Mybatis源码分析——运行原理及流程

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1、配置配置文件，扫描mapper文件 
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="dataSource" ref="dataSource"/>
    <property name="configLocation" value="classpath:XXX.xml"/>
</bean>


2、配置SqlSessionFactoryBean(创建mybatis的工厂类)，这里dataSource的配置就不贴出来了 
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
     <property name="basePackage" value="XXX.mapper" />
     <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
</bean>
SqlSessionFactoryBean类afterPropertiesSet方法在执行之前所有属性都已设置完成，其功能是校验dataSource(配置文件配置的)和sqlSessionFactoryBuilder(默认new出来的一个新对象)，然后创建sqlSessionFactory(后面再讲这个)  。
启动服务时，就会将所有的mappedStatements加载到sqlSession中了 

3、将扫描的mapper接口都一一进行注册代理，调用流程如下: MapperFactoryBean.getObject() --> SqlSessionTemplate.getMapper(Class<T> type) --> Configuration.getMapper(Class<T> type, SqlSession sqlSession) --> MapperRegistry.getMapper(Class<T> type, SqlSession sqlSession)  
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
  if (!knownMappers.contains(type))
    throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
  try {
    return MapperProxy.newMapperProxy(type, sqlSession);
  } catch (Exception e) {
    throw new BindingException("Error getting mapper instance. Cause: " + e, e);
  }
}
MapperProxy类代理代码：MapperProxy.newMapperProxy(type, sqlSession);`
public static <T> T newMapperProxy(Class<T> mapperInterface, SqlSession sqlSession) {
  ClassLoader classLoader = mapperInterface.getClassLoader();
  Class<?>[] interfaces = new Class[]{mapperInterface};
  MapperProxy proxy = new MapperProxy(sqlSession);
  return (T) Proxy.newProxyInstance(classLoader, interfaces, proxy);
}
将所有Mapper接口都进行代理后，后续调用接口中的方法时，都会进入到MapperProxy.invoke()中执行相应逻辑 。

4、启动服务完成后，调用mapper接口时，MapperProxy会自动进行代理，调用invoke方法（因为在服务启动时，就将所有的Mapper接口设置了MapperProxy代理） 
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  if (method.getDeclaringClass() == Object.class) {
    return method.invoke(this, args);
  }
  //通过代理类和方法名(全路径方法名) 获取Mapper接口类
  final Class<?> declaringInterface = findDeclaringInterface(proxy, method);
  //设置statement、sqlSession、declaringInterface等，为执行sql做准备
  final MapperMethod mapperMethod = new MapperMethod(declaringInterface, method, sqlSession);
  //执行sql，并返回结果 
  final Object result = mapperMethod.execute(args);
  if (result == null && method.getReturnType().isPrimitive() && !method.getReturnType().equals(Void.TYPE)) {
    throw new BindingException("Mapper method '" + method.getName() + "' (" + method.getDeclaringClass() + ") attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
  }
  return result;
}
MapperMethod类中execute方法，具体执行操作类；如果返回的是list,则对应的是returnsMany，返回的是Map，则对应的是returnsMap 
public Object execute(Object[] args) {
  Object result = null;
  if (SqlCommandType.INSERT == type) {
    Object param = getParam(args);
    result = sqlSession.insert(commandName, param);
  } else if (SqlCommandType.UPDATE == type) {
    Object param = getParam(args);
    result = sqlSession.update(commandName, param);
  } else if (SqlCommandType.DELETE == type) {
    Object param = getParam(args);
    result = sqlSession.delete(commandName, param);
  } else if (SqlCommandType.SELECT == type) {
    if (returnsVoid && resultHandlerIndex != null) {
      executeWithResultHandler(args);
    } else if (returnsMany) {
      result = executeForMany(args);
    } else if (returnsMap) {
      result = executeForMap(args);
    } else {
      Object param = getParam(args);
      result = sqlSession.selectOne(commandName, param);
    }
  } else {
    throw new BindingException("Unknown execution method for: " + commandName);
  }
  return result;
}
最后具体执行sql语句都是调用sqlSession中的方法：sqlSession.insert、sqlSession.update、sqlSession.delete、sqlSession.select、sqlSession.<E>selectList、sqlSession.selectOne等。

5、sqlSession执行具体sql操作过程 
执行sqlSession.<E>selectList时，需要到SqlSession的实现类(SqlSessionTemplate)中调用方法，注意看下面代码，这里面是通过sqlSessionProxy来调用selectList，说明在此做了一个代理 
public <E> List<E> selectList(String statement, Object parameter) {
  return this.sqlSessionProxy.<E> selectList(statement, parameter);
}
进入SqlSessionTemplate类后，此类有个构造方法
public SqlSessionTemplate(SqlSessionFactory sqlSessionFactory, ExecutorType executorType,
    PersistenceExceptionTranslator exceptionTranslator) {

  notNull(sqlSessionFactory, "Property 'sqlSessionFactory' is required");
  notNull(executorType, "Property 'executorType' is required");

  this.sqlSessionFactory = sqlSessionFactory;
  this.executorType = executorType;
  this.exceptionTranslator = exceptionTranslator;
  this.sqlSessionProxy = (SqlSession) newProxyInstance(
      SqlSessionFactory.class.getClassLoader(),
      new Class[] { SqlSession.class },
      new SqlSessionInterceptor());
}
看最后一句代码，sqlSessionProxy新创建了一个代理实例，执行具体sqlSessionProxy中的方法(sqlSessionProxy.<E> selectList)时，进入到SqlSessionInterceptor.invoke()方法中  
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
   //获取一个sqlSession 
  final SqlSession sqlSession = getSqlSession(
      SqlSessionTemplate.this.sqlSessionFactory,
      SqlSessionTemplate.this.executorType,
      SqlSessionTemplate.this.exceptionTranslator);
  try {
    //真正执行selectList方法     
    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);
    }
    return result;
  } catch (Throwable t) {
    Throwable unwrapped = unwrapThrowable(t);
    if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
      Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);
      if (translated != null) {
        unwrapped = translated;
      }
    }
    throw unwrapped;
  } finally {
    //执行完成后，需要关闭SqlSession   
    closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
  }
}
在invoke方法中，首先需要获取一个SqlSession,通过sessionFactory.openSession(executorType)获取，该代码在SqlSessionUtils类中
SqlSession session = sessionFactory.openSession(executorType);
debug到上面的Object result = method.invoke(sqlSession, args);后，继续往下运行，就进入了DefaultSqlSession中的selectList中 
public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  try {
    MappedStatement ms = configuration.getMappedStatement(statement);
    List<E> result = executor.<E>query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    return result;
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}
此处代码`executor.<E>query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);`继续执行。如果在配置文件中配置有Plugin，则会进入到你配置的插件类中, 在进入配置的插件类中之前，会到代理工具类Plugin(org.apache.ibatis.plugin.Plugin.java)里面  
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  try {
    Set<Method> methods = signatureMap.get(method.getDeclaringClass());
    if (methods != null && methods.contains(method)) {
      return interceptor.intercept(new Invocation(target, method, args));
    }
    return method.invoke(target, args);
  } catch (Exception e) {
    throw ExceptionUtil.unwrapThrowable(e);
  }
}
然后才通过interceptor.intercept(new Invocation(target, method, args))进入到你配置的插件类里面去，直到配置的插件类代码执行完毕后，才真正执行`executor.<E>query()`方法，此时继续运行，则进入到BaseExecutor中的query()方法
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();
    }
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      clearLocalCache(); // issue #482
    }
  }
  return list;
}
注意看，这里会先从本地缓存中获取你想要的结果，这个key的值对应的就是namespace+statementId+sql语句，如果缓存中没有(需要将缓存功能开启后，才会将结果缓存到本地缓存中)，再去查询数据库queryFromDatabase()
在queryFromDatabase()中doQuery()进行查询,跳到SimpleExecutor类中的doQuery()中 
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(this, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.<E>query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}
在执行handler.<E>query时，会将其委托给PreparedStatementHandler执行query() 
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
  //执行查询操作
  ps.execute();
  return resultSetHandler.<E> handleResultSets(ps);
}
执行操作ps.execute()时，进入到PreparedStatement中的execute()方法，该方法中有句代码
rs = executeInternal(this.maxRows, sendPacket, doStreaming, (this.firstCharOfStmt == 'S'),
      metadataFromCache, false);
进入到executeInternal方法后，具体执行代码是
rs = locallyScopedConnection.execSQL(this, null, maxRowsToRetrieve, sendPacket,
   this.resultSetType, this.resultSetConcurrency,
   createStreamingResultSet, this.currentCatalog,
   metadataFromCache, isBatch);
继续执行此句代码，进入到ConnectionImpl类中的execSQL方法中，该方法中return执行的结果
return this.io.sqlQueryDirect(callingStatement, null, null,
      packet, maxRows, resultSetType,
      resultSetConcurrency, streamResults, catalog,
      cachedMetadata);
然后进入到MysqlIO类中的sqlQueryDirect方法，下面代码就是发送执行语句
Buffer resultPacket = sendCommand(MysqlDefs.QUERY, null, queryPacket,
      false, null, 0);
继续往下到sendCommand方法中
send(this.sendPacket, this.sendPacket.getPosition());
进入到send()方法，最后通过socket连接数据库，将需要执行的语句发给数据库服务器，让其执行，然后将结果返回给mysqlOutput，最后将结果回填回去。

到此，mybatis运行流程基本上就接受了。

6、总结 
通过对mybatis源码的分析，可以看出其整个流程很多地方都用了动态代理，注册Mapper接口是使用了代理，执行sqlSession中的具体操作时，也使用了代理，其他地方也有用到代理；从Mybatis的源码分析，其实其整个运行流程也不难理解，只要理解其原理后，在后续的使用中才能得心应手。

有理解不到位的地方，还望各位大佬指正