CodeQL documentation

Direct call to a run() method

ID: java/call-to-thread-run
Kind: problem
Severity: recommendation
Precision: high
Tags:
   - reliability
   - correctness
   - concurrency
   - external/cwe/cwe-572
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

A direct call of a Thread object’s run method does not start a separate thread. The method is executed within the current thread. This is an unusual use because Thread.run() is normally intended to be called from within a separate thread.

Recommendation

To execute Runnable.run from within a separate thread, do one of the following:

  • Construct a Thread object using the Runnable object, and call start on the Thread object.

  • Define a subclass of a Thread object, and override the definition of its run method. Then construct an instance of this subclass and call start on that instance directly.

Example

In the following example, the main thread, ThreadDemo, calls the child thread, NewThread, using run. This causes the child thread to run to completion before the rest of the main thread is executed, so that “Child thread activity” is printed before “Main thread activity”.

public class ThreadDemo {
    public static void main(String args[]) {
        NewThread runnable = new NewThread();

        runnable.run();    // Call to 'run' does not start a separate thread

        System.out.println("Main thread activity.");
    }
}

class NewThread extends Thread {
    public void run() {
        try {
            Thread.sleep(10000);
        }
        catch (InterruptedException e) {
            System.out.println("Child interrupted.");
        }
        System.out.println("Child thread activity.");
    }
}

To enable the two threads to run concurrently, create the child thread and call start, as shown below. This causes the main thread to continue while the child thread is waiting, so that “Main thread activity” is printed before “Child thread activity”.

public class ThreadDemo {
    public static void main(String args[]) {
    	NewThread runnable = new NewThread();
    	
        runnable.start();                                         // Call 'start' method
        
        System.out.println("Main thread activity.");
    }
}

References