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Easy Learn Java: Programming Articles, Examples and Tips - Page 371


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Concurrent Programming with J2SE 5.0

Go to all tips in Good To Know
Java is a multithreaded programming language that makes programming with threads easier, by providing built-in language support for threads. The built-in primitives however, such as synchronized blocks, Object.wait(), and Object.notify() are insufficient for many programming tasks. This leads developers to implement their own high-level synchronization facilities, but given the difficulty of concurrency issues, their implementations may not be correct, efficient, or high quality.

The Java 2 Platform, Standard Edition release 5.0 (J2SE 5.0), which is also known as Tiger, has provided a new path to multithreading in the Java programming language. The original mechanisms for coordinating threads with wait() and notify() are now enhanced with new and sophisticated mechanisms for working with threads. The new mechanisms are part of the java.util.concurrent package, which aims to offer a standard set of concurrency utilities that will ease the task of developing multithreaded applications and servers. In addition, such standards will improve the quality of such applications. The package has been defined through the Java Community Process as JSR 166: Concurrency Utilities.

This article provides an overview and an introductory tutorial to the new concurrency mechanisms that have been added to J2SE 5.0. It helps developers get started using the new java.util.concurrent package and its subpackages.
Limitations of Built-in Synchronization in J2SE 1.4.x
When writing multithread applications, the issues that may create difficulties are related to data synchronization; these are the errors that make design harder, and such errors are hard to detect. Built-in synchronization (methods and blocks) are fine for many lock-based applications, but they do have their own limitations, such as:

  • No way to back off from an attempt to acquire a lock that is already held, or to give up after waiting for a specified period of time, or to cancel a lock attempt after an interrupt.
  • No way to alter the semantics of a lock, for example, with respect to reentrancy, read versus write protection, or fairness.
  • No access control for synchronization. Any method can perform synchronized(obj) for any accessible object.
  • Synchronization is done within methods and blocks, thus limiting use to strict block-structured locking. In other words, you cannot acquire a lock in one method and release it in another.
Such problems can be overcome by using utility classes to control locking, such as Mutex, which is another term for a lock. A mutex, however, doesn't nest like synchronization methods or blocks.

Please read full article here



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Posted by jalex on Friday, June 10, 2005 (00:00:00) (2968 reads)

Pattern and Matcher

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In addition to the regular expression methods that are available in the String class , there are two classes that are specifically user for regular expression matching.

  • java.util.regex.Pattern precompiles regular expressions so they can be executed more efficiently. It also has a few utility functions. This same pattern can be reused by many Matcher objects.
        Pattern pat = Pattern.compile(regexString);
  • java.util.regex.Matcher objects are created from a Pattern object and a subject string to scan. This class provides a full set of methods to do the sacnning.
        Matcher m = pat.matcher(subject);

Common methods

The following variables represent elements of their declared type in the prototypes below.

import java.util.regex.*;
. . .
    boolean b;       // may be used in if statement.
    int i;           // index or character position.
    int g;           // group number
    int n;           // number of groups
    CharSequence cs; // effectively either String or StringBuffer
    String s;        // Subject string.
    String regex;    // Regular expression string.
    StringBuffer sb; // Used to build up string from repeated scans.
    Pattern p = Pattern.compile(regex); // Compiles regular expression into Pattern.
    Matcher m = p.matcher(s); // Creates Matcher with subject s and Pattern p.
Result Method Description
Creating a Pattern
p = Pattern.compile(regex); Creates Pattern based on regex. May throw PatternSyntaxException.
p = Pattern.compile(regex, f); As above. Flag f can be Pattern.CASE_INSENSITIVE, ....
Finding pattern matches
b = m.find(); True if pattern can be found in the subject string. First call starts trying at beginning of string. Subsequent calls start after last character previously matched, which makes it good for a while loop.
b = m.find(i); True if pattern can match somewhere at or after position i.
b = m.matches(); True if pattern matches entire subject string.
b = Pattern.matches(regex, s); As above, but less efficient if regex used more than once.
b = m.lookingAt(); True if pattern matches starting at first char.
Getting results of last pattern match. Corresponds to group 0, then entire match.
s = m.group(); String which was matched.
i = m.start(); Index of first character of match.
i = m.end(); Index of last character plus 1 of match.
Getting group results of last match
s = m.group(g); String which was matched by group g.
i = m.start(g); Index of first character of group g.
i = m.end(g); Index of last character plus 1 of group g.
n = m.groupCount(); Number of groups that were matched.
Misc
m = m.reset(); Resets Matcher m so that next find starts at beginning.
m = m.reset(cs); Resets Matcher m to match subject cs.
Replacing text
s = m.replaceFirst(rep); Returns string which is subject with first match replaced by rep.
s = m.replaceAll(rep); Returns string with all matches replaced by rep.
Building replacement in a StringBuffer
m = m.appendReplacement(sb, s); When it matches, (1) everything skipped before the match is appended to the StringBuffer sb, then (2) it appends s, with "$n" replaced by group n.
sb = m.appendTail(sb); Appends last part that m didn't match to StringBuffer. Useful after loop calling appendReplacement() to finish.

Other Resources



1 comment | Printer Friendly Page  Send to a Friend | Score: 0
Posted by jalex on Wednesday, June 08, 2005 (00:00:00) (2851 reads)

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