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当定义好一个Mapper接口(UserDao)里,我们并不需要去实现这个类,但sqlSession.getMapper()最终会返回一个实现该接口的对象。这个对象是Mybatis利用jdk的动态代理实现的。这里将介绍这个代理对象的生成过程及其方法的实现过程。
Mapper代码对象的生成过程
DefaultSqlSession.getMapp()方法最终会调用MapperRegistry.getMapper()方法
public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
//这个MapperProxyFactory是调用addMapper方法时加到knownMappers中的,
final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
if (mapperProxyFactory == null)
//说明这个Mapper接口没有注册
throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
try {
//生成一个MapperProxy对象
return mapperProxyFactory.newInstance(sqlSession);
} catch (Exception e) {
throw new BindingException("Error getting mapper instance. Cause: " + e, e);
}
}
下面是MapperProxyFactory的newInstance方法
public T newInstance(SqlSession sqlSession) {
//创建一个Mapperxy对象,这个方法实现了JDK动态代理中的InvocationHandler接口
final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
return newInstance(mapperProxy);
}
protected T newInstance(MapperProxy<T> mapperProxy) {
//mapperInterface,说明Mapper接口被代理了,这样子返回的对象就是Mapper接口的子类,方法被调用时会被mapperProxy拦截,也就是执行mapperProxy.invoke()方法
return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
}
MapperProxy
详细分析一下MapperProxy类
public class MapperProxy<T> implements InvocationHandler, Serializable {
private static final long serialVersionUID = -6424540398559729838L;
private final SqlSession sqlSession;
//Mapper接口
private final Class<T> mapperInterface;
//Mapper接口中的每个方法都会生成一个MapperMethod对象, methodCache维护着他们的对应关系
//这个methodCache是在MapperProxyFactory中持有的,MapperProxyFactory又是在Configuration中持有的
//所以每个Mapper接口类对应的MapperProxyFactory和methodCache在整个应用中是共享的,一般只会有一个实例
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;
}
//这里会拦截Mapper接口(UserDao)的所有方法
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//如果是Object中定义的方法,直接执行。如toString(),hashCode()等待。
if (Object.class.equals(method.getDeclaringClass())) {
try {
return method.invoke(this, args);
} catch (Throwable t) {
throw ExceptionUtil.unwrapThrowable(t);
}
}
//其他Mapper接口定义的方法交由mapperMethod来执行。
final MapperMethod mapperMethod = cachedMapperMethod(method);
return mapperMethod.execute(sqlSession, args);
}
private MapperMethod cachedMapperMethod(Method method) {
MapperMethod mapperMethod = methodCache.get(method);
if (mapperMethod == null) {
mapperMethod = new MapperMethod(mapperInterface, method, sqlSession.getConfiguration());
methodCache.put(method, mapperMethod);
}
return mapperMethod;
}
}
MapperMethod
//所有Mapper接口中方法被调用里,都会执行这个方法.这里实际上是调用SqlSession中的相关方法,
public Object execute(SqlSession sqlSession, Object[] args) {
Object result;
//判断这个方法被注解里的Sql类型
if (SqlCommandType.INSERT == command.getType()) {
//执行insert
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.insert(command.getName(), param));
} else if (SqlCommandType.UPDATE == command.getType()) {
//执行update
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.update(command.getName(), param));
} else if (SqlCommandType.DELETE == command.getType()) {
/delete
Object param = method.convertArgsToSqlCommandParam(args);
result = rowCountResult(sqlSession.delete(command.getName(), param));
} else if (SqlCommandType.SELECT == command.getType()) {
//select ,查询
if (method.returnsVoid() && method.hasResultHandler()) {
//没有返回值,并且有ResultHandler的情况
executeWithResultHandler(sqlSession, args);
result = null;
} else if (method.returnsMany()) {
//返回一个List
result = executeForMany(sqlSession, args);
} else if (method.returnsMap()) {
//返回一个Map
result = executeForMap(sqlSession, args);
} else {
//返回一个对象
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;
}
小结
- 在Mybatis提供的编程接口中,开发人员只需要定义好Mapper接口(如:UserDao),开发人员无需去实现。Mybatis会利用JDK的动态代理实现 Mapper接口。
- 在Mybatis中,每个Mapper接口都会对应一个MapperProxyFactory对象实例,这个对应关系在Configuration.mapperRegistry.knownMappers中。
- 当getMapper()方法被调用时,Mybatis会找到相对应的MapperProxyFactory对象实例,利用这个工厂来创建一个jdk动态代理对象,是这个Mapper接口的实现类,当Mapper定义的方法被调用时,会调用MapperProxy来处理。
- MapperProxy会根据方法找到对应的MapperMethod对象来实现这次调用。
- MapperMethod对应会读取方法中的注解,从Configuration中找到相对应的MappedStatement对象,再执行。