mybatis 拦截器

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());     }   }  }

 mybatis 拦截器

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