MyBatis源码解析（三）：初始化SqlSession

电玩女神 2022-02-26 13:50 246阅读 0赞

前言：在[MyBatis源码解析（二）：初始化SqlSessionFactory][MyBatis_SqlSessionFactory]中，已经成功解析XML配置文件，并初始化SqlSessionFactory；这一篇将对 SqlSessionFactory.openSession() 进行解析

一，SqlSessionFactory.openSession() 执行流程

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTE5NzYzODg_size_16_color_FFFFFF_t_70][]

二，初始化步骤

1，触发 openSession()，内部调用 openSessionFromDataSource() 完成初始化动作

public SqlSession openSession() {
        return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
      }

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
        Transaction tx = null;
        try {
          // 从Configuration中获取Environment配置信息
          final Environment environment = configuration.getEnvironment();
          // 从Environment配置信息获取事务工厂
          final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
          // 初始化事务, 此时事务持有dataSource对象
          tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
          // 初始化Executor, Executor通过Transaction间接持有DataSource对象
          final Executor executor = configuration.newExecutor(tx, execType);
          // 初始化SqlSession对象, SqlSession持有configuration和executor引用
          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();
        }
      }

2，初始化数据库事务 -- JdbcTransactionFactory.newTransaction

public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
        // 此时数据库事务持有DataSource信息
        return new JdbcTransaction(ds, level, autoCommit);
      }

3，初始化 Executor 执行对象 -- Configuration.newExecutor

\* ExecuteType配置信息和CacheEnabled配置信息官网解释

![20190329235618512.png][]

![2019032923563271.png][]

\* 初始化Executor方法；在开启缓存处理的情况下，注意此处通过装饰者模式对 SimpleExecutor 进行了二次装饰 CachingExecutor

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
        // executorType如果未配置, 默认为Simple
        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);
        }
        // 未配置默认开启
        if (cacheEnabled) {
          executor = new CachingExecutor(executor);
        }
        // 通过拦截器加载plugin, 返回对象为代理对象
        executor = (Executor) interceptorChain.pluginAll(executor);
        return executor;
      }

\* Executor 初始化时， 关联加载 Plugin 插件，并生成Executor代理对象，此处注意：代理对象可以被再次代理

public Object pluginAll(Object target) {
        // 此处遍历Plugins标签内所有Plugin进行加载
        // 通过代理对象再代理模式, 保证每一个拦截器都能被执行
        // 此处注意点 : 代理对象可以被再次代理
        for (Interceptor interceptor : interceptors) {
          target = interceptor.plugin(target);
        }
        return target;
      }

\* 生成 Executor 对象的代理对象，如存在多个，则后续传入的对象已经是代理对象，类似装饰者模式方式

public static Object wrap(Object target, Interceptor interceptor) {
        Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
        Class<?> type = target.getClass();
        Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
        // 存在有效插件
        if (interfaces.length > 0) {
          // 对target即executor进行代理
          // 外部循环调用, 则此时executor对象可能已经是代理对象
          // 允许对代理对象再次进行代理
          // 最终执行时, 装箱代理, 拆箱调用, 类似装饰者模式一层层进行处理
          return Proxy.newProxyInstance(
              type.getClassLoader(),
              interfaces,
              new Plugin(target, interceptor, signatureMap));
        }
        return target;
      }

4，初始化SqlSession成功，获取的 SqlSession 为 DefaultSqlSession，此时 SqlSession 持有配置对象 Configuration，执行对象 CachingExecetor，已经持有 DataSource 配置的数据库事务对象 Transaction

SqlSession sqlSession = sqlSessionFactory.openSession();

[MyBatis_SqlSessionFactory]: https://mp.csdn.net/postedit/88903330
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[20190329235618512.png]: /images/20220226/16989a43e8814384a7e8bfbe26638043.png
[2019032923563271.png]: /images/20220226/2959f4b7dbbd4e7aa8bfa0411ee3a643.png