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It is critical to ensure that threads are started correctly. Thread start-up Thread startup can be misleading because sometimes the code appears can appear to be performing the its function correctly , when it is actually executing in being executed by the wrong thread.
The Invoking the Thread.start() method starts executing a instructs the Java runtime 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 Thread object. When 's run() method is invoked directly, the statements in the run() method execute in are executed by the 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 constructed from a Runnable object, any calls to the subclass's run() method invokes would invoke Thread.run(), which does nothing. Consequently, programs must not directly invoke a Thread object's run() method.

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 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 runtime 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

THI02THI00-J-EX1EX0: The run() method may be directly invoked when during unit testing functionality. Note that this method cannot be used to test a class for multithreaded use.

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

public void sampleRunTest() {

  Thread thread = new Thread(new Runnable() {
      @Override public void run() {
        // ...
      }
    });
  
  ((Runnable) thread).run();  // ExceptionTHI00-J-EX0: ThisDoes 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.

THI00-J-EX1: 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 Failure to start threads correctly can cause unexpected behavior.

References

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

Automated Detection

Automated detection of direct invocations of Thread.run() methods is straightforward. Sound automated determination of which specific invocations are permitted may be infeasible. Heuristic approaches may be useful.

Related Guidelines

Android Implementation Details

Android provides a couple of solutions for threading. The Android Developers Blog's article "Painless Threading" discusses those solutions.

Bibliography

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