Mybatis一級緩存和結合Spring Framework后失效的源碼探

　　1.在下面的案例中，執行兩次查詢(xún)控制臺只會(huì )輸出一次 SQL 查詢(xún)：

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();//開(kāi)啟會(huì )話(huà)
        PersonMapper mapper = sqlSession.getMapper(PersonMapper.class);
        mapper.list();
        mapper.list();

　　之所以會(huì )出現這種情況，是因為 Mybatis 存在一級緩存導致的，下面 debug 探究下內部流程：

　?。?）mapper.list() 會(huì )進(jìn)入 MapperProxy#invoke()：參數proxy是一個(gè)代理對象（每個(gè) Mapper 接口都會(huì )被轉換成一個(gè)代理對象），里面包含會(huì )話(huà) sqlSession、接口信息、方法信息；method是目標方法（當前執行的方法），它里面包含了所屬的哪個(gè)類(lèi)（接口）、方法名、返回類(lèi)型（List、Map、void 或其他）、參數類(lèi)型等；args是參數；

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);
    }
    //從方法緩存methodCache中獲取到方法的信息：比如方法名、類(lèi)型（select、update等）、返回類(lèi)型
    //如果獲取中沒(méi)有MapperMethod，則創(chuàng  )建一個(gè)并放入methodCache中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //執行查詢(xún)SQL并返回結果
    return mapperMethod.execute(sqlSession, args);
  }

　　cacheMapperMethod：MapperMethod 包含方法名、類(lèi)型（select、update等）、返回類(lèi)型等信息

private MapperMethod cachedMapperMethod(Method method) {
    //緩存中獲取
    MapperMethod mapperMethod = methodCache.get(method);
    //沒(méi)有則創(chuàng  )建一個(gè)對象并放入緩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;
  }

　?。?）MapperMethod#execute()根據 SQL 類(lèi)型進(jìn)入不同的查詢(xún)方法

public Object execute(SqlSession sqlSession, Object[] args) {
    //返回結果
    Object result;
    //判斷語(yǔ)句類(lèi)型
    switch (command.getType()) {
      case INSERT: {//插入語(yǔ)句
      Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {//更新語(yǔ)句
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {//刪除語(yǔ)句
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT://查詢(xún)語(yǔ)句
        //返回空的查詢(xún)
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
          //返回List的查詢(xún)
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
          //返回Map的查詢(xún)
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
          //返回游標的查詢(xún)
        } 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;
  }

　?。?）上面的案例是 select 語(yǔ)句，返回結果是List集合，所以進(jìn)入 MapperMethod#executeForMany()：

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    //獲取參數
    Object param = method.convertArgsToSqlCommandParam(args);
    //是否有分頁(yè)查詢(xún)
    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中的泛型跟結果類(lèi)型不一致，進(jìn)行轉換
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }

　?。?）selectList執行了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執行的信息：resource（xxMapper.xml）、id、sql、返回類(lèi)型等
      MappedStatement ms = configuration.getMappedStatement(statement);
      //執行查詢(xún)
      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();
    }
  }

　?。?）接下來(lái)調用緩存執行器的方法：CachingExecutor#query()

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    //獲取到執行SQL
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    //將SQL包裝成一個(gè)緩存對對象，該對象和結果集組成鍵值對存儲到緩存中，方便下次直接從緩存中拿而不需要再次查詢(xún)
    //createCacheKey：調用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 {
    //獲取緩存
    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;
      }
    }
    //沒(méi)有緩存連接查詢(xún)
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

　?。?）接下來(lái)執行 BaseExecutor#query()：從下面可以看到將結果緩存到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.");
    }
    //如果不是嵌套查詢(xún)（默認為0），且 <select> 的 flushCache=true 時(shí)清空緩存
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
     //嵌套查詢(xún)層數+1
      queryStack++;
      //從localCache緩存中獲取
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        //連接查詢(xún)
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    //下面是延遲加載邏輯
    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;
    //緩存中添加占位符
    localCache.putObject(key, EXECUTION_PLACEHOLDER);
    try {
      //連接查詢(xún)獲取到數據結果    
      list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
      //刪除占位符
      localCache.removeObject(key);
    }
    //將結果緩存起來(lái)
    localCache.putObject(key, list);
    //處理存儲過(guò)程
    if (ms.getStatementType() == StatementType.CALLABLE) {
      localOutputParameterCache.putObject(key, parameter);
    }
    return list;
  }

　　2.但當 Spring Framework + Mybatis 時(shí)，情況就不一樣了，每次查詢(xún)都會(huì )連接數據庫查詢(xún)，控制臺都會(huì )打印 SQL 出來(lái)，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進(jìn)入的步驟跟上面的（1）、（2）、（3）是一致的，但第四步卻是進(jìn)入SqlSessionTemplate#selectList()中【SqlSessionTemplate是mybatis-spring-xx.jar的，上文的DefaultSqlSession是屬于mybatis-xx.jar的】：

  public <E> List<E> selectList(String statement, Object parameter) {
    return this.selectList(statement, parameter, RowBounds.DEFAULT);
  }

　　接下來(lái)的selectList() 會(huì )被方法攔截：method.invoke() 會(huì )執行到 DefaultSqlSession#selectList()，重新回到上文的第四步并且繼續下去，也就是在上文的（1）~（6）中插入了前后文，在其中做了關(guān)閉會(huì )話(huà)的操作；

private class SqlSessionInterceptor implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      //得到會(huì )話(huà)
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        //執行方法查詢(xún)
        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);//在關(guān)閉會(huì )話(huà)前提交和回滾
        }
        return result;
        
      } catch (Throwable t) {//有異常拋出異常并結束會(huì )話(huà)
        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 {
        //關(guān)閉會(huì )話(huà)
        if (sqlSession != null) {
          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
        }
      }
    }
  }

總結：

　　Mybatis 的一級緩存是會(huì )話(huà)級別的緩存（單線(xiàn)程的，特別雞肋），Mybatis 每創(chuàng )建一個(gè) SqlSession 會(huì )話(huà)對象，就表示打開(kāi)一次數據庫會(huì )話(huà)，在一次會(huì )話(huà)中，應用程序很可能在短時(shí)間內反復執行相同的查詢(xún)語(yǔ)句，如果不對數據進(jìn)行緩存，則每查詢(xún)一次就要執行一次數據庫查詢(xún)，這就造成數據庫資源的浪費。又因為通過(guò) SqlSession 執行的操作，實(shí)際上由 Executor 來(lái)完成數據庫操作的，所以在 Executor 中會(huì )建立一個(gè)簡(jiǎn)單的緩存，即一級緩存；將每次的查詢(xún)結果緩存起來(lái)，再次執行查詢(xún)的時(shí)候，會(huì )先查詢(xún)一級緩存（默認開(kāi)啟的），如果命中，則直接返回，否則再去查詢(xún)數據庫并放入緩存中。

　　一級緩存的生命周期與 SqlSession 的生命周期相同，因此當 Mybatis 和Spring Framework 的集成包中擴展了一個(gè) SqlSessionTemplate 類(lèi)（它是一個(gè)代理類(lèi)，增強了查詢(xún)方法），所有的查詢(xún)經(jīng)過(guò) SqlSessionTemplate 代理攔截后再進(jìn)入到 DefaultSqlSession#selectList() 中，結束查詢(xún)后把會(huì )話(huà)SqlSession 關(guān)了，所以導致了緩存失效。

　　那為什么要這么操作呢？

　　原始的 Mybatis 有暴露 SqlSession 接口，因此有 close 方法暴露出來(lái)供你選擇使用，你可以選擇關(guān)與不關(guān)，但在Mybatis 和Spring Framework 的集成包中，SqlSession 是交給了Spring Framework 管理的，沒(méi)有暴露出來(lái)，為了穩妥決定，直接給你關(guān)了。

到此這篇關(guān)于Mybatis一級緩存和結合Spring Framework后失效的源碼探究的文章就介紹到這了,更多相關(guān)Mybatis一級緩存Spring Framework失效內容請搜索腳本之家以前的文章或繼續瀏覽下面的相關(guān)文章希望大家以后多多支持腳本之家！