RunMethodCalledOnJavaLangThreadDirectly.md

J-D-001: Method call to java.lang.Thread or its subclasses may indicate a logical bug

Calling run() on java.lang.Thread or its subclasses may indicate misunderstanding on the programmer's part.

Overview

The java.lang package provides class Thread for multithreading. This class provides a method named start, to begin executing its code in a separate thread, that calls another public API called run. However, directly calling run does not result in this multithreading behavior; rather, it executes the code in the context of the current thread.

Meanwhile, the argument of the call to the constructor of Thread should implement java.lang.Runnable which provides the public method run. Calling this method directly also does not create a separate thread, however, this rule does not prohibit such calls.

Recommendation

For instances of Thread and its subclasses, use start instead of run to start the thread and begin executing the run method of the Runnable instance used to construct the instance of Thread.

Example

The following example creates an instance of java.lang.Thread and intends to execute it by calling the run method on it instead of start.

import java.lang.Thread;
import java.lang.Runnable;

class Job implements Runnable {
  public void run() {
    /* ... */
  }
}

class AnotherThread extends Thread {
  AnotherThread(Runnable runnable) {
    super(runnable);
  }

  public void run() {
    /* ... */
  }
}

class ThreadExample {
  public void f() {
    Thread thread = new Thread(new Job());
    thread.run();          // NON_COMPLIANT
    thread.start();        // COMPLIANT

    AnotherThread anotherThread = new AnotherThread(new Job());
    anotherThread.run();   // NON_COMPLIANT
    anotherThread.start(); // COMPLIANT

    Job job = new Job();
    job.run();             // COMPLIANT
  }
}

References

• Oracle: Documentation of java.lang.Thread.
• Oracle: Documentation of java.lang.Runnable.