网络异常，图片无法展示

我们在使用 Mybatis 的时候、只需要定义一个 Mapper xml 文件和一个对应的 Mapper 接口、并需要实现该接口、即可在程序中使用该 Mapper 接口、调用里面的方法对其进行查询(当然 xml 中的 namespace 要关联接口)。

那么这个功能是怎么实现的呢？

MapperRegistry

我们知道在使用 Mybatis 的时候都会去创建一个 Configuration 类、而在这个类中、则会创建一个 MapperRegistry

public class MapperRegistry {
  private final Configuration config;
  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);
    }
  }
  public <T> boolean hasMapper(Class<T> type) {
    return knownMappers.containsKey(type);
  }
  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }
  public Collection<Class<?>> getMappers() {
    return Collections.unmodifiableCollection(knownMappers.keySet());
  }
  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }
}
复制代码

我们知道注册一个 Mapper 类会加入到 knownMappers 这个 Map 里面、key 为对应的 Mapper 类型、value 为 MapperProxyFactory

在 getMapper 方法中我们看到 mapperProxyFactory.newInstance(sqlSession);

MapperProxyFactory

public class MapperProxyFactory<T> {
  private final Class<T> mapperInterface;
  private final Map<Method, MapperMethodInvoker> methodCache = new ConcurrentHashMap<>();
  public MapperProxyFactory(Class<T> mapperInterface) {
    this.mapperInterface = mapperInterface;
  }
  public Class<T> getMapperInterface() {
    return mapperInterface;
  }
  public Map<Method, MapperMethodInvoker> getMethodCache() {
    return methodCache;
  }
  @SuppressWarnings("unchecked")
  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }
  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }
}
复制代码

我们看到这里创建了一个名为 MapperProxy 的代理类、哦吼、这里也是代理模式

MapperProxy

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
  try {
    if (Object.class.equals(method.getDeclaringClass())) {
      return method.invoke(this, args);
    } else {
      return cachedInvoker(method).invoke(proxy, method, args, sqlSession);
    }
  } catch (Throwable t) {
    throw ExceptionUtil.unwrapThrowable(t);
  }
}
复制代码

private MapperMethodInvoker cachedInvoker(Method method) throws Throwable {
  try {
    // A workaround for https://bugs.openjdk.java.net/browse/JDK-8161372
    // It should be removed once the fix is backported to Java 8 or
    // MyBatis drops Java 8 support. See gh-1929
    MapperMethodInvoker invoker = methodCache.get(method);
    if (invoker != null) {
      return invoker;
    }
    return methodCache.computeIfAbsent(method, m -> {
      if (m.isDefault()) {
        try {
          if (privateLookupInMethod == null) {
            return new DefaultMethodInvoker(getMethodHandleJava8(method));
          } else {
            return new DefaultMethodInvoker(getMethodHandleJava9(method));
          }
        } catch (IllegalAccessException | InstantiationException | InvocationTargetException
            | NoSuchMethodException e) {
          throw new RuntimeException(e);
        }
      } else {
        return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
      }
    });
  } catch (RuntimeException re) {
    Throwable cause = re.getCause();
    throw cause == null ? re : cause;
  }
}
复制代码

正常来说我们创建的接口都不使用 default 修饰、如果使用了、那就代表其有方法的实现、那么就直接调用该方法、而不会再走 Mybatis 的逻辑。如果走正常逻辑的话就会创建 MapperMethod

MapperMethod

public class MapperMethod {
  private final SqlCommand command;
  private final MethodSignature method;
  public MapperMethod(Class<?> mapperInterface, Method method, Configuration config) {
    this.command = new SqlCommand(config, mapperInterface, method);
    this.method = new MethodSignature(config, mapperInterface, method);
  }
.......
}
复制代码

我们首先来看看 SqlCommand

public static class SqlCommand {
  private final String name;
  private final SqlCommandType type;
  public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
    final String methodName = method.getName();
    final Class<?> declaringClass = method.getDeclaringClass();
    MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
        configuration);
    if (ms == null) {
      if (method.getAnnotation(Flush.class) != null) {
        name = null;
        type = SqlCommandType.FLUSH;
      } else {
        throw new BindingException("Invalid bound statement (not found): "
            + mapperInterface.getName() + "." + methodName);
      }
    } else {
      name = ms.getId();
      type = ms.getSqlCommandType();
      if (type == SqlCommandType.UNKNOWN) {
        throw new BindingException("Unknown execution method for: " + name);
      }
    }
  }
......
  private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
      Class<?> declaringClass, Configuration configuration) {
    String statementId = mapperInterface.getName() + "." + methodName;
    if (configuration.hasStatement(statementId)) {
      return configuration.getMappedStatement(statementId);
    } else if (mapperInterface.equals(declaringClass)) {
      return null;
    }
    for (Class<?> superInterface : mapperInterface.getInterfaces()) {
      if (declaringClass.isAssignableFrom(superInterface)) {
        MappedStatement ms = resolveMappedStatement(superInterface, methodName,
            declaringClass, configuration);
        if (ms != null) {
          return ms;
        }
      }
    }
    return null;
  }
}
复制代码

非常简单的逻辑、根据被代理的接口的名称和调用方法名称、组装成一个 statementId(namespace + sqlId)、去 Configuration 对象中查找是否存在对应的 MappedStatement

SqlCommandType 则是

public enum SqlCommandType {
  UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH
}
复制代码

再来看看 MethodSignature

public static class MethodSignature {
  private final boolean returnsMany;
  private final boolean returnsMap;
  private final boolean returnsVoid;
  private final boolean returnsCursor;
  private final boolean returnsOptional;
  private final Class<?> returnType;
  private final String mapKey;
  private final Integer resultHandlerIndex;
  private final Integer rowBoundsIndex;
  private final ParamNameResolver paramNameResolver;
  ......
复制代码

主要是对方法进行一系列的解释、返回值啊、对参数的解释

再回到 MapperMethod 类中、我们知道它的 invoke 方法被 MapperProxy 调用

public Object execute(SqlSession sqlSession, Object[] args) {
  Object result;
  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()) {
        executeWithResultHandler(sqlSession, args);
        result = null;
      } else if (method.returnsMany()) {
        result = executeForMany(sqlSession, args);
      } else if (method.returnsMap()) {
        result = executeForMap(sqlSession, args);
      } else if (method.returnsCursor()) {
        result = executeForCursor(sqlSession, args);
      } else {
        Object param = method.convertArgsToSqlCommandParam(args);
        result = sqlSession.selectOne(command.getName(), param);
        if (method.returnsOptional()
            && (result == null || !method.getReturnType().equals(result.getClass()))) {
          result = Optional.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;
}
复制代码

如果是 Insert、Update、Delete 的话、那么则先进行 convertArgsToSqlCommandParam 然后则调用 rowCountResult 处理参数

public Object convertArgsToSqlCommandParam(Object[] args) {
  return paramNameResolver.getNamedParams(args);
}
public Object getNamedParams(Object[] args) {
  final int paramCount = names.size();
  if (args == null || paramCount == 0) {
    return null;
  } else if (!hasParamAnnotation && paramCount == 1) {
    Object value = args[names.firstKey()];
    return wrapToMapIfCollection(value, useActualParamName ? names.get(0) : null);
  } else {
    final Map<String, Object> param = new ParamMap<>();
    int i = 0;
    for (Map.Entry<Integer, String> entry : names.entrySet()) {
      param.put(entry.getValue(), args[entry.getKey()]);
      // add generic param names (param1, param2, ...)
      final String genericParamName = GENERIC_NAME_PREFIX + (i + 1);
      // ensure not to overwrite parameter named with @Param
      if (!names.containsValue(genericParamName)) {
        param.put(genericParamName, args[entry.getKey()]);
      }
      i++;
    }
    return param;
  }
}
复制代码

这里主要是将参数放进到 Map 中、这里还额外增加了一些参数、比如我们经常在 mapper 文件中使用的 param1 、param2....

就是在这里实现的

wrapToMapIfCollection 则是对集合数组进行处理的

public static Object wrapToMapIfCollection(Object object, String actualParamName) {
  if (object instanceof Collection) {
    ParamMap<Object> map = new ParamMap<>();
    map.put("collection", object);
    if (object instanceof List) {
      map.put("list", object);
    }
    Optional.ofNullable(actualParamName).ifPresent(name -> map.put(name, object));
    return map;
  } else if (object != null && object.getClass().isArray()) {
    ParamMap<Object> map = new ParamMap<>();
    map.put("array", object);
    Optional.ofNullable(actualParamName).ifPresent(name -> map.put(name, object));
    return map;
  }
  return object;
}
复制代码

再回到 rowCountResult、则是对返回值进行处理的、逻辑比较简单。比如说返回布尔值的、则判断其影响值是否大于 0

private Object rowCountResult(int rowCount) {
  final Object result;
  if (method.returnsVoid()) {
    result = null;
  } else if (Integer.class.equals(method.getReturnType()) || Integer.TYPE.equals(method.getReturnType())) {
    result = rowCount;
  } else if (Long.class.equals(method.getReturnType()) || Long.TYPE.equals(method.getReturnType())) {
    result = (long) rowCount;
  } else if (Boolean.class.equals(method.getReturnType()) || Boolean.TYPE.equals(method.getReturnType())) {
    result = rowCount > 0;
  } else {
    throw new BindingException("Mapper method '" + command.getName() + "' has an unsupported return type: " + method.getReturnType());
  }
  return result;
}
复制代码

至于如果 sql 类型是 Select 的话

if (method.returnsVoid() && method.hasResultHandler()) {
    executeWithResultHandler(sqlSession, args);
    result = null;
  } else if (method.returnsMany()) {
    result = executeForMany(sqlSession, args);
  } else if (method.returnsMap()) {
    result = executeForMap(sqlSession, args);
  } else if (method.returnsCursor()) {
    result = executeForCursor(sqlSession, args);
  } else {
    Object param = method.convertArgsToSqlCommandParam(args);
    result = sqlSession.selectOne(command.getName(), param);
    if (method.returnsOptional()
        && (result == null || !method.getReturnType().equals(result.getClass()))) {
      result = Optional.ofNullable(result);
    }
  }
  break;
复制代码

针对存在 ResultHandler 的、判断是否存在 RowBounds

private void executeWithResultHandler(SqlSession sqlSession, Object[] args) {
  MappedStatement ms = sqlSession.getConfiguration().getMappedStatement(command.getName());
  if (!StatementType.CALLABLE.equals(ms.getStatementType())
      && void.class.equals(ms.getResultMaps().get(0).getType())) {
    throw new BindingException("method " + command.getName()
        + " needs either a @ResultMap annotation, a @ResultType annotation,"
        + " or a resultType attribute in XML so a ResultHandler can be used as a parameter.");
  }
  Object param = method.convertArgsToSqlCommandParam(args);
  if (method.hasRowBounds()) {
    RowBounds rowBounds = method.extractRowBounds(args);
    sqlSession.select(command.getName(), param, rowBounds, method.extractResultHandler(args));
  } else {
    sqlSession.select(command.getName(), param, method.extractResultHandler(args));
  }
}
复制代码

针对返回集合的

private <E> Object executeForMany(SqlSession sqlSession, Object[] args) {
  List<E> result;
  Object param = method.convertArgsToSqlCommandParam(args);
  if (method.hasRowBounds()) {
    RowBounds rowBounds = method.extractRowBounds(args);
    result = sqlSession.selectList(command.getName(), param, rowBounds);
  } else {
    result = sqlSession.selectList(command.getName(), param);
  }
  // issue #510 Collections & arrays support
  if (!method.getReturnType().isAssignableFrom(result.getClass())) {
    if (method.getReturnType().isArray()) {
      return convertToArray(result);
    } else {
      return convertToDeclaredCollection(sqlSession.getConfiguration(), result);
    }
  }
  return result;
}
复制代码

针对返回 Map 的

private <K, V> Map<K, V> executeForMap(SqlSession sqlSession, Object[] args) {
  Map<K, V> result;
  Object param = method.convertArgsToSqlCommandParam(args);
  if (method.hasRowBounds()) {
    RowBounds rowBounds = method.extractRowBounds(args);
    result = sqlSession.selectMap(command.getName(), param, method.getMapKey(), rowBounds);
  } else {
    result = sqlSession.selectMap(command.getName(), param, method.getMapKey());
  }
  return result;
}
复制代码

针对返回游标的

private <T> Cursor<T> executeForCursor(SqlSession sqlSession, Object[] args) {
  Cursor<T> result;
  Object param = method.convertArgsToSqlCommandParam(args);
  if (method.hasRowBounds()) {
    RowBounds rowBounds = method.extractRowBounds(args);
    result = sqlSession.selectCursor(command.getName(), param, rowBounds);
  } else {
    result = sqlSession.selectCursor(command.getName(), param);
  }
  return result;
}
复制代码

针对 selectOne 的

Object param = method.convertArgsToSqlCommandParam(args);
result = sqlSession.selectOne(command.getName(), param);
if (method.returnsOptional()
    && (result == null || !method.getReturnType().equals(result.getClass()))) {
  result = Optional.ofNullable(result);
}
复制代码

可以看到 MapperMethod 也是依赖 Mybatis 的 SqlSession 去帮我们执行而已、并没有太高端的技术。

Mapper 文件与接口的映射

MapperRegistry

MapperProxyFactory

MapperProxy

MapperMethod

热门文章

最新文章

相关电子书

相关实验场景

热门

活动广场

任务中心

开发者评测

高校计划

乘风者计划

训练营

阿里云MVP

话题

直播

下载

镜像站

技术资料

插件

Mapper 文件与接口的映射

MapperRegistry

MapperProxyFactory

MapperProxy

MapperMethod

热门文章

最新文章

相关电子书

相关实验场景