Jersey 为什么evaluatePreconditions不'；行不通

下面显示的RESTful方法代码，我想使用cache提供快速响应，但是来自

请求我总是得到
null

@GET
@Path("e_tag")
@Produces(MediaType.TEXT_PLAIN)
public Response getItWithETag(@Context Request request) {
    Response.ResponseBuilder rb;
    CacheControl cacheControl = new CacheControl();
    cacheControl.setMaxAge(1200);
    EntityTag tag = new EntityTag(GOT_IT.hashCode() + "");
    rb = request.evaluatePreconditions(tag);
    if (rb != null) {
        return rb.cacheControl(cacheControl).tag(tag).build();
    } else {
        return Response.ok(GOT_IT).cacheControl(cacheControl).tag(tag).build();
    }
}

测试代码：

@Test
public void testETag() throws InterruptedException {
    WebTarget webTarget = target("rest").path("e_tag");

    Response head = webTarget.request().get();
    System.out.println(head.getStatus() + "\t" + head.getEntityTag());
    Assert.assertEquals(200, head.getStatus());
    Thread.sleep(1000);

    Response head1 = webTarget.request().get();
    System.out.println(head1.getStatus() + "\t" + head1.getEntityTag());
    Assert.assertEquals(304, head1.getStatus());
}

我无法获得所需的结果。
如果http头中没有匹配的测试代码，则测试代码将丢失。正确的方法：

@Test
public void testETag() throws InterruptedException {
    WebTarget webTarget = target("rest").path("e_tag").queryParam("userId", "eric");

    Response head = webTarget.request().get();
    EntityTag eTag = head.getEntityTag();
    System.out.println(head.getStatus() + "\t" + eTag);
    Assert.assertEquals(200, head.getStatus());
    Thread.sleep(1000);

    Response head1 = webTarget.request().header("If-None-Match", eTag).get();
    System.out.println(head1.getStatus() + "\t" + head1.getEntityTag());
    Assert.assertEquals(304, head1.getStatus());
}

您没有在第二个请求上设置电子标签。