The balking pattern is a software design pattern

Design pattern (computer science)

In software engineering, a design pattern is a general reusable solution to a commonly occurring problem within a given context in software design. A design pattern is not a finished design that can be transformed directly into code. It is a description or template for how to solve a problem that...

 that only executes an action on an object

Object (computer science)

In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...

 when the object is in a particular state. For example, if an object reads ZIP files and a calling method invokes a get method on the object when the ZIP file is not open, the object would "balk" at the request. In the Java

Java (programming language)

Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...

 programming language, for example, an IllegalStateException might be thrown under these circumstances.

There are some specialists in this field who think this is more of an anti-pattern

Anti-pattern

In software engineering, an anti-pattern is a pattern that may be commonly used but is ineffective and/or counterproductive in practice.The term was coined in 1995 by Andrew Koenig,...

, than a design pattern. If an object cannot support its API, it should either limit the API so that the offending call is not available or so that the call can be made without limitation, it should:

• Be created in a sane state
• Not make itself available until it is in a sane state
• Become a facade
  Façade pattern
  The facade pattern is a software engineering design pattern commonly used with Object-oriented programming. The name is by analogy to an architectural facade....
  
   and answer back an object that is in a sane state

Usage

Objects that use this pattern are generally only in a state that is prone to balking temporarily but for an unknown amount of time. If objects are to remain in a state which is prone to balking for a known, finite period of time, then the guarded suspension pattern may be preferred.

Implementation

Below is a general, simple example for an implementation of the balking pattern as originally seen in . As demonstrated by the definition above, notice how the "synchronized" line is utilized. If there are multiple calls to the job method, only one will proceed while the other calls will return with nothing. Another thing to note is the jobCompleted method. The reason it is synchronized is because the only way to guarantee another thread will see a change to a field is to synchronize access all access to it or declare it as volatile.


public class Example {
private boolean jobInProgress = false;

public void job {
synchronized(this) {
if (jobInProgress) {
return;
}
jobInProgress = true;
}
// Code to execute job goes here
// ...
}

void jobCompleted {
synchronized(this) {
jobInProgress = false;
}
}
}

See also

• Read and write lock pattern
• Guarded suspension pattern
  Guarded suspension
  In concurrent programming, guarded suspension is a software design pattern for managing operations that require both a lock to be acquired and a precondition to be satisfied before the operation can be executed...