1.mybatis拦截器介绍

拦截器可在mybatis进行sql底层处理的时候执行额外的逻辑，最常见的就是分页逻辑、对结果集进行处理过滤敏感信息等。

  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }

  public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,
      ResultHandler resultHandler, BoundSql boundSql) {
    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }

  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {
    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

  public Executor newExecutor(Transaction transaction) {
    return newExecutor(transaction, defaultExecutorType);
  }

  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
    executorType = executorType == null ? defaultExecutorType : executorType;
    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;
    Executor executor;
    if (ExecutorType.BATCH == executorType) {
      executor = new BatchExecutor(this, transaction);
    } else if (ExecutorType.REUSE == executorType) {
      executor = new ReuseExecutor(this, transaction);
    } else {
      executor = new SimpleExecutor(this, transaction);
    }
    if (cacheEnabled) {
      executor = new CachingExecutor(executor);
    }
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }

从上面的代码可以看到mybatis支持的拦截类型只有四种（按拦截顺序）

1.Executor 执行器接口

2.StatementHandler sql构建处理器

3.ParameterHandler 参数处理器

4.ResultSetHandler 结果集处理器

2.拦截器原理

public class InterceptorChain {

  private final List<Interceptor> interceptors = new ArrayList<>();

  // 遍历定义的拦截器，对拦截的对象进行包装
  public Object pluginAll(Object target) {
    for (Interceptor interceptor : interceptors) {
      target = interceptor.plugin(target);
    }
    return target;
  }

  public void addInterceptor(Interceptor interceptor) {
    interceptors.add(interceptor);
  }

  public List<Interceptor> getInterceptors() {
    return Collections.unmodifiableList(interceptors);
  }

}


#Interceptor
public interface Interceptor {

  Object intercept(Invocation invocation) throws Throwable;

  default Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }

  default void setProperties(Properties properties) {
    // NOP
  }

}

mybatis拦截器本质上使用了jdk动态代理，interceptorChain拦截器链中存储了用户定义的拦截器，会遍历进行对目标对象代理包装。

用户自定义拦截器类需要实现Interceptor接口，以及实现intercept方法，plugin和setProperties方法可重写，plugin方法一般不会改动，该方法调用了Plugin的静态方法wrap实现了对目标对象的代理

public class Plugin implements InvocationHandler {

  // 拦截目标对象
  private final Object target;

  // 拦截器对象-执行逻辑
  private final Interceptor interceptor;

  // 拦截接口和拦截方法的映射
  private final Map<Class<?>, Set<Method>> signatureMap;

  private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {
    this.target = target;
    this.interceptor = interceptor;
    this.signatureMap = signatureMap;
  }

  // 获取jdk代理对象
  public static Object wrap(Object target, Interceptor interceptor) {
    // 存储拦截接口和拦截方法的映射
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    // 获取拦截目标对象实现的接口，若为空则不代理
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
      return Proxy.newProxyInstance(
          type.getClassLoader(),
          interfaces,
          new Plugin(target, interceptor, signatureMap));
    }
    return target;
  }

  @Override
  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    try {
      // 获取需要拦截的方法集合，若不存在则使用目标对象执行
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      if (methods != null && methods.contains(method)) {
        // Invocation存储了目标对象、拦截方法以及方法参数
        return interceptor.intercept(new Invocation(target, method, args));
      }
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }

  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {
    // 获取Intercepts注解值不能为空
    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);
    // issue #251
    if (interceptsAnnotation == null) {
      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());
    }
    Signature[] sigs = interceptsAnnotation.value();
    // key 拦截的类型
    Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
    for (Signature sig : sigs) {
      Set<Method> methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
      try {
        // 获取拦截的方法
        Method method = sig.type().getMethod(sig.method(), sig.args());
        methods.add(method);
      } catch (NoSuchMethodException e) {
        throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);
      }
    }
    return signatureMap;
  }

  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {
    Set<Class<?>> interfaces = new HashSet<>();
    while (type != null) {
      for (Class<?> c : type.getInterfaces()) {
        if (signatureMap.containsKey(c)) {
          interfaces.add(c);
        }
      }
      type = type.getSuperclass();
    }
    return interfaces.toArray(new Class<?>[interfaces.size()]);
  }

}

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface Intercepts {
  /**
   * Returns method signatures to intercept.
   *
   * @return method signatures
   */
  Signature[] value();
}

@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target({})
public @interface Signature {
  /**
   * Returns the java type.
   *
   * @return the java type
   */
  Class<?> type();

  /**
   * Returns the method name.
   *
   * @return the method name
   */
  String method();

  /**
   * Returns java types for method argument.
   * @return java types for method argument
   */
  Class<?>[] args();
}

可以看到，当被拦截的方法被执行时主要调用自定义拦截器的intercept方法，把拦截对象、方法以及方法参数封装成Invocation对象传递过去。

在getSignatureMap方法中可以看到，自定义的拦截器类上需要添加Intercepts注解并且Signature需要有值，Signature注解中的type为需要拦截对象的接口（Executor.class/StatementHandler/ParameterHandler/ResultSetHandler），method为需要拦截的方法的方法名，args为拦截方法的方法参数类型。

3.参考例子

接下来举一个拦截器实现对结果集下划线转驼峰的例子来简要说明

/**
 * @author dxu2
 * @date 2022/7/14
 * map结果转驼峰
 */
@Intercepts(value = {@Signature(type = ResultSetHandler.class, method = "handleResultSets", args = {Statement.class})})
public class MyInterceptor implements Interceptor {

  @SuppressWarnings("unchecked")
  @Override
  public Object intercept(Invocation invocation) throws Throwable {
    // 调用目标方法
    List<Object> result = (List<Object>) invocation.proceed();
    for (Object o : result) {
      if (o instanceof Map) {
        processMap((Map<String, Object>) o);
      } else {
        break;
      }
    }
    return result;
  }

  @Override
  public Object plugin(Object target) {
    return Plugin.wrap(target, this);
  }

  @Override
  public void setProperties(Properties properties) {

  }


  private void processMap(Map<String, Object> map) {
    Set<String> keySet = new HashSet<>(map.keySet());
    for (String key : keySet) {
      if ((key.charAt(0) >= 'A' && key.charAt(0) <= 'Z') || key.indexOf("_") > 0) {
        Object value = map.get(key);
        map.remove(key);
        map.put(camel(key), value);
      }
    }
  }

  // 下划线转驼峰
  private String camel(String fieldName) {
    StringBuffer stringBuffer = new StringBuffer();
    boolean flag = false;
    for (int i = 0; i < fieldName.length(); i++) {
      if (fieldName.charAt(i) == '_') {
        if (stringBuffer.length() > 0) {
          flag = true;
        }
      } else {
        if (flag) {
          stringBuffer.append(Character.toUpperCase(fieldName.charAt(i)));
          flag = false;
        } else {
          stringBuffer.append(Character.toLowerCase(fieldName.charAt(i)));
        }
      }
    }
    return stringBuffer.toString();
  }
}

这个例子拦截的是ResultSetHandler的handleResultSets方法，这个方法是用来对结果集处理的，看intercept方法首先调用了目标对象的方法接着强转为List<Object>类型，这里为什么可以强转呢，因为我们可以看到handleResultSets方法定义<E> List<E> handleResultSets(Statement stmt) throws SQLException; 返回的是List类型，然后遍历列表，若元素是map类型的再进行处理把key值转化为驼峰形式重新put到map中。

最后不要忘了把自定义的拦截器添加到配置中，这边是使用xml配置的，添加完后接着运行测试代码，可以看到列user_id已经转换成驼峰形式了。

<plugins>
  <plugin interceptor="org.apache.ibatis.study.interceptor.MyInterceptor">
  </plugin>
</plugins>

#mapper接口
List<Map> selectAllUsers();

#mapper.xml
<select id="selectAllUsers" resultType="map">
    select user_id, username, password, nickname
    from user
</select>
  
      
#java测试类
public class Test {

  public static void main(String[] args) throws IOException {

    try (InputStream inputStream = Resources.getResourceAsStream("mybatis-config.xml")) {
      // 构建session工厂 DefaultSqlSessionFactory
      SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
      SqlSession sqlSession = sqlSessionFactory.openSession();
      UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
      System.out.println(userMapper.selectAllUsers());
    }
  }

}