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前言
面试官：你说一下为什么Mapper映射器是一个interface，而我们却可以直接调用它的方法，还能执行对应的SQL。额…也许你不知道，也许你知道个大概，本篇文章将带你从源码的角度彻彻底底理解Mybatis的Mapper映射器
Mapper的注册
我们在执行Mybatis的时候可以使用 sqlSession.selectOne("cn.whale.mapper.StudentMapper.selectById",1L)这种最原生的方式，这种方式的弊端是太麻烦，每次都要去拼接 statementId。所以我们在项目中通常是使用Mapper映射器来执行。如下：
StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
Student student = mapper.selectById(1L);
下面我们就来分析一下通过Mapper映射器是如何工作的。在之前的文章中我们有分析到，在SqlSessionFactoryBuilder.buid的时候会通过XMLConfigBuilder对mybatis-config.xml进行解析，其中有一个步骤就是对Mapper.xml的解析 ，如：<mapper resource="mapper/StudentMapper.xml"/> 。代码直接来到XMLConfigBuilder#mapperElement
private void mapperElement(XNode parent) throws Exception {
if (parent != null) {
for (XNode child : parent.getChildren()) {
if ("package".equals(child.getName())) {
//package配置方式
String mapperPackage = child.getStringAttribute("name");
configuration.addMappers(mapperPackage);
} else {
//拿到配置的资源 如： mapper/studentMapper.xml
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);
//把资源加载为流
InputStream inputStream = Resources.getResourceAsStream(resource);
//mapper.xml的解析器
XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
//【重点】解析mapper.xml
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.");
}
}
}
}
}
上面代码会加载mapper.xml文件然后使用XMLMapperBuilder去解析xml,代码来到
org.apache.ibatis.builder.xml.XMLMapperBuilder#parse
public void parse() {
if (!configuration.isResourceLoaded(resource)) {
//解析mapper.xml中的 <cache ,<resultMap ,<parameterMap,<sql ,select|insert|update|delete 等元素
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
//【重点】我们的重点在这，为当前namespace绑定mapper接口
bindMapperForNamespace();
}
parsePendingResultMaps();
parsePendingCacheRefs();
parsePendingStatements();
}
我们看到上面解析完mapper.xml后执行了这样一个方法bindMapperForNamespace，看名字能猜到他的作用是为当前namespace绑定mapper接口
private void bindMapperForNamespace() {
//拿到namespace
String namespace = builderAssistant.getCurrentNamespace();
if (namespace != null) {
Class<?> boundType = null;
try {
//拿到namespace对应的mapper接口的class：比如：cn.whale.mapper.StudentMapper
boundType = Resources.classForName(namespace);
} catch (ClassNotFoundException e) {
//ignore, bound type is not required
}
if (boundType != null) {
//判断configuration中是否包含 当前 mapper
if (!configuration.hasMapper(boundType)) {
// Spring may not know the real resource name so we set a flag
// to prevent loading again this resource from the mapper interface
// look at MapperAnnotationBuilder#loadXmlResource
configuration.addLoadedResource("namespace:" + namespace);
//把Mapper接口添加到configuration中
configuration.addMapper(boundType);
}
}
}
}
我们看到方法中拿到 namespace 后转换为 Class 。然后调用 configuration.addMapper把Mapper接口添加到configuration中，继续跟进org.apache.ibatis.session.Configuration#addMapper
public class Configuration {
//mapper的注册器
protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
public <T> void addMapper(Class<T> type) {
//把mapper的class添加到 MapperRegistry
mapperRegistry.addMapper(type);
}
mapper的class被添加到了Configuration#mapperRegistry中，那么mapperRegistry又是个什么东西呢,见org.apache.ibatis.binding.MapperRegistry#addMapper
public class MapperRegistry {
private final Configuration config;
//保存Mapper接口的真正结构是一个HashMap
private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<Class<?>, MapperProxyFactory<?>>();
public <T> void addMapper(Class<T> type) {
if (type.isInterface()) {
//必须是一个接口
if (hasMapper(type)) {
//判断是否重复添加
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
//【重点】 ，以mapper的class为key， 把mapper的class封装到MapperProxyFactory作为Value。
//存储到knownMappers 一个HashMap中。
knownMappers.put(type, new MapperProxyFactory<T>(type));
// It's important that the type is added before the parser is run
// otherwise the binding may automatically be attempted by the
// mapper parser. If the type is already known, it won't try.
//映射器注释生成器,Mapper注解上的注解
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
//该方法会解析Mapper接口上的注解，比如： @Select("select * from student")
//然后会把这些SQL语句封装成MappedStatement
parser.parse();
loadCompleted = true;
} finally {
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
我们重点看这行代码，knownMappers.put(type, new MapperProxyFactory<T>(type)); 以mapper的class为key， 把mapper的class封装到MapperProxyFactory作为Value。存储到knownMappers 一个HashMap中，MapperProxyFactory我们可以看做是mapper的代理工厂，它封装了mapperInterface成class和接口中的方法，同时提供了创建mapper接口代理的方法。
public class MapperProxyFactory<T> {
private final Class<T> mapperInterface;
private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<Method, MapperMethod>();
public MapperProxyFactory(Class<T> mapperInterface) {
this.mapperInterface = mapperInterface;
}
public Class<T> getMapperInterface() {
return mapperInterface;
}
public Map<Method, MapperMethod> getMethodCache() {
return methodCache;
}
@SuppressWarnings("unchecked")
protected T newInstance(MapperProxy<T> mapperProxy) {
//反射创建代理
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
public T newInstance(SqlSession sqlSession) {
//反射创建代理
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
}
到了到这里就差不多了，总结一下，就是在sqlSessionFactoryBuilder.buid 时候就会解析mapper.xml，然后根据namespace找到对应的mapper接口，把mapper接口的clazz封装到一个 MapperProxyFactory 代理工厂里面，然后以mapper的class为key把MapperProxyFactory存储到MapperRegistry中的 knownMappers 属性中。而MapperRegistry本身又是存储在 Configuration对象中。
Mapper的代理
上面我们分析了Mapper映射器的注册流程，我们接下来分析它的代理。入口就是我们执行sqlSession.getMapper(StudentMapper.class);的时候，该方法会调用org.apache.ibatis.session.defaults.DefaultSqlSession#getMapper获取Mapper
@Override
public <T> T getMapper(Class<T> type) {
return configuration.<T>getMapper(type, this);
}
DefaultSqlSession#getMapper又调用了org.apache.ibatis.session.Configuration#getMapper，最终从mapperRegistry中获取Mpaper见：org.apache.ibatis.session.Configuration#getMapper
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
return mapperRegistry.getMapper(type, sqlSession);
}
下面是org.apache.ibatis.binding.MapperRegistry#getMapper获取Mapper的方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//先从knownMappers拿到Mapper，Mapper被封装到MapperProxyFactory代理工厂年
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
//【重要重要】 这里在调用MapperProxyFactory#newInstance创建Mapper的实例了
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
MapperRegistry#getMapper方法中从 knownMappers 中获取到Mapper,mapper被封装成了MapperProxyFactory，然后调用mapperProxyFactory.newInstance创建Mapper的代理类，见：org.apache.ibatis.binding.MapperProxyFactory#newInstance(org.apache.ibatis.session.SqlSession)
public T newInstance(SqlSession sqlSession) {
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
protected T newInstance(MapperProxy<T> mapperProxy) {
//JDK动态代理 ， mapperProxy 是一个 InvocationHandler
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
MapperProxyFactory#newInstance方法中依然是使用了JDK动态代理为Mapper接口创建代理类，需要注意的是 MapperProxy ，这个类，它是一个 InvocationHandler ，也就是说当Mapper接口的方法被调用(其实是代理类被调用)，请求会被 InvocationHandler#invoke 拦截。
到这里，sqlSession.getMapper方法的源码分析完了，其实就是从Configuration的MapperRegistry中取出封装了Mapper接口的MapperProxyFactory代理工厂类，然后执行 mapperProxyFactory.newInstance为接口生成代理类。
Mapper接口的执行
上面我们知道了，Mapper接口是通过 MapperProxyFactory.newInstance 生成的代理，当Mapper的方法被调用的时候就会被 MapperProxy#invoke 拦截器，见：org.apache.ibatis.binding.MapperProxy
public class MapperProxy<T> implements InvocationHandler, Serializable {
private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
//mapper接口的class
private final Class<T> mapperInterface;
//方法的缓存
private final Map<Method, MapperMethod> methodCache;
public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
this.sqlSession = sqlSession;
this.mapperInterface = mapperInterface;
this.methodCache = methodCache;
}
//invoke会拦截mapper的方法执行
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
try {
if (Object.class.equals(method.getDeclaringClass())) {
return method.invoke(this, args);
} else if (isDefaultMethod(method)) {
return invokeDefaultMethod(proxy, method, args);
}
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
//这里会尝试从缓存中获取MapperMethod，如果没有就会把Method封装为MapperMethod，写入缓存
//在MapperMethod的构造器中会把 mapperInterface.getName() + "." + methodName; 即：namespace加上方法名作为 statementId
final MapperMethod mapperMethod = cachedMapperMethod(method);
//[重点]执行mapper方法了
return mapperMethod.execute(sqlSession, args);
}
上面invoke方法中会把method封装为mapperMethod，在MapperMethod的构造器中会以mapperInterface.getName() + "." + methodName得到statementId，然后会把MapperMethod进行缓存，然后执行 org.apache.ibatis.binding.MapperMethod#execute
public class MapperMethod {
private final SqlCommand command;
private final MethodSignature method;
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
//处理SQL命令，SqlCommand的name属性就是： mapperInterface.getName() + "." + methodName
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, mapperInterface, method);
}
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//判断类型
switch (command.getType()) {
//是执行insert操作
case INSERT: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
break;
}
//是执行update操作
case UPDATE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
break;
}
//是执行delete操作
case DELETE: {
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
break;
}
//是执行select操作
case SELECT:
if (method.returnsVoid() && method.hasResultHandler()) {
//有指定结果处理器
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
//有多个返回值
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
//返回一个map
result = executeForMap(sqlSession, args);
} else if (method.returnsCursor()) {
//通过游标查询
result = executeForCursor(sqlSession, args);
} else {
//查询一个对象走这
//转换参数
Object param = method.convertArgsToSqlCommandParam(args);
//最终调用了sqlSession#selectOne方法
result = sqlSession.selectOne(command.getName(), param);
}
break;
case FLUSH:
result = sqlSession.flushStatements();
break;
default:
throw new BindingException("Unknown execution method for: " + command.getName());
}
if (result == null && method.getReturnType().isPrimitive() && !method.returnsVoid()) {
throw new BindingException("Mapper method '" + command.getName()
+ " attempted to return null from a method with a primitive return type (" + method.getReturnType() + ").");
}
return result;
}
在 MapperMethod构造器中会创建一个SqlCommand ， 根据 mapperInterface.getName() + "." + methodName 得到statementid，作为 SqlCommand的name 。当方法被执行，也就是execute被执行，会根据执行的SQL的类型做不同的处理，最终还是会调用SqlSession#selectOne方法去执行SQL。而statementid就是 mapperInterface.getName() + "." + methodName。
总结
到这里就分析的差不多了我们总结一下整体流程
- sqlSessionFactoryBuilder.buid 的时候就会解析mybatis-config.xml ,然后解析 mapper.xml，然后根据namespace找到对应的mapper接口，把
mapper接口的clazz封装到一个 MapperProxyFactory代理工厂里面，然后以mapper的class为key把MapperProxyFactory存储到MapperRegistry中的 knownMappers 属性中。而MapperRegistry本身又是存储在 Configuration对象中。 - 当执行 sqlSession.getMapper的时候，就从
Configuration的MapperRegistry中取出封装了Mapper接口的MapperProxyFactory代理工厂类，然后执行mapperProxyFactory.newInstance为接口生成代理类。 - 最后在执行mapper接口的方法的时候，请求会被MapperProxy#invoke方法拦截器，在该方法中会把Method封装成MapperMethod后缓存，然后再执行MapperMethod#execute 。最终以
mapperInterface.getName() + "." + methodName为 statement ，调用sqlSession去执行查询。
所以最开始的面试题你会答了吗？
文章结束，喜欢就给我去点个五星好评吧。2021一路有你，2022我们继续加油！你的肯定是我最大的动力
博主在参加博客之星评比，点击链接 ， https://bbs.csdn.net/topics/603957267 疯狂打Call！五星好评 感谢
