Mybatis 攔截器

1.mybatis攔截器介紹

攔截器可在mybatis進行sql底層處理的時候執行額外的邏輯，最常見的就是分頁邏輯、對結果集進行處理過濾敏感資訊等。

  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;
  }

從上面的程式碼可以看到mybatis支援的攔截型別只有四種（按攔截順序）

1.Executor 執行器介面

2.StatementHandler sql構建處理器

3.ParameterHandler 引數處理器

4.ResultSetHandler 結果集處理器

 

2.攔截器原理

public class InterceptorChain {

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

  // 遍歷定義的攔截器，對攔截的物件進行包裝
  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攔截器本質上使用了jdk動態代理，interceptorChain攔截器鏈中儲存了使用者定義的攔截器，會遍歷進行對目標物件代理包裝。

使用者自定義攔截器類需要實現Interceptor介面，以及實現intercept方法，plugin和setProperties方法可重寫，plugin方法一般不會改動，該方法呼叫了Plugin的靜態方法wrap實現了對目標物件的代理

public class Plugin implements InvocationHandler {

  // 攔截目標物件
  private final Object target;

  // 攔截器物件-執行邏輯
  private final Interceptor interceptor;

  // 攔截介面和攔截方法的對映
  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;
  }

  // 獲取jdk代理物件
  public static Object wrap(Object target, Interceptor interceptor) {
    // 儲存攔截介面和攔截方法的對映
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    Class<?> type = target.getClass();
    // 獲取攔截目標物件實現的介面，若為空則不代理
    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 {
      // 獲取需要攔截的方法集合，若不存在則使用目標物件執行
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      if (methods != null && methods.contains(method)) {
        // Invocation儲存了目標物件、攔截方法以及方法引數
        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) {
    // 獲取Intercepts註解值不能為空
    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 攔截的型別
    Map<Class<?>, Set<Method>> signatureMap = new HashMap<>();
    for (Signature sig : sigs) {
      Set<Method> methods = signatureMap.computeIfAbsent(sig.type(), k -> new HashSet<>());
      try {
        // 獲取攔截的方法
        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，當被攔截的方法被執行時主要呼叫自定義攔截器的intercept方法，把攔截物件、方法以及方法引數封裝成Invocation物件傳遞過去。

在getSignatureMap方法中可以看到，自定義的攔截器類上需要新增Intercepts註解並且Signature需要有值，Signature註解中的type為需要攔截物件的介面（Executor.class/StatementHandler/ParameterHandler/ResultSetHandler），method為需要攔截的方法的方法名，args為攔截方法的方法引數型別。

3.參考例子

接下來舉一個攔截器實現對結果集下劃線轉駝峰的例子來簡要說明

/**
 * @author dxu2
 * @date 2022/7/14
 * map結果轉駝峰
 */
@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 {
    // 呼叫目標方法
    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);
      }
    }
  }

  // 下劃線轉駝峰
  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();
  }
}

這個例子攔截的是ResultSetHandler的handleResultSets方法，這個方法是用來對結果集處理的，看intercept方法首先呼叫了目標物件的方法接著強轉為List<Object>型別，這裡為什麼可以強轉呢，因為我們可以看到handleResultSets方法定義<E> List<E> handleResultSets(Statement stmt) throws SQLException; 返回的是List型別，然後遍歷列表，若元素是map型別的再進行處理把key值轉化為駝峰形式重新put到map中。

最後不要忘了把自定義的攔截器新增到配置中，這邊是使用xml配置的，新增完後接著執行測試程式碼，可以看到列user_id已經轉換成駝峰形式了。

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

#mapper介面
List<Map> selectAllUsers();

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

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

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

}