Java concurrency package

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The Java concurrency package is a library supporting threading and parallel programming in the Java programming language.

Synchronizers

Mutilthread Synchronization before Java 5

Before Java 5, multithreads are supported using the lock mechanism for synchronization. Locks are implemented in Synchronized method. This mechanism “ensures that only one Java thread can execute an object's synchronized methods at a time” and “also allows threads to wait for resources to become available, and allows the thread that makes resources available to notify other threads that are waiting for the resources”. . When the synchronized keyword is used, the thread which invokes the synchronized method must obtain a lock for the object which makes this thread the lock holder. The rule of thumb of synchronized method is that only one thread can hold this lock at a time.

Three most commonly used methods including, wait(), notify(), and notifyAll() are used for resource communication between threads.

“The wait() method can only be invoked by the object's lock holder. It causes current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object” .

The notify() method wakes up one thread and only the notified thread can go ahead and do something. If there are more than one thread waiting on this object’s waiting queue, one of them is selected to be woke up. notify() wakes up the first thread in the waiting queue.

The notifyAll() method wakes up all threads in the wait set. notifyAll() is normally used when there are many threads to wake up simultaneously. Which thread gets the right to go ahead to execute depends upon thread property such as their priority.

One of the biggest issues of synchronized method is that it is an “all-or-nothing thing” . “Once a thread attempts to enter a synchronized block, it will hang until the lock is available.” which causes low performance because all other threads that need the same object have to wait. Another issue is that missing appropriate notifications such as notify() or notifyAll() while programming probably results in deadlock.

Java 5: Synchronizers

In Java 5, the Java concurrent package provides four new classes including: semaphore, countdownlatch, cyclickbarrier, and exchanger, used for data synchronization .

Semaphore is a counting signal. It maintains a set of permits. The “parking garage” can be a good analogy as an example to explain it. We can think of that the permits in semaphore equals to the capability in a garage. If the permits that have been issued reach the maximum garage capacity, the garage is full. No parking permit can be issued. The garage will have some space for other cars when some permits are returned.

“CountDownLatch is a synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes. It is initialized with a given count” . Something here that needs to be mentioned that “CountDownLatch is a one-shot phenomenon” meaning that the counter cannot be reused. If you need a counter that can be reset, consider using the CyclicBarrier method stating below.

CyclicBarriers is an aid “which allows a set of threads to wait for each other to reach a common barrier point” . CyclicBarriers is useful when a fixed sized group of thread is occasionally but must wait for each other in a program. “The barrier is called cyclic because it can be re-used after the waiting threads are released” .

Exchanger is actually what the word looks like. “It is a synchronization point where two threads can exchange objects. Each thread presents some object on entry to the exchange method and receives the object presented by the other thread on return” .

Concurrent Collections

As it is mentioned in the beginning of synchronizer, the lock mechanism is implemented in “synchronized” classes before Java 5 and “synchronized” method will lock the object that’s been accessed by a thread. When an object is locked by one thread, other threads which need to access the same object are going to wait until the object is available. Also, only the resources in the object are protected. Other resources outside the object are not.

"Synchronized classes can be useful when you need to prevent all access to a collection through a single lock, at the expense of poorer scalability”.

Java realized this issue and therefore enhanced it by supplying some more collection implementations such as ConcurrentHashMap, CopyOnWriteArrayList , and CopyOnWriteArraySet. Here only basic concepts will be introduced here due to that this component is a very advanced one in java concurrent package. For more information, please refer to J2SE 5.0 official website Package java.util.concurrenct.

ConcurrentHashMap: “It synchronizes different segments in a hash table, not the whole object. Therefore, other threads can still access other segments that are not in synchronization” . So other threads don’t have to wait. This provides the thread safety and also improves balances of the performance.

CopyOnWriteArrayList: “It uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible” . This means no modification will be allowed when using CopyOnWriteArrayList. Therefore this operation is immutable and also guarantees thread-safety.

CopyOnWriteArraySet: It’s a data structure to protect the data during traversal for modification of the Array. With CopyOnWriteArraySet, any changes in the data structure during traversal results in a copy being made for the modification. However, there is something we have to keep in mind. This is used assuming in that there won’t be too many changes being made. If a lot of possible modifications need to be performed, we are talking a lot of copies as well which could be bad.

Links

Java Concurrency in Practice

Lesson: Concurrency

Java Technology