Mybatis配置和接口映射原理

Mybatis可以把Mapper.xml文件直接映射到对应的接口，调用接口方法会自动去Mapper.xml文件中找到对应的标签，这个功能就是利用Java的动态代理在binding包中实现的。

一、注册Mapper

在初始化时会把获取到的Mapper接口注册到MapperRegistry，注册的时候创建一个Mapper代理工厂，这个工厂通过JDK的代理创建一个执行对象，创建代理需要的InvocationHandler为MapperProxy

//接口注册public class MapperRegistry {public <T> void addMapper(Class<T> type) {//如果是接口if (type.isInterface()) {if (hasMapper(type)) {throw new BindingException("Type " + type + " is already known to the MapperRegistry.");}boolean loadCompleted = false;try {//放到map中, value为创建代理的工厂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.//这里是解析Mapper接口里面的注解MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);parser.parse();loadCompleted = true;} finally {if (!loadCompleted) {knownMappers.remove(type);}}}}}

二、获取接口对象

从knownMappers中根据接口类型取出对应的代理创建工厂，用该工厂创建代理。

public class MapperRegistry {public <T> T getMapper(Class<T> type, SqlSession sqlSession) {                //取出MapperProxyFactoryfinal MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);if (mapperProxyFactory == null)throw new BindingException("Type " + type + " is not known to the MapperRegistry.");try {                        //创建代理return mapperProxyFactory.newInstance(sqlSession);} catch (Exception e) {throw new BindingException("Error getting mapper instance. Cause: " + e, e);}}}//创建代理的工厂public class MapperProxyFactory<T> {/** * 需要创建代理的接口 */private final Class<T> mapperInterface;/** * 执行方法的缓存,不需要每次都创建MapperMethod */private 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) {//创建代理, InvocationHanderl是MapperProxyreturn (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface },mapperProxy);}/** * 传人sqlSession创建代理 * @param sqlSession * @return */public T newInstance(SqlSession sqlSession) {//把代理执行需要用到的对象传入final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);return newInstance(mapperProxy);}}

三、调用接口方法

调用代理方法会进入到MapperProxy的public Object invoke(Object proxy, Method method, Object[] args)方法

public class MapperProxy<T> implements InvocationHandler, Serializable {private static final long serialVersionUID = -6424540398559729838L;private final SqlSession sqlSession;private final Class<T> mapperInterface;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;}public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {//如果方法是Object里面的则直接调用方法if (Object.class.equals(method.getDeclaringClass())) {try {return method.invoke(this, args);} catch (Throwable t) {throw ExceptionUtil.unwrapThrowable(t);}}//获取执行方法的封装对象final MapperMethod mapperMethod = cachedMapperMethod(method);//里面就是找到对应的sql 执行sql语句return mapperMethod.execute(sqlSession, args);}//缓存, 不需要每次都创建private MapperMethod cachedMapperMethod(Method method) {MapperMethod mapperMethod = methodCache.get(method);if (mapperMethod == null) {//传人配置参数mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());methodCache.put(method, mapperMethod);}return mapperMethod;}}

最终执行sql会进入到MapperMethod中execute方法：

//具体的根据接口找到配置文件标签的类public class MapperMethod {private final SqlCommand command;private final MethodSignature method;public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {//SqlCommand封装该接口方法需要执行sql的相关属性，如：id(name), 类型this.command = new SqlCommand(config, mapperInterface, method);//执行方法特性进行封装，用于构造sql参数，判断执行sql逻辑走哪条分支this.method = new MethodSignature(config, method);}public Object execute(SqlSession sqlSession, Object[] args) {Object result;//先找到对应的执行sql类型, sqlSession会调用不同方法if (SqlCommandType.INSERT == command.getType()) {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.insert(command.getName(), param));} else if (SqlCommandType.UPDATE == command.getType()) {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.update(command.getName(), param));} else if (SqlCommandType.DELETE == command.getType()) {Object param = method.convertArgsToSqlCommandParam(args);result = rowCountResult(sqlSession.delete(command.getName(), param));} else if (SqlCommandType.SELECT == command.getType()) {//如果是查询, 需要对返回做判断处理//根据方法的特性判断进入哪个执行分支if (method.returnsVoid() && method.hasResultHandler()) {executeWithResultHandler(sqlSession, args);result = null;} else if (method.returnsMany()) {result = executeForMany(sqlSession, args);} else if (method.returnsMap()) {result = executeForMap(sqlSession, args);} else {//只查一条数据Object param = method.convertArgsToSqlCommandParam(args);result = sqlSession.selectOne(command.getName(), param);}} else {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;}}

上面就是根据接口、方法、配置参数找到对应的执行sql，并构造参数，解析执行结果，具体sql执行在sqlSession流程里面，后面再看。