1.Mybatis中Mapper的使用

Mybatis除了使用SqlSession直接执行Mapper配置文件中的方法外，还可以生成Mapper接口的对象，通过Mapper对象调用接口方法也可以执行数据库操作。

在获得SqlSession之后，通过getMapper方法可以获得Mapper接口的实例，传入的参数是对应Mapper的class对象

public static void main(String[] args) {
	...
	session = factory.openSession();
	MybatisUserMapper mybatisUserMapper = session.getMapper(MybatisUserMapper.class);
	user = mybatisUserMapper.select(3);
	...
}

2.SqlSession获取Mapper

从SqlSession的创建过程中了解到SqlSession的实例是DefaultSqlSession

查看DefaultSqlSession的getMapper方法，方法中实际是通过Configuration的getMapper方法获得Mapper对象，传入的参数是Mapper的class类型和SqlSession的引用

Configuration是XMLConfigBuilder解析Mybatis配置文件得到的

    public <T> T getMapper(Class<T> type) {
        return this.configuration.getMapper(type, this);
    }

3.Configuration中获取Mapper

查看Configuration的getMapper方法，方法中通过mapperRegistry对象的getMapper方法获取Mapper对象

    public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        return this.mapperRegistry.getMapper(type, sqlSession);
    }

mapperRegistry是MapperRegistry的对象，它在Configuration的构造方法中进行初始化

protected final MapperRegistry mapperRegistry;

public Configuration() {
        ...
        this.mapperRegistry = new MapperRegistry(this);
        ...
}

4.MapperRegistry中获取Mapper

MapperRegistry构造方法中给Configuration对象进行赋值，它还有另一个Map对象的属性knownMappers

public class MapperRegistry {
    private final Configuration config;
    private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap();

    public MapperRegistry(Configuration config) {
        this.config = config;
    }
}

在MapperRegistry的getMapper方法中，通过传入的Mapper接口的class对象到knownMappers中获取对应的MapperProxyFactory对象

然后通过MapperProxyFactory的newInstance方法创建对应的接口的实例

	public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
        MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory)this.knownMappers.get(type);
        if (mapperProxyFactory == null) {
            throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
        } else {
            try {
                return mapperProxyFactory.newInstance(sqlSession);
            } catch (Exception var5) {
                throw new BindingException("Error getting mapper instance. Cause: " + var5, var5);
            }
        }
    }

5.MapperProxyFactory中生成Mapper对象

MapperProxyFactory的newInstance方法中，首先创建一个MapperProxy对象，然后调用newInstance(MapperProxy)方法创建Mapper对象

public class MapperProxyFactory<T> {
	...
    public T newInstance(SqlSession sqlSession) {
        MapperProxy<T> mapperProxy = new MapperProxy(sqlSession, this.mapperInterface, this.methodCache);
        return this.newInstance(mapperProxy);
    }
	...
}

MapperProxy实现InvocationHandler接口，是Java动态代理中的代理类，在调用对象的方法的时候会回调invoke方法

public class MapperProxy<T> implements InvocationHandler, Serializable {
	...
	public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            return Object.class.equals(method.getDeclaringClass()) ? method.invoke(this, args) : this.cachedInvoker(method).invoke(proxy, method, args, this.sqlSession);
        } catch (Throwable var5) {
            throw ExceptionUtil.unwrapThrowable(var5);
        }
    }
	...
}

newInstance(MapperProxy)方法中使用动态代理模式创建Mapper实例

public class MapperProxyFactory<T> {
	...
    protected T newInstance(MapperProxy<T> mapperProxy) {
        return Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);
    }
	...
}

6.总结

SqlSession中获取Mapper实际上是通过SqlSession的configuration对象来获取，在Configuration类中有一个MapperRegistry对象，MapperRegistry中有一个knownMappers集合对象，里面存储了Mapper的class对象和MapperProxyFactory的对应关系，通过Mapper的class对象可以从集合中获取对应的MapperProxyFactory对象，在MapperProxyFactory中通过动态代理的模式来创建Mapper的实例