• 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

        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:

        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

    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:

        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:

        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:

        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:

        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。
        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();
        }
    
    1. 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
        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

        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。

        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

        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;
        }
    

    创建 SqlSession

    通过调用SqlSessionFactory.openSession()创建SqlSession

        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()

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

    openSession()方法最终实现代码如下:

        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);
              // 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)

        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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  • 原文地址:https://www.cnblogs.com/ytao-blog/p/11756456.html
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