基于Jersey开发的一个操作OpenStack的REST服务，利用Jersey的Test Framework编写单元测试类如下：
public class RestAddressTest extends JerseyTest { Integer autoId = 1; @BeforeClass public void before() throws Exception { super.setUp(); } @AfterClass public void after() throws Exception { super.tearDown(); } @Override protected Application configure() { return new ResourceConfig(RestAddress.class); } @Test(priority = 0) public void testAdd() { Address ac = new Address(); ac.setGateway("1.1.1.1"); ac.setName("CLOUD_TEST_BJ"); ac.setNicName("ipv4"); Response res = target("address").request(MediaType.APPLICATION_JSON).post(Entity.entity(ac, MediaType.APPLICATION_JSON), Response.class); Assert.assertEquals(200, res.getStatus()); } @Test(priority = 1) public void testGet() { Response res = target("address").request(MediaType.APPLICATION_JSON).get(); List<Address> acl = res.readEntity(new GenericType<List<Address>>() {}); this.autoId = acl.get(0).getAutoId(); Assert.assertEquals(1, acl.size()); } @Test(priority = 2) public void testDelete() { Response response = target("address").path(String.valueOf(autoId)).request(MediaType.APPLICATION_JSON).delete(); Assert.assertEquals(200, response.getStatus()); } }但是在测试时，却始终遇到如下异常：
... java.lang.IllegalArgumentException: attempt to create delete event with null entity at org.hibernate.event.spi.DeleteEvent.<init>(DeleteEvent.java:31) at org.hibernate.internal.SessionImpl.delete(SessionImpl.java:911) ... [03:39:54,116 ERROR] rollbackTxn - Transaction.rollback [03:39:54,143 INFO ] Stopped ServerConnector@54089484{HTTP/1.1}{0.0.0.0:9998} [03:39:54,146 WARN ] /address/1 java.lang.RuntimeException: java.lang.IllegalArgumentException: Status code of the supplied response [500] is not from the required status code family "CLIENT_ERROR". at org.glassfish.jersey.jetty.JettyHttpContainer.handle(JettyHttpContainer.java:197) ... [03:39:54,147 WARN ] Could not send response error 500: java.lang.RuntimeException: java.lang.IllegalArgumentException: Status code of the supplied response [500] is not from the required status code family "CLIENT_ERROR". Oct 24, 2018 5:39:54 AM org.glassfish.jersey.test.jetty.JettyTestContainerFactory$JettyTestContainer <init> INFO: Creating JettyTestContainer configured at the base URI http://localhost:9998/ [03:39:54,275 INFO ] jetty-9.2.14.v20151106 [03:39:54,279 INFO ] Started ServerConnector@5042e3d0{HTTP/1.1}{0.0.0.0:9998} [03:39:54,280 INFO ] Started @5991ms [03:39:54,312 INFO ] Stopped ServerConnector@5042e3d0{HTTP/1.1}{0.0.0.0:9998} ... Tests run: 4, Failures: 3, Errors: 0, Skipped: 0, Time elapsed: 4.734 sec <<< FAILURE! Results : Failed tests: testAdd(com.mycompany.myapp.rest.RestAddressTest): RESTEASY004655: Unable to invoke request testGet(com.mycompany.myapp.rest.RestAddressTest): RESTEASY003145: Unable to find a MessageBodyReader of content-type application/json and type interface java.util.List testDelete(com.mycompany.myapp.rest.RestAddressTest): expected [500] but found [200] ...分析异常，可以发现在执行测试用例testDelete()时异常，这里的信息有很强的迷惑性。事实上，在测试用例testDelete()之前执行的testAdd()和testGet()都已经发生了异常，但是却没有输出错误信息。
继续分析发现这应该是个“CLIENT_ERROR”。
在最后的测试结果中，终于暴露了错误的根源。原来，发生错误的是RESTEASY，等等，我们利用Jersey的测试框架进行单元测试，哪里来的RestEasy呢？
检查项目依赖，发现果然同时存在Jersey-Client和RestEasy-Client。突然想到，存在OpenStack的OpenStack4j就是依赖了RestEasy。查看OpenStack4j 3.1.0果然是其引入了RestEasy。
那么RestEasy为什么会在这里被调用呢？感谢StackOverflow，我很快找到了答案。
原来，同样作为JAX-RS的实现，Jersey和RestEasy都扩展了javax.ws.rs.client.ClientBuilder类，并实现了javax.ws.rs.client.Client接口。
其中，Jersey提供的是org.glassfish.jersey.client.JerseyClientBuilder和org.glassfish.jersey.client.JerseyClient，
而RestEasy提供的是org.jboss.resteasy.client.jaxrs.ResteasyClientBuilder和org.jboss.resteasy.client.jaxrs.ResteasyClient。
在同时存在两个JAX-RS实现的时候，由于JAX-RS采用了Java SPI的服务实现注入机制。RestEasy响应了这种机制，声明了ResteasyClientBuilder实现如下：
而Jersey对此机制却无动于衷，实现中根本没有给出JerseyClientBuilder的实现，如下所示：
所以，ResteasyClientBuilder的优先级高于JerseyClientBuilder而被采用。这样，在测试过程中，客户端事实上使用的是RestEasy的ResteasyClient，而非Jersey测试框架期望的JerseyClient。
找到问题的根源，解决起来也很容易了，只要在测试类中重写JerseyTest的getClient()方法，明确指定JerseyClient即可，修改测试类如下：
@Override public Client getClient() { return JerseyClientBuilder.createClient(); }
参考链接：
https://jersey.github.io/documentation/latest/test-framework.html
https://github.com/jersey/jersey/tree/master/test-framework
https://stackoverflow.com/questions/36348675/unable-to-test-jax-rs-with-json-entity
https://stackoverflow.com/questions/48337023/eclipse-jerseytest-getclient-returns-resteasyclient
https://github.com/ContainX/openstack4j/blob/3.1.0/connectors/resteasy/pom.xml
https://github.com/resteasy/Resteasy/blob/master/resteasy-client/src/main/resources/META-INF/services/javax.ws.rs.client.ClientBuilder
https://github.com/jersey/jersey/tree/master/core-client/src/main
