MyBatis工作流程

朴灿烈づ我的快乐病毒、 2021-09-25 10:06 489阅读 0赞

# mybatis工作流程 #

## 基本使用方法 ##

// 获取mybatis配置文件
    InputStream is = Resources.getResourceAsStream("mybatis-config.xml");
    // 新建SqlSessionFactory创建器
    SqlSessionFactoryBuilder sfb =  new SqlSessionFactoryBuilder();
    // 通过SqlSessionFactoryBuilder 创建SqlSessionFactory ，并传入配置文件流
    SqlSessionFactory ssf= sfb.build(is);
    // 通过工厂获取sqlSession
    SqlSession sqlSession = ssf.openSession();
    // sqlSession获取mapper
    DatasourceMapper mapper = sqlSession.getMapper(xxxMapper.class);
    // 在通过mapper调用相关方法
    String s = mapper.getXxx();
    System.out.println(s);

## 解析 ##

`SqlSessionFactoryBuilder sfb = new SqlSessionFactoryBuilder();`  
`SqlSessionFactory ssf= sfb.build(is);`

public SqlSessionFactory build(InputStream inputStream) { 
      	return build(inputStream, null, null);
    }
    
    public SqlSessionFactory build(Reader reader, String environment, Properties properties) { 
        try { 
        	// xml解析器
          	XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
          	// parser.parse() 获取配置文件信息，并封装成Configuration对象
          	return build(parser.parse());
        } catch (Exception e) { 
          	throw ExceptionFactory.wrapException("Error building SqlSession.", e);
        } finally { 
       		ErrorContext.instance().reset();
    	    try { 
    	      reader.close();
    	    } catch (IOException e) { 
    	      // Intentionally ignore. Prefer previous error.
    	    }
        }
    }
    
    public SqlSessionFactory build(Configuration config) { 
    	return new DefaultSqlSessionFactory(config);
    }
    // DefaultSqlSessionFactory 是 SqlSessionFactory的一个实现类
    public DefaultSqlSessionFactory(Configuration configuration) { 
    	this.configuration = configuration;
    }

`SqlSession sqlSession = ssf.openSession();`

public SqlSession openSession() { 
        return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
    }
    
    /** * @Param execType 执行器 默认SIMPLE （SIMPLE, REUSE, BATCH） * @Param level 事务等级 * @Param autoCommit 自动提交事务 false */
    private SqlSession openSessionFromDataSource(ExecutorType execType, 
    						TransactionIsolationLevel level, 	
    						boolean autoCommit) { 
        Transaction tx = null;
        try { 
        	// 通过配置文件中的<Encironment>标签中的数据，获取数据库配置信息。
    	    final Environment environment = configuration.getEnvironment();
    	    // 通过environment中创建事务工厂
    	    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    	    // 通过事务工厂创建事务 【数据源，数据库隔离等级，是否自动提交】
    	    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    	    // 创建执行器 相当于JDBC中的statement
    	    final Executor executor = configuration.newExecutor(tx, execType);
    	    // 返回SqlSession
    	    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();
        }
    }

`DatasourceMapper mapper = sqlSession.getMapper(xxxMapper.class);`

<T> T getMapper(Class<T> type);
    
    @Override
    public <T> T getMapper(Class<T> type) { 
      	return getConfiguration().getMapper(type, this);
    }
    
    // 实际上就是通过 mapperRegistry 帮我们存储了所有的mapper对象，mapperRegistry 底层封装了HashMap。
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) { 
        return mapperRegistry.getMapper(type, sqlSession);
    }
    
    public <T> T getMapper(Class<T> type, SqlSession sqlSession) { 
    	// private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();
    	// 
    	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);
    	}
    }
    
    // class MapperProxyFactory<T>
    public T newInstance(SqlSession sqlSession) { 
        final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
    }
    
    // 类下的成员变量
    // private final Class<T> mapperInterface;
    // private final Map<Method, MapperMethodInvoker> methodCache = new ConcurrentHashMap<>();
    protected T newInstance(MapperProxy<T> mapperProxy) { 
    	// JDK 动态代理
        return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] {  mapperInterface }, mapperProxy);
    }

`String s = mapper.getXxx();`  
其实是通过调用 `MapperProxy`的invoke方法。

public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable { 
        return mapperMethod.execute(sqlSession, args);
    }

`execute` 方法

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);
              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;
      }

### 查询流程 ###

public <E> List<E> query(MappedStatement ms, Object parameterObject,
     			RowBounds rowBounds, ResultHandler resultHandler,
     			CacheKey key, BoundSql boundSql)
        throws SQLException { 
        // 从二级缓存中获取
      Cache cache = ms.getCache();
      // 如果有 则从缓存中取
      if (cache != null) { 
        flushCacheIfRequired(ms);
        if (ms.isUseCache() && resultHandler == null) { 
          ensureNoOutParams(ms, boundSql);
          @SuppressWarnings("unchecked")
          List<E> list = (List<E>) 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;
        }
      }
      // 查询一级缓存
      return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
    }

## 总结 ##

*  获取配置文件，转成流。(`InputStream is = Resources.getResourceAsStream("mybatis-config.xml")`)
 *  解析流得到 configuration
 *  通过SqlSessionFactoryBuilder 创建 SqlSessionFactory
 *  通过SqlSessionFactory 获取 SqlSession
 *  使用SqlSession 执行SQL (MapperProxy)
 *  CachingExecutor 判断是否开启二级缓存
    
     *  如果没有开启二级缓存，则调用 BaseExecutor 调用一级缓存
     *  如果一级缓存有，则返回，否则，请往下
 *  RountingStatementHandle 选中sql执行方式
 *  选则预编译方式，PreparedStatementHandle
 *  最后，通过DefaultResultSetHandle返回结果。

参考：https://blog.csdn.net/qq\_38270106/article/details/93398694