Page History

 to Section 17.9, "Sleep and Yield," of the Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\]

{quote}
It is important to note that neither {{Thread.sleep}} nor {{Thread.yield}} have any synchronization semantics. In particular, the compiler does not have to flush writes cached in registers out to shared memory before a call to {{Thread.sleep}} or {{Thread.yield}}, nor does the compiler have to reload values cached in registers after a call to {{Thread.sleep}} or {{Thread.yield}}.
{quote}

Incorrectly assuming that thread suspension and yielding do any of the following can result in unexpected behavior:
* flush cached registers 
* reload any values 
* provide any [happens-before|BB. Definitions#happens-before order] relationships when execution resumes


h2. Noncompliant Code Example ({{sleep()}})

This noncompliant code attempts to use a non-volatile Boolean {{done}} as a flag to terminate execution of a thread. A separate thread sets {{done}} to true by calling the {{shutdown()}} method.

{code:bgColor=#FFCCCC}
final class ControlledStop implements Runnable {
  private boolean done = false;

  @Override public void run() {
    while (!done) {
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {
        Thread.currentThread().interrupt(); // Reset interrupted status
        Thread.currentThread().interrupt(); }
    }
  }

  public void shutdown() {
    this.done = true;
  }
}
{code}

However, the compiler is free to read the field {{this.done}} once and reuse the cached value in each execution of the loop. Consequently, the while loop might not terminate, even if another thread calls the {{shutdown()}} method to change the value of {{this.done}} \[[JLS 2005|AA. Bibliography#JLS 05]\].  This error could have resulted from the programmer incorrectly assuming that the call to {{Thread.sleep()}} would cause cached values to be reloaded.


h2. Compliant Solution (Volatile Flag)

This compliant solution declares the flag volatile to ensure that updates to it are made visible across multiple threads.

{code:bgColor=#ccccff}

final class ControlledStop implements Runnable {
  private volatile boolean done = false;

  @Override public void run() {
    // ...
  }
{code

  // ...
}

{code}


h2. Noncompliant Code Example ({{getState()}})

This noncompliant code example contains a {{doSomething()}} method that starts a thread. The thread supports interruption by checking a volatile flag and blocks waiting until notified. The {{stop()}} method notifies the thread if it is blocked on the wait and sets the flag to true so that the thread can terminate.

{code:bgColor=#FFCCCC}
public class Waiter {
  private Thread thread;
  private volatile boolean flag;
  private final Object lock = new Object();

  public void doSomething() {
    thread = new Thread(new Runnable() {
      @Override public void run() {
        synchronized(lock) {
          while (!flag) {
            try {
              lock.wait();
              // ...
            } catch (InterruptedException e) {
              // Forward to handler
            }
          }
        }
      }
    });
    thread.start();
  }

  public boolean stop() {
    if (thread != null) {
      if (thread.getState() == Thread.State.WAITING) {
        flagsynchronized = true;
(lock) {
         synchronized (lock) { flag = true;
          lock.notifyAll();
        }
        return true;
      }
    }
    return false;
  }
}
{code}

 inappropriate.This is true because a blocked thread is not always required to enter the {{WAITING}} or {{TIMED_WAITING}} state in cases where the JVM implements blocking using spin-waiting \[[Goetz 2006|AA. Bibliography#Goetz 06]\]. Because the thread may never enter the {{WAITING}} state, the {{stop()}} method might not terminate the thread.


h2. Compliant Solution

This compliant solution removes the check for determining whether the thread is in the {{WAITING}} state. This check is unnecessary because invoking {{notifyAll()}} on a thread that is not blocked on a {{wait()}} invocation has no effect.

{code:bgColor=#ccccff}
public class Waiter {
  // ...
  private Thread thread;
  private volatile boolean flag;
  private final Object lock = new Object();

  public boolean stop() {
    if (thread != null) {
      flagsynchronized = true;
(lock) {
       synchronized (lock) { flag = true;
        lock.notifyAll();
      }
      return true;
    }
    return false;
  }
}
{code}


{mc} This does not talk about invoking getState() and comparing with TERMINATE, RUNNABLE and other states. Should we? {mc}

h2. Risk Assessment

Relying on the {{Thread}} class's {{sleep()}}, {{yield()}} and {{getState()}} methods for synchronization control can cause unexpected behavior.

|| Guideline || Severity || Likelihood || Remediation Cost || Priority || Level ||
| THI00- J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |


h3. Automated Detection 

TODO 

h3. Related Vulnerabilities 

Search for vulnerabilities resulting from the violation of this guideline on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON44-J].


h2. Bibliography

\[[JLS 2005|AA. Bibliography#JLS 05]\] section 17.9 "Sleep and Yield"


h2. Issue Tracking 

{tasklist:Review List} 
||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name|| 
{tasklist}

----
[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|13. Thread APIs (THI)]      [!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|13. Thread APIs (THI)]      [!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|THI01-J. Do not invoke ThreadGroup methods]