本文作者：鍾昕靈，叩丁狼高階講師。原創文章，轉載請註明出處。

相信只要是使用過MyBatis開發的同學,都有使用其中的Mapper介面來開發,因為確實是很方便,方便到我們只需要編寫介面而不需要寫實現類,就能夠完成對資料庫的CRUD操作,但是不知道大家有沒有去思考過,如果我們真的只有Mapper介面的話,程式又是如何去完成實際的業務的呢?來看看下面的程式碼

cn.wolfcode.mybatis.mapper.UserMapper介面

public interface UserMapper {
    void save(User u);
}

UserMapper.xml對映檔案

<mapper namespace="cn.wolfcode.mybatis.mapper.UserMapper">
    <insert id="save">
        INSERT INTO user (id, username, password) VALUES (NULL, #{username}, #{password})
    </insert>
</mapper>
 

UserServiceImpl業務方法

public void save(User u) throws IOException {
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(Resources.getResourceAsStream("mybatis-config.xml"));
        SqlSession session = factory.openSession();
        UserMapper mapper = session.getMapper(UserMapper.class);
        mapper.save(u);
        session.commit();
        session.close();
    }
 

從上面的程式碼中可以看出,我們只需要給MyBatis提供Mapper介面和與之匹配的對映檔案,就能夠讓MyBatis按照我們的需求執行到對應的SQL

這裡的實現原理就是我們前面所講過的 動態代理,接下來我們看一波原始碼

通過debug斷點除錯,我們可以依次看到下面的程式碼

DefaultSqlSession:

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

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

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);
            }
        }
    }
 

該方法中最關鍵程式碼:mapperProxyFactory.newInstance(sqlSession); MapperProxyFactory是一個建立MapperProxy的工廠類,呼叫其中的newInstance方法可以獲取到一個代理物件,繼續往下看

MapperProxyFactory:

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

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

在該類中可以看到,

Proxy.newProxyInstance(this.mapperInterface.getClassLoader(), new Class[]{this.mapperInterface}, mapperProxy);

最終由JDK的動態代理,動態的為我們在記憶體中建立了一個代理物件

到此,我們已經看到了一部分真相,就是我們為mybatis提供Mapper介面,而mybatis使用JDK的動態代理為我們生成實現類

相信大家和我一樣,還想繼續瞭解一下,在這個代理類中具體為我們做了什麼,那好,我們繼續

如果大家瞭解JDK的動態代理的話,那麼就應該知道我們現在最關心的應該是InvocationHandler的實現,從上面的程式碼中可以看到,它叫做MapperProxy MapperProxy:

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 {
        if(Object.class.equals(method.getDeclaringClass())) {
            return method.invoke(this, args);
        } else {
            MapperMethod mapperMethod = this.cachedMapperMethod(method);
            return mapperMethod.execute(this.sqlSession, args);
        }
    }

    private MapperMethod cachedMapperMethod(Method method) {
        MapperMethod mapperMethod = (MapperMethod)this.methodCache.get(method);
        if(mapperMethod == null) {
            mapperMethod = new MapperMethod(this.mapperInterface, method, this.sqlSession.getConfiguration());
            this.methodCache.put(method, mapperMethod);
        }

        return mapperMethod;
    }
}

在invoke方法中可以看到,如果我們呼叫的是Object中的方法,不做任何處理,直接呼叫,否則執行: mapperMethod.execute(this.sqlSession, args);

MapperMethod:

public Object execute(SqlSession sqlSession, Object[] args) {
        Object param;
        Object result;
        if(SqlCommandType.INSERT == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.insert(this.command.getName(), param));
        } else if(SqlCommandType.UPDATE == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.update(this.command.getName(), param));
        } else if(SqlCommandType.DELETE == this.command.getType()) {
            param = this.method.convertArgsToSqlCommandParam(args);
            result = this.rowCountResult(sqlSession.delete(this.command.getName(), param));
        } else {
            if(SqlCommandType.SELECT != this.command.getType()) {
                throw new BindingException("Unknown execution method for: " + this.command.getName());
            }

            if(this.method.returnsVoid() && this.method.hasResultHandler()) {
                this.executeWithResultHandler(sqlSession, args);
                result = null;
            } else if(this.method.returnsMany()) {
                result = this.executeForMany(sqlSession, args);
            } else if(this.method.returnsMap()) {
                result = this.executeForMap(sqlSession, args);
            } else {
                param = this.method.convertArgsToSqlCommandParam(args);
                result = sqlSession.selectOne(this.command.getName(), param);
            }
        }

        if(result == null && this.method.getReturnType().isPrimitive() && !this.method.returnsVoid()) {
            throw new BindingException("Mapper method '" + this.command.getName() 
  + " attempted to return null from a method with a primitive return type (" + this.method.getReturnType() + ").");
        } else {
            return result;
        }
    }

主要將SQL分為兩類執行,DML和SQL 如果是DML,呼叫SQLSession中對應的方法執行,並且使用rowCountResult方法根據方法的返回值和受影響的行數做處理

如果是查詢,則要根據方法的返回值的型別來執行不同的方法 如果Collection系的集合獲取陣列來接收,使用selectList方法執行查詢 如果使用Map集合,呼叫selectMap方法執行查詢 否則,呼叫selectOne執行查詢

相信,原始碼看到這裡,大家心裡應該很清楚MyBatis中Mapper介面的使用原理了

最後總結一下: MapperProxyFactory中,使用JDK的動態代理生成Mapper介面的代理代理類 由動態處理器MapperProxy中呼叫MapperMethod中的方法處理執行SQL 最後,在MapperMethod中根據執行的方法返回值決定呼叫SqlSession中的對應方法執行SQL