mybatis笔记（1）---加载过程

先写个小demo,如下。相关配置文件就不贴出来了。官网上直接可以复制

public static void main(String[] args) throws IOException {
        Reader reader = Resources.getResourceAsReader("com/forcht/testMybatis/mybatis-config.xml");
        SqlSessionFactory sessionFactory=new SqlSessionFactoryBuilder().build(reader);
        SqlSession session = sessionFactory.openSession();
        PaperMapper mapper = session.getMapper(PaperMapper.class);
        System.out.println(mapper.selectPaper(1));
    }

1、如何加载配置文件
使用Resources类将配置文件加载为Reader。

第一步
//调用getResourceAsReader。
Reader reader = Resources.getResourceAsReader("com/forcht/testMybatis/mybatis-config.xml");
第二步
//进入getResourceAsReader方法。使用InputStreamReader将InputStream包装成Reader
public static Reader getResourceAsReader(String resource) throws IOException {
    Reader reader;
    if (charset == null) {
      reader = new InputStreamReader(getResourceAsStream(resource));
    } else {
      reader = new InputStreamReader(getResourceAsStream(resource), charset);
    }
    return reader;
  }
  第三步
  //进入getResourceAsStream加载InputStream 
   public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
   //classLoaderWrapper包装了几种类加载器。
   //每次回按顺序选择第一个可用的加载器
    InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
    if (in == null) {
      throw new IOException("Could not find resource " + resource);
    }
    return in;
  }
  第四步
  //选择一个类加载器来加载文件，包装成InputStream 
  //选择第一个可以用的加载器，这里不明白为什么要这么做
  //（直接都指定为其中一个不就好了？有大佬看到可以解答下）
   InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
    for (ClassLoader cl : classLoader) {
      if (null != cl) {

        // try to find the resource as passed
        InputStream returnValue = cl.getResourceAsStream(resource);

        // now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource
        if (null == returnValue) {
          returnValue = cl.getResourceAsStream("/" + resource);
        }

        if (null != returnValue) {
          return returnValue;
        }
      }
    }
    return null;
  }

2、创建SqlSessionFactory

SqlSessionFactory sessionFactory=new SqlSessionFactoryBuilder().build(reader);
第一步
//创建一个XMLConfigBuilder，再调用XMLConfigBuilder的parse()方法获取一个Configuration对象
//并使用该对象去build一个SqlSessionFactory
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.
      }
    }
  }
  第二步
  //创建XMLConfigBuilder时，会先创建一个XPathParser和XMLMapperEntityResolver
  public XMLConfigBuilder(Reader reader, String environment, Properties props) {
    this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
  }
  第三步
  //进入XMLConfigBuilder的parse()方法
   public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    //获取<configuration>根节点,进行解析
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }
 第四步
 //这里就是解析所有的子节点的过程
 private void parseConfiguration(XNode root) {
    try {
      //issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      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);
    }
  }