MyBatis binding 模塊分析

MyBatis binding 模塊分析

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binding功能代碼所在包

org.apache.ibatis.binding

binding模塊做用

封裝ibatis編程模型 ibatis編程模型中,SqlSession做爲sql執行的入口,實用方法爲sqlSession.selectOne(namespace+id, 參數列表)的形式(例:sqlSession.selectOne("com.enjoylearning.mybatis.mapper.TUserMapper.selectByPrimaryKey", 2)),不易於維護和閱讀

須要解決的問題

• 找到SqlSession中對應的方法(insert|update|select)
• 找到命名空間和方法名(兩維座標)
• 傳遞參數

核心類

• MapperRegistry:mapper接口和對應的代理工廠的註冊中心;是Configuration的成員變量
• MapperProxyFactory:用於生成mapper接口動態代理的實例對象;保證mapper實例對象是局部變量(爲何不緩存?見文章最後)
• MapperMethod:封裝了Mapper接口中對應的方法信息,易級對應sql語句的信息.MapperMethod對象不記錄任何狀態,因此它能夠再多個代理對象之間共享;sqlCommand:封裝sql語句;MtehodSignature:封裝mapper接口的入參和返回類型

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我的理解

• MapperRegistry的Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>(); key:mapper的Class對象 value:生產這個mapper的動態代理示例的工廠
• MapperProxyFactory:Map<Method, MapperMethod> methodCache key:方法的反射類 value 方法的信息(sql語句,入參,返回值)

相關代碼

public class MapperRegistry {

  private final Configuration config;//config對象，mybatis全局惟一的
  //記錄了mapper接口與對應MapperProxyFactory之間的關係
  private final Map<Class<?>, MapperProxyFactory<?>> knownMappers = new HashMap<>();

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

  @SuppressWarnings("unchecked")
  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);
    }

/**
 * 
 * 用於生成mapper接口動態代理的實例對象；
 * @author Lasse Voss
 */
public class MapperProxyFactory<T> {

  //mapper接口的class對象
  private final Class<T> mapperInterface;
//key是mapper接口中的某個方法的method對象，value是對應的MapperMethod，MapperMethod對象不記錄任何狀態信息，因此它能夠在多個代理對象之間共享
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

public class MapperMethod {
  //從configuration中獲取方法的命名空間.方法名以及SQL語句的類型
  private final SqlCommand command;
  //封裝mapper接口方法的相關信息（入參，返回類型）；
  private final MethodSignature method;

getMapper的過程

public class MapperProxyFactory<T> {

  //mapper接口的class對象
  private final Class<T> mapperInterface;
//key是mapper接口中的某個方法的method對象，value是對應的MapperMethod，MapperMethod對象不記錄任何狀態信息，因此它能夠在多個代理對象之間共享
  private final Map<Method, MapperMethod> methodCache = new ConcurrentHashMap<>();

  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }

  public Class<T> getMapperInterface() {
    return mapperInterface;
  }

  public Map<Method, MapperMethod> getMethodCache() {
    return methodCache;
  }

  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
	//建立實現了mapper接口的動態代理對象
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

  public T newInstance(SqlSession sqlSession) {
	 //每次調用都會建立新的MapperProxy對象
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

}

兩個newInstance方法生成mapper的動態代理對象MapperProxy

public class MapperProxy<T> implements InvocationHandler, Serializable {
.......
@Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      if (Object.class.equals(method.getDeclaringClass())) {//若是是Object自己的方法不加強
        return method.invoke(this, args);
      } else if (isDefaultMethod(method)) {
        return invokeDefaultMethod(proxy, method, args);
      }
    } catch (Throwable t) {
      throw ExceptionUtil.unwrapThrowable(t);
    }
    //從緩存中獲取mapperMethod對象，若是緩存中沒有，則建立一個，並添加到緩存中
    final MapperMethod mapperMethod = cachedMapperMethod(method);
    //調用execute方法執行sql
    return mapperMethod.execute(sqlSession, args);
  }
  
  private MapperMethod cachedMapperMethod(Method method) {
    return methodCache.computeIfAbsent(method, k -> new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
  }

從invoke方法能夠看出 是mapperMethod調用了execute方法

public class MapperMethod {
 ......
  public Object execute(SqlSession sqlSession, Object[] args) {
    Object result;
    //根據sql語句類型以及接口返回的參數選擇調用不一樣的
    switch (command.getType()) {
      case INSERT: {
    	Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.insert(command.getName(), param));
        break;
      }
      case UPDATE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.update(command.getName(), param));
        break;
      }
      case DELETE: {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = rowCountResult(sqlSession.delete(command.getName(), param));
        break;
      }
      case SELECT:
        if (method.returnsVoid() && method.hasResultHandler()) {//返回值爲void
          executeWithResultHandler(sqlSession, args);
          result = null;
        } else if (method.returnsMany()) {//返回值爲集合或者數組
          result = executeForMany(sqlSession, args);
        } else if (method.returnsMap()) {//返回值爲map
          result = executeForMap(sqlSession, args);
        } else if (method.returnsCursor()) {//返回值爲遊標
          result = executeForCursor(sqlSession, args);
        } else {//處理返回爲單一對象的狀況
          //經過參數解析器解析解析參數
          Object param = method.convertArgsToSqlCommandParam(args);
          result = sqlSession.selectOne(command.getName(), param);
          if (method.returnsOptional() &&
              (result == null || !method.getReturnType().equals(result.getClass()))) {
            result = OptionalUtil.ofNullable(result);
          }
        }
        break;
      case FLUSH:
        result = sqlSession.flushStatements();
        break;
      default:
        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;
  }

重點看 result = sqlSession.selectOne(command.getName(), param); 這裏使用了ibatis的用法

問題解答

mapperProxy作查詢依賴的仍是sqlSession.按照如今的代碼結構來講,若是緩存了mapperProxy,當sqlSession失效,mapperProxy也就失效了. 若是修改代結構,那麼sqlSession就要做爲查詢參數

// 本來
TUser user = mapper.selectByPrimaryKey(2);
// 修改後的寫法
TUser user = mapper.selectByPrimaryKey(sqslSession,2);