Is there a good way to forcefully stop a Java thread?

Question

I want to stop a Java thread immediately but when I try the thread always takes some time before stopping. Is there a good way to force a Java thread to stop instantly?

Answer 1

There is no good way to stop a thread instantly.

• There is Thread.stop(), but it is dangerous and deprecated. Don't use it, unless:

  1. you fully understand the problems that make it dangerous,

  2. you have thoroughly analyzed your code and determined that the problems do not apply and / or the risks are acceptable, and

  3. you don't care that your application may end up stuck at a some version of Java if / when the method is removed¹.

• There was Thread.stop(Throwable) but it was equally dangerous², deprecated and was removed³ in Java 11.

• There is Thread.interrupt(), but there is no guarantee that the thread will stop quickly, or even stop at all. The behavior will depend on whether the application thread code has been designed to notice and correctly respond to interrupts. (See below.)

• There is the approach of writing the thread to periodically check a flag; see this answer. (See below.)

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The "check a flag" and interrupt() approaches are largely the same. In both cases, the thread that expects to be interrupted needs to regularly check; e.g. by calling interrupted() or isInterrupted() or by checking a flag.

The difference between the approaches is that that the interrupt() mechanism will work when the code is waiting for a notify(), join(), etcetera, or blocked in an I/O operation. Because of this, and because interrupt is an accepted application independent way of doing this, it is usually preferable to implementing an application specific flag mechanism.

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^{1 - In the Java 18 prerelease javadocs, Thread.stop() is now shown as "flagged for removal".
2 - Possibly more so. If you did a Thread.stop(new IOException()) for example, some code further up the call stack could easily catch the exception and subvert your "stop".
3 - You can still use the old "stop with an exception" functionality by calling the private Thread.stop0(Throwable) method reflectively; see https://stackoverflow.com/a/67077495/139985. It would be better to avoid having to do that.}