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{{Short description|Programming language}}
{{primarysources|date=March 2011}}
{{Infobox programming language
 | name                   = Join Java
 | paradigm               = [[Procedural programming|Procedural]], [[Reflective programming|Reflective]]
 | released               = {{Start date|2000}}
 | designer               = [http://joinjava.unisa.edu.au/members.htm G. Stewart von Itzstein]
 | influenced by          = [[Java (programming language)|Java]]
 | website                = [http://joinjava.unisa.edu.au/ joinjava.unisa.edu.au]
}}
'''Join Java''' is a [[programming language]] based on the [[join-pattern]] that extends the standard [[Java (programming language)|Java programming language]] with the join semantics of the [[join-calculus]]. It was written at the [[University of South Australia]] within the Reconfigurable Computing Lab by Dr. Von Itzstein.

== Language characteristics ==
The Join Java extension introduces three new language constructs:
* Join methods
* Asynchronous methods
* Order class modifiers for determining the order that patterns are matched

Concurrency in most popular programming languages is implemented using constructs such as semaphores and monitors. Libraries are emerging (such as the Java concurrency library JSR-166) that provide higher-level concurrency semantics. [[communicating sequential processes]] (CSP), Calculus of Communicating Systems (CCS) and Pi have higher-level synchronization behaviours defined implicitly through the composition of events at the interfaces of concurrent processes. Join calculus, in contrast, has explicit synchronization based on a localized conjunction of events defined as reduction rules. Join semantics try to provide explicit expressions of synchronization without breaching the object-oriented idea of modularization, including dynamic creation and destruction of processes and channels.

The Join Java language can express virtually all published concurrency patterns without explicit recourse to low-level monitor calls. In general, Join Java programs are more concise than their Java equivalents. The overhead introduced in Join Java by the higher-level expressions derived from the Join calculus is manageable. The synchronization expressions associated with monitors (wait and notify) which are normally located in the body of methods can be replaced by Join Java expressions (the Join methods) which form part of the method signature.

=== Join methods ===
A Join method is defined by two or more Join fragments. A Join method will execute once all the fragments of the Join pattern have been called. If the return type is a standard Java type then the leading fragment will block the caller until the Join pattern is complete and the method has executed. If the return type is of type <STRONG>signal</STRONG> then the leading fragment will return immediately. All trailing fragments are asynchronous so will not block the caller.

Example:
<syntaxhighlight lang="java">
 class JoinExample {
    int fragment1() & fragment2(int x) {
       //will return value of x
       //to caller of fragment1
       return x;
    }
 }
</syntaxhighlight>
=== Ordering modifiers ===
Join fragments can be repeated in multiple Join patterns so there can be a case when multiple Join patterns are completed when a fragment is called. Such a case could occur in the example below if B(), C() and D() then A() are called. The final A() fragment completes three of the patterns so there are three possible methods that may be called. The <STRONG>ordered</STRONG> class modifier is used here to determine which Join method will be called. The default and when using the <STRONG>unordered</STRONG> class modifier is to pick one of the methods at random. With the <STRONG>ordered</STRONG> modifier the methods are prioritised according to the order they are declared.

Example:
<syntaxhighlight lang="java">
 class ordered SimpleJoinPattern {
    void A() & B() {
    }
    void A() & C() {
    }
    void A() & D() {
    }
    signal D() & E() {
    }
 }
</syntaxhighlight>
=== Asynchronous methods ===
Asynchronous methods are defined by using the <STRONG>signal</STRONG> return type. This has the same characteristics as the <STRONG>void</STRONG> type except that the method will return immediately. When an asynchronous method is called a new thread is created to execute the body of the method.

Example:
<syntaxhighlight lang="java">
 class ThreadExample {
    signal thread(SomeObject x) {
       //this code will execute in a new thread
    }
 }
</syntaxhighlight>

== Related languages ==
[[Polyphonic C sharp]] is the closest related language.<br>
[[Cω]] the successor of Polyphonic C sharp.

[[Hardware Join Java]] language further extended Join Java to implement Hardware semantics. This language extended the semantics of Join Java to [[FPGA]] applications.

== References ==
* von Itzstein, G, Stewart. and Jasiunas, M (2003). On Implementing High Level Concurrency in Java. Advances in Computer Systems Architecture 2003, Aizu Japan, Springer Verlag.
* von Itzstein, G, Stewart. and D. Kearney (2002). Applications of Join Java. Proceedings of the Seventh Asia Pacific Computer Systems Architecture Conference ACSAC'2002. Melbourne, Australia, Australian Computer Society: 1-20.
* von Itzstein, G, Stewart. and D. Kearney (2004). The Expression of Common Concurrency Patterns in Join Java. International Conference on Parallel and Distributed Processing Techniques and Applications, Las Vegas.
* Hopf, J., von Itzstein, G, Stewart, et al. (2002). Hardware Join Java: A High Level Language For Reconfigurable Hardware Development. International Conference on Field Programmable Technology, Hong Kong.

== External links ==
* [http://joinjava.unisa.edu.au/ Website]

[[Category:Concurrent programming languages]]
[[Category:Programming languages]]
[[Category:Programming languages created in 2000]]
[[Category:2000 software]]
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