Mybatis一级缓存和结合Spring Framework后失效的源码探究

Mybatis一级缓存和结合Spring Framework后失效的源码探究

1.在下面的案例中，执行两次查询控制台只会输出一次 SQL 查询：

mybatis-config.xml

PUBLIC "-//mybatis.org//DTD Config 3.0//EN"

"http://mybatis.org/dtd/mybatis-3-config.dtd">

PersonMapper.xml

id, name, age

select

from tab_person

public interface PersonMapper {

List list();

}

String resource = "mybatis-config2.xml";

InputStream inputStream = Resources.getResourceAsStream(resource);

SqlSessionFactory sqlSessionFactory =

new SqlSessionFactoryBuilder().build(inputStream);

SqlSession sqlSession = sqlSessionFactory.openSession();//开启会话

PersonMapper mapper = sqlSession.getMapper(PersonMapper.class);

mapper.list();

mapper.list();

之所以会出现这种情况，是因为 Mybatis 存在一级缓存导致的，下面 debug 探究下内部流程：

（1）mapper.list() 会进入 MapperProxy#invoke()：参数proxy是一个代理对象（每个 Mapper 接口都会被转换成一个代理对象），里面包含会话 sqlSession、接口信息、方法信息；method是目标方法（当前执行的方法），它里面包含了所属的哪个类（接口）、方法名、返回类型（List、Map、void 或其他）、参数类型等；args是参数；

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

try {

if (Object.class.equals(method.getDeclaringClass())) {

return method.invoke(this, args);

} else if (isDefaultMethod(method)) {

return invokeDefaultMethod(proxy, method, args);

}

} catch (Throwable t) {

throw ExceptionUtil.unwrapThrowable(t);

}

//从方法缓存methodCache中获取到方法的信息：比如方法名、类型（select、update等）、返回类型

//如果获取中没有MapperMethod，则创建一个并放入methodCache中

final MapperMethod mapperMethod = cachedMapperMethod(method);

//执行查询SQL并返回结果

return mapperMethod.execute(sqlSession, args);

}

cacheMapperMethod：MapperMethod 包含方法名、类型（select、update等）、返回类型等信息

private MapperMethod cachedMapperMethod(Method method) {

//缓存中获取

MapperMethod mapperMethod = methodCache.get(method);

//没有则创建一个对象并放入缓存中供下次方便取用

if (mapperMethod == null) {

mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());

methodCache.put(method, mapperMethod);

}

return mapperMethod;

}

（2）MapperMethod#execute()根据 SQL 类型进入不同的查询方法

public Object execute(SqlSession sqlSession, Object[] args) {

//返回结果

Object result;

//判断语句类型

switch (command.getType()) {

case INSERT: {//插入语句

Object param = method.convertArgsToSqlCommandParam(args);

result = rowCountResult(sqlSession.insert(command.getName(), param));

break;

}

case UPDATE: {//更新语句

Object param = method.convertArgsToSqlCommandParam(args);

result = rowCountResult(sqlSession.update(command.getName(), param));

break;

}

case DELETE: {//删除语句

Object param = method.convertArgsToSqlCommandParam(args);

result = rowCountResult(sqlSession.delete(command.getName(), param));

break;

}

case SELECT://查询语句

//返回空的查询

if (method.returnsVoid() && method.hasResultHandler()) {

executeWithResultHandler(sqlSession, args);

result = null;

//返回List的查询

} else if (method.returnsMany()) {

result = executeForMany(sqlSession, args);

//返回Map的查询

} else if (method.returnsMap()) {

result = executeForMap(sqlSession, args);

//返回游标的查询

} else if (method.returnsCursor()) {

result = executeForCursor(sqlSession, args);

} else {

Object param = method.convertArgsToSqlCommandParam(args);

result = sqlSession.selectOne(command.getName(), param);

}

break;

case FLUSH:

result = sqlSession.flushStatements();

break;

default:

throw new BindingException("Unknown execution method for: " + command.getName());

}

if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {

throw new BindingException("Mapper method '" + command.getName()

+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");

}

return result;

}

（3）上面的案例是 select 语句，返回结果是List集合，所以进入 MapperMethod#executeForMany()：

private Object executeForMany(SqlSession sqlSession, Object[] args) {

List result;

//获取参数

Object param = method.convertArgsToSqlCommandParam(args);

//是否有分页查询

if (method.hasRowBounds()) {

RowBounds rowBounds = method.extractRowBounds(args);

result = sqlSession.selectList(command.getName(), param, rowBounds);

} else {

result = sqlSession.selectList(command.getName(), param);

}

// issue #510 Collections & arrays support

//如果list中的泛型跟结果类型不一致，进行转换

if (!method.getReturnType().isAssignableFrom(result.getClass())) {

if (method.getReturnType().isArray()) {

return convertToArray(result);

} else {

return convertToDeclaredCollection(sqlSession.getConfiguration(), result);

}

}

return result;

}

（4）selectList执行了DefaultSqlSession#selectList()：

public List selectList(String statement, Object parameter) {

return this.selectList(statement, parameter, RowBounds.DEFAULT);

}

public List selectList(String statement, Object parameter, RowBounds rowBounds) {

try {

//SQL执行的信息：resource（xxMapper.xml）、id、sql、返回类型等

MappedStatement ms = configuration.getMappedStatement(statement);

//执行查询

return exNEgjwqZecutor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);

} catch (Exception e) {

throw ExceptionFactory.wrapException("Error querying database. Cause: " + e, e);

} finally {

ErrorContext.instance().reset();

}

}

（5）接下来调用缓存执行器的方法：CachingExecutor#query()

public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {

//获取到执行SQL

BoundSql boundSql = ms.getBoundSql(parameterObject);

//将SQL包装成一个缓存对对象，该对象和结果集组成键值对存储到缓存中，方便下次直接从缓存中拿而不需要再次查询

//createCacheKey：调用BaseExecutor#createCacheKey

CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);

return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

}

public List query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)

throws SQLException {

//获取缓存

Cache cache = ms.getCache();

if (cache != null) {

flushCacheIfRequired(ms);

if (ms.isUseCache() && resultHandler == null) {

ensureNoOutParams(ms, boundSql);

@SuppressWarnings("unchecked")

List list = (List) tcm.getObject(cache, key);

if (list == null) {

list = delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

tcm.putObject(cache, key, list); // issue #578 and #116

}

return list;

}

}

//没有缓存连接查询

return delegate. query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);

}

（6）接下来执行 BaseExecutor#query()：从下面可以看到将结果缓存到localCache 中了

public List 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.");

}

//如果不是嵌套查询（默认为0），且 的 flushCache=true 时清空缓存

if (queryStack == 0 && ms.isFlushCacheRequired()) {

clearLocalCache();

}

List list;

try {

//嵌套查询层数+1

queryStack++;

//从localCache缓存中获取

list = resultHandler == null ? (List) localCache.getObject(key) : null;

if (list != null) {

handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);

} else {

//连接查询

list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);

}

} finally {

queryStack--;

}

//下面是延迟加载逻辑

if (queryStack == 0) {

for (DeferredLoad deferredLoad : deferredLoads) {

deferredLoad.load();

}

// issue #601

deferredLoads.clear();

if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {

// issue #482

clearLocalCache();

}

}

return list;

}

private List queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {

List list;

//缓存中添加占位符

localCache.putObject(key, EXECUTION_PLACEHOLDER);

try {

//连接查询获取到数据结果

list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);

} finally {

//删除占位符

localCache.removeObject(key);

}

//将结果缓存起来

localCache.putObject(key, list);

//处理存储过程

if (ms.getStatementType() == StatementType.CALLABLE) {

localOutputParameterCache.putObject(key, parameter);

}

return list;

}

2.但当 Spring Framework + Mybatis 时，情况就不一样了，每次查询都会连接数据库查询，控制台都会打印 SQL 出来，如下案例：

@Service

public class PersonService {

@Autowired

PersonMapper personMapper;

public List getList() {

personMapper.list();

personMapper.list();

return personMapper.list();

}

}

@Configuration

@ComponentScan("com.hrh")

@MapperScan("com.hrh.mapper")

public class MyBatisConfig {

@Bean

public SqlSessionFactoryBean sqlSessionFactory() throws Exception {

SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();

factoryBean.setDataSource(dataSource());

factoryBean.setMapperLocations(resolveMapperLocations());

return factoryBean;

}

public Resource[] resolveMapperLocations() {

ResourcePatternResolver resourceResolver = new PathMatchingResourcePatternResolver();

List mapperLocations = new ArrayList<>();

mapperLocations.add("classpath*:com/hrh/mapper/*Mapper*.xml");

List resources = new ArrayList();

if (mapperLocations != null) {

for (String mapperLocation : mapperLocations) {

try {

Resource[] mappers = resourceResolver.getResources(mapperLocation);

resources.addAll(Arrays.asList(mappers));

} catch (IOException e) {

// ignore

}

}

}

return resources.toArray(new Resource[resources.size()]);

}

@Bean

public DataSource dataSource() {

DriverManagerDataSource driverManagerDataSource = new DriverManagerDataSource();

driverManagerDataSource.setDriverClassName("com.mysql.jdbc.Driver");

driverManagerDataSource.setUsername("xxx");

driverManagerDataSource.setPassword("xxx");

driverManagerDataSource.setUrl("jdbc:mysql://localhost:3306/xxx?useUnicode=true&characterEncodihttp://ng=utf-8&autoReconnect=true");

return driverManagerDataSource;

}

}

AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(MyBatisConfig.class);

PersonService bean = context.getBean(PersonService.class);

bean.getList();

下面debug进入的步骤跟上面的（1）、（2）、（3）是一致的，但第四步却是进入SqlSessionTemplate#selectList()中【SqlSessionTemplate是mybatis-spring-xx.jar的，上文的DefaultSqlSession是属于mybatis-xx.jar的】：

public List selectList(String statement, Object parameter) {

return this.selectList(statement, parameter, RowBounds.DEFAULT);

}

接下来的selectList() 会被方法拦截：method.invoke() 会执行到 DefaultSqlSession#selectList()，重新回到上文的第四步并且继续下去，也就是在上文的（1）~（6）中插入了前后文，在其中做了关闭会话的操作；

private class SqlSessionInterceptor implements InvocationHandler {

@Override

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

//得到会话

SqlSession sqlSession = getSqlSession(

SqlSessionTemplate.this.sqlSessionFactory,

SqlSessionTemplate.this.executorType,

SqlSessionTemplate.this.exceptionTranslator);

try {

//执行方法查询

Object result = method.invoke(sqlSession, args);

if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {

// force commit even on non-dirty sessions because some databases require

// a commit/rollback before calling close()

sqlSession.commit(true);//在关闭会话前提交和回滚

}

return result;

} catch (Throwable t) {//有异常抛出异常并结束会话

Throwable unwrapped = unwrapThrowable(t);

if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {

// release the connection to avoid a deadlock if the translator is no loaded. See issue #22

closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);

sqlSession = null;

Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);

if (translated != null) {

unwrapped = translated;

}

}

throw unwrapped;

} finally {

//关闭会话

if (sqlSession != null) {

closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);

}

}

}

}

总结：

Mybatis 的一级缓存是会话级别的缓存（单线程的，特别鸡肋），Mybatis 每创建一个 SqlSession 会话对象，就表示打开一次数据库会话，在一次会话中，应用程序很可能在短时间内反复执行相同的查询语句，如果不对数据进行缓存，则每查询一次就要执行一次数据库查询，这就造成数据库资源的浪费。又因为通过 SqlSession 执行的操作，实际上由 Executor 来完成数据库操作的，所以在 Executor 中会建立一个简单的缓存，即一级缓存；将每次的查询结果缓存起来，再次执行查询的时候，会先查询一级缓存（默认开启的），如果命中，则直接返回，否则再去查询数据库并放入缓存中。

一级缓存的生命周期与 SqlSession 的生命周期相同，因此当 Mybatis 和Spring Framework 的集成包中扩展了一个 SqlSessionTemplate 类（它是一个代理类，增强了查询方法），所有的查询经过 SqlSessionTemplate 代理拦截后再进入到 DefaultSqlSession#selectList() 中，结束查询后把会话SqlSession 关了，所以导致了缓存失效。

那为什么要这么操作呢？

原始的 Mybatis 有暴露 SqlSession 接口，因此有 close 方法暴露出来供你选择使用，你可以选择关与不关，但在Mybatis 和Spring Framework 的集成包中，SqlSession 是交给了Spring Framework 管理的，没有暴露出来，为了稳妥决定，直接给你关了。

版权声明：本文内容由网络用户投稿，版权归原作者所有，本站不拥有其著作权，亦不承担相应法律责任。如果您发现本站中有涉嫌抄袭或描述失实的内容，请联系我们jiasou666@gmail.com 处理，核实后本网站将在24小时内删除侵权内容。