THI00-J. Do not invoke Thread.run()

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

Invoking the Thread.start() method tells 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 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 being constructed from a Runnable object, calls to the subclass's run() method 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 runtimes 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-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 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.

public void sampleRunTest(){

  Thread thread = new Thread(new Runnable() {
    @Override public void run() {
        // ...
      }
    });
  
  ((Runnable) thread).run();  // THI00-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-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 to start threads correctly can cause unexpected behavior.

Automated Detection

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 Guidelines

Bibliography

09. Thread APIs (THI)      09. Thread APIs (THI)