面试官：讲一讲Mybatis插件的原理及如何实现？

public interface Interceptor {
  //拦截的方法
  Object intercept(Invocation invocation) throws Throwable;
  //返回拦截器的代理对象
  Object plugin(Object target);
  //设置一些属性
  void setProperties(Properties properties);
}

/**
 * @Intercepts 注解标记这是一个拦截器,其中可以指定多个@Signature
 * @Signature 指定该拦截器拦截的是四大对象中的哪个方法
 *      type：拦截器的四大对象的类型
 *      method：拦截器的方法，方法名
 *      args：入参的类型，可以是多个，根据方法的参数指定，以此来区分方法的重载
 */
@Intercepts(
        {
                @Signature(type = ParameterHandler.class,method ="setParameters",args = {PreparedStatement.class})
        }
)
public class ParameterInterceptor implements Interceptor {
    @Override
    public Object intercept(Invocation invocation) throws Throwable {
        System.out.println("拦截器执行："+invocation.getTarget());
        //目标对象
        Object target = invocation.getTarget();
        //获取目标对象中所有属性的值，因为ParameterHandler使用的是DefaultParameterHandler，因此里面的所有的属性都封装在其中
        MetaObject metaObject = SystemMetaObject.forObject(target);
        //使用xxx.xxx.xx的方式可以层层获取属性值，这里获取的是mappedStatement中的id值
        String value = (String) metaObject.getValue("mappedStatement.id");
        //如果是指定的查询方法
        if ("cn.cb.demo.dao.UserMapper.selectByUserId".equals(value)){
            //设置参数的值是admin_1，即是设置id=admin_1，因为这里只有一个参数，可以这么设置，如果有多个需要需要循环
            metaObject.setValue("parameterObject", "admin_1");
        }
        //执行目标方法
        return invocation.proceed();
    }
    @Override
    public Object plugin(Object target) {
        //如果没有特殊定制，直接使用Plugin这个工具类返回一个代理对象即可
        return Plugin.wrap(target, this);
    }
    @Override
    public void setProperties(Properties properties) {
    }
}

/**
 * @Configuration：这个注解标注该类是一个配置类
 */
@Configuration
public class MybatisConfig{
    /**
     * @Bean ： 该注解用于向容器中注入一个Bean
     * 注入Interceptor[]这个Bean
     * @return
     */
    @Bean
    public Interceptor[] interceptors(){
        //创建ParameterInterceptor这个插件
        ParameterInterceptor parameterInterceptor = new ParameterInterceptor();
        //放入数组返回
        return new Interceptor[]{parameterInterceptor};
    }
}

@Test
    void contextLoads() {
      //传入的是1222
        UserInfo userInfo = userMapper.selectByUserId("1222");
        System.out.println(userInfo);
    }

//创建Executor
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);
    }
    //调用pluginAll方法，生成代理对象
    executor = (Executor) interceptorChain.pluginAll(executor);
    return executor;
  }
  //创建ParameterHandler
  public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {
    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);
    //调用pluginAll方法，生成代理对象
    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);
    return parameterHandler;
  }
//创建ResultSetHandler
  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);
    //调用pluginAll方法，生成代理对象
    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);
    return resultSetHandler;
  }
  //创建StatementHandler
  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);
    //调用pluginAll方法，生成代理对象
    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);
    return statementHandler;
  }

public Object pluginAll(Object target) {
    //循环遍历插件
    for (Interceptor interceptor : interceptors) {
      //调用插件的plugin()方法
      target = interceptor.plugin(target);
    }
    //返回
    return target;
  }

public static Object wrap(Object target, Interceptor interceptor) {
    //获取注解的@signature的定义
    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);
    //目标类
    Class<?> type = target.getClass();
    //获取需要拦截的接口
    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);
    if (interfaces.length > 0) {
      //生成代理对象
      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@signature标注的方法
      Set<Method> methods = signatureMap.get(method.getDeclaringClass());
      //如果这个方法被拦截了
      if (methods != null && methods.contains(method)) {
      //直接执行插件的intercept()这个方法
        return interceptor.intercept(new Invocation(target, method, args));
      }
      //没有被拦截，执行原方法
      return method.invoke(target, args);
    } catch (Exception e) {
      throw ExceptionUtil.unwrapThrowable(e);
    }
  }

<dependency>
            <groupId>com.github.pagehelper</groupId>
            <artifactId>pagehelper</artifactId>
            <version>5.1.6</version>
        </dependency>

@Intercepts(
        {
                @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class}),
                @Signature(type = Executor.class, method = "query", args = {MappedStatement.class, Object.class, RowBounds.class, ResultHandler.class, CacheKey.class, BoundSql.class}),
        }
)
public class PageInterceptor implements Interceptor {}