domingo, abril 19, 2015

Effective Java (Remarked) - Concurrency

Item 66: Synchronize access to shared mutable data (stability)

In summary, when multiple threads share mutable data, each thread that reads or writes the data must perform synchronization. Without synchronization, there is no guarantee that one thread’s changes will be visible to another. The penalties for failing to synchronize shared mutable data are liveness and safety failures. These failures are among the most difficult to debug. 

When use? when you need to avoid dirty-reads.

Item 67: Avoid excessive synchronization (stability, performance)

As a rule, you should do as little work as possible inside synchronized regions. Obtain the lock, examine the shared data, transform it as necessary, and drop the lock. 

When use? always in threads that you do not need to avoid dirty-reads.

Item 68: Prefer executors and tasks to threads (design, stability)

The Executor Framework also has a replacement for java.util.Timer, which is ScheduledThreadPoolExecutor. While it is easier to use a timer, a scheduled thread pool executor is much more flexible. A timer uses only a single thread for task execution, which can hurt timing accuracy in the presence of long- running tasks. If a timer’s sole thread throws an uncaught exception, the timer ceases to operate. A scheduled thread pool executor supports multiple threads and recovers gracefully from tasks that throw unchecked exceptions.

When use? always.

Item 69: Prefer concurrency utilities to wait and notify (design, stability)

In summary, using wait and notify directly is like programming in “concurrency assembly language,” as compared to the higher-level language provided by java.util.concurrent. There is seldom, if ever, a reason to use wait and notify in new code. If you maintain code that uses wait and notify, make sure that it always invokes wait from within a while loop using the standard idiom. The notifyAll method should generally be used in preference to notify. If notify is used, great care must be taken to ensure liveness.

When use? always.

Item 70: Document thread safety (design, readability)

To summarize, every class should clearly document its thread safety proper- ties with a carefully worded prose description or a thread safety annotation. The synchronized modifier plays no part in this documentation. Conditionally thread-safe classes must document which method invocation sequences require external synchronization, and which lock to acquire when executing these sequences. If you write an unconditionally thread-safe class, consider using a private lock object in place of synchronized methods. This protects you against synchronization interference by clients and subclasses and gives you the flexibility to adopt a more sophisticated approach to concurrency control in a later release.

When use? always.

Item 71: Use lazy initialization judiciously (performance, stability, scalability)

In summary, you should initialize most fields normally, not lazily. If you must initialize a field lazily in order to achieve your performance goals, or to break a harmful initialization circularity, then use the appropriate lazy initialization technique. For instance fields, it is the double-check idiom; for static fields, the lazy initialization holder class idiom. For instance fields that can tolerate repeated ini- tialization, you may also consider the single-check idiom.

When use? it depends pretty of context. Is hard to define when.

Item 72: Don’t depend on the thread scheduler (design, stability, readability)

In summary, do not depend on the thread scheduler for the correctness of your program. The resulting program will be neither robust nor portable. As a corollary, do not rely on Thread.yield or thread priorities. These facilities are merely hints to the scheduler. Thread priorities may be used sparingly to improve the quality of service of an already working program, but they should never be used to “fix” a program that barely works.

When use? always.

Item 73: Avoid thread groups (design)

To summarize, thread groups don’t provide much in the way of useful functionality, and much of the functionality they do provide is flawed. Thread groups are best viewed as an unsuccessful experiment, and you should simply ignore their existence. If you design a class that deals with logical groups of threads, you should probably use thread pool executors.


When use? always.

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