MyBatis之启动分析（一）

时间 2020-01-27

原文原文链接

前言

MyBatis 做为目前最经常使用的持久层框架之一，分析其源码，对咱们的使用过程当中可更好的运用它。本系列基于mybatis-3.4.6进行分析。 MyBatis 的初始化工做就是解析主配置文件，映射配置文件以及注解信息。而后保存在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。html

启动

public static void main(String[] args) throws IOException {
        // 获取配置文件
        Reader reader = Resources.getResourceAsReader("mybatis-config.xml");
        // 经过 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。经过查看该类，使用方法重载，有如下9个方法：java

方法重载最终实现处理的方法源码以下：sql

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。

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

经过 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：apache

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

parsed 是否解析，一次解析便可。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 用于建立和缓存Reflector对象，一个类对应一个Reflector。由于参数处理、结果映射等操做时，会涉及大量的反射操做。DefaultReflectorFactory实现类比较简单，这里再也不进行讲解。

XMLConfigBuilder构建函数实现：api

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:缓存

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<properties>的值
xpath Xpath对象，用于对XML文件节点的操做

XPathParser 对象构造函数有：session

函数里面都处理了两件事：mybatis

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

初始化赋值，和建立XPath对象，用于对XML文件节点的操做。

private void commonConstructor(boolean validation, Properties variables, EntityResolver entityResolver) {
        this.validation = validation;
        this.entityResolver = entityResolver;
        this.variables = variables;
        // 建立Xpath对象，用于对XML文件节点的操做
        XPathFactory factory = XPathFactory.newInstance();
        this.xpath = factory.newXPath();
    }

建立Document对象并赋值到document变量, 这里属于Document建立的操做,再也不详细讲述，不懂能够点击这里查看API

private Document createDocument(InputSource inputSource) {
        // important: this must only be called AFTER common constructor
        try {
          // 实例化 DocumentBuilderFactory 对象，用于建立 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);
          // 建立 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

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;
    }

初始化父类BaseBuilder的值。
将外部值赋值给对象。
将实例化的XPathParser赋值给parser。

最后返回XMLConfigBuilder对象。oracle

解析 XMLConfigBuilder 对象

经过 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。app

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;
    }

建立 SqlSessionFactory

DefaultSqlSessionFactory实现了SqlSessionFactory接口。经过上面解析获得的Configuration，调用SqlSessionFactoryBuilder.build(Configuration config)建立一个 DefaultSqlSessionFactory。

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。

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

建立 SqlSession

经过调用SqlSessionFactory.openSession()建立SqlSession。

public interface SqlSessionFactory {
      // 默认建立
      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 链接
execType 执行器的类型：SIMPLE(不作特殊处理), REUSE（复用预处理语句）, BATCH（会批量执行）

由于上面DefaultSqlSessionFactory实现了SqlSessionFactory接口，因此进入到DefaultSqlSessionFactory查看openSession()。

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

openSession()方法最终实现代码以下：

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);
          // 建立一个事务
          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) {
          // 异常状况下关闭事务
          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)：

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;
    }

以ExecutorType.SIMPLE为例, BatchExecutor, ReuseExecutor同理：

至此，mybatis的启动流程大体简单的介绍到这里，对mybatis的启动初始化有个大体了解。接下将会针对单独模块进行详细分析。

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