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Mybatis一级缓存和结合Spring Framework后失效的源码探究
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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 <?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/xxx?useUnicode=true&characterEncoding=utf-8&autoReconnect=true"/>
<property name="username" value="xxx"/>
<property name="password" value="xxx"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="com/hrh/mapper/PersonMapper.xml"/>
</mappers>
</configuration>
PersonMapper.xml
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd" >
<mapper namespace="com.hrh.mapper.PersonMapper">
<resultMap id="BaseResultMap" type="com.hrh.bean.Person">
<id column="id" property="id" jdbcType="BIGINT"/>
<result column="name" property="name" jdbcType="VARCHAR"/>
<result column="age" property="age" jdbcType="BIGINT"/>
</resultMap>
<sql id="Base_Column_List">
id, name, age
</sql>
<select id="list" resultType="com.hrh.bean.Person">
select
<include refid="Base_Column_List"/>
from tab_person
</select>
</mapper>
public interface PersonMapper {
List<Person> 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 <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
List<E> 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.<E>selectList(command.getName(), param, rowBounds);
} else {
result = sqlSession.<E>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 <E> List<E> selectList(String statement, Object parameter) {
return this.selectList(statement, parameter, RowBounds.DEFAULT);
}
public <E> List<E> 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 executor.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 <E> List<E> 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 <E> List<E> 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<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); // issue #578 and #116
}
return list;
}
}
//没有缓存连接查询
return delegate.<E> 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 <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.");
}
//如果不是嵌套查询（默认为0），且 <select> 的 flushCache=true 时清空缓存
if (queryStack == 0 && ms.isFlushCacheRequired()) {
clearLocalCache();
}
List<E> list;
try {
//嵌套查询层数+1
queryStack++;
//从localCache缓存中获取
list = resultHandler == null ? (List<E>) 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 <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
List<E> 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<Person> 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<String> mapperLocations = new ArrayList<>();
mapperLocations.add("classpath*:com/hrh/mapper/*Mapper*.xml");
List<Resource> 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&characterEncoding=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 <E> List<E> 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。
