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Thread startup can be misleading because the code can appear to be performing its function correctly when it is actually being executed by the wrong thread.
Invoking the Thread.start() method instructs the Java runtime to start executing the thread's run() method using the started thread. Invoking a Thread object's run() method directly is incorrect. When a Thread object's run() method is invoked directly, the statements in the run() method are executed by the current thread rather than by the newly created thread. Furthermore, if the Thread object was constructed by instantiating a subclass of Thread that fails to override the run() method rather than constructed from a Runnable object, any calls to the subclass's run() method 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 newly created thread is never started because of the incorrect assumption that run() starts the new thread. Consequently, the statements in the run() method are 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:

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

THI00-J-EX0: The run() method may be directly invoked during unit testing. Note that this method cannot be used to test a class for multithreaded use.

Given a Thread object that 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();  // THI00-J-EX0: 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

Failure to start threads correctly can cause unexpected behavior.

Rule

Severity

Likelihood

Remediation Cost

Priority

Level

THI00-J

Low

Probable

Medium

P4

L3

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.

Tool
Version
Checker
Description
CodeSonar4.2FB.MT_CORRECTNESS.RU_INVOKE_RUNInvokes run on a thread (did you mean to start it instead?)
Coverity7.5DC.THREADING.thread_runImplemented
Parasoft Jtest
10.3
TRS.IRUNImplemented
SonarQube
6.7
S1217Thread.run() should not be called directly

Related Guidelines

MITRE CWE

CWE-572, Call to Thread run() instead of start()

Android Implementation Details

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

Bibliography



6 Comments

  1. This is a good rule, so far, simple, yet realistic. (I think I've made this mistake in the past.)

    I wonder if we should be stronger with this rule. In particular, perhaps this rule should never invoke the run() method on a Thread object. (A side note is that if a thread was not constructed from a Runnable object, than Thread.run() does nothing.) If you have a Thread object and you must run it in this thread, and you know it is a Runnable, then cast it to a Runnable b/f calling run().

    1. This change is in now as an exception. Thanks.

  2. IIRC there used to be two exceptions to this rule: one saying "unit testing is ok" and one saying "cast a Thread object to a Runnable first". They seem to have been merged into one exception...why?

    1. Because it is assumed that the run method should be used for unit testing its functionality only. Casting it to Runnable just documents such as exception and does not serve as a valid exception on its own.

  3. Automated Detection is not done yet. It's marked as TODO.

  4. Obvious heuristic for checking this rule:

    • Calls are OK when the Thread object is cast to a Runnable first.
    • All other invocations are in error. Well... almost all...
    • Use an annotation like @COSCGExcept("THI02-EX2") to suppress errors for the vanishingly few instances where the call is legitimate.