According 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}
Code that depends on thread suspension or yielding to:
* flush cached registers
* reload any values
* provide any [happens-before|BB. Definitions#happens-before order] relationships when execution resumes
is incorrect, and is consequently forbidden. Programs must ensure that communication between threads has proper synchronization, happens-before, and visibility semantics.
h2. Noncompliant Code Example ({{sleep()}})
This noncompliant code attempts to use the non-volatile primitive Boolean member {{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
}
}
}
public void shutdown() {
this.done = true;
}
}
{code}
However, the compiler is free to read the field {{this.done}} once and to reuse the cached value in each execution of the loop. Consequently, the while loop might never terminate, even when 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 field volatile to ensure that updates to its value are made visible across multiple threads.
{code:bgColor=#ccccff}
final class ControlledStop implements Runnable {
private volatile boolean done = false;
// ...
@Override public void run() {
//...
}
}
{code}
The volatile flag establishes a [happens-before|BB. Definitions#happens-before order] relationship between this thread and any other thread that sets {{done}}.
h2. Compliant Solution ({{Thread.interrupt()}})
A better solution for methods that call {{sleep()}} is to use thread interruption, which causes the sleeping thread to wake immediately and handle the interruption. Note that the interrupting thread must know which thread to interrupt; logic for tracking this relationship has been elided.
{code:bgColor=#ccccff}
final class ControlledStop implements Runnable {
@Override public void run() {
// record current thread, so others can interrupt it
myThread = currentThread();
while (!Thread.interrupted()) {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
Thread.currentThread().interrupt();
}
}
}
public void shutdown(Thread th) {
th.interrupt();
}
}
{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 checks to see whether the thread is blocked on the wait; if so, it sets the flag to true and notifies the thread 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) {
flag = true;
synchronized (lock) {
lock.notifyAll();
}
return true;
}
}
return false;
}
}
{code}
Unfortunately, the {{stop()}} method incorrectly uses the {{Thread.getState()}} method to check whether the thread is blocked and has not terminated before delivering the notification. Using the {{Thread.getState()}} method for synchronization control, such as checking whether a thread is blocked on a wait, is inappropriate. JVMs are permitted to implements blocking using spin-waiting; consequently, a blocked thread may never enter the {{WAITING}} or {{TIMED_WAITING}} state \[[Goetz 2006|AA. Bibliography#Goetz 06]\]. Because the thread may never enter the {{WAITING}} state, the {{stop()}} method might fail to 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()}} effects only threads that are blocked on a {{wait()}} invocation.
{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) {
flag = true;
synchronized (lock) {
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.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| THI00-J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |
h3. Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON44-J].
h2. Related Guidelines
| [MITRE CWE|http://cwe.mitre.org/] | [CWE ID 821|http://cwe.mitre.org/data/definitions/821.html], "Incorrect Synchronization" |
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!|09. Thread APIs (THI)] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|09. Thread APIs (THI)] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|THI01-J. Do not invoke ThreadGroup methods]
| 