When start a JMXMP client (probably JMXRMI too), sometime will get:
java.lang.NullPointerException
        at javax.management.remote.generic.ClientIntermediary.access$000(ClientI
ntermediary.java:64)
        at javax.management.remote.generic.ClientIntermediary$GenericClientCommu
nicatorAdmin.checkConnection(ClientIntermediary.java:1525)
        at com.sun.jmx.remote.opt.internal.ClientCommunicatorAdmin$Checker.run(C
lientCommunicatorAdmin.java:162)
        at java.lang.Thread.run(Thread.java:479)

and it stops the heartbeat at client side.

It happens by using jdk1.3, but not 1.4.2