MyBatis-一次Select查询过程

1 前提：

MyBatis初始化完后，org.apache.ibatis.session.Configuration中，会有已经初始化完的数据，供后续的执行：

1.1 mapperRegistry

MapperRegistry的实例，有一个属性Map<Class<?>, MapperProxyFactory<?>> knownMappers:

• key: Mapper类，比如接口'com.xxx.yyy.model.UserMapper';
• value: MapperProxyFactory对象，是Mapper代理类MapperProxy的工厂，创建MapperProxy对象执行Mapper类中定义的方法。

1.2 mappedStatements

类型是Map<String, MappedStatement>：

• key: MappedStatement对象的id，如'com.xxx.yyy.model.UserMapper.selectList';
• value: MappedStatement对象。

2 查询过程

MyBatis分三个步骤执行查询过程：

• 1 创建SqlSession，默认实现类是DefaultSqlSession；
• 1 获取Mapper，如session.getMapper(UserMapper.class);
• 2 用Mapper执行查询，如userMapper.findList()。

2.1 创建SqlSession

DefaultSqlSessionFactory.openSession:

  public SqlSession openSession(boolean autoCommit) {
    // 第一步
    // Configuration中：defaultExecutorType = ExecutorType.SIMPLE
    return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, autoCommit);
  }
  
    // 第二步
    //openSessionFromDataSource关键代码：
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
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Executor从Configuration.newExecutor方法中创建：

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;
  }
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newExecutor的执行过程如下：

• 根据不同的ExecutorType返回不同Executor;
• 因Configuration中默认是ExecutorType.SIMPLE，所以默认使用SimpleExecutor;
• 如果开启了缓存，则返回CachingExecutor对象;
• 通过interceptorChain.pluginAll加入拦截器Interceptor列表。

2.2 获取Mapper

DefaultSqlSession从Configuration.mapperRegistry中获取Mapper的实现对象：

// 第一步
// DefaultSqlSesison:
 public <T> T getMapper(Class<T> type) {
    return configuration.<T>getMapper(type, this);
  }

// 第二步
// Configuration:
 public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }
  
// 第三步
// MapperRegistry:
// 从属性knownMappers中获取MapperProxyFactory对象，获取到后执行newInstance获取MapperProxy，即Mapper类的实现对象。
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);
    }
  }
  
 // 第四步
 // MapperProxyFactory 创建MapperProxy对象
   protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

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由上面MapperProxyFactory代码可知，每次执行session.getMapper都会创建MapperProxy对象及其代理对象，所以应避免多次调用session.getMapper。

2.3 执行查询

MyBatis使用JDK代理方式，MapperProxy实现了InvocationHandler接口，所以Mapper接口类的实现方法，是在MapperProxy.invoke方法里执行。

invoke中，获取或创建一个MapperMethod对象，然后执行MapperMethod.execute方法。

//// 获取MapperMethod对象
  private MapperMethod cachedMapperMethod(Method method) {
    MapperMethod mapperMethod = methodCache.get(method);
    if (mapperMethod == null) {
      mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
      methodCache.put(method, mapperMethod);
    }
    return mapperMethod;
  }
  
//// 执行execute
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
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2.3.1 MapperMethod创建

MapperMethod有两个属性：

  private final SqlCommand command;
  private final MethodSignature method;
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1) SqlCommand

SqlCommand有两个属性：

• String name: MappedStatement的id；
• SqlCommandType type: MappedStatement的sqlCommandType， UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH。

SqlCommand创建时，会从Configuration中获取MappedStatement对象，获取到则用于name和type的赋值：

String statementId = mapperInterface.getName() + "." + methodName;

if (configuration.hasStatement(statementId)) {
    return configuration.getMappedStatement(statementId);
}
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2) MethodSignature

    private final boolean returnsMany; // configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray()
    private final boolean returnsMap; // 
    private final boolean returnsVoid; // void.class.equals(this.returnType)
    private final boolean returnsCursor; // org.apache.ibatis.cursor.Cursor.class.equals(this.returnType)
    private final Class<?> returnType;
    //////
    private final ParamNameResolver paramNameResolver; //  new ParamNameResolver(configuration, method)
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2.3.2 MapperMethod.execute

根据command.type，判断执行的操作：

 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;
  }
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在execute调用的executeFor*方法中，最终调用的是sqlSession.select*方法。

2.3.3 session.selectList

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
    try {
      // 第一步，从Configuration中取出MappedStatement对象
      // statement : interfaceName + "." + methodName
      MappedStatement ms = configuration.getMappedStatement(statement);
      
      // 第二步，执行查询
      // executor : 前面Configuration.newExecutor创建的executor，默认SimpleExecutor
      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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executor.query的调用链：

BaseExecutor.query -> BaseExecutor.queryFromDatabase -> SimpleExecutor.doQuery

SimpleExecutor.doQuery:

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();
      
      // 第一步 创建RoutingStatementHandler
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
      
      // 第二步 调用StatementHandler.prepare创建Statement
      stmt = prepareStatement(handler, ms.getStatementLog());
      
      // 第三步 执行StatementHandler.query
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }
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2.3.4 StatementHandler

1) RoutingStatementHandler

configuration.newStatementHandler中创建的是RoutingStatementHandler，然后设置拦截器:

  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;
  }
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RoutingStatementHandler是一种委派模式，根据MappedStatement.statementType的不同，返回不同的StatementHandler实现类：

  // 代理
  private final StatementHandler delegate;
  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());
    }
  }
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2) StatementHandler.prepare

创建java.sql.Statement对象，调用链：

RoutingStatementHandler.prepare -> BaseStatementHandler.prepare -> PreparedStatementHandler.instantiateStatement

instantiateStatement是BaseStatementHandler的抽象方法，供子类实现。

  // PreparedStatementHandler
  protected Statement instantiateStatement(Connection connection) throws SQLException {
    if (mappedStatement.getResultSetType() != null) {
      return connection.createStatement(mappedStatement.getResultSetType().getValue(), ResultSet.CONCUR_READ_ONLY);
    } else {
      return connection.createStatement();
    }
  }
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3) StatementHandler.query

有了Statement之后，就可以拿来进行查询了：

  @Override
  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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