。看到这儿你们会发现一级缓存是如此简单，以致于没有任何的淘汰，过时策略! 若是一直执行查询的话，一级缓存会不断增长。那么何时一级缓存会清空呢？两种状况:缓存
看到这儿你们会发现，一级缓存并很差用。bash
这边还有个细节就是当mybatis与spring整合的时候，mybatis在SqlSessionTemplate类中给session封装了一层SqlSessionInterceptor，而这个类中有这样一个逻辑。session
private class SqlSessionInterceptor implements InvocationHandler {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
...
Object result = method.invoke(sqlSession, args);
if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
sqlSession.commit(true);
}
return result;
...
}
}
复制代码
若是该线程没有开启事务则执行sqlSession.commit(true)。也就是清空一级缓存。因此你们在spring中使用mybatis的话，是不用担忧一级缓存的问题的，由于每次操做后都会删掉。固然在spring中你也别想使用一级缓存。mybatis
二级缓存使用须要开启一下配置，首先在mapper文件中添加cache配置。app
<mapper namespace="***">
<cache/>
</mapper>
复制代码
其次数据bean还须要实现Serializable接口。这样就能够开启二级缓存了。二级缓存代码以下:ide
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
try {
MappedStatement ms = configuration.getMappedStatement(statement);
return executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
} ...
}
}
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
BoundSql boundSql = ms.getBoundSql(parameterObject);
CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
throws SQLException {
Cache cache = ms.getCache();
...
List<E> list = (List<E>) tcm.getObject(cache, key);
if (list == null) {
list = delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
tcm.putObject(cache, key, list);
}
return list;
}
...
}
复制代码
这个地方cache就是二级缓存。固然二级缓存在使用上和一级缓存稍有区别。一级缓存执行session.commit()以后，缓存就清空了。二级缓存则必须执行session.commit()数据才会被真正的缓存下来。这边能够看一下 tcm.putObject(cache, key, list); 这个方法post
public void putObject(Cache cache, CacheKey key, Object value) {
getTransactionalCache(cache).putObject(key, value);
}
public void putObject(Object key, Object object) {
数据仅仅是被存到了entriesToAddOnCommit这个里面
entriesToAddOnCommit.put(key, object);
}
session.commit();
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();
}
}
public void commit(boolean required) throws SQLException {
delegate.commit(required);
//这个方法才是真正存数据的方法
tcm.commit();
}
public void commit() {
for (TransactionalCache txCache : transactionalCaches.values()) {
txCache.commit();
}
}
public void commit() {
if (clearOnCommit) {
delegate.clear();
}
flushPendingEntries();
reset();
}
private void flushPendingEntries() {
for (Map.Entry<Object, Object> entry : entriesToAddOnCommit.entrySet()) {
delegate.putObject(entry.getKey(), entry.getValue());
}
for (Object entry : entriesMissedInCache) {
if (!entriesToAddOnCommit.containsKey(entry)) {
delegate.putObject(entry, null);
}
}
}
复制代码
二级缓存是带有过时和淘汰策略的。进入XMLMapperBuilder的cacheElement方法:
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);
}
}
复制代码
这个地方能够看到和缓存相关的配置以及默认值。这边逻辑比较简单你们本身看看就好。
那么相比于一级缓存而言，拥有了过时，淘汰等策略，还能够自定义的二级缓存是否是就好用一点了呢？至少我以为不是，由于翻看代码能够发现，在默认状况下，每次执行增删改等操做的时候，二级缓存也会被清空。这等于手动触发缓存雪崩啊。固然你能够本身定义，可是由于CacheKey是固定生成模式，想要本身定义，得本身解析相应的key或者结果集，仍是比较麻烦的。
因此综上所述，我我的认为mybatis的缓存体系比较鸡肋，不实用。