mybatis源码探索之代理封装阶段

曾经终败给现在 2023-09-28 20:38 8阅读 0赞

## 版本 ##

mybatis版本3.5.7

## 背景 ##

我们在使用mybatis进行数据库crud时通常都是通过\*Mapper.java对象的方法进行操作的，那sql语句怎么和对应的方法绑定的，mybatis是怎么知道返回结果是多个、单个或者是其他的，\*Mapper是个接口，它是怎么实例化调用方法的。这些一系列问题背后怎么实现的，都是跟binding模块相关的。

## 实现步骤 ##

要实现以上操作有三个步骤：  
1、根据sql语句类型和参数选择调用不同的方法  
2、通过找到命名空间和方法名  
3、传递参数  
完成以上三个主要步骤就绑定成功了，要完成以上步骤需要下面几个核心类。

## 核心类 ##

### MapperRegistry ###

MapperRegistry 是mapper 接口和对应的代理对象工厂的注册中心。MapperRegistry 内部维护着一个Map对象，key是mapper接口的Class对象，value是MapperProxyFactory。

当需要获取mapper接口对象时就需要调用SqlSession\#getMapper方法，getMapper是从configuration对象的MapperRegistry获取一个代理对象工厂。

SqlSession\#getMapper：

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

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);
        }
      }
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在配置加载阶段会往Configuration对象的属性mapperRegistry中添加mapper接口和代理工厂映射， 添加入口是Configuration\#addMappers和Configuration\#addMapper，前者是把某个包下的mapper接口添加进去，后者只是添加单独一个mapper接口，最终都是调用MapperRegistry\#addMapper。 MapperRegistry\#addMapper

/**
       * 把mapper 接口添加到 knownMappers 中注册中心
       * @param type
       * @param <T>
       */
      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 {
            // 建立mapper 和 MapperProxyFactory 的连接
            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.
    
            // 解析接口上的注解信息并添加到configuration对象中
            MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
            parser.parse();
    
            loadCompleted = true;
          } finally {
            if (!loadCompleted) {
              knownMappers.remove(type);
            }
          }
        }
      }
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### MapperProxyFactory ###

MapperProxyFactory是MapperProxy的工厂类，专门用来生产MapperProxy的。 MapperProxyFactory:

public class MapperProxyFactory<T> {
    
      /**
       * mapper接口的Class对象
       */
      private final Class<T> mapperInterface;
      /**
       * key: mapper接口的一个方法的Method对象
       * value: 对应的MapperMethodInvoker对象
       */
      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;
      }
    
      /**
       * 创建mapperInterface接口的实现类的代理对象
       * @param mapperProxy
       * @return
       */
      @SuppressWarnings("unchecked")
      protected T newInstance(MapperProxy<T> mapperProxy) {
        T proxyInstance = (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[]{mapperInterface}, mapperProxy);
        return proxyInstance;
      }
    
      public T newInstance(SqlSession sqlSession) {
        //创建MapperProxy对象
        final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
        return newInstance(mapperProxy);
      }
    }
    
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mapper接口的代理类形如org.apache.ibatis.binding.MapperProxy@3d5c822d

![format_png][]

### MapperProxy ###

MapperProxy是在MapperProxyFactory\#newInstance(SqlSession sqlSession)方法中生成实例对象的。 MapperProxy实现了InvocationHandler，使用的jdk的动态代理，对mapper接口进行了增强。 既然MapperProxy实现了InvocationHandler接口，那就从invoke开始看起。

@Override
      public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
          //如果是Object类声明的方法
          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);
        }
      }
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接下来看cachedInvoker方法：

/**
       * 缓存method对应的MapperMethodInvoker对象
       * @param method
       * @return
       * @throws Throwable
       */
      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 -> {
            //如果是接口的default方法生成DefaultMethodInvoker
            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 {
              //生成非default方法MethodInvoker
              return new PlainMethodInvoker(new MapperMethod(mapperInterface, method, sqlSession.getConfiguration()));
            }
          });
        } catch (RuntimeException re) {
          Throwable cause = re.getCause();
          throw cause == null ? re : cause;
        }
      }
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#### MapperMethodInvoker ####

MapperMethodInvoker是MapperProxy的内部类。 MapperMethodInvoker对于接口中非default方法来说它是MapperMethod的调用者，它的实现PlainMethodInvoker里面维护着一个MapperMethod，真正的执行者还是MapperMethod。 DefaultMethodInvoker内部就直接调用了default方法。重点还是要看PlainMethodInvoker PlainMethodInvoker

/**
       * 接口中非default方法的调用器
       */
      private static class PlainMethodInvoker implements MapperMethodInvoker {
        private final MapperMethod mapperMethod;
    
        public PlainMethodInvoker(MapperMethod mapperMethod) {
          super();
          this.mapperMethod = mapperMethod;
        }
    
        @Override
        public Object invoke(Object proxy, Method method, Object[] args, SqlSession sqlSession) throws Throwable {
          //调用mapperMethod对象执行sql
          return mapperMethod.execute(sqlSession, args);
        }
      }
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PlainMethodInvoker的invoke方法内部也没有对参数做什么别的逻辑操作而是直接转发给MapperMethod\#execute方法，让MapperMethod来执行，MapperMethod也是最后真正的执行者。

### MapperMethod ###

MapperMethod才是binding模块真正的执行者，上述的三个步骤也都是在MapperMethod中实现的。 三个步骤：  
1、根据sql语句类型和参数选择调用不同的方法  
2、找到命名空间和方法名  
3、传递参数

#### 属性 ####

MapperMethod内部有两个属性，SqlCommand和MethodSignature对象，它们也都是MapperMethod的内部类。

//mapper接口中方法的命名空间和sql类型
      private final SqlCommand command;
      //mapper接口中的方法签名，包括参数、返回类型等信息
      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);
      }
     
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SqlCommand

SqlCommand的作用是获取sql命名空间和sql语句的类型。 SqlCommand源码，删除了一些getter方法。

public static class SqlCommand {
        //sql的命名空间
        private final String name;
        //sql语句类型，UNKNOWN, INSERT, UPDATE, DELETE, SELECT, FLUSH
        private final SqlCommandType type;
    
        public SqlCommand(Configuration configuration, Class<?> mapperInterface, Method method) {
          final String methodName = method.getName();
          final Class<?> declaringClass = method.getDeclaringClass();
          //从configuration对象获取MappedStatement对象
          MappedStatement ms = resolveMappedStatement(mapperInterface, methodName, declaringClass,
              configuration);
          if (ms == null) {
            //判断没有Flush注解
            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);
            }
          }
        }
    
        //从configuration对象获取MappedStatement对象
        private MappedStatement resolveMappedStatement(Class<?> mapperInterface, String methodName,
            Class<?> declaringClass, Configuration configuration) {
            //接口全限定名假方法名组成statementId,如 mapperClasses.UserInfoMapper.selectByPrimaryKey
          String statementId = mapperInterface.getName() + "." + methodName;
          if (configuration.hasStatement(statementId)) {
            return configuration.getMappedStatement(statementId);
          } else if (mapperInterface.equals(declaringClass)) {
            //如果是方法的所在的接口
            return null;
          }
          for (Class<?> superInterface : mapperInterface.getInterfaces()) {
            //递归查找直到找到对应MappedStatement对象
            if (declaringClass.isAssignableFrom(superInterface)) {
              MappedStatement ms = resolveMappedStatement(superInterface, methodName,
                  declaringClass, configuration);
              if (ms != null) {
                return ms;
              }
            }
          }
          return null;
        }
      }
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MethodSignature

MethodSignature 作用是解析方法的参数、返回值类型等信息。 MethodSignature源码，删除了一些getter方法。

public static class MethodSignature {
        //返回结果是集合或者数组
        private final boolean returnsMany;
        //返回结果是map对象
        private final boolean returnsMap;
        private final boolean returnsVoid;
        private final boolean returnsCursor;
        private final boolean returnsOptional;
        //返回类型
        private final Class<?> returnType;
        //@MapKey注解的value，也就是属性名或者列名
        private final String mapKey;
        private final Integer resultHandlerIndex;
        private final Integer rowBoundsIndex;
        //方法的参数解析器，将参数转为map
        private final ParamNameResolver paramNameResolver;
    
        public MethodSignature(Configuration configuration, Class<?> mapperInterface, Method method) {
          //获取方法返回值类型
          Type resolvedReturnType = TypeParameterResolver.resolveReturnType(method, mapperInterface);
          if (resolvedReturnType instanceof Class<?>) {
            this.returnType = (Class<?>) resolvedReturnType;
          } else if (resolvedReturnType instanceof ParameterizedType) {
            this.returnType = (Class<?>) ((ParameterizedType) resolvedReturnType).getRawType();
          } else {
            this.returnType = method.getReturnType();
          }
          this.returnsVoid = void.class.equals(this.returnType);
          this.returnsMany = configuration.getObjectFactory().isCollection(this.returnType) || this.returnType.isArray();
          this.returnsCursor = Cursor.class.equals(this.returnType);
          this.returnsOptional = Optional.class.equals(this.returnType);
          this.mapKey = getMapKey(method);
          this.returnsMap = this.mapKey != null;
          this.rowBoundsIndex = getUniqueParamIndex(method, RowBounds.class);
          this.resultHandlerIndex = getUniqueParamIndex(method, ResultHandler.class);
          this.paramNameResolver = new ParamNameResolver(configuration, method);
        }
    
        public Object convertArgsToSqlCommandParam(Object[] args) {
          return paramNameResolver.getNamedParams(args);
        }
    
        public boolean hasRowBounds() {
          return rowBoundsIndex != null;
        }
    
        public RowBounds extractRowBounds(Object[] args) {
          return hasRowBounds() ? (RowBounds) args[rowBoundsIndex] : null;
        }
    
        public boolean hasResultHandler() {
          return resultHandlerIndex != null;
        }
    
        public ResultHandler extractResultHandler(Object[] args) {
          return hasResultHandler() ? (ResultHandler) args[resultHandlerIndex] : null;
        }
      
        /**
         * return whether return type is {@code java.util.Optional}.
         *
         * @return return {@code true}, if return type is {@code java.util.Optional}
         * @since 3.5.0
         */
        public boolean returnsOptional() {
          return returnsOptional;
        }
    
        /**
         * 获取参数唯一下标
         * @param method
         * @param paramType
         * @return
         */
        private Integer getUniqueParamIndex(Method method, Class<?> paramType) {
          Integer index = null;
          final Class<?>[] argTypes = method.getParameterTypes();
          for (int i = 0; i < argTypes.length; i++) {
            //判断paramType是否是参数的父类或者superinterface ，true返回下标
            if (paramType.isAssignableFrom(argTypes[i])) {
              if (index == null) {
                index = i;
              } else {
                throw new BindingException(method.getName() + " cannot have multiple " + paramType.getSimpleName() + " parameters");
              }
            }
          }
          return index;
        }
    
        /**
         * 获取MapKey注解value
         * @param method
         * @return
         */
        private String getMapKey(Method method) {
          String mapKey = null;
          if (Map.class.isAssignableFrom(method.getReturnType())) {
            final MapKey mapKeyAnnotation = method.getAnnotation(MapKey.class);
            if (mapKeyAnnotation != null) {
              mapKey = mapKeyAnnotation.value();
            }
          }
          return mapKey;
        }
      }
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#### 方法 ####

MapperMethod核心方法是MapperMethod\#execute方法

execute

execute方法中的executeFor\*方法最终都会根据返回值是集合、void、map等调用sqlSession对象中相应的方法。

public Object execute(SqlSession sqlSession, Object[] args) {
        Object result;
        //步骤一：根据sql语句类型和参数选择调用不同的方法
        switch (command.getType()) {
          case INSERT: {
            //步骤三：将参数转为map
            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()) { //返回类型是void
              executeWithResultHandler(sqlSession, args);
              result = null;
            } else if (method.returnsMany()) { //返回类型是集合或者数组
              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;
      }
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convertArgsToSqlCommandParam

convertArgsToSqlCommandParam是代理封装阶段的最后一个步骤，把参数转为map然后进行传递。它是MethodSignature的对象方法，最终调用的是ParamNameResolver\#getNamedParams方法，

/**
       * <p>
       * A single non-special parameter is returned without a name.
       * Multiple parameters are named using the naming rule.
       * In addition to the default names, this method also adds the generic names (param1, param2,
       * ...).
       * </p>
       *
       * @param args
       *          the args
       * @return the named params
       */
      public Object getNamedParams(Object[] args) {
        final int paramCount = names.size();
        if (args == null || paramCount == 0) {
          return null;
        } else if (!hasParamAnnotation && paramCount == 1) {
          //没有@Param注解并且是单个参数，直接返回参数值
          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()]);
            // 生成key是GENERIC_NAME_PREFIX前缀的参数 (param1, param2, ...)，value是参数的entry.getValue()
            final String genericParamName = GENERIC_NAME_PREFIX + (i + 1);
            // 确保不覆盖以@Param命名的参数
            if (!names.containsValue(genericParamName)) {
              param.put(genericParamName, args[entry.getKey()]);
            }
            i++;
          }
          return param;
        }
      }
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## 总结 ##

代理封装阶段三个步骤对应的方法

1、根据sql语句类型和参数选择调用不同的方法 MapperMethod\#execute中根据sql语句类型和参数选择调用不同的方法

2、找到命名空间和方法名 SqlCommand\#resolveMappedStatement中根据方法名和接口获取MappedStatement对象，从而获取到命名空间

3、传递参数 MethodSignature\#convertArgsToSqlCommandParam方法调用ParamNameResolver\#getNamedParams将参数转为map进行传递

能力一般，水平有限，如有错误，请多指出。

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[format_png]: https://img-blog.csdnimg.cn/img_convert/b2ab349e5c42468d8a96f9d95d22a15a.webp?x-oss-process=image/format,png
[0ce5db0357694c26add5a15aafdd5e70.jpeg]: https://img-blog.csdnimg.cn/0ce5db0357694c26add5a15aafdd5e70.jpeg