Concurrent computing is a form of computing in which programs are designed as collections of interacting computational processes that may be executed in parallel...
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...
In computer science, a lock is a synchronization mechanism for enforcing limits on access to a resource in an environment where there are many threads of execution. Locks are one way of enforcing concurrency control policies.-Types:...
In computer programming, a precondition is a condition or predicate that must always be true just prior to the execution of some section of code or before an operation in a formal specification....
to be satisfied before the operation can be executed. The guarded suspension pattern is typically applied to method calls in object-oriented programs, and involves suspending the method call, and the calling thread, until the precondition (acting as a guard
Guard (computing)
In computer programming, a guard is a boolean expression that must evaluate to true if the program execution is to continue in the branch in question. The term is used at least in Haskell, Clean, Erlang, occam, Promela, OCaml and Scala programming languages. In Mathematica, guards are called...
Blocking occurs when a subroutine does not return until it either completes its task or fails with an error or exception. A process that is blocked is one that waits for some event, such as a resource becoming available or the completion of an I/O operation.In a multitasking computer system,...
, the guarded suspension pattern is generally only used when the developer knows that a method call will be suspended for a finite and reasonable period of time. If a method call is suspended for too long, then the overall program will slow down or stop, waiting for the precondition to be satisfied. If the developer knows that the method call suspension will be indefinite or for an unacceptably long period, then the balking pattern
Balking pattern
The balking pattern is a software design pattern that only executes an action on an object 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...
may be preferred.
Implementation
In Java, the Object class provides the wait and notify methods to assist with guarded suspension. In the implementation below, originally found in , if there is no precondition satisfied for the method call to be successful, then the method will wait until it finally enters a valid state.
public class Example {
synchronized void guardedMethod {
while (!preCondition) {
try {
//Continue to wait
wait;
//…
} catch (InterruptedException e) {
//…
}
}
//Actual task implementation
}
synchronized void alterObjectStateMethod {
//Change the object state
//…..
//Inform waiting threads
notify;
}
}
An example of an actual implementation would be a queue object with a get method that has a guard to detect when there are no items in the queue. Once the "put" method notifies the other methods (for example, a get method), then the get method can exit its guarded state and proceed with a call. Once the queue is empty, then the get method will enter a guarded state once again.
The balking pattern is a software design pattern that only executes an action on an object 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...
is an alternative pattern for dealing with a precondition
Guarded commands includes a similar language construct
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