1. getById 的执行

前面一篇提到过, Mapper.java 创建的时候, 会通过 jdk 代理的方式来创建, 且代理处理类为: MapperProxy .

所以当执行 UserMapper 的 getById 方法的时候, 就会去 MapperProxy 中执行 invoke 方法.

  //MapperProxy.java

  @Override

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

    try {

      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);

  }

cachedMapperMethod 方法是做了一层缓存处理. 先从缓存中获取, 如果获取不到, 再创建 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;

  }

接下来看一下 MapperMethod.execute() 方法. 这个方法中, 会看到我们期待的东西.

  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 {
　　　　　　//对参数进行处理

          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;

  }

getById 在这里会调用 SqlSessionTemplate 的 selectOne 方法.

  //SqlSessionTemplate.java

  @Override

  public <T> T selectOne(String statement, Object parameter) {

    return this.sqlSessionProxy.<T> selectOne(statement, parameter);

  }

前面解析过, sqlSessionProxy.selectOne 的时候, 会去 SqlSessionInterceptor 中执行 invoke() 方法.

  private class SqlSessionInterceptor implements InvocationHandler {

    @Override

    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

      SqlSession sqlSession = getSqlSession(

          SqlSessionTemplate.this.sqlSessionFactory,

          SqlSessionTemplate.this.executorType,

          SqlSessionTemplate.this.exceptionTranslator);

      try {

        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) {

          // release the connection to avoid a deadlock if the translator is no loaded. See issue #22

          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);

          sqlSession = null;

          Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException) unwrapped);

          if (translated != null) {

            unwrapped = translated;

          }

        }

        throw unwrapped;

      } finally {

        if (sqlSession != null) {

          closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);

        }

      }

    }

  }

这里有两个方法需要重点关注, 一个是 getSqlSession() , 另一个是 invoke() 方法.

1.1 getSqlSession()

  public static SqlSession getSqlSession(SqlSessionFactory sessionFactory, ExecutorType executorType, PersistenceExceptionTranslator exceptionTranslator) {

    notNull(sessionFactory, NO_SQL_SESSION_FACTORY_SPECIFIED);

    notNull(executorType, NO_EXECUTOR_TYPE_SPECIFIED);

    SqlSessionHolder holder = (SqlSessionHolder) TransactionSynchronizationManager.getResource(sessionFactory);

    SqlSession session = sessionHolder(executorType, holder);

    if (session != null) {

      return session;

    }

    if (LOGGER.isDebugEnabled()) {

      LOGGER.debug("Creating a new SqlSession");

    }

    session = sessionFactory.openSession(executorType);

    registerSessionHolder(sessionFactory, executorType, exceptionTranslator, session);

    return session;

  }

这里的 SqlSessionFactory 就是配置类中创建的默认的实现类: DefaultSqlSessionFactory

  //DefaultSqlSessionFactory.java

  @Override

  public SqlSession openSession(ExecutorType execType) {

    return openSessionFromDataSource(execType, null, false);

  }

 |

 |

\|/

  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();

    }

  }

这里先不管别的代码, 直接看返回值, 创建了一个 DefaultSqlSession 的实例返回.

1.2 method.invoke()

回到 SqlSessionInterceptor 中来, 继续看下面的执行代码:

Object result = method.invoke(sqlSession, args);

sqlSession 就是上面创建的 DefaultSqlSesion 实例. 我们知道 Method.invoke(obj, args) 是反射调用方法的一种方式.

此处的Method是 getById 的方法反射类型. 所以, 会调用 obj 的 getById() 方法, 即 DefaultSqlSession 的 getById() 方法.

  @Override

  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;

    }

  }

  @Override

  public <E> List<E> selectList(String statement, Object parameter) {

    return this.selectList(statement, parameter, RowBounds.DEFAULT);

  }

  @Override

  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();

    }

  }

此例中, statement = "com.study.demo.mybatis.mapper.UserMapper.getById"

这里是通过statement 拿 MappedStatement, 而 MappedStatement 中, 就有 mapper.xml 中对应的 sql 语句.

后续过程中, 就可以拿着 sql 去数据库执行了.

到这里, 主体流程其实已经走通了.

巴特西

mybatis - 执行 getById

1. getById 的执行

1.1 getSqlSession()

1.2 method.invoke()

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