mybatis原理解析

前提：

mybatis是当下最流行的ORM框架，结合一个查询例子来跟踪下mybatis的执行流程以及mybatis的四大对象创建流程，再结合画图跟大家分享下，如有理解有误，欢迎指教。

首先说下mybatis与hibernate区别：

hibernate是全自动ORM框架；皆在消除sql，面向对象开发，使用javaBean与数据库打交道；

如图所示：java与数据库打交道需要经过以下5个步骤：

1. 编写sql
2. sql预编译
3. 参数设置
4. 执行sql
5. 封装结果

hibernate将这5部全部封装起来，对于开发人员是透明的，相当于黑箱操作，无法编写sql以及优化sql；如果需要编写复杂sql需要学习hql语言，最后会加重学习负担。

而mybatis是一款半自动框架，sql与编码分离，sql交给开发人员控制，功能边界清晰，一个专注于业务，一个专注于数据。对于开发人员来说，核心sql还是需要自己优化。

通过一个例子讲解mybatis查询流程

@Test
    public void test() throws IOException {
    
    // 1、获取sqlSessionFactory
        SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
        
        // 2、获取sqlSession
        SqlSession sqlSession = sqlSessionFactory.openSession();

		// 3、获取mapper
        RefundBillMapper refundBillMapper = sqlSession.getMapper(RefundBillMapper.class);
        
        // 4、执行查询
        refundBillMapper.selectOne(1);
    }

    public SqlSessionFactory getSqlSessionFactory() throws IOException {
        String source = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(source);
        return new SqlSessionFactoryBuilder().build(inputStream);
    }

上述例子为使用mabatis进行一个普通的查询，那么总共要经历4步：

1. 获取sqlSessionFactory
2. 获取sqlSession
3. 获取mapper
4. 执行查询

下面我们结合源码以及图解来详细看下每一步的执行过程

1、获取sqlSessionFactory

 new SqlSessionFactoryBuilder().build(inputStream)

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

这步主要是创建XMLConfigBuilder对象，创建XMLConfigBuilder对象时会初始化configuration对象，XMLConfigBuilder用于解析mybatis-config.xml中配置的各种标签属性；来看下parser.parse()方法

public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

从configuration标签开始解析，因为configuration是mybatis-config.xml中的顶级标签，parseConfiguration(parser.evalNode("/configuration"))方法如下：

private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

这里解析了mybatis-config.xml文件中的所有标签；并将解析后的属性全部添加到configuration中；注意mapper映射类会加入mapperRegistry中；

最后返回build(parser.parse())，此方法中返回new DefaultSqlSessionFactory

public SqlSessionFactory build(Configuration config) {
    return new DefaultSqlSessionFactory(config);
  }

总结：创建解析器，解析配置文件中所有标签，并将解析的结果放到configuration中

configuration属性如下

2、获取sqlSession

SqlSession sqlSession = sqlSessionFactory.openSession();

sqlSession是从sqlSessionFactory中获取，重点看下openSession()方法：

 @Override
  public SqlSession openSession() {
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
  }

openSession调用了DefaultSqlSessionFactory中的openSessionFromDataSource方法

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();
    }
  }

前面为创建tx，重点为Executor executor = configuration.newExecutor(tx, execType)，来看下四大对象之一的Executor是怎么创建的：

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

1. 如果没有配置executorType的话，默认为SimpleExecutor类型的Executor(executor = new SimpleExecutor(this, transaction))；
2. 如果配置了二级缓存cacheEnabled，则会创建CachingExecutor(executor = new CachingExecutor(executor);)，CachingExecutor实则对Executor做了一层包装，最终调用的都是BaseExecutor
3. executor = (Executor) interceptorChain.pluginAll(executor)插件开发，可以对Executor做动态代理，再次包装
4. 初始化SimpleExecutor最终会初始化BaseExecutor，BaseExecutor初始化如下：

protected BaseExecutor(Configuration configuration, Transaction transaction) {
    this.transaction = transaction;
    this.deferredLoads = new ConcurrentLinkedQueue<DeferredLoad>();
    this.localCache = new PerpetualCache("LocalCache");
    this.localOutputParameterCache = new PerpetualCache("LocalOutputParameterCache");
    this.closed = false;
    this.configuration = configuration;
    this.wrapper = this;
  }

Executor创建完会创建DefaultSqlSession；DefaultSqlSession里会包含configuration以及executor

new DefaultSqlSession(configuration, executor, autoCommit)

Executor类型及区别：

1. SimpleExecutor：每执行一次update或select，就开启一个Statement对象，用完立刻关闭Statement对象。
2. ReuseExecutor：执行update或select，以sql作为key查找Statement对象，存在就使用，不存在就创建，用完后，不关闭Statement对象，而是放置于Map内，供下一次使用。简言之，就是重复使用Statement对象。
3. BatchExecutor：执行update（没有select，JDBC批处理不支持select），将所有sql都添加到批处理中（addBatch()），等待统一执行（executeBatch()），它缓存了多个Statement对象，每个Statement对象都是addBatch()完毕后，等待逐一执行executeBatch()批处理。与JDBC批处理相同。
4. CachingExecutor：CachingExecutor是一个Executor接口的装饰器，它为Executor对象增加了二级缓存的相关功能，委托的执行器对象可以是SimpleExecutor、ReuseExecutor、BatchExecutor中任一一个。执行 update 方法前判断是否清空二级缓存；执行 query 方法前先在二级缓存中查询，命中失败再通过被代理类查询。

总结：返回SqlSession的实现类DefaultSqlSession，里面包含了configuration以及executor对象；

3、获取mapper + 4、执行查询

先看图吧，这部是核心，内容比较多；

sqlSession.getMapper(RefundBillMapper.class)

 @Override
  public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

调用DefaultSqlSession中的getMapper

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

调用Configuration中的getMapper

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

先从缓存中获取mapperProxyFactory，knownMappers是个map，获取不到则直接创建；直接看mapperProxyFactory.newInstance(sqlSession)

public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

创建mapperProxy，直接看newInstance(mapperProxy)

protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

这里使用了jdk动态代理，代理类为MapperProxy，直接看MapperProxy的invoke方法：

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
    // 放行继承与Object的原生方法
      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);
  }

从缓存中获取MapperMethod，获取不到则创建，创建完加入缓存；创建MapperMethod时会给SqlCommand、MethodSignature赋值；然后执行mapperMethod.execute方法：

SqlCommand结构如下：

MethodSignature结构如下：

public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    switch (command.getType()) {
      case INSERT: {
    	Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      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;
      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;
  }

按操作类型进行匹配，如果是查询类型，还会根据返回结果类型匹配；看下sqlSession.selectOne方法：

public <T> T selectOne(String statement, Object parameter) {
    // Popular vote was to return null on 0 results and throw exception on too many.
    List<T> list = this.<T>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;
    }
  }

调用了selectList并取了第一条，看下selectList

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();
    }
  }

从configuration中获取MappedStatement，MappedStatement是第一步解析mapper.xml然后添加到configuration的，看下MappedStatement结构：

紧接着执行了executor.query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER)

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);
  }

获取BoundSql，BoundSql里存储了原生sql，以及参数；BoundSql结构如下：

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);
        @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;
      }
    }
    return delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

Cache cache = ms.getCache();为二级缓存，我们没有配置二级缓存，直接看delegate.<E> query(ms, parameterObject, rowBounds, resultHandler, key, 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 (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    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--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

localCache.getObject(key)先从一级缓存中获取，获取不到直接queryFromDatabase从数据库中查找；

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;
  }

localCache.putObject(key, EXECUTION_PLACEHOLDER)，缓存占位；然后执行doQuery查询，将查询结果放到localCache缓存

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      stmt = prepareStatement(handler, ms.getStatementLog());
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

创建了4大对象之一的StatementHandler

public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

StatementHandler创建时会根据statementType类型创建不同类型的StatementHandler，创建完会执行interceptorChain.pluginAll插件代理

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

    switch (ms.getStatementType()) {
      case STATEMENT:
        delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case PREPARED:
        delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      case CALLABLE:
        delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
        break;
      default:
        throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
    }

  }

创建不同类型的StatementHandler；不同类型StatementHandler区别

protected BaseStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    this.configuration = mappedStatement.getConfiguration();
    this.executor = executor;
    this.mappedStatement = mappedStatement;
    this.rowBounds = rowBounds;

    this.typeHandlerRegistry = configuration.getTypeHandlerRegistry();
    this.objectFactory = configuration.getObjectFactory();

    if (boundSql == null) { // issue #435, get the key before calculating the statement
      generateKeys(parameterObject);
      boundSql = mappedStatement.getBoundSql(parameterObject);
    }

    this.boundSql = boundSql;

    this.parameterHandler = configuration.newParameterHandler(mappedStatement, parameterObject, boundSql);
    this.resultSetHandler = configuration.newResultSetHandler(executor, mappedStatement, rowBounds, parameterHandler, resultHandler, boundSql);
  }

创建完StatementHandler会调用父类的构造方法BaseStatementHandler(),该方法里创建了ParameterHandler和ResultSetHandler；

 public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

创建ParameterHandler和ResultSetHandler时，同样会调用插件代理；

再回到doQuery()方法中看prepareStatement(handler, ms.getStatementLog())方法，

private Statement prepareStatement(StatementHandler handler, Log statementLog) throws SQLException {
    Statement stmt;
    Connection connection = getConnection(statementLog);
    stmt = handler.prepare(connection, transaction.getTimeout());
    handler.parameterize(stmt);
    return stmt;
  }

获取Connection连接；handler.prepare(connection, transaction.getTimeout())会预编译sql，handler.parameterize(stmt)会给sql设置参数值；

connection.prepareStatement(sql, keyColumnNames);

预编译sql

typeHandler.setParameter(ps, i + 1, value, jdbcType);

给sql设置参数值，借助typeHandler给不同类型的参数设值

再回到doQuery()方法中看handler.<E>query(stmt, resultHandler)方法，

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

ps.execute()执行数据库查询，与原生的jdbc是一样的，然后将查询结构传给resultSetHandler，由resultSetHandler去将数据库结果集跟我们在mapper里配置的javaBean类型进行转换

public List<Object> handleResultSets(Statement stmt) throws SQLException {
    ErrorContext.instance().activity("handling results").object(mappedStatement.getId());
 
    final List<Object> multipleResults = new ArrayList<Object>();
 
    int resultSetCount = 0;
	//获取第一个结果值
    ResultSetWrapper rsw = getFirstResultSet(stmt);
	//获得resultMap
    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
	//这边应该为1吧，一般resultMap为一个
    int resultMapCount = resultMaps.size();
	//判断是否有resultMap,没有的话抛出异常
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
	  //获得resultMap，实体类和表中数据字段的对应关系
      ResultMap resultMap = resultMaps.get(resultSetCount);
	  //将值设置成对应的resultmap对象
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }
 
    String[] resultSets = mappedStatement.getResulSets();
    if (resultSets != null) {
      while (rsw != null && resultSetCount < resultSets.length) {
        ResultMapping parentMapping = nextResultMaps.get(resultSets[resultSetCount]);
        if (parentMapping != null) {
          String nestedResultMapId = parentMapping.getNestedResultMapId();
          ResultMap resultMap = configuration.getResultMap(nestedResultMapId);
          handleResultSet(rsw, resultMap, null, parentMapping);
        }
        rsw = getNextResultSet(stmt);
        cleanUpAfterHandlingResultSet();
        resultSetCount++;
      }
    }
 
    return collapseSingleResultList(multipleResults);
  }
  //获得第一个值，并将值打包
  private ResultSetWrapper getFirstResultSet(Statement stmt) throws SQLException {
    ResultSet rs = stmt.getResultSet();
	//结果集不为空
    while (rs == null) {
      // move forward to get the first resultset in case the driver
      // doesn't return the resultset as the first result (HSQLDB 2.1)
      if (stmt.getMoreResults()) {
        rs = stmt.getResultSet();
      } else {
        if (stmt.getUpdateCount() == -1) {
          // no more results. Must be no resultset
          break;
        }
      }
    }
	//将数据打包
    return rs != null ? new ResultSetWrapper(rs, configuration) : null;
  }