An in-depth explanation of the Mybatis series (3) - Detailed configuration of properties and environments (mybatis source code)

In the previous article "In-depth introduction to the Mybatis series (2)---Configuration introduction (mybatis source code)", through a simple analysis of the mybatis source code, we can see that In the mybatis configuration file, in the configuration root node Below, you can configure the properties, typeAliases, plugins, objectFactory, objectWrapperFactory, settings, environments, databaseIdProvider, typeHandlers, and mappers nodes. So this time, we will first introduce the properties node and environments node.

In order to allow everyone to better read the mybatis source code, I will give you a simple example of how to use properties.

1 <configuration> 
2 <!-- 方法一： 从外部指定properties配置文件, 除了使用resource属性指定外，还可通过url属性指定url  
 3   <properties resource="dbConfig.properties"></properties> 
 4   --> 
 5   <!-- 方法二： 直接配置为xml --> 
 6   <properties> 
 7       <property name="driver" value="com.mysql.jdbc.Driver"/> 
 8       <property name="url" value="jdbc:mysql://localhost:3306/test1"/> 
 9       <property name="username" value="root"/>
 10       <property name="password" value="root"/>
 11   </properties>

So, if I use both methods at the same time, which method takes precedence?

When the above two methods are used, xml configuration takes priority, and externally specified properties configuration takes second place. As for why, the following source code analysis will mention it, please pay attention to it.

Let’s take a look at how to use the environments element node:

<environments default="development">
    <environment id="development">
      <transactionManager type="JDBC"/>
      <dataSource type="POOLED">
          <!--
          如果上面没有指定数据库配置的properties文件，那么此处可以这样直接配置 
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <property name="url" value="jdbc:mysql://localhost:3306/test1"/>
        <property name="username" value="root"/>
        <property name="password" value="root"/>         -->
         
         <!-- 上面指定了数据库配置文件， 配置文件里面也是对应的这四个属性 -->
         <property name="driver" value="${driver}"/>
         <property name="url" value="${url}"/>
         <property name="username" value="${username}"/>
         <property name="password" value="${password}"/>
         
      </dataSource>
    </environment>
    
    <!-- 我再指定一个environment -->
    <environment id="test">
      <transactionManager type="JDBC"/>
      <dataSource type="POOLED">
        <property name="driver" value="com.mysql.jdbc.Driver"/>
        <!-- 与上面的url不一样 -->
        <property name="url" value="jdbc:mysql://localhost:3306/demo"/>
        <property name="username" value="root"/>
        <property name="password" value="root"/>
      </dataSource>
    </environment>
    
  </environments>

The environments element node can be configured with multiple environment sub-nodes. How to understand it? ?

If the database used in the development environment of our system is different from the database used in the formal environment (this is for sure), then you can set up two environments, and the two IDs correspond to the development environment (dev) respectively. and formal environment (final), then you can select the corresponding environment by configuring the default attribute of environments. For example, if I configure the value of the default attribute of environments to dev, then the environment of dev will be selected. As for how this is implemented, the source code will be discussed below.

Okay, I briefly introduced the configuration of properties and environments above, and then we officially started to look at the source code:

Last time we said that mybatis is through XMLConfigBuilder. The class is parsing the mybatis configuration file, so this time we will take a look at XMLConfigBuilder’s parsing of properties and environments:

XMLConfigBuilder:

  1 public class XMLConfigBuilder extends BaseBuilder {  
  2   
  3     private boolean parsed;  
  4     //xml解析器  
  5     private XPathParser parser;  
  6     private String environment;  
  7     
  8     //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  
  9     //今天首先要看的就是properties节点和environments节点 
  10     private void parseConfiguration(XNode root) { 
  11         try { 
  12           //解析properties元素 
  13           propertiesElement(root.evalNode("properties")); //issue #117 read properties first 
  14           typeAliasesElement(root.evalNode("typeAliases")); 
  15           pluginElement(root.evalNode("plugins")); 
  16           objectFactoryElement(root.evalNode("objectFactory")); 
  17           objectWrapperFactoryElement(root.evalNode("objectWrapperFactory")); 
  18           settingsElement(root.evalNode("settings")); 
  19           //解析environments元素 
  20           environmentsElement(root.evalNode("environments")); // read it after objectFactory and objectWrapperFactory issue #631 
  21           databaseIdProviderElement(root.evalNode("databaseIdProvider")); 
  22           typeHandlerElement(root.evalNode("typeHandlers")); 
  23           mapperElement(root.evalNode("mappers")); 
  24         } catch (Exception e) { 
  25           throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e); 
  26         } 
  27     } 
  28    
  29      
  30     //下面就看看解析properties的具体方法 
  31     private void propertiesElement(XNode context) throws Exception { 
  32         if (context != null) { 
  33           //将子节点的 name 以及value属性set进properties对象 
  34           //这儿可以注意一下顺序，xml配置优先， 外部指定properties配置其次 
  35           Properties defaults = context.getChildrenAsProperties(); 
  36           //获取properties节点上 resource属性的值 
  37           String resource = context.getStringAttribute("resource"); 
  38           //获取properties节点上 url属性的值, resource和url不能同时配置 
  39           String url = context.getStringAttribute("url"); 
  40           if (resource != null && url != null) { 
  41             throw new BuilderException("The properties element cannot specify both a URL and a resource based property file reference.  Please specify one or the other."); 
  42           } 
  43           //把解析出的properties文件set进Properties对象 
  44           if (resource != null) { 
  45             defaults.putAll(Resources.getResourceAsProperties(resource)); 
  46           } else if (url != null) { 
  47             defaults.putAll(Resources.getUrlAsProperties(url)); 
  48           } 
  49           //将configuration对象中已配置的Properties属性与刚刚解析的融合 
  50           //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 
  51           //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？ 
 52           //答案是肯定的， 不过使用配置文件进行配置，优势不言而喻 
 53           Properties vars = configuration.getVariables(); 
 54           if (vars != null) { 
 55             defaults.putAll(vars); 
 56           } 
 57           //把装有解析配置propertis对象set进解析器， 因为后面可能会用到 
 58           parser.setVariables(defaults); 
 59           //set进configuration对象 
 60           configuration.setVariables(defaults); 
 61         } 
 62     } 
 63      
 64     //下面再看看解析enviroments元素节点的方法 
 65     private void environmentsElement(XNode context) throws Exception { 
 66         if (context != null) { 
 67             if (environment == null) { 
 68                 //解析environments节点的default属性的值 
 69                 //例如: <environments default="development"> 
 70                 environment = context.getStringAttribute("default"); 
 71             } 
 72             //递归解析environments子节点 
 73             for (XNode child : context.getChildren()) { 
 74                 //<environment id="development">, 只有enviroment节点有id属性，那么这个属性有何作用？ 
 75                 //environments 节点下可以拥有多个 environment子节点 
 76                 //类似于这样： <environments default="development"><environment id="development">...</environment><environment id="test">...</environments> 
 77                 //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 
 78                 String id = child.getStringAttribute("id"); 
 79                 //isSpecial就是根据由environments的default属性去选择对应的enviroment 
 80                 if (isSpecifiedEnvironment(id)) { 
 81                     //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，  
 82                     TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager")); 
 83                     //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） 
 84                     DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource")); 
 85                     DataSource dataSource = dsFactory.getDataSource(); 
 86                     Environment.Builder environmentBuilder = new Environment.Builder(id) 
 87                           .transactionFactory(txFactory) 
 88                           .dataSource(dataSource); 
 89                     //老规矩，会将dataSource设置进configuration对象 
 90                     configuration.setEnvironment(environmentBuilder.build()); 
 91                 } 
 92             } 
 93         } 
 94     } 
 95      
 96     //下面看看dataSource的解析方法 
 97     private DataSourceFactory dataSourceElement(XNode context) throws Exception { 
 98         if (context != null) { 
 99             //dataSource的连接池
 100             String type = context.getStringAttribute("type");
 101             //子节点 name, value属性set进一个properties对象
 102             Properties props = context.getChildrenAsProperties();
 103             //创建dataSourceFactory
 104             DataSourceFactory factory = (DataSourceFactory) resolveClass(type).newInstance();
 105             factory.setProperties(props);
 106             return factory;
 107         }
 108         throw new BuilderException("Environment declaration requires a DataSourceFactory.");
 109     } 
110 }

Through the above Interpretation of mybatis source code, I believe everyone has an in-depth understanding of the configuration of mybatis.

There is another question. We saw above that the expression ${driver} was used when configuring dataSource. How is this form parsed? In fact, it is parsed through the PropertyParser class:

PropertyParser:

/**
 * 这个类解析${}这种形式的表达式 */public class PropertyParser {  public static String parse(String string, Properties variables) {
    VariableTokenHandler handler = new VariableTokenHandler(variables);
    GenericTokenParser parser = new GenericTokenParser("${", "}", handler);    return parser.parse(string);
  }  private static class VariableTokenHandler implements TokenHandler {    private Properties variables;    public VariableTokenHandler(Properties variables) {      this.variables = variables;
    }    public String handleToken(String content) {      if (variables != null && variables.containsKey(content)) {        return variables.getProperty(content);
      }      return "${" + content + "}";
    }
  }
}

Okay, the above is the analysis of the properties and environments element nodes, which is more important. They are all marked in the comments on the source code. This article ends here, and the following articles will continue to analyze the configuration of other nodes.

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