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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 the wrong thread.

The Thread.start() method starts executing a thread's run() method in the respective thread. It is a mistake to directly invoke the run() method on a Thread object. When invoked directly, the statements in the run() method execute in the current thread instead of the newly created thread. Furthermore, if the Thread object is not constructed from a Runnable object but rather by instantiating a subclass of Thread that does not override the run() method, a call to the subclass's run() method invokes Thread.run(), which does nothing.

Noncompliant Code Example

This noncompliant code example explicitly invokes run() in the context of the current thread.

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 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 one.

Compliant Solution

This compliant solution correctly uses the start() method 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();
  }
}

Exceptions

THI02-EX1: The run() method may be invoked when unit testing functionality. Note that this method cannot be used to test a class for multithreaded use.

Given a Thread object that has been constructed with a runnable argument, when invoking the Thread.run() method, the Thread object may be cast to Runnable to eliminate analyzer diagnostics.

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

((Runnable) thread).run();  // Exception: This does not start a new thread

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.

Risk Assessment

Failing to start threads correctly can cause unexpected behavior.

Automated Detection

TODO

Related Vulnerabilities

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

Bibliography

\[[API 2006|AA. Bibliography#API 06]\] Interface Runnable and class {{Thread}}

      13. Thread APIs (THI)