（一）MyBatis源码解析之配置文件
使用mybatis进行数据库查询的代码如下：
public class MessageDaoDemo {
@Test
public void queryMessageList() throws IOException{
SqlSession sqlSession=getSessionFactory().openSession();
MessageMapperDao messageMapper = sqlSession.getMapper(MessageMapperDao.class);
Message msg = new Message();
msg.setCommand("555");
msg.setDescription("2");
List message = messageMapper.queryMessageList(msg.getCommand(),msg.getDescription());
for(Message m :message){
System.out.println(m.toString());
}
}
private SqlSessionFactory getSessionFactory() {
SqlSessionFactory sessionFactory = null;
String resource = "mybatis-config.xml";
try {
sessionFactory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsReader(resource));
// String resource1 = "src/main/resources/mybatis-config.xml";
// File file = new File(resource1);
// sessionFactory = new SqlSessionFactoryBuilder().build(new FileInputStream(file));
} catch (IOException e) {
e.printStackTrace();
}
return sessionFactory;
}
} 图1
Mybatis使用dom来解析xml配置文件。在Mybatis中由XmlConfigBuilder负责总的配置文件的解析工作，XmlConfigBuilder的继承结构图如图1所示。mybatis总配置文件共有11个标签（全部在代码中），这11个标签的顺序是固定的，标签的顺序在mybatis-3-config.dtd文件中定义，在mybatis配置文件的头部引用了mybatis-3-config.dtd，其中最常用的标签有4个，分别是properties，typeAliases，environments，mapper标签，
public Configuration parse() {
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
private void parseConfiguration(XNode root) {
try {
Properties settings = settingsAsPropertiess(root.evalNode("settings"));
//issue #117 read properties first
propertiesElement(root.evalNode("properties"));
loadCustomVfs(settings);
typeAliasesElement(root.evalNode("typeAliases"));
pluginElement(root.evalNode("plugins"));
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectionFactoryElement(root.evalNode("reflectionFactory"));
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);
}
}
下面来分别看看这些标签的作用然后通过源码来看看mybatis是如何使用这些标签的
properties标签：
properties配置如下
dbConfig.properties配置如下
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://127.0.0.1:3306/mybatis?characterEncoding=utf8
username=root
password=root解析properties标签的代码在XMLConfigBuilder类的propertiesElement(...)方法中（参数省略），代码如下，在代码1 处将properties中配置的属性以键值对的形式存储到defaults中，这个时候value中的值还是以${...}的形式存储，接下来因为resource属性的值不为空，所以会用外部的properties文件中的内容直接覆盖defaults中对应的记录，所以如果我们将数据库连接的信息是从外部引入的，property标签中的value配置成什么样都是无所谓的，但是为了规范最好还是按照大家都熟悉的方式来写，用外部文件进行替换之后，mybatis会将这些信息存储到configuration的variables属性中，configuration是一个十分重要的对象，mybatis将配置文件中的所有信息都存放到该对象中
private void propertiesElement(XNode context) throws Exception {
if (context != null) {
//将properties中配置的属性以键值对的形式存储到Properties中
Properties defaults = context.getChildrenAsProperties();// 1
String resource = context.getStringAttribute("resource");
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.");
}
if (resource != null) {
//用外部的peoperties文件覆盖property属性的值
defaults.putAll(Resources.getResourceAsProperties(resource)); // 2
} else if (url != null) {
defaults.putAll(Resources.getUrlAsProperties(url));
}
Properties vars = configuration.getVariables(); // 3
if (vars != null) {
defaults.putAll(vars);
}
parser.setVariables(defaults);
//将property属性中配置的键值对全部存储到defaults中
configuration.setVariables(defaults); //4
}
}
typeAliases标签：
typeAliases配置如下
private void typeAliasesElement(XNode parent) {
if (parent != null) {
for (XNode child : parent.getChildren()) {
//为某个包下的所有类生成别名,如果包名为net.snail,该包下有个Message类,则该类生成的别名为message
if ("package".equals(child.getName())) {
String typeAliasPackage = child.getStringAttribute("name");
configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
} else {
String alias = child.getStringAttribute("alias");
String type = child.getStringAttribute("type");
try {
Class clazz = Resources.classForName(type);
if (alias == null) {
typeAliasRegistry.registerAlias(clazz);
} else {
//将类的全路径名和别名的对应信息存到typeAliasRegistry对象中
typeAliasRegistry.registerAlias(alias, clazz);
}
} catch (ClassNotFoundException e) {
throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
}
}
}
}
}在创建TypeAliasRegistry对象时
public TypeAliasRegistry() {
registerAlias("string", String.class);
registerAlias("byte", Byte.class);
registerAlias("long", Long.class);
registerAlias("short", Short.class);
registerAlias("int", Integer.class);
registerAlias("integer", Integer.class);
registerAlias("double", Double.class);
registerAlias("float", Float.class);
registerAlias("boolean", Boolean.class);
registerAlias("byte[]", Byte[].class);
registerAlias("long[]", Long[].class);
registerAlias("short[]", Short[].class);
registerAlias("int[]", Integer[].class);
registerAlias("integer[]", Integer[].class);
registerAlias("double[]", Double[].class);
registerAlias("float[]", Float[].class);
registerAlias("boolean[]", Boolean[].class);
registerAlias("_byte", byte.class);
registerAlias("_long", long.class);
registerAlias("_short", short.class);
registerAlias("_int", int.class);
registerAlias("_integer", int.class);
registerAlias("_double", double.class);
registerAlias("_float", float.class);
registerAlias("_boolean", boolean.class);
registerAlias("_byte[]", byte[].class);
registerAlias("_long[]", long[].class);
registerAlias("_short[]", short[].class);
registerAlias("_int[]", int[].class);
registerAlias("_integer[]", int[].class);
registerAlias("_double[]", double[].class);
registerAlias("_float[]", float[].class);
registerAlias("_boolean[]", boolean[].class);
registerAlias("date", Date.class);
registerAlias("decimal", BigDecimal.class);
registerAlias("bigdecimal", BigDecimal.class);
registerAlias("biginteger", BigInteger.class);
registerAlias("object", Object.class);
registerAlias("date[]", Date[].class);
registerAlias("decimal[]", BigDecimal[].class);
registerAlias("bigdecimal[]", BigDecimal[].class);
registerAlias("biginteger[]", BigInteger[].class);
registerAlias("object[]", Object[].class);
registerAlias("map", Map.class);
registerAlias("hashmap", HashMap.class);
registerAlias("list", List.class);
registerAlias("arraylist", ArrayList.class);
registerAlias("collection", Collection.class);
registerAlias("iterator", Iterator.class);
registerAlias("ResultSet", ResultSet.class);
} public Configuration() {
typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);
typeAliasRegistry.registerAlias("MANAGED", ManagedTransactionFactory.class);
typeAliasRegistry.registerAlias("JNDI", JndiDataSourceFactory.class);
typeAliasRegistry.registerAlias("POOLED", PooledDataSourceFactory.class);
typeAliasRegistry.registerAlias("UNPOOLED", UnpooledDataSourceFactory.class);
typeAliasRegistry.registerAlias("PERPETUAL", PerpetualCache.class);
typeAliasRegistry.registerAlias("FIFO", FifoCache.class);
typeAliasRegistry.registerAlias("LRU", LruCache.class);
typeAliasRegistry.registerAlias("SOFT", SoftCache.class);
typeAliasRegistry.registerAlias("WEAK", WeakCache.class);
typeAliasRegistry.registerAlias("DB_VENDOR", VendorDatabaseIdProvider.class);
typeAliasRegistry.registerAlias("XML", XMLLanguageDriver.class);
typeAliasRegistry.registerAlias("RAW", RawLanguageDriver.class);
typeAliasRegistry.registerAlias("SLF4J", Slf4jImpl.class);
typeAliasRegistry.registerAlias("COMMONS_LOGGING", JakartaCommonsLoggingImpl.class);
typeAliasRegistry.registerAlias("LOG4J", Log4jImpl.class);
typeAliasRegistry.registerAlias("LOG4J2", Log4j2Impl.class);
typeAliasRegistry.registerAlias("JDK_LOGGING", Jdk14LoggingImpl.class);
typeAliasRegistry.registerAlias("STDOUT_LOGGING", StdOutImpl.class);
typeAliasRegistry.registerAlias("NO_LOGGING", NoLoggingImpl.class);
typeAliasRegistry.registerAlias("CGLIB", CglibProxyFactory.class);
typeAliasRegistry.registerAlias("JAVASSIST", JavassistProxyFactory.class);
languageRegistry.setDefaultDriverClass(XMLLanguageDriver.class);
languageRegistry.register(RawLanguageDriver.class);
}environments标签：
environmets标签配置如下：
environments标签可以有多个environment子标签，在environments标签中我们通过default属性来让那个标签生效，这为我们在多套不同的环境中切换提供了方便。我们还可以通过在调用SqlSessionFactoryBuilder的build(...)方法时指定让那个environment生效
public SqlSessionFactory build(Reader reader, String environment) {
return build(reader, environment, null);
}来看看源码在解析environments标签时为我们做了哪些工作，解析environments源码如下
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");
if (isSpecifiedEnvironment(id)) {
TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));// 1
DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource")); // 2
DataSource dataSource = dsFactory.getDataSource();
Environment.Builder environmentBuilder = new Environment.Builder(id)
.transactionFactory(txFactory)
.dataSource(dataSource);
configuration.setEnvironment(environmentBuilder.build());
}
}
}
} private TransactionFactory transactionManagerElement(XNode context) throws Exception {
if (context != null) {
String type = context.getStringAttribute("type");
Properties props = context.getChildrenAsProperties();
TransactionFactory factory = (TransactionFactory) resolveClass(type).newInstance();
factory.setProperties(props);
return factory;
}
throw new BuilderException("Environment declaration requires a TransactionFactory.");
}防止篇幅过长，本篇文章我们就讲述这三个标签，其余标签会在接下来的文章中继续分析！！！