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POSingleton Provider vs Regular Provider
 

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  1. POSingleton Provider vs Regular Provider
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    <p>I'm attempting (for the first time) to build up a token using dependency injection. I have a diamond hierarchy problem, where I need to prepend a signed hash of my data to the actual data itself. </p> <p>As a result, I find myself calling get() on the original data provider multiple times. This would be fine, but further down the graph, there are time-based components and as a result, the data returned on subsequent calls is different.</p> <p>A solution was suggested to use the Singleton Provider model. For this Provider, subsequent calls to get() return the same object (reference). This Provider caches the result of the first call to get() and continually returns that. When injecting dependencies, I inject the same instance of this provider twice, and as a result it returns the same data twice.</p> <p>This got me thinking, is there any situation where you wouldn't want to use a Singleton Provider? If you expect a different result, shouldn't a new Provider instance be created every time?</p> <pre><code>public MyUnderscoreStringSingletonProvider implements Provider&lt;String&gt; { private final Provider&lt;String&gt; mySomeOtherStringProvider; private String myCachedString; public MyUnderscoreStringSingletonProvider( Provider&lt;String&gt; someOtherStringProvider ) { mySomeOtherStringProvider = someOtherStringProvider; myCachedString = null; } @Override public String get() { if( myCachedString == null ) { myCachedString = create(); } return myCachedString; } private String create() { return "_" + mySomeOtherStringProvider.get(); } } // ... public class SomeCoolService { // ... public Provider&lt;String&gt; injectStringDoubler() { final Provider&lt;String&gt; stringProvider = injectUnderScoreStringProvider(); return new TwoConcatendatedStringsProvider( stringProvider, stringProvider ); // This would not work if Singleton Provider was not used. // Why should we ever use non-Singleton Providers? } protected Provider&lt;String&gt; injectUnderScoreStringProvider() { return new MyUnderscoreStringSingletonProvider( injectMyTimebasedStringProvider() // returns different result // depending // on time. ); } // ... } </code></pre>
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    1. USstudro
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    1. COWhy did you say you have a diamond hierarchy problem in Java?
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    2. CO@SenJacob - it's a diamond problem in the dependency injection graph. The same object needs to be used twice, and without the Singleton Provider, it will be a different object provided by the get() call each time. The question is, why would you not use a Singleton? Are there any examples of this that can be provided?
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      1. USstudro
    3. COThere are two ways on initializing an object one it to create it from scratch. The other is to reset an existing object to it's initial state. If the cost of creating the object is significantly more expensive than resetting it then using a pool of pre-created instances makes sense.
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