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## Package javolution.context

Provides real-time `Context` to facilitate separation of concerns and achieve higher level of performance and code predictability.

See:
Description

Class Summary
Allocator<T> | This class represents an object allocator; instances of this class are generated by `AllocatorContext`.
AllocatorContext | This class represents an allocator context; it defines the the allocation policy of `ObjectFactory` products.
AllocatorContext.Reference<T> | This class represents a `reference` allocated from the current `AllocatorContext`.
ArrayFactory<T> | This class holds factories to produces arrays of variable length.
ConcurrentContext | This class represents a context to take advantage of concurrent algorithms on multi-processors systems.
Context | This class represents an execution context; they can be associated to particular threads or objects.
HeapContext | This class represents the default allocator context.
ImmortalContext | This class represents an allocator from immortal memory (RTSJ).
LocalContext | This class represents a context to define locally scoped environment settings.
LocalContext.Reference<T> | This class represents a reference whose setting is local to the current `LocalContext`.
LogContext | This class represents a context for object-based/thread-based logging capabilities.
ObjectFactory<T> | This class represents an object factory; it allows for object recycling, pre-allocation and stack allocations.
PersistentContext | This class represents a context persistent accross multiple program executions.
PersistentContext.Reference<T> | This class represents a reference over an object which can be kept persistent accross multiple program executions.
PoolContext | This class represents a shared pool context for object allocation/recycling.
SecurityContext | This class represents a high-level security context (low level security being addressed by the system security manager).
StackContext | This class represents a stack `allocator context`; (using thread-local pools or RTSJ `ScopedMemory`).

Exception Summary
ConcurrentException | This class encapsulates errors or exceptions raised during the execution of concurrent threads (`ConcurrentException` are raised upon exit of the `ConcurrentContext`).

## Package javolution.context Description

Provides real-time `Context` to facilitate separation of concerns and achieve higher level of performance and code predictability.

## Separation of Concerns

Separation of concerns is an important design principle greatly misunderstood. Most developers think it is limited to modularity and encapsulation or it requires special programming tools (e.g. Aspect programming).

Separation of concerns is very powerful and easier than it looks. Basically, it could be summarized as the "pass the buck principle". If you don't know what to do with some information, just give it to someone else who might know.

A frequent example is the catching of exceptions too early (with some logging processing) instead of throwing a checked exception. Unfortunately, they are still plenty of cases where the separation of concerns is not as good as it could be. For example logging! Why low-level code need to know which logging facility is being used (e.g. standard logging, Log4J library or anything else)? Furthermore, why logging should have to be based upon the class hierarchy (standard logging)?

Separation of concerns can be addressed through "Aspect Programming", but there is a rather simpler solution "Context Programming"!

It does not require any particular tool, it basically says that every threads has a context which can be customized by someone else (the one who knows what to do). Then, your code looks a lot cleaner and is way more flexible as you don't have to worry about logging, security, performance etc. in your low level methods. For example:``

           void myMethod() {
               ...
               LogContext.info("Don't know where this is going to be logged to");
               ...
           }

Used properly Javolution's `contexts` greatly facilitate the separation of concerns. Contexts are complemented by others classes such as for example the `Configurable` class to reduce dependency between configuration and application code.

## Predefined Contexts:

This package provides few predefined contexts:

- `LocalContext` - To define locally scoped environment settings.
- `ConcurrentContext` - To take advantage of concurrent algorithms on multi-processors systems.
- `AllocatorContext` - To control object allocation, e.g. `StackContext` to allocate on the stack (or RTSJ ScopedMemory).
- `LogContext` - For thread-based or object-based logging capability, e.g. `StandardLog` to leverage standard logging capabilities. Note: `java.util.logging` provides class-based logging (based upon class hierarchy).
- `PersistentContext` - To achieve persistency across multiple program execution.
- `SecurityContext` - To address application-level security concerns.
- `TestContext` - To address varied aspect of testing such as performance and regression.

## FAQ:

1. I am writing an application using third party libraries. I cannot avoid GC unless I get the source and patch it to Javolution. Can I still make my application real-time using `StackContext`?

   You cannot get determinism using "any" library (including Java standard library) regardless of the garbage collector issue. Array resizing, lazy initialization, map rehashing (...) would all introduce unexpected delays (this is why Javolution comes with its own `real-time` collections implementation). Still, you may use incremental/real-time collectors (if few milliseconds delays are acceptable). These collectors work even faster if you limit the amount of garbage produced onto the heap through `stack allocations`.

2. Can you explain a little how objects can be "stack" allocated?

   It all depends upon the StackContext `default` implementation. The default implementation use thread-local queues (no synchronization required); but if you run on a RTSJ virtual machine entering a `StackContext` could mean using `ScopedMemory`.

3. As a rule, I am skeptical of classes that pool small objects. At one time (5 years ago) it was a big win. Over time, the advantage has diminished as garbage collectors improve. Object pools can make it much more difficult for the garbage collector to do its job efficiently, and can have adverse effects on footprint. (Joshua Bloch)

   Stack allocation is different from object pooling, it is a simple and transparent way to make your methods "clean" (no garbage generated), it has also the side effect of making your methods faster and more time-predictable. If all your methods are "clean" then your whole application is "clean", faster and more time-predictable (aka real-time).

   In practice very few methods need to enter a `StackContext`, only the one generating a significant number of temporary objects (these methods are made "cleaner" and faster through stack allocation). For example:``

               public final class DenseVector<F extends Field<F>> extends Vector<F> {
                   ...
                   public F times(Vector<F> that) {
                       final int n = this.getDimension();
                       if (that.getDimension() != n) throw new DimensionException();
                       StackContext.enter();
                       try { // Reduces memory allocation / garbage collection.
                           F sum = this.get(0).times(that.get(0));
                           for (int i = 1; i < n; i++) {
                               sum = sum.plus(this.get(i).times(that.get(i)));
                           }
                           return StackContext.outerCopy(sum); // Stack object exported through copy.
                       } finally {
                           StackContext.exit(); // Resets stack.
                       }
                   }
                   ...
               }

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