Mybatis 源码分析（二）之 Mybatis 操作数据库的流程

Mybatis系列： Mybatis 基础介绍与逆向工程的构建：https://www.jianshu.com/p/1c18db4d7a38 Mybatis 源码分析（一）之 Mybatis 的Executor的初始化：https://www.jianshu.com/p/c7425c841337 Mybatis 源码分析（二）之 Mybatis 操作数据库的流程：https://www.jianshu.com/p/11d354ec3612 Mybatis 源码分析（三）之 Mybatis 的一级缓存和二级缓存：https://www.jianshu.com/p/5515640d14fe

承接之前的 Mybatis 源码分析（一）之 Mybatis 的Executor的初始化继续进行之后的源码分析。在之前的分析中，我们知道初始化Executor的流程。

SqlSessionFactoryBuilder->parse->Configuation->build->SqlSessionFactory->openSession->SqlSession->query->Executor

接下来则是mybatis操作数据库的流程

执行查询

org.apache.ibatis.session.defaults.DefaultSqlSession#selectList(java.lang.String, java.lang.Object, org.apache.ibatis.session.RowBounds)
    
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    List var5;
    try {
        MappedStatement ms = this.configuration.getMappedStatement(statement);
        var5 = this.executor.query(ms, this.wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    } catch (Exception var9) {
        throw ExceptionFactory.wrapException("Error querying database.  Cause: " + var9, var9);
    } finally {
        ErrorContext.instance().reset();
    }

    return var5;
}

其中

MappedStatement ms = this.configuration.getMappedStatement(statement);

getMappedStatement是个map进行get，statement是我们传入的selectByPrimaryKey等参数，如下中的id。

<select id="selectByPrimaryKey" parameterType="java.lang.Integer" resultMap="BaseResultMap">
    select 
    <include refid="Base_Column_List" />
    from food
    where id = #{id,jdbcType=INTEGER}
</select>

进行进入query

org.apache.ibatis.executor.BaseExecutor#query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler, org.apache.ibatis.cache.CacheKey, org.apache.ibatis.mapping.BoundSql)

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 (this.closed) {
        throw new ExecutorException("Executor was closed.");
    } else {
        if (this.queryStack == 0 && ms.isFlushCacheRequired()) {
            this.clearLocalCache();
        }

        List list;
        try {
            ++this.queryStack;
            list = resultHandler == null ? (List)this.localCache.getObject(key) : null;
            if (list != null) {
                this.handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
            } else {
                list = this.queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
            }
        } finally {
            --this.queryStack;
        }

        if (this.queryStack == 0) {
            Iterator i$ = this.deferredLoads.iterator();

            while(i$.hasNext()) {
                BaseExecutor.DeferredLoad deferredLoad = (BaseExecutor.DeferredLoad)i$.next();
                deferredLoad.load();
            }

            this.deferredLoads.clear();
            if (this.configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
                this.clearLocalCache();
            }
        }

        return list;
    }
}

其中

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

传入的参数key，就是我们拼出来的cache key ： cache key: id + sql + limit + offset

例如：

-2061571805:4913609797:com.demo.mybatis.mapper.UserMapper.selectByPrimaryKey:0:2147483647:select 
 
id, name, age

from user
where id = ?:1:development

假如第一次查询没有缓存，则查询数据库

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

继续进去

org.apache.ibatis.executor.BaseExecutor#queryFromDatabase

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    //先放一个值占位
    this.localCache.putObject(key, ExecutionPlaceholder.EXECUTION_PLACEHOLDER);

    List list;
    try {
         //查询数据
        list = this.doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
    } finally {
        //删除占位的
        this.localCache.removeObject(key);
    }

    //将查询出来的结果存入一级缓存
    this.localCache.putObject(key, list);
    if (ms.getStatementType() == StatementType.CALLABLE) {
        //如果是存储过程把参数存入localOutputParameterCache
        this.localOutputParameterCache.putObject(key, parameter);
    }

    return list;
}

进行数据查询

org.apache.ibatis.executor.SimpleExecutor#doQuery

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;

    List var9;
    try {
        Configuration configuration = ms.getConfiguration();
        StatementHandler handler = configuration.newStatementHandler(this.wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
        stmt = this.prepareStatement(handler, ms.getStatementLog());
        var9 = handler.query(stmt, resultHandler);
    } finally {
        this.closeStatement(stmt);
    }

    return var9;
}

进入query

org.apache.ibatis.executor.statement.PreparedStatementHandler#query

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    PreparedStatement ps = (PreparedStatement)statement;
    ps.execute();
    return this.resultSetHandler.handleResultSets(ps);
}

其中

 ps.execute();

底层仍然是我们熟悉的JDBC

java.sql.PreparedStatement#execute

然后进入handleResultSets对我们的结果信息进封装，最后返回给我们的，就是我们定义的pojo

org.apache.ibatis.executor.resultset.DefaultResultSetHandler#handleResultSets

public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(this.mappedStatement.getId());
    List<Object> multipleResults = new ArrayList();
    int resultSetCount = 0;
    ResultSetWrapper rsw = this.getFirstResultSet(stmt);
    List<ResultMap> resultMaps = this.mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    this.validateResultMapsCount(rsw, resultMapCount);

    while(rsw != null && resultMapCount > resultSetCount) {
        ResultMap resultMap = (ResultMap)resultMaps.get(resultSetCount);
        this.handleResultSet(rsw, resultMap, multipleResults, (ResultMapping)null);
        rsw = this.getNextResultSet(stmt);
        this.cleanUpAfterHandlingResultSet();
        ++resultSetCount;
    }

    String[] resultSets = this.mappedStatement.getResulSets();
    if (resultSets != null) {
        while(rsw != null && resultSetCount < resultSets.length) {
            ResultMapping parentMapping = (ResultMapping)this.nextResultMaps.get(resultSets[resultSetCount]);
            if (parentMapping != null) {
                String nestedResultMapId = parentMapping.getNestedResultMapId();
                ResultMap resultMap = this.configuration.getResultMap(nestedResultMapId);
                this.handleResultSet(rsw, resultMap, (List)null, parentMapping);
            }

            rsw = this.getNextResultSet(stmt);
            this.cleanUpAfterHandlingResultSet();
            ++resultSetCount;
        }
    }

    return this.collapseSingleResultList(multipleResults);
}

其中

private List<Object> collapseSingleResultList(List<Object> multipleResults) {
    return multipleResults.size() == 1 ? (List)multipleResults.get(0) : multipleResults;
}

调用链

org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(java.lang.String, java.lang.Object)
 >org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(java.lang.String, java.lang.Object)
   >org.apache.ibatis.session.Configuration.getMappedStatement(java.lang.String)
    >org.apache.ibatis.executor.CachingExecutor.query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler)
     >org.apache.ibatis.executor.CachingExecutor.createCacheKey 创建缓存的key
      >org.apache.ibatis.executor.CachingExecutor.query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler, org.apache.ibatis.cache.CacheKey, org.apache.ibatis.mapping.BoundSql)
         >org.apache.ibatis.executor.BaseExecutor.queryFromDatabase
           >org.apache.ibatis.executor.BaseExecutor.doQuery
             >org.apache.ibatis.executor.statement.PreparedStatementHandler.query
               >org.apache.ibatis.executor.resultset.ResultSetHandler.handleResultSets
                >org.apache.ibatis.executor.resultset.DefaultResultSetHandler

上面是操作数据库的流程，其中真正干活的是handler，mybatis底层操作数据库的仍然是我们熟悉的JDBC。

其中获取mapper的过程

org.apache.ibatis.session.defaults.DefaultSqlSessionFactory#openSessionFromDataSource

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;

    DefaultSqlSession var8;
    try {
        Environment environment = this.configuration.getEnvironment();
        TransactionFactory transactionFactory = this.getTransactionFactoryFromEnvironment(environment);
        tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
        Executor executor = this.configuration.newExecutor(tx, execType);
        var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);
    } catch (Exception var12) {
        this.closeTransaction(tx);
        throw ExceptionFactory.wrapException("Error opening session.  Cause: " + var12, var12);
    } finally {
        ErrorContext.instance().reset();
    }

    return var8;
}

其中

var8 = new DefaultSqlSession(this.configuration, executor, autoCommit);

然后

org.apache.ibatis.session.defaults.DefaultSqlSession#getMapper

public <T> T getMapper(Class<T> type) {
        return this.configuration.getMapper(type, this);
    }
    
    
org.apache.ibatis.session.Configuration#getMapper

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return this.mapperRegistry.getMapper(type, sqlSession);
}

org.apache.ibatis.binding.MapperRegistry#getMapper

public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
    if (mapperProxyFactory == null) {
        throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    } else {
        try {
            return mapperProxyFactory.newInstance(sqlSession);
        } catch (Exception var5) {
            throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
        }
    }
}

调用链

org.apache.ibatis.session.defaults.DefaultSqlSession.getMapper
   >org.apache.ibatis.session.Configuration.getMapper
    >org.apache.ibatis.binding.MapperRegistry.getMapper
      >org.apache.ibatis.binding.MapperProxyFactory.newInstance(org.apache.ibatis.session.SqlSession)
        >org.apache.ibatis.binding.MapperMethod.execute

其中获取sql的过程

org.apache.ibatis.binding.MapperMethod#execute

public Object execute(SqlSession sqlSession, Object[] args) {
    Object param;
    Object result;
    if (SqlCommandType.INSERT == this.command.getType()) {
        param = this.method.convertArgsToSqlCommandParam(args);
        result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
    } else if (SqlCommandType.UPDATE == this.command.getType()) {
        param = this.method.convertArgsToSqlCommandParam(args);
        result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
    } else if (SqlCommandType.DELETE == this.command.getType()) {
        param = this.method.convertArgsToSqlCommandParam(args);
        result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
    } else if (SqlCommandType.SELECT == this.command.getType()) {
        if (this.method.returnsVoid() && this.method.hasResultHandler()) {
            this.executeWithResultHandler(sqlSession, args);
            result = null;
        } else if (this.method.returnsMany()) {
            result = this.executeForMany(sqlSession, args);
        } else if (this.method.returnsMap()) {
            result = this.executeForMap(sqlSession, args);
        } else {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = sqlSession.selectOne(this.command.getName(), param);
        }
    } else {
        if (SqlCommandType.FLUSH != this.command.getType()) {
            throw new BindingException("Unknown execution method for: " + this.command.getName());
        }

        result = sqlSession.flushStatements();
    }

    if (result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
        throw new BindingException("Mapper method '" + this.command.getName() + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
    } else {
        return result;
    }
}

然后

org.apache.ibatis.session.defaults.DefaultSqlSession#selectOne(java.lang.String, java.lang.Object)

public <T> T selectOne(String statement, Object parameter) {
    List<T> list = this.selectList(statement, parameter);
    if (list.size() == 1) {
        return list.get(0);
    } else if (list.size() > 1) {
        throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
    } else {
        return null;
    }
}

org.apache.ibatis.executor.BaseExecutor#query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler)

public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameter);
    CacheKey key = this.createCacheKey(ms, parameter, rowBounds, boundSql);
    return this.query(ms, parameter, rowBounds, resultHandler, key, boundSql);
}

最后

org.apache.ibatis.mapping.MappedStatement#getBoundSql

public BoundSql getBoundSql(Object parameterObject) {
    BoundSql boundSql = this.sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.isEmpty()) {
        boundSql = new BoundSql(this.configuration, boundSql.getSql(), this.parameterMap.getParameterMappings(), parameterObject);
    }

    Iterator i$ = boundSql.getParameterMappings().iterator();

    while(i$.hasNext()) {
        ParameterMapping pm = (ParameterMapping)i$.next();
        String rmId = pm.getResultMapId();
        if (rmId != null) {
            ResultMap rm = this.configuration.getResultMap(rmId);
            if (rm != null) {
                this.hasNestedResultMaps |= rm.hasNestedResultMaps();
            }
        }
    }

    return boundSql;
}

调用链

org.apache.ibatis.binding.MapperMethod.execute
>org.apache.ibatis.session.SqlSession.selectOne(java.lang.String, java.lang.Object)
  >org.apache.ibatis.executor.BaseExecutor.query(org.apache.ibatis.mapping.MappedStatement, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler)
    >org.apache.ibatis.mapping.MappedStatement.getBoundSql

总结

整个流程如下：

SqlSessionFactoryBuilder->parse->Configuation->build->SqlSessionFactory
->openSession->SqlSession->query->Executor->newStatementHandler->
StatementHandler->handlerResultSets->ResultSetHandler

感觉还有些没有写完整，不定期进行补充：）如有问题，欢迎留言：）