mybatis源码之我见

以前一直想看mybatis的源代码，但是一直没找到入口（傻），最近看教程，有些感悟。

和起以前一样，关键代码我会用红色标记。

首先，先贴下我的dao和mapper，代码很简单，和平时写的hello world 差不多。代码如下：

  <select id="select" parameterType="INTEGER" resultMap="BaseResultMap">
    SELECT  * from t_user where id=#{id}
  </select>

@Mapper
public interface UserMapper {
    int insert(User record);

    int insertSelective(User record);

    int update(User user);

    User select(int id);
}

service层代码：

@Service
public class UserService {

    @Autowired
    private UserMapper userMapper;

    public User getUser(int id){
        return userMapper.select(id);
    }
}

这个 userMapper.select(id) 就是入口了。以前一直没有在运行的时候调试过这个，还以为是直接就调用了 User select(int id)，傻逼了。跟踪进入代码：

  @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)) {          //是否是构造函数
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    return mapperMethod.execute(sqlSession, args);
  }

从代码可以看出，这里使用了MapperProxy 代理（启动程序的时候已经生成），这是一个jdk代理（实现了InvocationHandler 接口），因为这个调用的userMapper是一个接口,，接口的默认实现是jdk代理。继续跟进

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()) {  //这里检查是否会有结果集处理
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {         //针对集合list进行处理
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {          //针对ResultMap处理
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {       //针对游标处理（这是什么？）
          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;
  }

上面进的代码，主要是区分了sql的类型：insert、update、delete、select、flush。继续跟进代码，下面的代码进入SqlSession模块：

#org\mybatis\mybatis-spring\1.3.1\mybatis-spring-1.3.1.jar!\org\mybatis\spring\SqlSessionTemplate.class

private class SqlSessionInterceptor implements InvocationHandler {
        private SqlSessionInterceptor() {
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            SqlSession sqlSession = SqlSessionUtils.getSqlSession(SqlSessionTemplate.this.sqlSessionFactory, SqlSessionTemplate.this.executorType, SqlSessionTemplate.this.exceptionTranslator);

            Object unwrapped;
            try {
                Object result = method.invoke(sqlSession, args);
                if (!SqlSessionUtils.isSqlSessionTransactional(sqlSession, SqlSessionTemplate.this.sqlSessionFactory)) {
                    sqlSession.commit(true);
                }

                unwrapped = result;
            } catch (Throwable var11) {
                unwrapped = ExceptionUtil.unwrapThrowable(var11);
                if (SqlSessionTemplate.this.exceptionTranslator != null && unwrapped instanceof PersistenceException) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                    sqlSession = null;
                    Throwable translated = SqlSessionTemplate.this.exceptionTranslator.translateExceptionIfPossible((PersistenceException)unwrapped);
                    if (translated != null) {
                        unwrapped = translated;
                    }
                }

                throw (Throwable)unwrapped;
            } finally {
                if (sqlSession != null) {
                    SqlSessionUtils.closeSqlSession(sqlSession, SqlSessionTemplate.this.sqlSessionFactory);
                }

            }

            return unwrapped;
        }
    }

#org\mybatis\mybatis\3.4.5\mybatis-3.4.5-sources.jar!\org\apache\ibatis\session\defaults\DefaultSqlSession.java  

@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, 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();
  }
}

 #org\mybatis\mybatis\3.4.5\mybatis-3.4.5-sources.jar!\org\apache\ibatis\plugin\Plugin.java
@Override
  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)) {      //注意这里，我们自定义插件的时候，就是在这里起作用的。如果有自定义插件，则会被拦截
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }

下面开始进入Executor模块，代码如下：

@Override
  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);
  }

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
    throws SQLException {
  Cache cache = ms.getCache();      //二级缓存
  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);
}

这个类是执行器类。可以看到，这里是一个装饰器模式，通过重写Excutor的方法，进行了自己的方法的一些内容的添加（装饰器模式，保留核心功能，同时又新加了一些功能）。

#org\mybatis\mybatis\3.4.5\mybatis-3.4.5-sources.jar!\org\apache\ibatis\executor\BaseExecutor.java
@SuppressWarnings("unchecked")
  @Override
  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;        //一级缓存?
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);              //从缓存中取出结果
      } else {
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);    //如果没有缓存，则读取数据库
      }
    } 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;
  }

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;
}

#org\mybatis\mybatis\3.4.5\mybatis-3.4.5-sources.jar!\org\apache\ibatis\executor\SimpleExecutor.java 

@Override
  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);
    }
  }

从SimpleExecutor.doQuery()方法可以看出，这个是流程是：1、获取链接Connection  2、调用StateMentHandler执行

#org\mybatis\mybatis\3.4.5\mybatis-3.4.5-sources.jar!\org\apache\ibatis\executor\statement\RoutingStatementHandler.java

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
    return delegate.<E>query(statement, resultHandler);
  }

org\mybatis\mybatis\3.4.5\mybatis-3.4.5-sources.jar!\org\apache\ibatis\executor\statement\PreparedStatementHandler.java

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

从上面可以看出，mybatis最终调用的是jdbc的PrepareStatement 的execute方法执行sql，并且进行结果映射。再看下PrepareStatement

的execute()方法

#mysql\mysql-connector-java\5.1.46\mysql-connector-java-5.1.46.jar!\com\mysql\jdbc\PreparedStatement.class
public boolean execute() throws SQLException {
        synchronized(this.checkClosed().getConnectionMutex()) {
            MySQLConnection locallyScopedConn = this.connection;
            if (!this.doPingInstead && !this.checkReadOnlySafeStatement()) {
                throw SQLError.createSQLException(Messages.getString("PreparedStatement.20") + Messages.getString("PreparedStatement.21"), "S1009", this.getExceptionInterceptor());
            } else {
                ResultSetInternalMethods rs = null;
                this.lastQueryIsOnDupKeyUpdate = false;
                if (this.retrieveGeneratedKeys) {
                    this.lastQueryIsOnDupKeyUpdate = this.containsOnDuplicateKeyUpdateInSQL();
                }

                this.batchedGeneratedKeys = null;
                this.resetCancelledState();
                this.implicitlyCloseAllOpenResults();
                this.clearWarnings();
                if (this.doPingInstead) {
                    this.doPingInstead();
                    return true;
                } else {
                    this.setupStreamingTimeout(locallyScopedConn);
                    Buffer sendPacket = this.fillSendPacket();
                    String oldCatalog = null;
                    if (!locallyScopedConn.getCatalog().equals(this.currentCatalog)) {
                        oldCatalog = locallyScopedConn.getCatalog();
                        locallyScopedConn.setCatalog(this.currentCatalog);
                    }

                    CachedResultSetMetaData cachedMetadata = null;
                    if (locallyScopedConn.getCacheResultSetMetadata()) {
                        cachedMetadata = locallyScopedConn.getCachedMetaData(this.originalSql);
                    }

                    Field[] metadataFromCache = null;
                    if (cachedMetadata != null) {
                        metadataFromCache = cachedMetadata.fields;
                    }

                    boolean oldInfoMsgState = false;
                    if (this.retrieveGeneratedKeys) {
                        oldInfoMsgState = locallyScopedConn.isReadInfoMsgEnabled();
                        locallyScopedConn.setReadInfoMsgEnabled(true);
                    }

                    locallyScopedConn.setSessionMaxRows(this.firstCharOfStmt == 'S' ? this.maxRows : -1);
                    rs = this.executeInternal(this.maxRows, sendPacket, this.createStreamingResultSet(), this.firstCharOfStmt == 'S', metadataFromCache, false);   //最终实际执行sql语句的地方
                    if (cachedMetadata != null) {
                        locallyScopedConn.initializeResultsMetadataFromCache(this.originalSql, cachedMetadata, rs);
                    } else if (rs.reallyResult() && locallyScopedConn.getCacheResultSetMetadata()) {
                        locallyScopedConn.initializeResultsMetadataFromCache(this.originalSql, (CachedResultSetMetaData)null, rs);
                    }

                    if (this.retrieveGeneratedKeys) {
                        locallyScopedConn.setReadInfoMsgEnabled(oldInfoMsgState);
                        rs.setFirstCharOfQuery(this.firstCharOfStmt);
                    }

                    if (oldCatalog != null) {
                        locallyScopedConn.setCatalog(oldCatalog);
                    }

                    if (rs != null) {
                        this.lastInsertId = rs.getUpdateID();
                        this.results = rs;
                    }

                    return rs != null && rs.reallyResult();
                }
            }
        }
    }

下面是结果映射的代码：

#org\mybatis\mybatis\3.4.5\mybatis-3.4.5-sources.jar!\org\apache\ibatis\executor\resultset\DefaultResultSetHandler.java
@Override
  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;
    ResultSetWrapper rsw = getFirstResultSet(stmt);

    List<ResultMap> resultMaps = mappedStatement.getResultMaps();
    int resultMapCount = resultMaps.size();
    validateResultMapsCount(rsw, resultMapCount);
    while (rsw != null && resultMapCount > resultSetCount) {
      ResultMap resultMap = resultMaps.get(resultSetCount);
      handleResultSet(rsw, resultMap, multipleResults, null);
      rsw = getNextResultSet(stmt);
      cleanUpAfterHandlingResultSet();
      resultSetCount++;
    }

    String[] resultSets = mappedStatement.getResultSets();
    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);
  }

注意：一个有意思的地方是，调用insert()的时候，最终调用的还是update()，代码在DefaultSqlSession中展现，如下：

  @Override
  public int insert(String statement) {
    return insert(statement, null);
  }

  @Override
  public int insert(String statement, Object parameter) {
    return update(statement, parameter);
  }

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------

总结：

mybatis的执行过程：

1、调用代理对象的execute方法执行

2、判断sql类型，分别处理增、删、改、查操作

3、转换参数成为sql参数。如果是单个参数，则直接返回，如果是多个参数，则用param1、param2 这样数组返回

4、如果一级缓存已经有缓存了这个方法，调用缓存的方法；如果没有则调用查询数据库的方法

5、获取connection

6、调用PrepareStateMent的execute方法

7、结果集映射

8、最后，调用sqlsession.commit()提交

select语句的类调用顺序：SqlSession 相关类->Executor 相关类 -> PrepareStatement 相关类