默认情况下，发生限流、降级、授权拦截时，都会抛出异常到调用方。如果要自定义异常时的返回结果，需要实现BlockExceptionHandler接口：

public class SentinelExceptionHandler implements BlockExceptionHandler {
    /**    
     * 处理请求被限流、降级、授权拦截时抛出的异常：BlockException
     */
    public void handle(HttpServletRequest request, HttpServletResponse response, BlockException e) throws Exception;
}

而BlockException包含很多个子类，分别对应不同的场景：

异常                       说明
FlowException             限流异常
ParamFlowException        热点参数限流的异常
DegradeException          降级异常
AuthorityException        授权规则异常
SystemBlockException      系统规则异常

自定义异常结果

创建SentinelExceptionHandler类实现BlockExceptionHandler接口，如下：

import com.alibaba.csp.sentinel.adapter.spring.webmvc.callback.BlockExceptionHandler;
import com.alibaba.csp.sentinel.slots.block.BlockException;
import com.alibaba.csp.sentinel.slots.block.authority.AuthorityException;
import com.alibaba.csp.sentinel.slots.block.degrade.DegradeException;
import com.alibaba.csp.sentinel.slots.block.flow.FlowException;
import com.alibaba.csp.sentinel.slots.block.flow.param.ParamFlowException;
import com.alibaba.csp.sentinel.slots.system.SystemBlockException;
import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.stereotype.Component;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@Component
public class SentinelExceptionHandler implements BlockExceptionHandler {
    @Override
    public void handle(HttpServletRequest request, HttpServletResponse response, BlockException ex) throws Exception {
            String msg = null;
            if (ex instanceof FlowException) {
                msg = "限流了";
            } else if (ex instanceof DegradeException) {
                msg = "降级了";
            } else if (ex instanceof ParamFlowException) {
                msg = "热点参数限流";
            } else if (ex instanceof SystemBlockException) {
                msg = "系统规则限流或降级";
            } else if (ex instanceof AuthorityException) {
                msg = "授权规则不通过";
            }
            // http状态码
            response.setStatus(500);
            response.setCharacterEncoding("utf-8");
            response.setHeader("Content-Type", "application/json;charset=utf-8");
            response.setContentType("application/json;charset=utf-8");
            // spring mvc自带的json操作工具，叫jackson
            String path = request.getServletPath();
            if (path != null) {
                msg = String.format("接口[%s]%s", path, msg);
            }
            new ObjectMapper().writeValue(response.getWriter(), msg);
        }
}