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POJava: give access for created objects to a shared resource
 

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    <p>The situation before:</p> <p>A Resource exists. Class Root is created, and during it's creation it creates Child classes (many implementations of one base). Those can, but don't have to, posses children. Basically, this is a typical tree structure, that builds itself.</p> <p>One of the Child classes "NeedyChild" requires access to a Resource that was opened during creation of Root. There was only one Resource possible, so I made it a singleton and accesed it from NeedyChild by Resource.getInstance();</p> <p>The situation now:</p> <p>I have to build many trees. Those are independent of each other and take quite some time, so I made it concurrent. Unfortunatelly, each tree has it's own Resource.</p> <p>Question: how to access the Resource specific to the current runnable now?</p> <p>My solutions:</p> <ol> <li><p>Make Child base class require Resource parameter in it's constructor. Pros: it's quick. It's fairly efficient (passing a reference is no big deal), it's simple. Cons: it requires me to change constructor calls and signatures throughout the application, possibly >100 places.</p></li> <li><p>Reflection to make base constructor find it's caller, save reference, then just give Resource to Root, and NeedyChild will go up "creators" hierarchy to get Root and then Resource. But I won't do that, it's evil.</p></li> <li><p>Didn't research it, but maybe I can access the parent Runnable the code is currently executing inside? Cast it back to my RunnableClass which could hold reference to Resource? The problems are: I don't know if it's possible, it seems a bit "wrong" too...</p></li> <li><p>(not really working) Make the tree traversable both ways, give Resource to Root and from NeedyChild getParent() long enough until you're at Root. It would be great, but the NeedyChild needs Resource during it's creation, and only after it is created, it is appended to the parent. I thought of delaying filling Resource-dependent fields, but it gets ugly very quick.</p></li> </ol> <p>TBH I probably will have to modify the base constructor and either pass "this" (so I can traverse back to the Root), or pass Resource. The runnable-specific context seems also quite decent, but I'm already doing the parallel procesing to make things faster, I don't really want to negate it by some very slow reflection lookups...</p> <p>So, any ideas?</p> <p>Addendum 1: Sample Code</p> <pre><code>abstract class BaseElement{ List&lt;BaseElement&gt; children = new ArrayList&lt;&gt;(); public void addChild(BaseElement child){ children.add(child); } } class Loader{ private PackageElement resultPackage; private Resource resource; public Loader(Resource resource){ this.resource = resource; } public callMeFromOutside(SourceTree source){ resultPackage = new Package(source.pack()); } } class PackageElement extends BaseElement { public PackageElement(PackageTree pack){ this.addChild(new Element(pack.element())); } } class Element extends BaseElement{ public Element(ElementTree element){ this.addChild(new NeedyChild(element.needyChild())); } } class NeedyChild extends BaseElement{ public NeedyChild(NeedyTree needy){ this.setSomethingImportant(resource.loadSomethingBasedOn(needy)); } } </code></pre>
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    1. COIs there a one-to-one correspondence between a thread and a tree? If so, you should be able to make modest changes to your singleton code s.t. `getInstance` returns an instance that is thread-local. See the official documentation on the `ThreadLocal<T>` class [here](http://docs.oracle.com/javase/7/docs/api/java/lang/ThreadLocal.html).
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    2. COSome sample code that shows how the tree and the nodes are related in your current code would make the question a lot clearer.
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    3. CO@Eiríkur - yeah, each Runnable gets it's own piece of data to build tree upon. I'll look into ThreadLocal tommorow at work and see if it helps, thanks!
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