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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Issues in Dependency Injection article","articleBody":"The [article on Dependency Injection](https://dev.java/learn/reflection/dependency-injection/) will turn one year tomorrow.\n\nThat being said, there are a few issues I see with that article:\n- The last codeblock at the end of the article declares an `BeanFactory beanFactory = BeanFactory.INSTANCE;` which is not used since it uses `BeanFactory.INSTANCE.getInstanceOf(...)` as opposed to `beanFactory.getInstanceOf(...)`.\n- In the \"Creating Singletons\" section, the text mentions \"Here is the class.\" but there are 6 paragraphs (not counting the \"Here is the class.\" paragraph) until the class actually starts.\n- (more of a suggestion) While the \"Creating Singletons\" section mentions that this approach could create the multiple objects of a singleton (can it really be called a singleton if the DI framework allows creating multiple instances of it even if only one is actually used), this problem could be avoided by using [`computeIfAbsent`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/concurrent/ConcurrentMap.html#computeIfAbsent(K,java.util.function.Function)) which ensures that the lambda is not executed concurrently.\n- The same section also contains \"For that, you can use a registry, implemented by a ConcurrentMap, that is a thread-safe _extension_ of the Map interface.\" - I think it should use _implementation_ instead of _extension_.\n- The code in the article introduces a potentially confusing side effect which is not mentioned at all: It is possible to create singleton instances with constructor parameters by calling the `getInstanceOf()` method with these parameters and these instances can then be used by later `@Inject`s. However, `@Inject` cannot use constructor parameters leading to the following issues:\n  - Assuming the singleton class has a constructor with arguments but no no-args constructor: If `getInstanceOf()` is called with the parameters before creating the objects, it can be used by `@Inject` but if the `@Inject` is executed first, it throws a `NoSuchMethodException` because the constructor is not found. This is dependent on the order of calls (and it's supposed to be thread-safe!).\n  - If the singleton class has both a no-args constructor and a parameterized constructor that's called via `getInstanceOf(parameters, here)` which is also used by `@Inject`, the called constructor depends on the number of arguments. This can lead to issues that are hard to debug (wrong constructor sometimes called), especially in multithreaded environments.","author":{"url":"https://github.com/danthe1st","@type":"Person","name":"danthe1st"},"datePublished":"2025-07-24T15:08:09.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/172/devjava-content/issues/172"}