Mybatis的配置文件入门介绍

系统管理员 2022-06-16 06:21 249阅读 0赞

从前面的几篇文章，我们看到了，如何简单的使用Mybatis。从这篇文章开始，我们将从其核心配置文件入手，对Mybatis支持的核心配置文件进行简单详细的描述。

从下面这段代码是我们在使用mybatis前的配置初始化过程，我们通过阅读其源码来逐步了解内部实现原理。

// Mybatis 通过SqlSessionFactory获取SqlSession, 然后才能通过SqlSession与数据库进行交互 private static SqlSessionFactory getSessionFactory() { SqlSessionFactory sessionFactory = null; String resource = "configuration.xml"; try { sessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource)); } catch (IOException e) { e.printStackTrace(); } return sessionFactory; }

我们进入到SqlSessionFactoryBuilder类里面，查看其源码：

/** * Copyright 2009-2016 the original author or authors. * * Licensed under the Apache License, Version 2.0 (the "License"); * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * * http://www.apache.org/licenses/LICENSE-2.0 * * Unless required by applicable law or agreed to in writing, software * distributed under the License is distributed on an "AS IS" BASIS, * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. * See the License for the specific language governing permissions and * limitations under the License. */ package org.apache.ibatis.session; import java.io.IOException; import java.io.InputStream; import java.io.Reader; import java.util.Properties; import org.apache.ibatis.builder.xml.XMLConfigBuilder; import org.apache.ibatis.exceptions.ExceptionFactory; import org.apache.ibatis.executor.ErrorContext; import org.apache.ibatis.session.defaults.DefaultSqlSessionFactory; /** * Builds {@link SqlSession} instances. * * @author Clinton Begin */ public class SqlSessionFactoryBuilder { public SqlSessionFactory build(Reader reader) { return build(reader, null, null); } public SqlSessionFactory build(Reader reader, String environment) { return build(reader, environment, null); } public SqlSessionFactory build(Reader reader, Properties properties) { return build(reader, null, properties); } public SqlSessionFactory build(Reader reader, String environment, Properties properties) { try { XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties); return build(parser.parse()); } catch (Exception e) { throw ExceptionFactory.wrapException("Error building SqlSession.", e); } finally { ErrorContext.instance().reset(); try { reader.close(); } catch (IOException e) { // Intentionally ignore. Prefer previous error. } } } public SqlSessionFactory build(InputStream inputStream) { return build(inputStream, null, null); } public SqlSessionFactory build(InputStream inputStream, String environment) { return build(inputStream, environment, null); } public SqlSessionFactory build(InputStream inputStream, Properties properties) { return build(inputStream, null, properties); } public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) { try { XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties); return build(parser.parse()); } catch (Exception e) { throw ExceptionFactory.wrapException("Error building SqlSession.", e); } finally { ErrorContext.instance().reset(); try { inputStream.close(); } catch (IOException e) { // Intentionally ignore. Prefer previous error. } } } public SqlSessionFactory build(Configuration config) { return new DefaultSqlSessionFactory(config); } }

在这个类中，支持多种构造SqlSessionFactory的方法。可以只传入mybatis配置文件，也可以同时传入properties配置文件替代mybatis配置文件中的<properties>元素标签，另外也支持传入环境参数envirmont参数。

我们跟随着源码继续往下看：

public SqlSessionFactory build(Reader reader, String environment, Properties properties) { try { XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties); return build(parser.parse()); } catch (Exception e) { throw ExceptionFactory.wrapException("Error building SqlSession.", e); } finally { ErrorContext.instance().reset(); try { reader.close(); } catch (IOException e) { // Intentionally ignore. Prefer previous error. } } }

这里创建了一个XMLConfigBuilder类实例，通过他来对mybatis配置文件（一个xml配置文件）进行解析。解析的代码入口如下所示：

public Configuration parse() { if (parsed) { throw new BuilderException("Each XMLConfigBuilder can only be used once."); } parsed = true; parseConfiguration(parser.evalNode("/configuration")); return configuration; } private void parseConfiguration(XNode root) { try { Properties settings = settingsAsPropertiess(root.evalNode("settings")); //issue #117 read properties first propertiesElement(root.evalNode("properties")); loadCustomVfs(settings); typeAliasesElement(root.evalNode("typeAliases")); pluginElement(root.evalNode("plugins")); objectFactoryElement(root.evalNode("objectFactory")); objectWrapperFactoryElement(root.evalNode("objectWrapperFactory")); reflectorFactoryElement(root.evalNode("reflectorFactory")); settingsElement(settings); // read it after objectFactory and objectWrapperFactory issue #631 environmentsElement(root.evalNode("environments")); databaseIdProviderElement(root.evalNode("databaseIdProvider")); typeHandlerElement(root.evalNode("typeHandlers")); mapperElement(root.evalNode("mappers")); } catch (Exception e) { throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); } }

从这里看出，配置文件是以configuration为根节点，在根节点之下有多个子节点，它们分别为：settings、properties、typeAliases、plugins、objectFactory、objectWrapperFactory、environments、databaseIdProvider、typeHandlers、mappers。

这篇文章不参与每个配置子项的说明，从下一篇开始，将陆续对这些元素进行详细讲解，敬请关注，如果对我的博客感兴趣，您可以添加关注哟。