Try to create a jmxmp server with an unallowed port like 1, and catch IOException caused by the creation, then try stop that server will get a NullPointerException.

see the following code:

JMXServiceURL u = null;
JMXConnectorServer server = null;

try {
u = new JMXServiceURL(proto, null, 1);

try {
server = JMXConnectorServerFactory.newJMXConnectorServer(u, null, mbs);
server.start();

} catch (Exception e) {}
server.stop();