MyBatis之启动分析(一)

ytao

前言

MyBatis 作为目前最常用的持久层框架之一,分析其源码,对我们的使用过程中可更好的运用它。本系列基于mybatis-3.4.6进行分析。
MyBatis 的初始化工作就是解析主配置文件,映射配置文件以及注解信息。然后保存在org.apache.ibatis.session.Configuration,供后期执行数据请求的相关调用。
Configuration 里有大量配置信息,在后面每涉及到一个相关配置,会进行详细的分析。

启动

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public static void main(String[] args) throws IOException {
// 获取配置文件
Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
// 通过 SqlSessionFactoryBuilder 构建 sqlSession 工厂
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(reader);
// 获取 sqlSession 实例
SqlSession sqlSession = sqlSessionFactory.openSession();

reader.close();
sqlSession.close();
}

分析

SqlSessionFactoryBuilder 类

SqlSessionFactoryBuilder 的build()是Mybatis启动的初始化入口,使用builder模式加载配置文件。
通过查看该类,使用方法重载,有以下9个方法:
SqlSessionFactoryBuilde类中的方法

方法重载最终实现处理的方法源码如下:

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public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
// 实例化 XMLConfigBuilder,用于读取配置文件信息
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
// 解析配置信息,保存到 Configuration
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.
}
}
}

  • environment 是指定加载环境,默认值为 null。
  • properties 是属性配置文件,默认值为 null。
    同时读取配置文件既可字符流读取,也支持字节流读取。
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    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.
    }
    }
    }

实例化 XMLConfigBuilder 类

通过 SqlSessionFactoryBuilder 中 XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties), 分析 XMLConfigBuilder实例化过程。
该类中有四个变量:

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private boolean parsed;
private final XPathParser parser;
private String environment;
private final ReflectorFactory localReflectorFactory = new DefaultReflectorFactory();

  • parsed 是否解析,一次解析即可。用于标志配置文件只解析一次,true为已解析过。
  • parser 解析配置的解析器
  • environment 加载环境,即 SqlSessionFactoryBuilder 中的 environment
  • localReflectorFactory 用于创建和缓存Reflector对象,一个类对应一个Reflector。因为参数处理、结果映射等操作时,会涉及大量的反射操作。DefaultReflectorFactory实现类比较简单,这里不再进行讲解。

XMLConfigBuilder构建函数实现:

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public XMLConfigBuilder(Reader reader, String environment, Properties props) {
this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
}

实例化 XPathParser 对象

首先实例化 XPathParser 对象,里面定义了5个变量:

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private final Document document;
private boolean validation;
private EntityResolver entityResolver;
private Properties variables;
private XPath xpath;

  • document 保存document对象
  • validation xml解析时是否验证文档
  • entityResolver 加载dtd文件
  • variables 配置文件定义的值
  • xpath Xpath对象,用于对XML文件节点的操作

XPathParser 对象构造函数有:
XPathParser构造方法
函数里面都处理了两件事:

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public XPathParser(Reader reader, boolean validation, Properties variables, EntityResolver entityResolver) {
commonConstructor(validation, variables, entityResolver);
this.document = createDocument(new InputSource(reader));
}

  1. 初始化赋值,和创建XPath对象,用于对XML文件节点的操作。

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    private void commonConstructor(boolean validation, Properties variables, EntityResolver entityResolver) {
    this.validation = validation;
    this.entityResolver = entityResolver;
    this.variables = variables;
    // 创建Xpath对象,用于对XML文件节点的操作
    XPathFactory factory = XPathFactory.newInstance();
    this.xpath = factory.newXPath();
    }
  2. 创建Document对象并赋值到document变量, 这里属于Document创建的操作,不再详细讲述,不懂可以点击这里查看API

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    private Document createDocument(InputSource inputSource) {
    // important: this must only be called AFTER common constructor
    try {
    // 实例化 DocumentBuilderFactory 对象,用于创建 DocumentBuilder 对象
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    // 是否校验文档
    factory.setValidating(validation);
    // 设置 DocumentBuilderFactory 的配置
    factory.setNamespaceAware(false);
    factory.setIgnoringComments(true);
    factory.setIgnoringElementContentWhitespace(false);
    factory.setCoalescing(false);
    factory.setExpandEntityReferences(true);
    // 创建 DocumentBuilder
    DocumentBuilder builder = factory.newDocumentBuilder();
    builder.setEntityResolver(entityResolver);
    builder.setErrorHandler(new ErrorHandler() {
    @Override
    public void error(SAXParseException exception) throws SAXException {
    throw exception;
    }

    @Override
    public void fatalError(SAXParseException exception) throws SAXException {
    throw exception;
    }

    @Override
    public void warning(SAXParseException exception) throws SAXException {
    }
    });
    // 加载文件
    return builder.parse(inputSource);
    } catch (Exception e) {
    throw new BuilderException("Error creating document instance. Cause: " + e, e);
    }
    }

XMLConfigBuilder构造函数赋值

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private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
super(new Configuration());
ErrorContext.instance().resource("SQL Mapper Configuration");
this.configuration.setVariables(props);
this.parsed = false;
this.environment = environment;
this.parser = parser;
}
  1. 初始化父类BaseBuilder的值。
  2. 将外部值赋值给对象。
  3. 将实例化的XPathParser赋值给parser

最后返回XMLConfigBuilder对象。

解析 XMLConfigBuilder 对象

通过 XMLConfigBuilder.parse() 解析配置信息,保存至Configuration。解析详解在后面文章中进行分析。

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public Configuration parse() {
// 是否解析过配置文件
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
// 标志解析过,定义为 true
parsed = true;
// 解析 configuration 节点中的信息
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}

创建 SqlSessionFactory

DefaultSqlSessionFactory实现了SqlSessionFactory接口。
通过上面解析得到的Configuration,调用SqlSessionFactoryBuilder.build(Configuration config)创建一个 DefaultSqlSessionFactory

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public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}

实例化DefaultSqlSessionFactory的过程,就是将Configuration传递给DefaultSqlSessionFactory成员变量configuration

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public DefaultSqlSessionFactory(Configuration configuration) {
this.configuration = configuration;
}

创建 SqlSession

通过调用SqlSessionFactory.openSession()创建SqlSession

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public interface SqlSessionFactory {
// 默认创建
SqlSession openSession();

SqlSession openSession(boolean autoCommit);
SqlSession openSession(Connection connection);
SqlSession openSession(TransactionIsolationLevel level);

SqlSession openSession(ExecutorType execType);
SqlSession openSession(ExecutorType execType, boolean autoCommit);
SqlSession openSession(ExecutorType execType, TransactionIsolationLevel level);
SqlSession openSession(ExecutorType execType, Connection connection);

Configuration getConfiguration();

}

  • autoCommit 是否自动提交事务,
  • level 事务隔离级别(共5个级别), 可查看相关源码
  • connection 连接
  • execType 执行器的类型:SIMPLE(不做特殊处理), REUSE(复用预处理语句), BATCH(会批量执行)

因为上面DefaultSqlSessionFactory实现了SqlSessionFactory接口,所以进入到DefaultSqlSessionFactory查看openSession()

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

openSession()方法最终实现代码如下:

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private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
Transaction tx = null;
try {
// 获取configuration中的加载环境
final Environment environment = configuration.getEnvironment();
// 获取事务工厂
final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
// 创建一个事务
tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
// 生成一个处理器,事务保存在处理器 BaseExecutor 中
final Executor executor = configuration.newExecutor(tx, execType);
// 实例化一个 DefaultSqlSession,DefaultSqlSession实现了SqlSession接口
return new DefaultSqlSession(configuration, executor, autoCommit);
} catch (Exception e) {
// 异常情况下关闭事务
closeTransaction(tx); // may have fetched a connection so lets call close()
throw ExceptionFactory.wrapException("Error opening session. Cause: " + e, e);
} finally {
// 充值错误实例上下文
ErrorContext.instance().reset();
}
}

生成处理器Configuration.newExecutor(Transaction transaction, ExecutorType executorType)

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public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
// 默认为 ExecutorType.SIMPLE
executorType = executorType == null ? defaultExecutorType : executorType;
executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
Executor executor;
if (ExecutorType.BATCH == executorType) {
executor = new BatchExecutor(this, transaction);
} else if (ExecutorType.REUSE == executorType) {
executor = new ReuseExecutor(this, transaction);
} else {
executor = new SimpleExecutor(this, transaction);
}
if (cacheEnabled) {
executor = new CachingExecutor(executor);
}
executor = (Executor) interceptorChain.pluginAll(executor);
return executor;
}

ExecutorType.SIMPLE为例, BatchExecutor, ReuseExecutor同理:
SimpleExecutor父类图
至此,mybatis的启动流程大致简单的介绍到这里,对mybatis的启动初始化有个大致了解。接下将会针对单独模块进行详细分析。

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