Holding locks while performing time-consuming or blocking operations can severely degrade system performance and can result in starvation. Furthermore, can result if interdependent threads block indefinitely. Blocking operations include network, file, and console I/O (for example,
Console.readLine()) and object serialization. Deferring a thread indefinitely also constitutes a blocking operation. Consequently, programs must not perform blocking operations while holding a lock.
When the Java Virtual Machine (JVM) interacts with a file system that operates over an unreliable network, file I/O might incur a large performance penalty. In such cases, avoid file I/O over the network while holding a lock. File operations (such as logging) that could block while waiting for the output stream lock or for I/O to complete could be performed in a dedicated thread to speed up task processing. Logging requests can be added to a queue, assuming that the queue's
put() operation incurs little overhead as compared to file I/O [Goetz 2006].
Noncompliant Code Example (Deferring a Thread)
This noncompliant code example defines a utility method that accepts a
Because the method is synchronized, when the thread is suspended, other threads cannot use the synchronized methods of the class. The current object's monitor continues to be held because the
Thread.sleep() method lacks synchronization semantics.
Compliant Solution (Intrinsic Lock)
This compliant solution defines the
doSomething() method with a
timeout parameter rather than the
time value. Using
Object.wait() instead of
Thread.sleep() allows setting a timeout period during which a notification may awaken the thread.
The current object's monitor is immediately released upon entering the wait state. When the timeout period elapses, the thread resumes execution after reacquiring the current object's monitor.
Note that the
waitmethod, as it places the current thread into the wait set for this object, unlocks only this object; any other objects on which the current thread may be synchronized remain locked while the thread waits.
This method should only be called by a thread that is the owner of this object's monitor.
Programs must ensure that threads that hold locks on other objects release those locks appropriately before entering the wait state. Additional guidance on waiting and notification is available in THI03-J. Always invoke wait() and await() methods inside a loop and THI02-J. Notify all waiting threads rather than a single thread.
Noncompliant Code Example (Network I/O)
This noncompliant code example defines a
sendPage() method that sends a
Page object from a server to a client. The method is synchronized to protect the
pageBuff array when multiple threads request concurrent access.
writeObject() within the synchronized
sendPage() method can result in delays and deadlock-like conditions in high-latency networks or when network connections are inherently lossy.
This compliant solution separates the process into a sequence of steps:
- Perform actions on data structures requiring synchronization.
- Create copies of the objects to be sent.
- Perform network calls in a separate unsynchronized method.
In this compliant solution, the unsynchronized
sendPage() method calls the synchronized
getPage() method to retrieve the requested
Page in the
pageBuff array. After the
Page is retrieved,
sendPage() calls the unsynchronized
deliverPage() method to deliver the
Page to the client.
LCK09-J-EX0: Classes that provide an appropriate termination mechanism to callers are permitted to violate this rule (see THI04-J. Ensure that threads performing blocking operations can be terminated).
LCK09-J-EX1: Methods that require multiple locks may hold several locks while waiting for the remaining locks to become available. This constitutes a valid exception, although the programmer must follow other applicable rules, especially LCK07-J. Avoid deadlock by requesting and releasing locks in the same order to avoid .
Blocking or lengthy operations performed within synchronized regions could result in a deadlocked or unresponsive system.
Some static analysis tools are capable of detecting violations of this rule.
Blocking in Critical Section (Java)
|Do not use blocking methods while holding a lock|
Do not call 'Thread.sleep()' while holding a lock since doing so can cause poor performance and deadlocks