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It is critical to ensure that threads are activated correctly. Thread activation can be misleading because sometimes the code appears to be performing the function correctly, whereas it may be operating in the presence of subtle concurrency issues.

It is usually a mistake to invoke the run() method on a Thread object. When invoked directly, the statements in the run() method execute, however, in the current thread instead of the Thread object itself. Furthermore, the Thread object was not constructed from a Runnable object, nor from a subclass of Thread that overrides run(), then the run() method invokes Thread.run() which itself does nothing.

It is recommended that if you have a Thread object that extends Runnable, and you wish to execute the object's run() method in the current thread, first cast the object to a Runnable and then invoke run().

Noncompliant Code Example

The run() method of interface Runnable must be invoked in its own thread, however, this noncompliant code example explicitly invokes it in the context of the current thread.

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The start() method is not invoked on the new thread because of the incorrect assumption that run() activates the thread. Consequently, the statements in the run() method execute, however, in the same thread instead of the new one.

Compliant Solution

This compliant solution correctly uses the start() method to start a new thread which then executes the run() method.

class Foo implements Runnable {
  public void run() {
    // ...
  }
  
  public static void main(String[] args) {
    Foo f = new foo();
    new Thread(f).start();
  }
}

Risk Assessment

Failing to activate threads correctly can cause unexpected behavior.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

\[[API 06|AA. Java References#API 06]\] Interface {{Runnable}} and class {{Thread}}

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