Mybatis SqlSession

1. 使用示例
2. SqlSession 源码
3. 异常测试

一直在使用Mybatis, 但是一直对Mybatis中的SqlSession的实际操作过程没有深入了解过, 今天在项目中引用了Mybatis-Guice模块, 很好奇Mybatis-Guice是如何做的SqlSeesion自动资源释放,因此今天就找时间好好研究一下`SqlSession`.

首先看一下Mybatis3官方文档中对SqlSessionFactoryBuilder, SqlSessionFactory和SqlSession的描述：

• SqlSessionFactoryBuilder : 用于构建SqlSessionFactory. 我们可以使用它来构建一个单数据源或者多数据源的应用程序.
• SqlSessionFactory : 创建SqlSession, 该实例的生命周期应该是整个应用程序.
• SqlSession : 包含了面向数据库执行 SQL 命令所需的所有方法, 但它不是线程安全的. 每当向数据库发起一个请求时都应该打开一个SqlSession,然后操作完之后再finally中关闭它.

使用示例

SqlSessionFactoryBuilder是通过xml配置文件或者Configuration建构出SqlSessionFactory, 然后SqlSessionFactory通过openSession()来获得一个SqlSession. SqlSession接口实现自Closeable接口. 按照官网所说, 我们应该这样使用SqlSession

SqlSession session = sqlSessionFactory.openSession();
try {
  // do work
} finally {
  session.close();
}

但其实我们可以使用Java7提供的AutoClose语法

try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
	// do work
}

这样代码就精简了很多.

SqlSession 源码

SqlSession接口中定义了大量的我们操作SQL提供的接口

• T selectOne(String statement);
• List selectList(String statement);
• void select(String statement, ResultHandler handler);
  等等. 我们看一下它的实现类DefaultSqlSession的实现:

  private Configuration configuration;
  private Executor executor;
  private boolean dirty;

• Configuration 是我们通过SqlSessionFactoryBuilder构建出SqlSessionFactory时使用的配置
• Executor 是真正的sql执行的部分

我们在DefaultSqlSessionFactory看一下SqlSession的真实创建过程

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    final Environment environment = configuration.getEnvironment();
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    final Executor executor = configuration.newExecutor(tx, execType, autoCommit);
    return new DefaultSqlSession(configuration, executor);
  } 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();
  }
}

在final Executor executor = configuration.newExecutor(tx, execType, autoCommit);会根据execType创建出不同类型的Executor

• BatchExecutor
• ReuseExecutor
• SimpleExecutor
• CachingExecutor

下来我们看一下selectList()实现

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  try {
    MappedStatement ms = configuration.getMappedStatement(statement);
    List<E> result = executor.<E>query(ms, wrapCollection(parameter), rowBounds, Executor.NO_RESULT_HANDLER);
    return result;
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

MappedStatement类在Mybatis框架中用于表示XML文件中一个sql语句节点,即一个<select />、<update />或者<insert />标签.Mybatis框架在初始化阶段会对XML配置文件进行读取,将其中的sql语句节点对象化为一个个MappedStatement对象.
从配置中拿到一个MappedStatement然后交给executor去真正的执行, 真正的有query逻辑的只有BaseExecutor和CachingExecutor, 为了简单起见,我们看一下BaseExecutor. 由于中间的过程还涉及到了Mybatis的本地存储, 我们也跳过这部分.
BaseExecutor#query() -> BaseExecutor#queryFromDatabase()-> SimpleExecutor#doQuery()

public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
    Statement stmt = null;
    try {
      Configuration configuration = ms.getConfiguration();
	  // StatementHandler用于管理java.sql.Statement, 执行真正的与数据库操作
      StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
	  // 通过StatementHandler实例化出一个Statement
      stmt = prepareStatement(handler, ms.getStatementLog());
	  // Statement开始执行查询逻辑
      return handler.<E>query(stmt, resultHandler);
    } finally {
      closeStatement(stmt);
    }
  }

我们看一下SimpleStatementHandler里的查询逻辑

public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
   String sql = boundSql.getSql();
   statement.execute(sql);
   return resultSetHandler.<E>handleResultSets(statement);
 }

SqlSource接口只有一个getBoundSql(Object parameterObject)方法,返回一个BoundSql对象.一个BoundSql对象,代表了一次sql语句的实际执行,而SqlSource对象的责任,就是根据传入的参数对象,动态计算出这个BoundSql,也就是说Mapper文件中的节点的计算,是由SqlSource对象完成的.SqlSource最常用的实现类是DynamicSqlSource

那么close()执行的是什么操作呢？

public void close() {
  try {
    executor.close(isCommitOrRollbackRequired(false));
    dirty = false;
  } finally {
    ErrorContext.instance().reset();
  }
}

我们还是看BaseExecutor的close()

public void close(boolean forceRollback) {
    try {
      try {
        rollback(forceRollback);
      } finally {
		// 真正关闭资源
        if (transaction != null) transaction.close();
      }
    } catch (SQLException e) {
      log.debug("Unexpected exception on closing transaction.  Cause: " + e);
    } finally {
      transaction = null;
      deferredLoads = null;
      localCache = null;
      localOutputParameterCache = null;
      closed = true;
    }
  }

我们看一下JdbcTransaction的close()

public void close() throws SQLException {
  if (connection != null) {
    resetAutoCommit();
    if (log.isDebugEnabled()) {
      log.debug("Closing JDBC Connection [" + connection + "]");
    }
    connection.close();
  }
}

• Transaction : 包装了一个数据库连接. 处理整个网络连接过程中的所有操作, 例如creation, preparation, commit/rollback and close.

异常测试

如果我们不关闭SqlSession会有什么情况发生呢?

import java.io.IOException;
import java.io.InputStream;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

import org.apache.ibatis.annotations.Select;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

public class TestMybatis {

    public static void main(String[] args) throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        List<SqlSession> list = new ArrayList<>();
        for (int i = 1; i < 10; i++) {
            SqlSession session = sqlSessionFactory.openSession();
            QueryMapper mapper = session.getMapper(QueryMapper.class);
            list.add(session);
            System.out.println(i + "  :  " + mapper.select() + " - " + new Date().toLocaleString());
        }
        list.forEach(session -> {
            try {
                System.out.println(session.getConnection().isClosed());
            } catch (SQLException e) {
                e.printStackTrace();
            }
        });
    }
}

interface QueryMapper {
    @Select("select name from user")
    public List<String> select();
}

下面是我们的配置文件

<?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="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/test?autoReconnect=true"/>
                <property name="username" value="root"/>
                <property name="password" value="root"/>
                <property name="poolMaximumActiveConnections" value="3"/>
                <property name="poolMaximumIdleConnections" value="1"/>
                <property name="poolTimeToWait" value="5"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper class="QueryMapper"/>
    </mappers>
</configuration>

我们配置了最大可用连接数为3, 最大的闲置连接为1. 结果为

1  :  [123] - 2016-4-12 18:37:28
2  :  [123] - 2016-4-12 18:37:28
3  :  [123] - 2016-4-12 18:37:28
4  :  [123] - 2016-4-12 18:37:48
5  :  [123] - 2016-4-12 18:37:48
6  :  [123] - 2016-4-12 18:37:48
7  :  [123] - 2016-4-12 18:38:08
8  :  [123] - 2016-4-12 18:38:08
9  :  [123] - 2016-4-12 18:38:08
java.sql.SQLException: Error accessing PooledConnection. Connection is invalid.
	at org.apache.ibatis.datasource.pooled.PooledConnection.checkConnection(PooledConnection.java:254)
	at org.apache.ibatis.datasource.pooled.PooledConnection.invoke(PooledConnection.java:243)
	at com.sun.proxy.$Proxy3.isClosed(Unknown Source)
	at TestMybatis.lambda$main$0(TestMybatis.java:29)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at TestMybatis.main(TestMybatis.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
java.sql.SQLException: Error accessing PooledConnection. Connection is invalid.
	at org.apache.ibatis.datasource.pooled.PooledConnection.checkConnection(PooledConnection.java:254)
	at org.apache.ibatis.datasource.pooled.PooledConnection.invoke(PooledConnection.java:243)
	at com.sun.proxy.$Proxy3.isClosed(Unknown Source)
	at TestMybatis.lambda$main$0(TestMybatis.java:29)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at TestMybatis.main(TestMybatis.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
java.sql.SQLException: Error accessing PooledConnection. Connection is invalid.
	at org.apache.ibatis.datasource.pooled.PooledConnection.checkConnection(PooledConnection.java:254)
	at org.apache.ibatis.datasource.pooled.PooledConnection.invoke(PooledConnection.java:243)
	at com.sun.proxy.$Proxy3.isClosed(Unknown Source)
	at TestMybatis.lambda$main$0(TestMybatis.java:29)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at TestMybatis.main(TestMybatis.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
java.sql.SQLException: Error accessing PooledConnection. Connection is invalid.
	at org.apache.ibatis.datasource.pooled.PooledConnection.checkConnection(PooledConnection.java:254)
	at org.apache.ibatis.datasource.pooled.PooledConnection.invoke(PooledConnection.java:243)
	at com.sun.proxy.$Proxy3.isClosed(Unknown Source)
	at TestMybatis.lambda$main$0(TestMybatis.java:29)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at TestMybatis.main(TestMybatis.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
java.sql.SQLException: Error accessing PooledConnection. Connection is invalid.
	at org.apache.ibatis.datasource.pooled.PooledConnection.checkConnection(PooledConnection.java:254)
	at org.apache.ibatis.datasource.pooled.PooledConnection.invoke(PooledConnection.java:243)
	at com.sun.proxy.$Proxy3.isClosed(Unknown Source)
	at TestMybatis.lambda$main$0(TestMybatis.java:29)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at TestMybatis.main(TestMybatis.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
java.sql.SQLException: Error accessing PooledConnection. Connection is invalid.
	at org.apache.ibatis.datasource.pooled.PooledConnection.checkConnection(PooledConnection.java:254)
	at org.apache.ibatis.datasource.pooled.PooledConnection.invoke(PooledConnection.java:243)
	at com.sun.proxy.$Proxy3.isClosed(Unknown Source)
	at TestMybatis.lambda$main$0(TestMybatis.java:29)
	at java.util.ArrayList.forEach(ArrayList.java:1249)
	at TestMybatis.main(TestMybatis.java:27)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
false
false
false