一文彻底吃透MyBatis源码！！
写在前面
随着互联网的发展，越来越多的公司摒弃了Hibernate，而选择拥抱了MyBatis。而且，很多大厂在面试的时候喜欢问MyBatis底层的原理和源码实现。总之，MyBatis几乎成为了Java开发人员必须深入掌握的框架技术，今天，我们就一起来深入分析MyBatis源码。文章有点长，建议先收藏后慢慢研究。整体三万字左右，全程高能，小伙伴们可慢慢研究。
文章已收录到：
https://github.com/sunshinelyz/technology-binghe
https://gitee.com/binghe001/technology-binghe
MyBatis源码解析
大家应该都知道Mybatis源码也是对Jbdc的再一次封装,不管怎么进行包装,还是会有获取链接、preparedStatement、封装参数、执行这些步骤的。
配置解析过程
String resource = "mybatis-config.xml";
//1.读取resources下面的mybatis-config.xml文件
InputStream inputStream = Resources.getResourceAsStream(resource);
//2.使用SqlSessionFactoryBuilder创建SqlSessionFactory
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
//3.通过sqlSessionFactory创建SqlSession
SqlSession sqlSession = sqlSessionFactory.openSession();
Resources.getResourceAsStream(resource)读取文件
public static InputStream getResourceAsStream(String resource) throws IOException {
return getResourceAsStream(null, resource);
}
//loader赋值为null
public static InputStream getResourceAsStream(ClassLoader loader, String resource) throws IOException {
InputStream in = classLoaderWrapper.getResourceAsStream(resource, loader);
if (in == null) {
throw new IOException("Could not find resource " + resource);
}
return in;
}
//classLoader为null
public InputStream getResourceAsStream(String resource, ClassLoader classLoader) {
return getResourceAsStream(resource, getClassLoaders(classLoader));
}
//classLoader类加载
InputStream getResourceAsStream(String resource, ClassLoader[] classLoader) {
for (ClassLoader cl : classLoader) {
if (null != cl) {
//加载指定路径文件流
InputStream returnValue = cl.getResourceAsStream(resource);
// now, some class loaders want this leading "/", so we'll add it and try again if we didn't find the resource
if (null == returnValue) {
returnValue = cl.getResourceAsStream("/" + resource);
}
if (null != returnValue) {
return returnValue;
}
}
}
return null;
}
总结：主要是通过ClassLoader.getResourceAsStream()方法获取指定的classpath路径下的Resource 。
通过SqlSessionFactoryBuilder创建SqlSessionFactory
//SqlSessionFactoryBuilder是一个建造者模式
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
public SqlSessionFactory build(InputStream inputStream) {
return build(inputStream, null, null);
}
//XMLConfigBuilder也是建造者模式
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.
}
}
}
//接下来进入XMLConfigBuilder构造函数
public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
}
//接下来进入this后,初始化Configuration
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;
}
//其中parser.parse()负责解析xml,build(configuration)创建SqlSessionFactory
return build(parser.parse());
parser.parse()解析xml
public Configuration parse() {
//判断是否重复解析
if (parsed) {
throw new BuilderException("Each XMLConfigBuilder can only be used once.");
}
parsed = true;
//读取配置文件一级节点configuration
parseConfiguration(parser.evalNode("/configuration"));
return configuration;
}
private void parseConfiguration(XNode root) {
try {
//properties 标签,用来配置参数信息，比如最常见的数据库连接信息
propertiesElement(root.evalNode("properties"));
Properties settings = settingsAsProperties(root.evalNode("settings"));
loadCustomVfs(settings);
loadCustomLogImpl(settings);
//实体别名两种方式:1.指定单个实体;2.指定包
typeAliasesElement(root.evalNode("typeAliases"));
//插件
pluginElement(root.evalNode("plugins"));
//用来创建对象(数据库数据映射成java对象时)
objectFactoryElement(root.evalNode("objectFactory"));
objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
reflectorFactoryElement(root.evalNode("reflectorFactory"));
settingsElement(settings);
// read it after objectFactory and objectWrapperFactory issue #631
//数据库环境
environmentsElement(root.evalNode("environments"));
databaseIdProviderElement(root.evalNode("databaseIdProvider"));
//数据库类型和Java数据类型的转换
typeHandlerElement(root.evalNode("typeHandlers"));
//这个是对数据库增删改查的解析
mapperElement(root.evalNode("mappers"));
} catch (Exception e) {
throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
}
}
总结:parseConfiguration完成的是解析configuration下的标签
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
//解析<package name=""/>
if ("package".equals(child.getName())) {
String mapperPackage = child.getStringAttribute("name");
//包路径存到mapperRegistry中
configuration.addMappers(mapperPackage);
} else {
//解析<mapper url="" class="" resource=""></mapper>
String resource = child.getStringAttribute("resource");
String url = child.getStringAttribute("url");
String mapperClass = child.getStringAttribute("class");
if (resource != null && url == null && mapperClass == null) {
ErrorContext.instance().resource(resource);
//读取Mapper.xml文件
InputStream inputStream = Resources.getResourceAsStream(resource);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream,
configuration, resource, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url != null && mapperClass == null) {
ErrorContext.instance().resource(url);
InputStream inputStream = Resources.getUrlAsStream(url);
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream,
configuration, url, configuration.getSqlFragments());
mapperParser.parse();
} else if (resource == null && url == null && mapperClass != null) {
Class<?> mapperInterface = Resources.classForName(mapperClass);
configuration.addMapper(mapperInterface);
} else {
throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
}
}
}
}
}
总结: 通过解析configuration.xml文件,获取其中的Environment、Setting,重要的是将下的所有解析出来之后添加到
Configuration,Configuration类似于配置中心,所有的配置信息都在这里。
mapperParser.parse()对 Mapper 映射器的解析
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
//解析所有的子标签
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
//把namespace（接口类型）和工厂类绑定起来
bindMapperForNamespace();
}
parsePendingResultMaps();
parsePendingCacheRefs();
parsePendingStatements();
}
//这里面解析的是Mapper.xml的标签
private void configurationElement(XNode context) {
try {
String namespace = context.getStringAttribute("namespace");
if (namespace == null || namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
builderAssistant.setCurrentNamespace(namespace);
//对其他命名空间缓存配置的引用
cacheRefElement(context.evalNode("cache-ref"));
//对给定命名空间的缓存配置
cacheElement(context.evalNode("cache"));
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
//是最复杂也是最强大的元素，用来描述如何从数据库结果集中来加载对象
resultMapElements(context.evalNodes("/mapper/resultMap"));
//可被其他语句引用的可重用语句块
sqlElement(context.evalNodes("/mapper/sql"));
//获得MappedStatement对象(增删改查标签)
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
}
}
//获得MappedStatement对象(增删改查标签)
private void buildStatementFromContext(List<XNode> list) {
if (configuration.getDatabaseId() != null) {
buildStatementFromContext(list, configuration.getDatabaseId());
}
buildStatementFromContext(list, null);
}
//获得MappedStatement对象(增删改查标签)
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
//循环增删改查标签
for (XNode context : list) {
final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
try {
//解析insert/update/select/del中的标签
statementParser.parseStatementNode();
} catch (IncompleteElementException e) {
configuration.addIncompleteStatement(statementParser);
}
}
}
public void parseStatementNode() {
//在命名空间中唯一的标识符,可以被用来引用这条语句
String id = context.getStringAttribute("id");
//数据库厂商标识
String databaseId = context.getStringAttribute("databaseId");
if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
String nodeName = context.getNode().getNodeName();
SqlCommandType sqlCommandType =
SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
//flushCache和useCache都和二级缓存有关
//将其设置为true后,只要语句被调用,都会导致本地缓存和二级缓存被清空,默认值:false
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
//将其设置为 true 后，将会导致本条语句的结果被二级缓存缓存起来，默认值：对 select 元素为 true
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);
// Include Fragments before parsing
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
includeParser.applyIncludes(context.getNode());
//会传入这条语句的参数类的完全限定名或别名
String parameterType = context.getStringAttribute("parameterType");
Class<?> parameterTypeClass = resolveClass(parameterType);
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
// Parse selectKey after includes and remove them.
processSelectKeyNodes(id, parameterTypeClass, langDriver);
// Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
KeyGenerator keyGenerator;
String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
if (configuration.hasKeyGenerator(keyStatementId)) {
keyGenerator = configuration.getKeyGenerator(keyStatementId);
} else {
keyGenerator = context.getBooleanAttribute("useGeneratedKeys", configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType)) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
}
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
StatementType statementType =
StatementType.valueOf(context.getStringAttribute("statementType",
StatementType.PREPARED.toString()));
Integer fetchSize = context.getIntAttribute("fetchSize");
Integer timeout = context.getIntAttribute("timeout");
String parameterMap = context.getStringAttribute("parameterMap");
//从这条语句中返回的期望类型的类的完全限定名或别名
String resultType = context.getStringAttribute("resultType");
Class<?> resultTypeClass = resolveClass(resultType);
//外部resultMap的命名引用
String resultMap = context.getStringAttribute("resultMap");
String resultSetType = context.getStringAttribute("resultSetType");
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
String keyProperty = context.getStringAttribute("keyProperty");
String keyColumn = context.getStringAttribute("keyColumn");
String resultSets = context.getStringAttribute("resultSets");
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
public MappedStatement addMappedStatement(
String id,
SqlSource sqlSource,
StatementType statementType,
SqlCommandType sqlCommandType,
Integer fetchSize,
Integer timeout,
String parameterMap,
Class<?> parameterType,
String resultMap,
Class<?> resultType,
ResultSetType resultSetType,
boolean flushCache,
boolean useCache,
boolean resultOrdered,
KeyGenerator keyGenerator,
String keyProperty,
String keyColumn,
String databaseId,
LanguageDriver lang,
String resultSets) {
if (unresolvedCacheRef) {
throw new IncompleteElementException("Cache-ref not yet resolved");
}
id = applyCurrentNamespace(id, false);
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration,
id, sqlSource, sqlCommandType)
.resource(resource)
.fetchSize(fetchSize)
.timeout(timeout)
.statementType(statementType)
.keyGenerator(keyGenerator)
.keyProperty(keyProperty)
.keyColumn(keyColumn)
.databaseId(databaseId)
.lang(lang)
.resultOrdered(resultOrdered)
.resultSets(resultSets)
.resultMaps(getStatementResultMaps(resultMap, resultType, id))
.resultSetType(resultSetType)
.flushCacheRequired(valueOrDefault(flushCache, !isSelect))
.useCache(valueOrDefault(useCache, isSelect))
.cache(currentCache);
ParameterMap statementParameterMap = getStatementParameterMap(parameterMap,
parameterType, id);
if (statementParameterMap != null) {
statementBuilder.parameterMap(statementParameterMap);
}
MappedStatement statement = statementBuilder.build();
//持有在configuration中
configuration.addMappedStatement(statement);
return statement;
}
public void addMappedStatement(MappedStatement ms){
//ms.getId = mapper.UserMapper.getUserById
//ms = MappedStatement等于每一个增删改查的标签的里的数据
mappedStatements.put(ms.getId(), ms);
}
//最终存放到mappedStatements中,mappedStatements存放的是一个个的增删改查
protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection").conflictMessageProducer((savedValue, targetValue) ->
". please check " + savedValue.getResource() + " and " + targetValue.getResource());
解析bindMapperForNamespace()方法
把 namespace（接口类型）和工厂类绑定起来
private void bindMapperForNamespace() {
//当前Mapper的命名空间
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
//interface mapper.UserMapper这种
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
}
if (boundType != null) {
if (!configuration.hasMapper(boundType)) {
configuration.addLoadedResource("namespace:" + namespace);
configuration.addMapper(boundType);
}
}
}
}
public <T> void addMapper(Class<T> type) {
mapperRegistry.addMapper(type);
}
public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
if (hasMapper(type)) {
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
//接口类型(key)->工厂类
knownMappers.put(type, new MapperProxyFactory<>(type));
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
生成SqlSessionFactory对象
XMLMapperBuilder.parse()方法,是对 Mapper 映射器的解析里面有两个方法：
（1）configurationElement()解析所有的子标签,最终解析Mapper.xml中的insert/update/delete/select标签的id(全路径)组成key和整个标签和数据连接组成MappedStatement存放到Configuration中的 mappedStatements这个map里面。
（2）bindMapperForNamespace()是把接口类型(interface mapper.UserMapper)和工厂类存到放MapperRegistry中的knownMappers里面。
