Mybaits 源码解析 (四)----- SqlSession的创建…

2019-10-30 09:44:33来源:博客园 阅读 ()

新老客户大回馈,云服务器低至5折

Mybaits 源码解析 (四)----- SqlSession的创建过程(看懂框架源码再也不用死记硬背面试题)

SqlSession是mybatis的核心接口之一,是myabtis接口层的主要组成部分,对外提供了mybatis常用的api。myabtis提供了两个SqlSesion接口的实现,常用的实现类是DefaultSqlSession。它相当于一个数据库连接对象,在一个SqlSession中可以执行多条SQL语句。

创建SqlSession

前面的两篇文章我们已经得到了SqlSessionFactory,那么SqlSession将由SqlSessionFactory进行创建。

SqlSession sqlSession=sqlSessionFactory.openSession();

我们就来看看这个SqlSessionFactoryopenSession方法是如何创建SqlSession对象的。根据上面的分析,这里的SqlSessionFactory类型对象其实是一个DefaultSqlSessionFactory对象,因此,需要到DefaultSqlSessionFactory类中去看openSession方法。

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

调用了openSessionFromDataSource方法,并且第一个参数获取了默认的执行器类型,第二个参数为null,第三个参数为false,看看这个默认的执行器类型是啥

  

默认的执行器类型SIMPLE,我们跟进openSessionFromDataSource方法

/**
 * ExecutorType 指定Executor的类型,分为三种:SIMPLE, REUSE, BATCH,默认使用的是SIMPLE
 * TransactionIsolationLevel 指定事务隔离级别,使用null,则表示使用数据库默认的事务隔离界别
 * autoCommit 是否自动提交,传过来的参数为false,表示不自动提交
 */
private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
    Transaction tx = null;
    try {
        // 获取配置中的环境信息,包括了数据源信息、事务等
        final Environment environment = configuration.getEnvironment();
        // 创建事务工厂
        final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
        // 创建事务,配置事务属性
        tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
        // 创建Executor,即执行器
        // 它是真正用来Java和数据库交互操作的类,后面会展开说。
        final Executor executor = configuration.newExecutor(tx, execType);
        // 创建DefaultSqlSession对象返回,其实现了SqlSession接口
        return new DefaultSqlSession(configuration, executor, autoCommit);
    } catch (Exception e) {
        closeTransaction(tx);
        throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
    } finally {
        ErrorContext.instance().reset();
    }
}

主要包含以下几个步骤:

  1. 首先从configuration获取Environment对象,里面主要包含了DataSource和TransactionFactory对象
  2. 创建TransactionFactory
  3. 创建Transaction
  4. 从configuration获取Executor
  5. 构造DefaultSqlSession对象

 我们先来看看常规的environment配置

//配置environment环境
<environments default="development">
    <environment id="development">
        /** 事务配置 type= JDBC、MANAGED 
         *  1.JDBC:这个配置直接简单使用了JDBC的提交和回滚设置。它依赖于从数据源得到的连接来管理事务范围。
         *  2.MANAGED:这个配置几乎没做什么。它从来不提交或回滚一个连接。
         */
        <transactionManager type="JDBC" />
        /** 数据源类型:type = UNPOOLED、POOLED、JNDI 
         *  1.UNPOOLED:这个数据源的实现是每次被请求时简单打开和关闭连接。
         *  2.POOLED:这是JDBC连接对象的数据源连接池的实现。 
         *  3.JNDI:这个数据源的实现是为了使用如Spring或应用服务器这类的容器
         */
        <dataSource type="POOLED">
            <property name="driver" value="com.mysql.jdbc.Driver" />
            <property name="url" value="jdbc:mysql://localhost:3306/xhm" />
            <property name="username" value="root" />
            <property name="password" value="root" />
            //默认连接事务隔离级别
            <property name="defaultTransactionIsolationLevel" value=""/> 
        </dataSource>
    </environment>
</environments>

还记得前面文章是怎么解析environments的吗,Mybaits 源码解析 (二)----- 根据配置文件创建SqlSessionFactory(Configuration的创建过程),我们简单的回顾一下

private void environmentsElement(XNode context) throws Exception {
    if (context != null) {
        if (environment == null) {
            // 获取 default 属性
            environment = context.getStringAttribute("default");
        }
        for (XNode child : context.getChildren()) {
            // 获取 id 属性
            String id = child.getStringAttribute("id");
            /*
             * 检测当前 environment 节点的 id 与其父节点 environments 的属性 default 
             * 内容是否一致,一致则返回 true,否则返回 false
             * 将其default属性值与子元素environment的id属性值相等的子元素设置为当前使用的Environment对象
             */
            if (isSpecifiedEnvironment(id)) {
                // 将environment中的transactionManager标签转换为TransactionFactory对象
                TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
                // 将environment中的dataSource标签转换为DataSourceFactory对象
                DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
                // 创建 DataSource 对象
                DataSource dataSource = dsFactory.getDataSource();
                Environment.Builder environmentBuilder = new Environment.Builder(id)
                    .transactionFactory(txFactory)
                    .dataSource(dataSource);
                // 构建 Environment 对象,并设置到 configuration 中
                configuration.setEnvironment(environmentBuilder.build());
            }
        }
    }
}

private TransactionFactory transactionManagerElement(XNode context) throws Exception {
    if (context != null) {
        String type = context.getStringAttribute("type");
        Properties props = context.getChildrenAsProperties();
        //通过别名获取Class,并实例化
        TransactionFactory factory = (TransactionFactory)this.resolveClass(type).newInstance();
        factory.setProperties(props);
        return factory;
    } else {
        throw new BuilderException("Environment declaration requires a TransactionFactory.");
    }
}

private DataSourceFactory dataSourceElement(XNode context) throws Exception {
    if (context != null) {
        String type = context.getStringAttribute("type");
        //通过别名获取Class,并实例化
        Properties props = context.getChildrenAsProperties();
        DataSourceFactory factory = (DataSourceFactory)this.resolveClass(type).newInstance();
        factory.setProperties(props);
        return factory;
    } else {
        throw new BuilderException("Environment declaration requires a DataSourceFactory.");
    }
}

获取TransactionFactory

我们的environment配置中transactionManager type="JDBC"和dataSource type="POOLED",则生成的transactionManager为JdbcTransactionFactory,DataSourceFactory为PooledDataSourceFactory

我们回到openSessionFromDataSource,接着看看getTransactionFactoryFromEnvironment方法

    private TransactionFactory getTransactionFactoryFromEnvironment(Environment environment) {
        return (TransactionFactory)(environment != null && environment.getTransactionFactory() != null ? environment.getTransactionFactory() : new ManagedTransactionFactory());
    }

创建Transaction

很明显 environment.getTransactionFactory() 就是JdbcTransactionFactory,看看这个工厂是如何创建Transaction的

public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new JdbcTransaction(ds, level, autoCommit);
}

直接通过工厂方法创建了一个JdbcTransaction对象,并传参DataSource ,事务隔离级别null,自动提交false三个参数,我们来看看JdbcTransaction

public class JdbcTransaction implements Transaction {
    //数据库连接对象
    protected Connection connection;
    //数据库DataSource
    protected DataSource dataSource;
    //数据库隔离级别
    protected TransactionIsolationLevel level;
    //是否自动提交
    protected boolean autoCommmit;

    public JdbcTransaction(DataSource ds, TransactionIsolationLevel desiredLevel, boolean desiredAutoCommit) {
        //设置dataSource和隔离级别,是否自动提交属性
        //这里隔离级别传过来的是null,表示使用数据库默认隔离级别,自动提交为false,表示不自动提交
        this.dataSource = ds;
        this.level = desiredLevel;
        this.autoCommmit = desiredAutoCommit;
    }

     public Connection getConnection() throws SQLException {
        if (this.connection == null) {
            this.openConnection();
        }

        return this.connection;
    }

    //提交功能是通过Connection去完成的
    public void commit() throws SQLException {
        if (this.connection != null && !this.connection.getAutoCommit()) {
            if (log.isDebugEnabled()) {
                log.debug("Committing JDBC Connection [" + this.connection + "]");
            }

            this.connection.commit();
        }

    }

    //回滚功能是通过Connection去完成的
    public void rollback() throws SQLException {
        if (this.connection != null && !this.connection.getAutoCommit()) {
            if (log.isDebugEnabled()) {
                log.debug("Rolling back JDBC Connection [" + this.connection + "]");
            }

            this.connection.rollback();
        }

    }

    //关闭功能是通过Connection去完成的
    public void close() throws SQLException {
        if (this.connection != null) {
            this.resetAutoCommit();
            if (log.isDebugEnabled()) {
                log.debug("Closing JDBC Connection [" + this.connection + "]");
            }

            this.connection.close();
        }

    }
    
    //获取连接是通过dataSource来完成的
    protected void openConnection() throws SQLException {
        if (log.isDebugEnabled()) {
            log.debug("Opening JDBC Connection");
        }

        this.connection = this.dataSource.getConnection();
        if (this.level != null) {
            this.connection.setTransactionIsolation(this.level.getLevel());
        }

        this.setDesiredAutoCommit(this.autoCommmit);
    }
}

JdbcTransaction主要维护了一个默认autoCommit为false的Connection对象,对事物的提交,回滚,关闭等都是接见通过Connection完成的。

创建Executor

//创建一个执行器,默认是SIMPLE
public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    //根据executorType来创建相应的执行器,Configuration默认是SIMPLE
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      //创建SimpleExecutor实例,并且包含Configuration和transaction属性
      executor = new SimpleExecutor(this, transaction);
    }
    
    //如果要求缓存,生成另一种CachingExecutor,装饰者模式,默认都是返回CachingExecutor
    /**
     * 二级缓存开关配置示例
     * <settings>
     *   <setting name="cacheEnabled" value="true"/>
     * </settings>
     */
    if (cacheEnabled) {
      //CachingExecutor使用装饰器模式,将executor的功能添加上了二级缓存的功能,二级缓存会单独文章来讲
      executor = new CachingExecutor(executor);
    }
    //此处调用插件,通过插件可以改变Executor行为,此处我们后面单独文章讲
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
}

executor包含了Configuration和刚刚创建的Transaction,默认的执行器为SimpleExecutor,如果开启了二级缓存(默认开启),则CachingExecutor会包装SimpleExecutor,然后依次调用拦截器的plugin方法返回一个被代理过的Executor对象。

CachingExecutor 对象里面包含了刚创建的SimpleExecutor,后面文章我们会及具体讲这个类

public class CachingExecutor implements Executor {
    private Executor delegate;
    private TransactionalCacheManager tcm = new TransactionalCacheManager();

    public CachingExecutor(Executor delegate) {
        this.delegate = delegate;
        delegate.setExecutorWrapper(this);
    }
    //
}

构造DefaultSqlSession对象

new DefaultSqlSession(this.configuration, executor, autoCommit);

传参configuration和刚生成的executor,我们来简单看看

public class DefaultSqlSession implements SqlSession {

  /**
   * mybatis全局配置新
   */
  private final Configuration configuration;
  /**
   * SQL执行器
   */
  private final Executor executor;

  /**
   * 是否自动提交
   */
  private final boolean autoCommit;

  private List<Cursor<?>> cursorList;
  
  public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
        this.configuration = configuration;
        this.executor = executor;
        this.dirty = false;
        this.autoCommit = autoCommit;
  }
  
  @Override
  public <T> T selectOne(String statement) {
    return this.<T>selectOne(statement, null);
  }

  @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) {
    return this.selectList(statement, 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();
    }
  }
  
  //略....update等方法
}

SqlSession的所有查询接口最后都归结位Exector的方法调用。后面文章我们来分析其调用流程

 

 


原文链接:https://www.cnblogs.com/java-chen-hao/p/11743506.html
如有疑问请与原作者联系

标签:

版权申明:本站文章部分自网络,如有侵权,请联系:west999com@outlook.com
特别注意:本站所有转载文章言论不代表本站观点,本站所提供的摄影照片,插画,设计作品,如需使用,请与原作者联系,版权归原作者所有

上一篇:java类在何时被加载

下一篇:Eureka源码解析系列文章汇总