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这次重点分析mybatis的事务管理器

仍旧从官网给的例子着手分析,配置文件mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
  "http://mybatis.org/dtd/mybatis-3-config.dtd">
  <configuration>
      <environments default="development">
        <environment id="development">
          <transactionManager type="JDBC"/>
          <dataSource type="POOLED">
            <property name="driver" value="${driver}"/>
            <property name="url" value="${url}"/>
            <property name="username" value="${username}"/>
            <property name="password" value="${password}"/>
          </dataSource>
        </environment>
      </environments>
      <mappers>
        <mapper resource="org/mybatis/example/BlogMapper.xml"/>
      </mappers>
  </configuration>

在根结点configuration下有environments和mappers两个结点.

environments:环境,mybatis可以配置多个环境即可以将 SQL 映射应用于多种数据库之中.如开发,测试,生产对应不同的数据库.

mappers:映射器,提供映射文件的地址.

在environment下的transactionManager就是现在要重点分析的对象.

mybatis中有两种类型的事务管理器,JDBC和MANAGED.

JDBC:直接使用了 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。

MANAGED:不提交或回滚一个连接，而是让容器来管理事务的整个生命周期。

对例子进行分析,之前分析过

String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

现在再来分析

SqlSession session = sqlSessionFactory.openSession();

DefaultSqlSessionFactory里的openSession

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

openSessionFromDataSource

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    //根据配置获取环境
    final Environment environment = configuration.getEnvironment();
    //构建事务工厂
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    //通过事务工厂创建事务
    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();
  }
}

看一下各个类之间的关系

JdbcTransaction和ManagedTransaction对应事务管理器的两种类型JDBC和MANAGED.

由于ManagedTransaction几乎没做什么,所以重点分析JdbcTransaction.

因为JdbcTransaction直接使用了 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。

那么直接来看看JDBC的事务

java.sql.Connection

/**
 * A constant indicating that transactions are not supported.
 */
int TRANSACTION_NONE             = 0;

/**
 * A constant indicating that
 * dirty reads, non-repeatable reads and phantom reads can occur.
 * This level allows a row changed by one transaction to be read
 * by another transaction before any changes in that row have been
 * committed (a "dirty read").  If any of the changes are rolled back,
 * the second transaction will have retrieved an invalid row.
 */
int TRANSACTION_READ_UNCOMMITTED = 1;

/**
 * A constant indicating that
 * dirty reads are prevented; non-repeatable reads and phantom
 * reads can occur.  This level only prohibits a transaction
 * from reading a row with uncommitted changes in it.
 */
int TRANSACTION_READ_COMMITTED   = 2;

/**
 * A constant indicating that
 * dirty reads and non-repeatable reads are prevented; phantom
 * reads can occur.  This level prohibits a transaction from
 * reading a row with uncommitted changes in it, and it also
 * prohibits the situation where one transaction reads a row,
 * a second transaction alters the row, and the first transaction
 * rereads the row, getting different values the second time
 * (a "non-repeatable read").
 */
int TRANSACTION_REPEATABLE_READ  = 4;

/**
 * A constant indicating that
 * dirty reads, non-repeatable reads and phantom reads are prevented.
 * This level includes the prohibitions in
 * <code>TRANSACTION_REPEATABLE_READ</code> and further prohibits the
 * situation where one transaction reads all rows that satisfy
 * a <code>WHERE</code> condition, a second transaction inserts a row that
 * satisfies that <code>WHERE</code> condition, and the first transaction
 * rereads for the same condition, retrieving the additional
 * "phantom" row in the second read.
 */
int TRANSACTION_SERIALIZABLE     = 8;

事务隔离级别

TRANSACTION_NONE:不支持事务

TRANSACTION_READ_UNCOMMITTED:允许脏读、不可重复的读和虚读.

TRANSACTION_READ_COMMITTED:不允许脏读,允许不可重复的读和虚读.

TRANSACTION_REPEATABLE_READ:不允许脏读和不可重复的读,允许虚读.

TRANSACTION_SERIALIZABLE:不允许脏读、不可重复的读和虚读.

脏读：一个事务对数据进行更新，但事务还没有提交，另一个事务就可以取到该事务没有提交的更新结果。

不可重复读：同一个事务在整个事务过程中对同一笔数据进行多次读取，每次读取结果都不同。

虚读：同一个查询在整个事务过程中多次执行后，查询所得的结果集是不一样的。虚读针对的是多条记录。

不可重复读:列值的不同; 虚读:记录数量的不同。

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如果你对 MyBatis 感兴趣，欢迎加入我的知识一起交流。