MyBatis核心源码深度剖析SQL执行过程

• 1 SQL语句的执行过程介绍
• 2 SQL执行的入口分析
• • 2.1 为Mapper接口创建代理对象
  • 2.2 执行代理逻辑
• 3 查询语句的执行过程分析
• • 3.1 selectOne方法分析
  • 3.2 sql获取
  • 3.3 参数设置
  • 3.4 SQL执行和结果集的封装
• 4 更新语句的执行过程分析
• • 4.1 sqlsession增删改方法分析
  • 4.2 sql获取
  • 4.3 参数设置
  • 4.4 SQL执行
• 5 小结

MyBatis核心执行组件：

// 方式1：
User user = session.selectOne("com.oldlu.dao.UserMapper.findUserById", 101);
// 方式2：
UserMapper mapper = session.getMapper(UserMapper.class);
List userList = mapper.findAll();

方式1入口分析：
session是DefaultSqlSession类型的，因为sqlSessionFactory默认生成的SqlSession是
DefaultSqlSession类型。
selectOne()会调用selectList()。

// DefaultSqlSession类
public  List selectList(String statement, Object parameter, RowBounds
rowBounds) {
  try {
    MappedStatement ms = configuration.getMappedStatement(statement);
    // CURD操作是交给Excetor去处理的
    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();
 }
}

方式2入口分析：
获取代理对象：

//DefaultSqlSession类 ====================>
@Override
public  T getMapper(Class type) {
  return configuration.getMapper(type, this);
  }
// Configuration类 ====================>
public  T getMapper(Class type, SqlSession sqlSession) {
  return mapperRegistry.getMapper(type, sqlSession);
}
//MapperRegistry ----> apperProxyFactory.newInstance ====================>
public  T getMapper(Class type, SqlSession sqlSession) {
  //从缓存中获取该Mapper接口的代理工厂对象
  final MapperProxyFactory mapperProxyFactory = (MapperProxyFactory)
knownMappers.get(type);
  //如果该Mapper接口没有注册过，则抛异常
  if (mapperProxyFactory == null) {
    throw new BindingException("Type " + type + " is not known to the
MapperRegistry.");
 }
  try {
    //【使用代理工厂创建Mapper接口的代理对象】
    return mapperProxyFactory.newInstance(sqlSession);
 } catch (Exception e) {
    throw new BindingException("Error getting mapper instance. Cause: " + e,
e);
 }
}
//MapperProxyFactory  --->此时生成代理对象 ====================>
protected T newInstance(MapperProxy mapperProxy) {
  //Mybatis底层是调用JDK的Proxy类来创建代理实例
  return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new
Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
  final MapperProxy mapperProxy = new MapperProxy<>(sqlSession,
mapperInterface, methodCache);
  return newInstance(mapperProxy);
}

代理对象执行逻辑：

//MapperProxy   ====================>

public Object invoke(Object proxy, Method method, Object[] args) throws
Throwable {
 try {
  if (Object.class.equals(method.getDeclaringClass())) {
   //如果是Object方法，则调用方法本身
   return method.invoke(this, args);
 } else {
   //调用接口方法：根据被调用接口的Method对象，从缓存中获取MapperMethodInvoker对象
   //apper接口中的每一个方法都对应一个MapperMethodInvoker对象，而MapperMethodInvoker
对象里面的MapperMethod保存着对应的SQL信息和返回类型以完成SQL调用 ...
   return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
 }
} catch (Throwable t) {
  throw ExceptionUtil.unwrapThrowable(t);
}
}

private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
  try {
    return methodCache.computeIfAbsent(method, m -> {
      if (m.isDefault()) {
        //如果调用接口的是默认方法（default方法）
        try {
          if (privateLookupInMethod == null) {
            return new
DefaultMethodInvoker(getMethodHandleJava8(method));
         } else {
            return new
DefaultMethodInvoker(getMethodHandleJava9(method));
         }
       } catch (IllegalAccessException | InstantiationException |
InvocationTargetException
            | NoSuchMethodException e) {
          throw new RuntimeException(e);
       }
     } else {
        //如果调用的普通方法（非default方法），则创建一个PlainMethodInvoker并放
入缓存，其中MapperMethod保存对应接口方法的SQL以及入参和出参的数据类型等信息
        return new PlainMethodInvoker(new MapperMethod(mapperInterface,
method, sqlSession.getConfiguration()));
     }
   });
 } catch (RuntimeException re) {
    Throwable cause = re.getCause();
throw cause == null ? re : cause;
 }
}
// MapperProxy内部类： PainMethodInvoker  ====================>
// 当cacheInvoker返回了PalinMethodInvoker实例之后，紧接着调用了这个实例的
PlainMethodInvoker:invoke方法
@Override
public Object invoke(Object proxy, Method method, Object[] args, SqlSession
sqlSession) throws Throwable {
 //Mybatis实现接口方法的核心: MapperMethod::execute方法：
 return mapperMethod.execute(sqlSession, args);
}
// MapperMethod  ====================>
public Object execute(SqlSession sqlSession, Object[] args) {
  Object result;
  switch (command.getType()) {
    case INSERT: {
      // 将args进行解析，如果是多个参数则，则根据@Param注解指定名称将参数转换为Map，
如果是封装实体则不转换
      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:
      //查询操作 
      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 {
        //解析参数，因为SqlSession::selectOne方法参数只能传入一个，但是我们
Mapper中可能传入多个参数，
        //有可能是通过@Param注解指定参数名，所以这里需要将Mapper接口方法中的多个参
数转化为一个ParamMap,
        //也就是说如果是传入的单个封装实体，那么直接返回出来；如果传入的是多个参数，
实际上都转换成了Map
        Object param = method.convertArgsToSqlCommandParam(args);
        //可以看到动态代理最后还是使用SqlSession操作数据库的
        result = sqlSession.selectOne(command.getName(), param);
        if (method.returnsOptional()
          && (result == null ||
!method.getReturnType().equals(result.getClass()))) {
          result = Optional.ofNullable(result);
       }
     }
      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;
}
// 此时我们发现： 回到了sqlsession中
private  Object executeForMany(SqlSession sqlSession, Object[] args) {
  List result;
  Object param = method.convertArgsToSqlCommandParam(args);
  if (method.hasRowBounds()) {
   RowBounds rowBounds = method.extractRowBounds(args);
   result = sqlSession.selectList(command.getName(), param, rowBounds);
 } else {
   result = sqlSession.selectList(command.getName(), param);
 }
 // ...
  return result;
}

// DefaultSqlSession类  ===============>
// selectOne
@Override
public  T selectOne(String statement, Object parameter) {
  // //selectOne()会调用selectList()。
  List 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;
 }
}
// selectList
public  List selectList(String statement, Object parameter, RowBounds
rowBounds) {
  try {
    MappedStatement ms = configuration.getMappedStatement(statement);
    // CURD操作是交给Excetor去处理的
    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();
 }
}

// CachingExecutor ===============>
public  List query(MappedStatement ms, Object parameterObject, RowBounds
rowBounds, ResultHandler resultHandler) throws SQLException {
  // 获取绑定的sql命令，比如"SELECT * FROM xxx"
  BoundSql boundSql = ms.getBoundSql(parameterObject); 
  CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
  return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}
@Override
public  List 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, boundSql);
      @SuppressWarnings("unchecked")
      List list = (List) tcm.getObject(cache, key);
      if (list == null) {
        list = delegate.query(ms, parameterObject, rowBounds,
resultHandler, key, boundSql);
        tcm.putObject(cache, key, list); // issue #578 and #116
    }
      return list;
   }
 }
  return delegate.query(ms, parameterObject, rowBounds, resultHandler, key,
boundSql);
}
//真正执行query操作的是SimplyExecutor代理来完成的，SimplyExecutor的父类BaseExecutor的
query方法中:
// BaseExecutor类:SimplyExecutor的父类 =================>
@Override
public  List 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 list;
  try {
    queryStack++;
    //localCache是一级缓存，如果找不到就调用queryFromDatabase从数据库中查找
    list = resultHandler == null ? (List) 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();
   }
    deferredLoads.clear();
    if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
      clearLocalCache();
   }
}
  return list;
}
//第一次，没有缓存，所以会调用queryFromDatabase方法来执行查询。
private  List queryFromDatabase(...) throws SQLException {
  List list;
  localCache.putObject(key, EXECUTION_PLACEHOLDER);
  try {
    // 查询
    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;
}
// SimpleExecutor类 ============================>
public  List doQuery(...) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(....);
    // 1：SQL查询参数的设置
    stmt = prepareStatement(handler, ms.getStatementLog());
    // StatementHandler封装了Statement
    // 2：SQL查询操作和结果集的封装
    return handler.query(stmt);
 } finally {
    closeStatement(stmt);
 }
}

// SimplyExecutor类 ============================>
// 【1】 参数设置： prepareStatement
private Statement prepareStatement(StatementHandler handler, Log statementLog)
throws SQLException {
  Statement stmt;
  // 通过getConnection方法来获取一个Connection，
  Connection connection = getConnection(statementLog);
  // 调用prepare方法来获取一个Statement
  stmt = handler.prepare(connection, transaction.getTimeout());
 
  // 设置SQL查询中的参数值 ***
  handler.parameterize(stmt);
  return stmt;
}
// RoutingStatementHandler ============================>
// PreparedStatementHandler ============================>
@Override
public void parameterize(Statement statement) throws SQLException {
  parameterHandler.setParameters((PreparedStatement) statement);
}
// DefaultParameterHandler ============================> 此时参数设置成功
@Override
public void setParameters(PreparedStatement ps) {
  ErrorContext.instance().activity("setting
parameters").object(mappedStatement.getParameterMap().getId());
  List parameterMappings = boundSql.getParameterMappings();
  if (parameterMappings != null) {
    for (int i = 0; i < parameterMappings.size(); i++) {
      ParameterMapping parameterMapping = parameterMappings.get(i);
      if (parameterMapping.getMode() != ParameterMode.OUT) {
        Object value;
        String propertyName = parameterMapping.getProperty();
        if (boundSql.hasAdditionalParameter(propertyName)) {
          value = boundSql.getAdditionalParameter(propertyName);
       } else if (parameterObject == null) {
          value = null;
       } else if
(typeHandlerRegistry.hasTypeHandler(parameterObject.getClass())) {
          value = parameterObject;
       } else {
          MetaObject metaObject =
configuration.newMetaObject(parameterObject);
          value = metaObject.getValue(propertyName);
       }
        TypeHandler typeHandler = parameterMapping.getTypeHandler();
        JdbcType jdbcType = parameterMapping.getJdbcType();
        if (value == null && jdbcType == null) {
          jdbcType = configuration.getJdbcTypeForNull();
       }
      try {
          typeHandler.setParameter(ps, i + 1, value, jdbcType);
       } catch (TypeException | SQLException e) {
          throw new TypeException("Could not set parameters for
mapping.....");
       }
     }
   }
 }
}

// RoutingStatementHandler ============================>
@Override
public  List query(Statement statement) throws SQLException {
  return delegate.query(statement);
}
// PreparedStatementHandler ============================>
@Override
public  List query(Statement statement, ResultHandler resultHandler)
throws SQLException {
  // 这里就到了熟悉的PreparedStatement了
  PreparedStatement ps = (PreparedStatement) statement;
  // 执行SQL查询操作
  ps.execute();
  // 结果交给ResultHandler来处理
  return resultSetHandler. handleResultSets(ps);
}
// DefaultResultSetHandler类（封装返回值，将查询结果封装成Object对象）
@Override
public List