mybatis在spring中的执行流程

简介

mybatis最终的 执行对象是一个代理对象，所以想要弄清mybatis的执行流程，就得从这个代理对象开始分析

MapperProxy

public class MapperProxy<T> implements InvocationHandler, Serializable {
	.....
	@Override
  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);
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }
	.....
}
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实现了InvocationHandler，使用的是jdk的动态代理，通过打断点继续往下看

  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: .....
      case UPDATE: .....
      case DELETE: .....
      case SELECT:
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        } else {
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      ......
    }
   
    return result;
  }
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execute方法中，针对不同的执行类型进行了不同的操作，这些执行 类型对应了sql
下面以select 的executeForMany进行分析

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
    List<E> result;
    Object param = method.convertArgsToSqlCommandParam(args);
    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
    if (!method.getReturnType().isAssignableFrom(result.getClass())) {
      if (method.getReturnType().isArray()) {
        return convertToArray(result);
      } else {
        return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
      }
    }
    return result;
  }
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1.将参数封装到RowBounds
2.执行sql

SqlSessionInterceptor.invoke(…)

SqlSessionTemplate.selectList(…)–>
SqlSessionInterceptor.invoke(…)

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
      SqlSession sqlSession = getSqlSession(
          SqlSessionTemplate.this.sqlSessionFactory,
          SqlSessionTemplate.this.executorType,
          SqlSessionTemplate.this.exceptionTranslator);
      try {
        Object result = method.invoke(sqlSession, args);
        if (!isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
          sqlSession.commit(true);
        }
        return result;
}```
这个invoke 方法中首先获取Sqlsession，然后执行，最后提交事物

### getSqlSession
```java
public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {

    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);

    SqlSession session = sessionHolder(executorType, holder);COCO
    if (session != null) {
      return session;
    }
    session = sessionFactory.openSession(executorType);
    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);
    return session;
  }
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这里就能看到非常熟悉的代码了sessionFactory.openSession
通过sessionFactory获取一个openSession

openSessionFromDataSource

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
      final Environment environment = configuration.getEnvironment();
      final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
      tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
      final Executor executor = configuration.newExecutor(tx, execType);
      return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
      closeTransaction(tx); // may have fetched a connection so lets call close()
      throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
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1.首先是获取了一个Transaction，就是开启一个事物，在spring中，这个事物类型是springManageTransactionFactory
2.这里看到了mybatis中第一个非常重要的对象获取一个Executor，Executor大家应该都了解，这里默认获取的是SIMPLE类型的
3.将Executor封装为DefaultSqlSession返回

selectList(…)

上面获取到了SqlSession方法，就要准备开始真正执行sql了，这个时候可能会有小伙伴要问，
执行sql之前不是还需要获取接口的代理对象么？？？其实这个接口的代理对象早就由spring帮忙生成了，然后我们就可以通过依赖注入使用，还不太了解的小伙伴可以先去了解下spring 与mybatis如何整合的

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);
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
    } finally {
      ErrorContext.instance().reset();
    }
  }
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这里看到了第二个非常重要的对象 MappedStatement

简单看一眼，MappedStatement里面存储的主要是执行sql可能会用到的信息

query(…)

  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);
  }
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这里看到了CacheKey，这个其实和二级缓存相关，接着往下看

public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
      throws SQLException {
    Cache cache = ms.getCache();
    if (cache != null) {
      flushCacheIfRequired(ms);
      if (ms.isUseCache() && resultHandler == null) {
        ensureNoOutParams(ms, parameterObject, boundSql);
        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;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }
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这里就是根据Cachekey来查看是否有缓存，如果有缓存，那就直接返回缓存内容了，第一次显然是没有的，这里也说明了，二级缓存的优先级比一级缓存高

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  	.......
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
   	.....
    return list;
  }
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localCache 就是我们常说的一级缓存，这里如果一级缓存有记录了，那么就会直接返回

queryFromDatabase

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;
  }
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这里可以看到，会将查询出来的结果缓存到一级缓存

query

  public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement) statement;
    ps.execute();
    return resultSetHandler.<E> handleResultSets(ps);
  }
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最后就可以看到，通过PreparedStatement执行sql，将执行的结果通过resultSetHandler进行处理，然后返回，至此，大体的查询流程就结束了

总结

根据不同sqlType进行不同的操作，先获取sqlSession，然后执行sql，如果开启了二级缓存，首先会在二级缓存里面找，找到了就返回。然后会在一级缓存里面找是否有对应缓存，找到了就返回，接着执行sql，将执行的结果添加缓存，最后结果通过ResultSetHandler封装返回