Mybatis3源码笔记（三）Configuration
1. XMLConfigBuilder
上一篇大致介绍了SqlSession的生成。在DefaultSqlSessionFactory的构造函数中就提到了Configuration这个对象。现在我们来看看Configuration的生成流程。
public SqlSessionFactory build(Reader reader, String environment, Properties properties) {
try {
XMLConfigBuilder parser = new XMLConfigBuilder(reader, environment, properties);
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.
}
}
}
代码比较简单，就是根据XMLConfigBuilder根据配置文件XML来parse生成的。其实不用看代码，我们脑海中应该也有一个大致的Configuration的构成细节，肯定是根据mybatis-config.xml具体生成对应的组成属性。一般的mybatis-config.xml文件如下：
但是我们都知道XML的解析都有DTD文件来约束和验证，那我们常用的mybatis-config.xml是DTD肯定也是有的。在哪呢？秘密就在XMLConfigBuilder构造函数中的XMLMapperEntityResolver。
public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
}
public class XMLMapperEntityResolver implements EntityResolver {
private static final String IBATIS_CONFIG_SYSTEM = "ibatis-3-config.dtd";
private static final String IBATIS_MAPPER_SYSTEM = "ibatis-3-mapper.dtd";
private static final String MYBATIS_CONFIG_SYSTEM = "mybatis-3-config.dtd";
private static final String MYBATIS_MAPPER_SYSTEM = "mybatis-3-mapper.dtd";
private static final String MYBATIS_CONFIG_DTD = "org/apache/ibatis/builder/xml/mybatis-3-config.dtd";
private static final String MYBATIS_MAPPER_DTD = "org/apache/ibatis/builder/xml/mybatis-3-mapper.dtd";
...
上面很明显就用到mybatis-3-config.dtd和mybatis-3-mapper.dtd，同时为了兼容旧版本的ibatis,还用到了ibatis-3-config.dtd和ibatis-3-mapper.dtd。
之前说过mybatis-3-mapper.dtd，那mybatis-3-mapper.dtd看名字，我们也能猜到是专门用来解析mapper的xml文件的，一般的样例如下：
<?xml version="1.0" encoding="UTF-8"?>
具体的dtd文件就不贴出来了，免的有凑字数的嫌疑。继续看代码：
public XPathParser(InputStream inputStream, boolean validation, Properties variables, EntityResolver entityResolver) {
commonConstructor(validation, variables, entityResolver);
//最终把XML文件生成document对象用来后面的解析工作
this.document = createDocument(new InputSource(inputStream));
}
private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
super(new Configuration());
//本地异常日志记录（ThreadLocal）单例模式来记录每一次的执行过程，用来详细定位异常信息。(为了防止内存泄露，在finally里有reset的操作)
ErrorContext.instance().resource("SQL Mapper Configuration");
//记录入参props
this.configuration.setVariables(props);
this.parsed = false;
//记录入参environment
this.environment = environment;
this.parser = parser;
}
public Configuration parse() {
//相同的XMLConfigBuilder对象只允许parse一次
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//开始解析configuration根节点
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
下面就到了重头戏Configuration的详细解析过程。其实前几项的解析相对来说比较简单，就是最后的mapper的解析比较复杂。
private void parseConfiguration(XNode root) {
try {
//解析properties节点（variables）
propertiesElement(root.evalNode("properties"));
//解析settings节点
Properties settings = settingsAsProperties(root.evalNode("settings"));
//根据settings内容解析VFS(虚拟文件系统vfsImpl)
loadCustomVfs(settings);
//根据settings内容解析log日志具体实现类（logImpl）
loadCustomLogImpl(settings);
//解析typeAliases节点（TypeAliasRegistry里注册对应的别名）
typeAliasesElement(root.evalNode("typeAliases"));
//解析plugins节点（注册interceptorChain里记录对应的拦截器）
pluginElement(root.evalNode("plugins"));
//解析objectFactory节点（自定义objectFactory）
objectFactoryElement(root.evalNode("objectFactory"));
//解析objectWrapperFactory节点（自定义objectWrapperFactory）
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
//解析reflectorFactory节点（自定义reflectorFactory）
reflectorFactoryElement(root.evalNode("reflectorFactory"));
//设置其它的setting参数
settingsElement(settings);
//解析environments节点(environment)解析放到objectFactory and objectWrapperFactory解析之后，具体原因参见issue117
environmentsElement(root.evalNode("environments"));
//解析databaseIdProvider节点（databaseId）
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
//解析typeHandlers节点，注册类型转换器（typeHandlerRegistry）
typeHandlerElement(root.evalNode("typeHandlers"));
//解析mappers节点（重中之重）
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
下面逐一解析下各个节点
1. propertiesElement(root.evalNode("properties"))
private void propertiesElement(XNode context) throws Exception {
if (context != null) {
//解析所有子节点property
Properties defaults = context.getChildrenAsProperties();
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.");
}
//根据url或者resource生成对应的property集合
if (resource != null) {
defaults.putAll(Resources.getResourceAsProperties(resource));
} else if (url != null) {
defaults.putAll(Resources.getUrlAsProperties(url));
}
//入参中的variables如果也存在的话，一并放入defaults
Properties vars = configuration.getVariables();
if (vars != null) {
defaults.putAll(vars);
}
parser.setVariables(defaults);
//设置variables
configuration.setVariables(defaults);
}
}
2. Properties settings = settingsAsProperties(root.evalNode("settings"))
private Properties settingsAsProperties(XNode context) {
if (context == null) {
return new Properties();
}
Properties props = context.getChildrenAsProperties();
// 利用MetaClass来检查Configuration
MetaClass metaConfig = MetaClass.forClass(Configuration.class, localReflectorFactory);
//检查下对应的setting的key值在configuration里存不存在
for (Object key : props.keySet()) {
if (!metaConfig.hasSetter(String.valueOf(key))) {
throw new BuilderException("The setting " + key + " is not known. Make sure you spelled it correctly (case sensitive).");
}
}
return props;
}
3. loadCustomVfs(settings)根据setting解析Vfs（代码比较简单，就不注释了）
private void loadCustomVfs(Properties props) throws ClassNotFoundException {
String value = props.getProperty("vfsImpl");
if (value != null) {
String[] clazzes = value.split(",");
for (String clazz : clazzes) {
if (!clazz.isEmpty()) {
@SuppressWarnings("unchecked")
Class<? extends VFS> vfsImpl = (Class<? extends VFS>) Resources.classForName(clazz);
configuration.setVfsImpl(vfsImpl);
}
}
}
}
4. loadCustomLogImpl(settings)根据setting解析log实现
private void loadCustomLogImpl(Properties props) {
Class<? extends Log> logImpl = resolveClass(props.getProperty("logImpl"));
configuration.setLogImpl(logImpl);
}
这个resolveClass方法会经常用到，我们跟一下看看。
public Class resolveAlias(String string) {
try {
if (string == null) {
return null;
}
// issue #748
String key = string.toLowerCase(Locale.ENGLISH);
Class value;
//根据typeAliases先去捞一波class，如果没有的话用Resources走classpath生成class
if (typeAliases.containsKey(key)) {
value = (Class) typeAliases.get(key);
} else {
value = (Class) Resources.classForName(string);
}
return value;
} catch (ClassNotFoundException e) {
throw new TypeException("Could not resolve type alias '" + string + "'. Cause: " + e, e);
}
}
public class TypeAliasRegistry {
private final Map> typeAliases = new HashMap<>();
//内置了一系列的类型别名
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);
...
5. typeAliasesElement(root.evalNode("typeAliases"))解析typeAliases节点（TypeAliasRegistry里注册对应的别名），正好跟上前的resolveClass联动起来。
private void typeAliasesElement(XNode parent) {
if (parent != null) {
for (XNode child : parent.getChildren()) {
//注册package包下面所有的Class,key:getSimpleName(),value:Class（不包括接口，内部类，匿名类）
if ("package".equals(child.getName())) {
String typeAliasPackage = child.getStringAttribute("name");
configuration.getTypeAliasRegistry().registerAliases(typeAliasPackage);
} else {
//普通加载
String alias = child.getStringAttribute("alias");
String type = child.getStringAttribute("type");
try {
Class<?> clazz = Resources.classForName(type);
if (alias == null) {
typeAliasRegistry.registerAlias(clazz);
} else {
typeAliasRegistry.registerAlias(alias, clazz);
}
} catch (ClassNotFoundException e) {
throw new BuilderException("Error registering typeAlias for '" + alias + "'. Cause: " + e, e);
}
}
}
}
}