volatile keyword in Java

Defining a variable with the volatile keyword means that the value of that variable can be changed by other threads. To understand what volatile does, it is helpful to understand how threads handle regular variables.

For performance reasons, the Java language specification allows the JRE to store a local copy of the variable for each thread that references it. These "local" copies of variables are like a cache and help a thread avoid accessing main memory every time it needs to retrieve the value of a variable. When two threads start, one of them reads variable A as 10, and the other as 20. If the value of variable A has changed from 10 to 20, then the first thread will not know about the change and will store the wrong value A. But if variable A is marked as volatile, then whenever the thread reads the value of A, it will access the master copy of A and read its current value. A thread-local cache makes sense if the variables in your applications will not be changed externally.

If a variable is declared volatile, it means that it can be changed by different threads. It is natural to expect the JRE to provide some form of synchronization for such volatile variables. The JRE does implicitly provide synchronization when accessing volatile variables, but with one very big caveat: reading a volatile variable and writing to a volatile variable are synchronized, but non-atomic operations are not.

The volatile modifier provides atomic reads and writes, and the Atomic* classes (from java.util.concurrent.atomic) allow compare-and-swap operations to atomically change values without fear of another write coming in between reads and writes.

Read also:


Comments

Post a Comment

Popular posts from this blog

Methods for reading XML in Java

Image
DOM (Document Object Model) - object - reads XML, recreating it in memory in the form of an object structure, while the XML document is represented as a set of tags - nodes. Each node can have an unlimited number of child nodes. Each child can also contain several levels of descendants or none at all. Thus, the result is a kind of tree. Pluses: Easy to program. If there are many objects in XML with cross-references to each other, it is enough to go through the document twice: first time create objects without references and fill in the name-object dictionary, the second time - restore links. If an error occurs in XML, a half-created XML structure remains in memory, which will be automatically destroyed. Suitable for both reading and writing. Minuses: Low speed. Wastes a lot of memory. SAX (Simple API for XML) - event-driven - reads an XML document, responding to emerging events (opening or closing tag, string, attribute) by calling the event handlers provided by the appl...
Read more

XML, well-formed XML and valid XML

Image
XML, the eXtensible Markup Language, is a language with a simple formal syntax, well suited for creating and manipulating documents with programs, and at the same time easy to read and create documents by humans. XML is extensible, it does not fix the markup used in documents, and the developer is free to create markup in accordance with the needs of a particular area, being limited only by the syntax rules of the language. DTD A DTD, Document Type Definition, is a predefined set of rules that defines the relationships between elements and attributes. For example, the DTD for HTML says that the DIV tag must be inside the BODY tag and can appear multiple times, TITLE - in HEAD and only once, and SCRIPT - both there and there as many times as you want. A DTD is usually described directly in the document as a formulation line starting with <!DOCTYPE ... > or in a separate file. Difference between well-formed XML and valid XML Depending on the level of compliance, the document...
Read more

ArrayList and LinkedList in Java, memory usage and speed

Image
How much extra memory is needed when calling ArrayList.add()? If there is enough space in the array to accommodate a new element, then no additional memory is required. Otherwise, a new array is created, 1.5 times larger than the existing one (this is true for JDK above 1.7, in earlier versions the size of the increase is different). How much extra memory is allocated when you call LinkedList.add()? One new instance of the nested Node class is created. How much memory to store one byte primitive in LinkedList? Each LinkedList element stores a link to the previous element, the next element, and a data link. private static class Node<E> { E item; Node<E> next; Node<E> prev; //... } For 32-bit systems, each link is 32 bits (4 bytes). The object (header) of the nested Node class itself takes 8 bytes. 4 + 4 + 4 + 8 = 20 bytes, and since the size of each object in Java is a multiple of 8, so we get 24 bytes. A byte primitive occupies 1 by...
Read more