THI03-J
4.4.2Compliant Solution
This compliant solution calls the wait() method from within a while loop to check the condition before and after wait() is called.
synchronized (object) {
while (<condition does not hold>) { object.wait();
}
// Proceed when condition holds
}
Similarly, invocations of the await() method of the java.util.concurrent.locks.Condition interface must be enclosed in a loop.
4.4.3Risk Assessment
To guarantee liveness and safety, the wait() and await() methods must always be invoked inside a while loop.
Guideline |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|
|
|
|
|
|
THI03- J |
low |
unlikely |
medium |
P2 |
L3 |
4.4.4References
[Bloch 2001] |
Item 50: “Never invoke wait outside a loop” |
[Goetz 2006] |
Section 14.2, “Using Condition Queues” |
[Lea 2000a] |
Section 3.2.2, “Monitor Mechanics” |
|
Section 1.3.2, “Liveness” |
[Sun 2009b] |
Class Object |
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4.5THI04-J. Notify all waiting threads instead of a single thread
A thread that invokes wait() expects to wake up and resume execution when its condition predicate becomes true. Waiting threads must test their condition predicates upon receiving notifications and resume waiting if the predicates are false, to be compliant with guideline “THI03-J. Always invoke wait() and await() methods inside a loop” on page 101.
The notify() and notifyAll() methods of the java.lang.Object package are used to wake up waiting thread(s). These methods must be invoked from code that holds the same object lock as the waiting thread(s). An IllegalMonitorStateException is thrown if the current thread does not acquire this object’s intrinsic lock before invoking these methods. The notifyAll() method wakes up all threads and allows threads whose condition predicate is true to resume execution. Furthermore, if all the threads whose condition predicate evaluates to true previously held a specific lock before going into the wait state, only one of them will reacquire the lock upon being notified. Presumably, the other threads will resume waiting. The notify() method wakes up only one thread and makes no guarantees as to which thread is notified. If the thread’s condition predicate doesn’t allow the thread to proceed, the chosen thread may resume waiting, defeating the purpose of the notification.
The notify() method may only be invoked if all of the following conditions are met:
•The condition predicate is identical for each waiting thread.
•All threads must perform the same set of operations after waking up. This means that any one thread can be selected to wake up and resume for a single invocation of notify().
•Only one thread is required to wake upon the notification.
These conditions are satisfied by threads that are identical and provide a stateless service or utility.
The java.util.concurrent utilities (Condition interface) provide the signal() and signalAll() methods to awaken threads that are blocked on an await() call. Condition objects are required when using Lock objects. A Lock object allows the use of the wait() and notify() methods. However, code that synchronizes using a Lock object does not use its own intrinsic lock. Instead, one or more Condition objects are associated with the Lock object. These objects interact directly with the locking policy enforced by the Lock object. Consequently, the Condition.await(), Condition.signal(), and Condition.signalAll() methods are used instead of Object.wait(), Object.notify(), and Object.notifyAll().
The use of the signal() method is insecure when multiple threads await the same Condition object unless all of the following conditions are met:
•The Condition object is identical for each waiting thread.
•All threads must perform the same set of operations after waking up. This means that any one thread can be selected to wake up and resume for a single invocation of signal().
•Only one thread is required to wake upon receiving the signal.
The signal() method may also be invoked when both of the following conditions are met:
•Each thread uses a unique Condition object.
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•Each Condition object is associated with a common Lock object.
The signal() method, if used securely, has better performance than signalAll().
4.5.1Noncompliant Code Example (notify())
This noncompliant code example shows a complex multistep process being undertaken by several threads. Each thread executes the step identified by the time field. Each thread waits for the time field to indicate that it is time to perform the corresponding thread’s step. After performing the step, each thread increments time and then notifies the thread that is responsible for performing the next step.
public final class ProcessStep implements Runnable { private static final Object lock = new Object(); private static int time = 0;
private final int step; // Do operations when field time reaches this value
public ProcessStep(int step) { this.step = step;
}
@Override public void run() { try {
synchronized (lock) { while (time != step) {
lock.wait();
}
// Perform operations
time++;
lock.notify();
}
} catch (InterruptedException ie) { Thread.currentThread().interrupt(); // Reset interrupted status
}
}
public static void main(String[] args) { for (int i = 4; i >= 0; i--) {
new Thread(new ProcessStep(i)).start();
}
}
}
This noncompliant code example violates the liveness property. Each thread has a different condition predicate, as each requires step to have a different value before proceeding. The Object.notify() method wakes up only one thread at a time. Unless it happens to wake up the thread that is required to perform the next step, the program will deadlock.
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4.5.2Compliant Solution (notifyAll())
In this compliant solution, each thread completes its step and then calls notifyAll() to notify the waiting threads. The thread that is ready can then perform its task, while all the threads whose condition predicates are false (loop condition expression is true) promptly resume waiting.
Only the run() method from the noncompliant code example is modified, as follows:
@Override public void run() { try {
synchronized (lock) { while (time != step) {
lock.wait();
}
// Perform operations
time++;
lock.notifyAll(); // Use notifyAll() instead of notify()
}
}catch (InterruptedException ie) { Thread.currentThread().interrupt(); // Reset interrupted status
}
}
4.5.3Noncompliant Code Example (Condition interface)
This noncompliant code example is similar to the noncompliant code example for notify() but uses the Condition interface for waiting and notification.
public class ProcessStep implements Runnable {
private static final Lock lock = new ReentrantLock();
private static final Condition condition = lock.newCondition(); private static int time = 0;
private final int step; // Do operations when field time reaches this value
public ProcessStep(int step) { this.step = step;
}
@Override public void run() { lock.lock();
try {
while (time != step) { condition.await();
}
// Perform operations
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time++;
condition.signal();
}catch (InterruptedException ie) { Thread.currentThread().interrupt(); // Reset interrupted status
}finally { lock.unlock();
}
}
public static void main(String[] args) { for (int i = 4; i >= 0; i--) {
new Thread(new ProcessStep(i)).start();
}
}
}
As with Object.notify(), the signal() method may awaken an arbitrary thread.
4.5.4Compliant Solution (signalAll())
This compliant solution uses the signalAll() method to notify all waiting threads. Before await() returns, the current thread reacquires the lock associated with this condition. When the thread returns, it is guaranteed to hold this lock [Sun 2009b]. The thread that is ready can perform its task, while all the threads whose condition predicates are false resume waiting.
Only the run() method from the noncompliant code example is modified, as follows:
@Override public void run() { lock.lock();
try {
while (time != step) { condition.await();
}
// Perform operations
time++;
condition.signalAll();
}catch (InterruptedException ie) { Thread.currentThread().interrupt(); // Reset interrupted status
}finally { lock.unlock();
}
}
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