深入浅出Mybatis系列（四）---配置详解之properties与environments

public class XMLConfigBuilder extends BaseBuilder {

    private boolean parsed;
    //xml解析器
    private XPathParser parser;
    private String environment;
  
    //上次说到这个方法是在解析mybatis配置文件中能配置的元素节点
    //今天首先要看的就是properties节点和environments节点
    private void parseConfiguration(XNode root) {
        try {
          //解析properties元素
          propertiesElement(root.evalNode("properties")); //issue #117 read properties first
          typeAliasesElement(root.evalNode("typeAliases"));
          pluginElement(root.evalNode("plugins"));
          objectFactoryElement(root.evalNode("objectFactory"));
          objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
          settingsElement(root.evalNode("settings"));
          //解析environments元素
          environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631
          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);
        }
    }
  
    
    //下面就看看解析properties的具体方法
    private void propertiesElement(XNode context) throws Exception {
        if (context != null) {
          //将子节点的 name 以及value属性set进properties对象
          //这儿可以注意一下顺序，xml配置优先， 外部指定properties配置其次
          Properties defaults = context.getChildrenAsProperties();
          //获取properties节点上 resource属性的值
          String resource = context.getStringAttribute("resource");
          //获取properties节点上 url属性的值, resource和url不能同时配置
          String url = context.getStringAttribute("url");
          if (resource != null && url != null) {
            throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other.");
          }
          //把解析出的properties文件set进Properties对象
          if (resource != null) {
            defaults.putAll(Resources.getResourceAsProperties(resource));
          } else if (url != null) {
            defaults.putAll(Resources.getUrlAsProperties(url));
          }
          //将configuration对象中已配置的Properties属性与刚刚解析的融合
          //configuration这个对象会装载所解析mybatis配置文件的所有节点元素，以后也会频频提到这个对象
          //既然configuration对象用有一系列的get/set方法， 那是否就标志着我们可以使用java代码直接配置？ 
          //答案是肯定的， 不过使用配置文件进行配置，优势不言而喻
          Properties vars = configuration.getVariables();
          if (vars != null) {
            defaults.putAll(vars);
          }
          //把装有解析配置propertis对象set进解析器， 因为后面可能会用到
          parser.setVariables(defaults);
          //set进configuration对象
          configuration.setVariables(defaults);
        }
    }
    
    //下面再看看解析enviroments元素节点的方法
    private void environmentsElement(XNode context) throws Exception {
        if (context != null) {
            if (environment == null) {
                //解析environments节点的default属性的值
                //例如: <environments default="development">
                environment = context.getStringAttribute("default");
            }
            //递归解析environments子节点
            for (XNode child : context.getChildren()) {
                //<environment id="development">, 只有enviroment节点有id属性，那么这个属性有何作用？
                //environments 节点下可以拥有多个 environment子节点
                //类似于这样： <environments default="development"><environment id="development">...</environment><environment id="test">...</environments>
                //意思就是我们可以对应多个环境，比如开发环境，测试环境等， 由environments的default属性去选择对应的enviroment
                String id = child.getStringAttribute("id");
                //isSpecial就是根据由environments的default属性去选择对应的enviroment
                if (isSpecifiedEnvironment(id)) {
                    //事务， mybatis有两种：JDBC 和 MANAGED, 配置为JDBC则直接使用JDBC的事务，配置为MANAGED则是将事务托管给容器， 
                    TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
                    //enviroment节点下面就是dataSource节点了，解析dataSource节点（下面会贴出解析dataSource的具体方法）
                    DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
                    DataSource dataSource = dsFactory.getDataSource();
                    Environment.Builder environmentBuilder = new Environment.Builder(id)
                          .transactionFactory(txFactory)
                          .dataSource(dataSource);
                    //老规矩，会将dataSource设置进configuration对象
                    configuration.setEnvironment(environmentBuilder.build());
                }
            }
        }
    }
    
    //下面看看dataSource的解析方法
    private DataSourceFactory dataSourceElement(XNode context) throws Exception {
        if (context != null) {
            //dataSource的连接池
            String type = context.getStringAttribute("type");
            //子节点 name, value属性set进一个properties对象
            Properties props = context.getChildrenAsProperties();
            //创建dataSourceFactory
            DataSourceFactory factory = (DataSourceFactory) resolveClass(type).newInstance();
            factory.setProperties(props);
            return factory;
        }
        throw new BuilderException("Environment declaration requires a DataSourceFactory.");
    } 
}