mybatis 拦截器

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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);  }}#Interceptorpublic 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());    }  }}