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The best way to handle exceptions at the application level is to use an exception handler. The handler can perform diagnostic actions, clean-up and shutdown the Java Virtual Machine (JVM) or simply log the details of the failure.
Noncompliant Code Example (Abnormal task termination)
This noncompliant code example consists of the PoolService class that encapsulates a thread pool and a runnable Task class. The Task.run() method can throw runtime exceptions such as NullPointerException.
| Code Block | ||
|---|---|---|
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final class PoolService {
private final ExecutorService pool = Executors.newFixedThreadPool(10);
public void doSomething() {
pool.execute(new Task());
}
}
final class Task implements Runnable {
@Override public void run() {
// ...
throw new NullPointerException();
// ...
}
}
|
The task does not notify the application when it terminates unexpectedly as a result of the runtime exception. Moreover, it does not use any recovery mechanism. Consequently, if Task throws a NullPointerException the exception is ignored.
Compliant Solution (ThreadPoolExecutor hooks)
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Task-specific recovery or clean-up actions can be performed by overriding the {{afterExecute()}} hook of class {{java.util.concurrent.ThreadPoolExecutor}}. This hook is called when a task successfully concludes by executing all statements in its {{run()}} method, or halts because of an exception ({{java.lang.Error}} might not be captured on specific implementations, see [Bug ID 6450211|http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6450211] \[[SDN 08|AA. Java References#SDN 08]\]). When using this approach, substitute the executor service with a custom {{ThreadPoolExecutor}} that overrides the {{afterExecute()}} hook as shown below: |
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|---|---|---|
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final class PoolService {
// The values have been hardcoded for brevity
ExecutorService pool = new CustomThreadPoolExecutor(10, 10, 10, TimeUnit.SECONDS,
new ArrayBlockingQueue<Runnable>(10));
// ...
}
class CustomThreadPoolExecutor extends ThreadPoolExecutor {
// ... Constructor ...
@Override
public void afterExecute(Runnable r, Throwable t) {
super.afterExecute(r, t);
if (t != null) {
// Exception occurred, forward to handler
}
// ... Perform task-specific clean-up actions
}
@Override
public void terminated() {
super.terminated();
// ... Perform final clean-up actions
}
}
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The terminated() hook is called after all the tasks have finished executing, and the Executor has terminated cleanly. This hook can be overridden to release resources acquired by the thread pool, much like a finally block.
Compliant Solution (Uncaught Exception Handler)
This compliant solution sets an uncaught exception handler on behalf of the thread pool. An argument of type ThreadFactory is passed to the thread pool during construction. The factory is responsible for creating new threads and setting the uncaught exception handler on their behalf. The Task class is unchanged from the noncompliant code example.
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final class PoolService {
private static final ThreadFactory factory = new
ExceptionThreadFactory(new MyExceptionHandler());
private static final ExecutorService pool =
Executors.newFixedThreadPool(10, factory);
public void doSomething() {
pool.execute(new Task()); // Task is a runnable class
}
public static class ExceptionThreadFactory implements ThreadFactory {
private static final ThreadFactory defaultFactory =
Executors.defaultThreadFactory();
private final Thread.UncaughtExceptionHandler handler;
public ExceptionThreadFactory(Thread.UncaughtExceptionHandler handler) {
this.handler = handler;
}
@Override public Thread newThread(Runnable run) {
Thread thread = defaultFactory.newThread(run);
thread.setUncaughtExceptionHandler(handler);
return thread;
}
}
public static class MyExceptionHandler extends ExceptionReporter
implements Thread.UncaughtExceptionHandler {
// ...
@Override public void uncaughtException(Thread thread, Throwable t) {
// Recovery or logging code
}
}
}
|
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The {{ExecutorService.submit()}} method can be used to submit a task to a thread pool instead of the {{execute()}} method to obtain a {{Future}} object. Note that the uncaught exception handler is not called if {{ExecutorService.submit()}} is invoked. This is because the thrown exception is considered to be part of the return status and is consequently wrapped in an {{ExecutionException}} and re-thrown by {{Future.get()}} \[[Goetz 06|AA. Java References#Goetz 06]\]. |
Compliant Solution (Future<V> and submit())
This compliant solution invokes the ExecutorService.submit() method to submit the task so that a Future object can be obtained. It uses the Future object to let the task re-throw the exception so that it can be handled locally.
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|---|---|---|
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final class PoolService {
private final ExecutorService pool = Executors.newFixedThreadPool(10);
public void doSomething() {
Future<?> future = pool.submit(new Task());
// ...
try {
future.get();
} catch (InterruptedException e) {
Thread.currentThread().interrupt(); // Reset interrupted status
} catch (ExecutionException e) {
Throwable exception = e.getCause();
// Forward to exception reporter
}
}
}
|
Furthermore, any exception that prevents doSomething() from obtaining the Future value can be handled as required.
Exceptions
CON32-EX1: This guideline may be violated if the code for all runnable and callable tasks has been audited to ensure that no exceptional conditions are possible. Nonetheless, it is usually a good practice to install a task-specific or global exception handler to initiate recovery or log the exceptional condition.
Risk Assessment
Failing to provide a mechanism to report that tasks in a thread pool failed as a result of an exceptional condition, can make it harder to find the source of the issue.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|---|---|---|---|---|
CON32- J | low | probable | medium | P4 | L3 |
To-Do List
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||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name|| |
Automated Detection
TODO
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
References
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\[[API 06|AA. Java References#API 06]\] interfaces {{ExecutorService}}, {{ThreadFactory}} and class {{Thread}}
\[[Goetz 06|AA. Java References#Goetz 06]\] Chapter 7.3: Handling abnormal thread termination |
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