引言
Mybtais是一种ORM框架，用于实现面向对象编程语言里不同类型系统的数据之间的转换。
本文我将从三个步骤进行Mybatis解读。
Mybatis 如何获取源
搭建Mybatis首先肯定要有配置文件mybatis-config.xml啦，但以下的配置文件只是独立使用Mybatis，并没有在Spring框架中集成MyBatis。
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<properties resource="jdbc.properties"/>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="org/mybatis/example/BlogMapper.xml"/>
</mappers>
</configuration>
测试代码
String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = SqlSessionFactory.openSession();
Blog blog = session.selectOne("org.mybatis.example.BlogMapper.selectBlog");
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
return build(parser.parse());
} catch (Exception e) {
......
}
}
在这时XMLConfigBuilder会用来解析xml并构建
进入parse();
public Configuration parse() {
//如果已经解析过了，报错
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//根节点是configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
如果此配置文件已经解析过了，就报错。
此时parser.evalNode("/configuration")是用来获取配置文件的根节点
<configuration>
<properties resource="jdbc.properties"/>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"/>
<dataSource type="POOLED">
<property name="driver" value="${driver}"/>
<property name="url" value="${url}"/>
<property name="username" value="${username}"/>
<property name="password" value="${password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="org/mybatis/example/BlogMapper.xml"/>
</mappers>
</configuration>
//解析配置
private void parseConfiguration(XNode root) {
try {
//分步骤解析
//issue #117 read properties first
//1.properties
propertiesElement(root.evalNode("properties"));
//2.类型别名
typeAliasesElement(root.evalNode("typeAliases"));
//3.插件
pluginElement(root.evalNode("plugins"));
//4.对象工厂
objectFactoryElement(root.evalNode("objectFactory"));
//5.对象包装工厂
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
//6.设置
settingsElement(root.evalNode("settings"));
// read it after objectFactory and objectWrapperFactory issue #631
//7.环境
environmentsElement(root.evalNode("environments"));
//8.databaseIdProvider
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
//9.类型处理器
typeHandlerElement(root.evalNode("typeHandlers"));
//10.映射器
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
通过evalNode()来解析标签
接下来进入environmentsElement()，可以查看context。
private void environmentsElement(XNode context) throws Exception {
if (context != null) {
if (environment == null) {
environment = context.getStringAttribute("default");
}
for (XNode child : context.getChildren()) {
String id = child.getStringAttribute("id");
//循环比较id是否就是指定的environment
if (isSpecifiedEnvironment(id)) {
//7.1事务管理器
TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
//7.2数据源
DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
DataSource dataSource = dsFactory.getDataSource();
Environment.Builder environmentBuilder = new Environment.Builder(id)
.transactionFactory(txFactory)
.dataSource(dataSource);
configuration.setEnvironment(environmentBuilder.build());
}
}
}
}
这时候会发现context已经讲数据源的连接取出来了。
<environments default="development">
<environment id="development">
<transactionManager type="JDBC">
<property name="..." value="..."/>
</transactionManager>
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Dirver"/>
<property name="url" value="jdbc:mysql://localhost:3306/mybatis"/>
<property name="username" value="root"/>
<property name="password" value="123456"/>
</dataSource>
</environment>
</environments>
但这时有一个问题，driver、url、username、password的值什么时候变化了呢？
<properties resource="jdbc.properties"/>
答案就在这，里面记录了driver、url、username、password之类的值，那这时候又有人要问了，能放在 下面吗，其实这时候你可以点击下面这个链接。
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
里面详细介绍了每个标签各自的属性、在它下面包括哪些标签，贴图为证。
<!ELEMENT configuration (properties?, settings?, typeAliases?, typeHandlers?, objectFactory?, objectWrapperFactory?, reflectorFactory?, plugins?, environments?, databaseIdProvider?, mappers?)>
<!ELEMENT databaseIdProvider (property*)>
<!ATTLIST databaseIdProvider
type CDATA #REQUIRED
>
<!ELEMENT properties (property*)>
<!ATTLIST properties
resource CDATA #IMPLIED
url CDATA #IMPLIED
>
之后在刚才的代码中有这样一句
DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
DataSource dataSource = dsFactory.getDataSource();
DataSource会取出driver、url、username、password，如下图。
接下来有一句非常关键的代码
configuration.setEnvironment(environmentBuilder.build());
这时候就引出Environment类，大家看代码应该就能明白了。
public final class Environment {
//环境id
private final String id;
//事务工厂
private final TransactionFactory transactionFactory;
//数据源
private final DataSource dataSource;
....
..
.
}
就是这个类将XML中的数据源存储进Mybatisy以及决定加载哪种环境(开发环境/生产环境)。
你可以发现数据源中的标签在Environment都一一对应。！！！ 划重点
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