上一篇:mybatis源码分析之Configuration

主要分析了构建SqlSessionFactory的过程中配置文件的读取.

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 configurationPUBLIC "-//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。