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The Java programming language and the Java virtual machine (JVM) is designed to support concurrent programming. All execution takes place in the context of threads. Objects and resources can be accessed by many separate threads. Each thread has its own path of execution, but can potentially access any object in the program.
For example, suppose that there are three processes, namely 1, 2, and 3. All three of them are concurrently executing, and they need to share a common resource (critical section) as shown in Figure 1. Synchronization should be used here to avoid any conflicts for accessing this shared resource.
This approach is characteristic of functional programming and is also used by the string implementations in Java, C#, and Python. (See Immutable object.) The second class of approaches are synchronization-related, and are used in situations where shared state cannot be avoided: Mutual exclusion
In concurrent programming, a monitor is a synchronization construct that prevents threads from concurrently accessing a shared object's state and allows them to wait for the state to change. They provide a mechanism for threads to temporarily give up exclusive access in order to wait for some condition to be met, before regaining exclusive ...
Concurrent components communicate by altering the contents of shared memory locations (exemplified by Java and C#). This style of concurrent programming usually needs the use of some form of locking (e.g., mutexes, semaphores, or monitors) to coordinate between threads. A program that properly implements any of these is said to be thread-safe.
In computer science, futures, promises, delays, and deferreds are constructs used for synchronizing program execution in some concurrent programming languages. Each is an object that acts as a proxy for a result that is initially unknown, usually because the computation of its value is not yet complete.
Concurrent data structures are significantly more difficult to design and to verify as being correct than their sequential counterparts. The primary source of this additional difficulty is concurrency, exacerbated by the fact that threads must be thought of as being completely asynchronous: they are subject to operating system preemption, page faults, interrupts, and so on.
Synchronization between threads is notoriously difficult for developers; this difficulty is compounded because Java applications can run on a wide range of processors and operating systems. To be able to draw conclusions about a program's behavior, Java's designers decided they had to clearly define possible behaviors of all Java programs.