Question

Question: I’m interested to know the best practice for killing a long standing operation that is running as a background thread (lets call this thread WorkerThread) in Java 1.2.

Scenario

Specifically, I’m developing an application for Blackberry devices whereby I make a HTTP connection. Big picture: a URL request if forwarded to a background thread (WorkerThread), the thread makes the request and returns the result using a call back.

Scenario Details

Now there exists a situation where at connection time, a data connection exists but then for whatever reason (drives through a tunnel) that connection no longer exists. Due to a limitation in Blackberry’s design architecture, that actual connection will hang as the time out is fixed to be 2 minutes. As a result, there’s a crucial need to kill a connection that has been hanging for a relatively (15 seconds) long period of time.

My Current Solution – 2 Theads?

Right now my current solution is to run WorkerThread inside another thread (lets call this new thread MonitorThead). MonitorThread starts WorkerThread, sleeps for 1000ms and then routinely checks if WorkerThread is still alive. If after 15 seconds WorkerThread is still alive, MonitorThread puts WorkerThread to sleep and exits. Is this really the best possible approach?

Summary of Question & Key points

In summary, below is the core question and key restraints associated with the question. Cheers!

How do I successful kill a java background thread that is stuck in a specific operation?

Scenario Restraints:

• No control of having operation pause
  and check the threads requested state
• Specific to Blackberry’s
  implementation of Java ME 1.2 and its
  Thread API so no explicit
  kill() method
• Most concerned about the best practice and how to
  most safely kill a holding thread.

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Follow Up/Edit
Neil Coffey recommended that I simply hold a reference to the connection object and instead call close() on that object. I am currently looking into this…

Answer 1

How to kill a Thread is a difficult question. There is no guaranteed way to be able to stop or interrupt a Thread. However, if you take your current architecture and upon timeout, just close the stream (not the Connection), that should cause an I/O Exception to occur on the thread that is stuck in I/O. If it doesn’t cause an IOException, then it should at least cause the read or write to return with EOF.

Note that closing the Connection doesn’t help, as the JavaDoc says:

Any open streams will cause the connection to be held open until
they themselves are closed.

You have to close the stream that was derived from the Connection.