Mybatis根据给出的mapper接口与对应的xml文件，利用jdk的动态代理来实现相应的逻辑。再调用具体的sqlsession，通过相应的excutor执行相关的sql逻辑。
关于Mapper代理，就从Configuration的addMappers()方法说起吧
public void addMappers(String packageName, Class<?> superType) {
mapperRegistry.addMappers(packageName, superType);
}
public void addMappers(String packageName) {
mapperRegistry.addMappers(packageName);
}
public <T> void addMapper(Class<T> type) {
mapperRegistry.addMapper(type);
}
无非扫描包，到每个类调用具体的addMapper。
public void addMappers(String packageName, Class<?> superType) {
//查找包下所有是superType的类
ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
for (Class<?> mapperClass : mapperSet) {
addMapper(mapperClass);
}
}
我们再来看addMapper的逻辑
public <T> void addMapper(Class<T> type) {
//mapper必须是接口！才会添加
if (type.isInterface()) {
if (hasMapper(type)) {
//如果重复添加了，报错
throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
}
boolean loadCompleted = false;
try {
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.
MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
parser.parse();
loadCompleted = true;
} finally {
//如果加载过程中出现异常需要再将这个mapper从mybatis中删除,这种方式比较丑陋吧，难道是不得已而为之？
if (!loadCompleted) {
knownMappers.remove(type);
}
}
}
}
mapper必须是接口，先判断有无重复添加，再根据类型创建相应的MapperProxyFactroy，并添加至knowMappers（存放mapper工厂的map），再之后创建MapperAnnotationbuilder对类中的相关注解进行解析。
若这个过程中出错，直接移除相应的MapperProxyFactroy。
public void parse() {
String resource = type.toString();
if (!configuration.isResourceLoaded(resource)) {
loadXmlResource();
configuration.addLoadedResource(resource);
assistant.setCurrentNamespace(type.getName());
parseCache();
parseCacheRef();
Method[] methods = type.getMethods();
for (Method method : methods) {
try {
// issue #237
if (!method.isBridge()) {
parseStatement(method);
}
} catch (IncompleteElementException e) {
configuration.addIncompleteMethod(new MethodResolver(this, method));
}
}
}
parsePendingMethods();
}
根据mapper中的注解进行配置，并且如果未加载相关xml资源的话在这里会重新加载，并且parseStatement处理mapper中的每一个方法（无非是配置相应参数加载到assistant）。
到这里为止~准备工作算是差不多了，剩下就是用了。
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null) {
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
}
try {
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
getMapper无非先得到对应的代理工厂，再返回newInstance(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) {
//用JDK自带的动态代理生成映射器
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
从上面代码中可以看出，通过jdk的动态代理，代理是MapperProxy类的对象，实现的是mapper接口，也就是说真正被代理的是mapperProxy。
jdk的动态代理，得到代理对象后调用的方法，都是通过invoke()；
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@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//代理以后，所有Mapper的方法调用时，都会调用这个invoke方法
//并不是任何一个方法都需要执行调用代理对象进行执行，如果这个方法是Object中通用的方法（toString、hashCode等）无需执行
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//这里优化了，去缓存中找MapperMethod
final MapperMethod mapperMethod = cachedMapperMethod(method);
//执行
return mapperMethod.execute(sqlSession, args);
}
//去缓存中找MapperMethod
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
//找不到才去new
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
思路很清晰，这里有个缓存methodCache的一个小的优化，只进行一次解析，空间换时间。
再进入MapperMethod的构造方法。
public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
this.command = new SqlCommand(config, mapperInterface, method);
this.method = new MethodSignature(config, method);
}
其实就是对方法的数据的一个封装，execute方法：
//执行
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//可以看到执行时就是4种情况，insert|update|delete|select，分别调用SqlSession的4大类方法
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()) {
if (method.returnsVoid() && method.hasResultHandler()) {
//如果有结果处理器
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
//如果结果有多条记录
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
//如果结果是map
result = executeForMap(sqlSession, args);
} else {
//否则就是一条记录
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;
}
根据command里的配置信息执行sqlSession不同的方法，convertArgsToSqlCommandParam是对传入参数的一个处理，rowCountResult无非是对返回值的一个合法性处理。
具体执行是通过sqlSession，已经相应的executor，执行bandSql。