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It is critical to ensure that threads are started correctly. Thread start-up can be misleading because sometimes the code appears to be performing the function correctly, when it is actually executing in being executed by the wrong thread. Ensure that threads are started correctly.

Invoking the The Thread.start() method starts executing a tells the Java runtimes to start executing the thread's run() method in using the respective started thread. It is a mistake to directly invoke the Invoking a Thread object's run() method on directly is incorrect. When a a Thread object. When 's run() method is invoked directly, the statements in the run() method execute in the are executed by current thread instead of , rather than by the newly created thread. Furthermore, if the Thread object is not constructed from a Runnable object but rather was constructed by instantiating a subclass of Thread that does not fails to override the run() method , a call rather than being constructed from a Runnable object, calls to the subclass's run() method invokes invoke Thread.run(), which does nothing.

Consequently, directly invoking a Thread object's run() method is forbidden.

Noncompliant Code Example

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public final class Foo implements Runnable {
  @Override public void run() {
    // ...
  }

  public static void main(String[] args) {
    Foo foo = new Foo();
    new Thread(foo).run();
  }
}

The start() method is not invoked on the new thread newly created thread is never started because of the incorrect assumption that run() starts the new thread. Consequently, the statements in the run() method execute in the same thread instead of the new oneare executed by the current thread rather than by the new thread.

Compliant Solution

This compliant solution correctly uses the start() method to tell the Java runtimes to start a new thread. Then, that method internally invokes the run() method in the new thread.

public final class Foo implements Runnable {
  @Override public void run() {
    // ...
  }

  public static void main(String[] args) {
    Foo foo = new Foo();
    new Thread(foo).start();
  }
}

...

Casting a thread to Runnable before calling the run() method documents that the explicit call to Thread.run() is intentional. Adding an explanatory comment alongside the invocation is highly recommended.

THI02-EX2: Runtime system code involved in starting new threads is permitted to invoke a Thread object's run() method directly; this is an obvious necessity for a working Java runtime system. Note that the likelihood that this exception applies to user-written code is vanishingly small.

Risk Assessment

Failing to start threads correctly can cause unexpected behavior.

Automated Detection

TODO Automated detection of direct invocations of Thread object's run() methods appears to be straightforward. Sound automated determination of which specific invocations are permitted may be infeasible. Heuristic approaches may be useful.

Related Vulnerabilities

Any vulnerabilities resulting from the violation of this rule are listed on the CERT website.

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