一.
SqlSessionFactoryBuilder源码片段：
SqlSessionFactoryBuilder.build()主要参数类型包括(Reader reader,String environment,Properties properties)/(InputStream inputStream,String environment, Properties properties)
public class SqlSessionFactoryBuilder {
//Reader读取mybatis配置文件，传入构造方法
//除了Reader外，其实还有对应的inputStream作为参数的构造方法，
//这也体现了mybatis配置的灵活性
public SqlSessionFactory build(Reader reader) {
return build(reader, null, null);
}
public SqlSessionFactory build(Reader reader, String environment) {
return build(reader, environment, null);
}
//mybatis配置文件 + properties, 此时mybatis配置文件中可以不配置properties，也能使用${}形式
public SqlSessionFactory build(Reader reader, Properties properties) {
return build(reader, null, properties);
}
//通过XMLConfigBuilder解析mybatis配置，然后创建SqlSessionFactory对象
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
//下面看看这个方法的源码
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.
}
}
}
//InputStream 读入
public SqlSessionFactory build(InputStream inputStream) {
return build(inputStream, null, null);
}
public SqlSessionFactory build(InputStream inputStream, String environment) {
return build(inputStream, environment, null);
}
public SqlSessionFactory build(InputStream inputStream, Properties properties) {
return build(inputStream, null, properties);
}
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.
}
}
}
public SqlSessionFactory build(Configuration config) {
return new DefaultSqlSessionFactory(config);
}
}
解析Mybatis主要用XMLConfigBuilder。
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
parser.parse();
XMLConfigBuilder的源码：
public class XMLConfigBuilder extends BaseBuilder {
//Reader读取mybatis配置文件
public XMLConfigBuilder(Reader reader, String environment, Properties props) {
this(new XPathParser(reader, true, props, new XMLMapperEntityResolver()), environment, props);
}
//InputStream读取mybatis配置文件
public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
}
//构造函数
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;
}
// //外部调用此方法对mybatis配置文件进行解析
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
////从根节点configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
//此方法就是依次解析configuration节点下的子节点
//由此也可看出，我们在configuration下面能配置的节点为以下10个节点
private void parseConfiguration(XNode root) {
try {
//issue #117 read properties first
propertiesElement(root.evalNode("properties"));//read properties first
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectionFactoryElement(root.evalNode("reflectionFactory"));
settingsElement(root.evalNode("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);
}
}
}
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
通过以上源码，我们就能看出，在mybatis的配置文件中：
1. configuration节点为根节点。
2. 在configuration节点之下，我们可以配置10个子节点， 分别为：properties、typeAliases、plugins、objectFactory、objectWrapperFactory、settings、environments、databaseIdProvider、typeHandlers、mappers。
二.properties节点和environments节点
那么，外部指定properties配置文件,直接配置为xmlp这两种方法都同时用了，那么哪种方法优先？当以上两种方法都用xml配置优先， 外部指定properties配置其次。
environments元素节点可以配置多个environment子节点， 怎么理解呢？
假如我们系统的开发环境和正式环境所用的数据库不一样（这是肯定的）， 那么可以设置两个environment, 两个id分别对应开发环境（dev）和正式环境（final），那么通过配置environments的default属性就能选择对应的environment了，
将environments的deault属性的值配置为dev, 那么就会选择dev的environment。
XMLConfigBuilder对于properties和environments的解析：
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 {
//优先解析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:
environment = context.getStringAttribute("default");
}
//递归解析environments子节点
for (XNode child : context.getChildren()) {
//
//environments 节点下可以拥有多个 environment子节点
//类似于这样：
//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.");
}
}
在配置dataSource的时候使用了 ${driver} 这种表达式， 这种形式是怎么解析的？其实，是通过PropertyParser这个类解析：
/**
* 这个类解析${}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 + "}";
}
}
}