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MyBatis源碼剖析之Mapper代理方式詳解

更新時間：2022年07月18日 08:17:39 作者：共飲一杯無

這篇文章主要為大家詳細介紹了MyBatis中Mapper代理的方式，文中將通過源碼為大家進行詳細的剖析，感興趣的小伙伴可以跟隨小編一起學(xué)習(xí)一下

//前三步都相同
InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
SqlSession sqlSession = sqlSessionFactory.openSession();
//這?不再調(diào)?SqlSession的api,?是獲得了接?對象，調(diào)?接?中的?法。
UserMapper mapper = sqlSession.getMapper(UserMapper.class);
//模擬ids的數(shù)據(jù)
List<Integer> ids = new ArrayList<Integer>();
ids.add(1);
ids.add(2);
List<User> userList = mapper.findByIds(ids);

思考?個問題，通常的Mapper接?我們都沒有實現(xiàn)的?法卻可以使?，是為什么呢？答案很簡單：動態(tài)代理

public class Configuration {  
    protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
}

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

開始之前介紹?下MyBatis初始化時對接?的處理：MapperRegistry是Configuration中的?個屬性，它內(nèi)部維護?個HashMap?于存放mapper接?的??類，每個接?對應(yīng)?個??類。mappers中可以配置接?的包路徑，或者某個具體的接?類。

<mappers>
  <mapper class="com.zjq.mapper.UserMapper"/>
  <package name="com.zjq.mapper"/>
</mappers>

當解析mappers標簽時，它會判斷解析到的是mapper配置?件時，會再將對應(yīng)配置?件中的增刪改查標簽封裝成MappedStatement對象，存?mappedStatements中。(上?介紹了)當判斷解析到接?時，會建此接?對應(yīng)的MapperProxyFactory對象，存?HashMap中，key =接?的字節(jié)碼對象，value =此接?對應(yīng)MapperProxyFactory對象。

源碼剖析-getmapper()

進? sqlSession.getMapper(UserMapper.class )中

public interface SqlSession extends Closeable {
      //調(diào)用configuration中的getMapper
      <T> T getMapper(Class<T> type);
}

public class DefaultSqlSession implements SqlSession {
  @Override
  public <T> T getMapper(Class<T> type) {
    //調(diào)用configuration 中的getMapper
    return configuration.<T>getMapper(type, this);
  }
}

public class Configuration {
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //調(diào)用MapperRegistry 中的getMapper
    return mapperRegistry.getMapper(type, sqlSession);
  }
}

public class MapperRegistry {
  @SuppressWarnings("unchecked")
  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    //從 MapperRegistry 中的 HashMap 中拿 MapperProxyFactory
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      //調(diào)用MapperProxyFactory的newInstance通過動態(tài)代理???成實例
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }
}


public class MapperProxyFactory<T> {
  public T newInstance(SqlSession sqlSession) {
    //創(chuàng)建了 JDK動態(tài)代理的Handler類
    final MapperProxy<T> mapperProxy = new MapperProxy<T>(sqlSession, mapperInterface, methodCache);
    //調(diào)?了重載?法
    return newInstance(mapperProxy);
  }
   
  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
}

//MapperProxy 類，實現(xiàn)了 InvocationHandler 接?
public class MapperProxy<T> implements InvocationHandler, Serializable {
  //省略部分源碼
  private final SqlSession sqlSession;
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethod> methodCache;
    
  //構(gòu)造，傳?了 SqlSession，說明每個session中的代理對象的不同的！  
  public MapperProxy(SqlSession sqlSession, Class<T> mapperInterface, Map<Method, MapperMethod> methodCache) {
    this.sqlSession = sqlSession;
    this.mapperInterface = mapperInterface;
    this.methodCache = methodCache;
  }
  
   //省略部分源碼
}

源碼剖析-invoke()

在動態(tài)代理返回了示例后，我們就可以直接調(diào)?mapper類中的?法了，但代理對象調(diào)??法，執(zhí)?是在MapperProxy中的invoke?法中。

public class MapperProxy<T> implements InvocationHandler, Serializable { 
  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      //如果是Object定義的?法，直接調(diào)?
      if (Object.class.equals(method.getDeclaringClass())) {
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    // 獲得 MapperMethod 對象
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //重點在這：最終調(diào)?了MapperMethod執(zhí)?的?法
    return mapperMethod.execute(sqlSession, args);
  }
}

進?execute?法：

public class MapperMethod { 
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //判斷mapper中的?法類型，最終調(diào)?的還是SqlSession中的?法 switch(command.getType()) 
    switch (command.getType()) {
      case INSERT: {
        //轉(zhuǎn)換參數(shù)
        Object param = method.convertArgsToSqlCommandParam(args);
        //執(zhí)?INSERT操作
        //轉(zhuǎn)換rowCount
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        //轉(zhuǎn)換參數(shù)
        Object param = method.convertArgsToSqlCommandParam(args);
        // 轉(zhuǎn)換 rowCount
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        //轉(zhuǎn)換參數(shù)
        Object param = method.convertArgsToSqlCommandParam(args);
        // 轉(zhuǎn)換 rowCount
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        //?返回，并且有ResultHandler?法參數(shù)，則將查詢的結(jié)果，提交給 ResultHandler 進?處理
        if (method.returnsVoid() && method.hasResultHandler()) {
          executeWithResultHandler(sqlSession, args);
          result = null;
        //執(zhí)?查詢，返回列表
        } else if (method.returnsMany()) {
          result = executeForMany(sqlSession, args);
        //執(zhí)?查詢，返回Map
        } else if (method.returnsMap()) {
          result = executeForMap(sqlSession, args);
        //執(zhí)?查詢，返回Cursor
        } else if (method.returnsCursor()) {
          result = executeForCursor(sqlSession, args);
        //執(zhí)?查詢，返回單個對象
        } else {
          //轉(zhuǎn)換參數(shù)
          Object param = method.convertArgsToSqlCommandParam(args);
          //查詢單條
          result = sqlSession.selectOne(command.getName(), param);
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        throw new BindingException("Unknown execution method for: " + command.getName());
    }
    //返回結(jié)果為null，并且返回類型為基本類型，則拋出BindingException異常
    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() + ").");
    }
    //返回結(jié)果
    return result;
  }
}

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