Thread passed where Runnable expected JAVA-S0056

A Thread object is passed as a parameter to a method where a Runnable is expected. This is rather unusual, and may indicate a logic error or cause unexpected behavior.

Because Thread inherits from Runnable, it has a public run method which any other code can freely call. In general, Thread wraps a Runnable instance, though it could be extended with a custom run method as well.

Calling Thread.run will not spawn a new thread; that is Thread.start's responsibility. Such usage is not harmful in and of itself, but it is likely to raise eyebrows in code review. It may be that Thread.run was called in place of Thread.start by accident.

Examples

Problematic Code

Thread a = new Thread(new Runnable() {
    @Override
    public void run() {
        // ...
    }
});

a.run();

Recommended

a.start();

Or, if you intended to use Runnable,

Runnable a = new Runnable() {
    @Override
    public void run() {
        // ...
    }
}

// ...

a.run(); // This is the same as Thread.run without the extra work.

Directly calling Thread.run will not spawn a new thread. If calling run is intentional, consider replacing the usage of Thread directly with Runnable instead, since that will allow for the same usage with less margin for error in future usage.

References

• Spotbugs - DMI_THREAD_PASSED_WHERE_RUNNABLE_EXPECTED