Likewise, a field can be annotated with to inform the Spring DI framework that the field should be populated using DI. In the case of Spring, a class can be annotated with to inform the Spring Dependency Injection (DI) framework that it can be used as a candidate for DI. In most cases, an annotation provides supplementary information about a class, method, or field. According to the Java Language Specification (JLS):Īnnotation is a marker which associates information with a program construct, but has no effect at run time. 5 Annotations Every Java Developer Should KnowĪn annotation is a special type of interface in Java that can be used to describe Java constructs, such as classes, methods, and fields.As well as describing the basic concepts surrounding meta-annotations, we will also look at pragmatic ways to process meta-annotations and examples of how these annotations are used in a few of the most popular frameworks.įor more information on annotations, see the other articles in this series: In this article, we will delve into one of these advanced techniques: Meta-annotations. While the annotations do not add any functionality in and of themselves, they allow for an application to process them and make decisions based on the presence of certain annotations.Ī basic understanding of annotations is essential to becoming a seasoned Java developer, but more and more, more advanced annotation techniques are required. If you want to register more processors write multiple paths, each in a new line.Annotations are an indispensable part of Java applications and have become nearly ubiquitous thanks to their use in frameworks such as Spring and JUnit. In this file write path to your annotation processor class. The last thing you need to do is register the processor.Ĭreate src/resources/META-INF/services/ file. I am saving the XML to a file descriptor.xml.Based on the elements found, I glue them in the XML format.I find all elements annotated with For each such element (class) I am looking for enclosed elements (setters) with annotation.I will not explain line by line what is happening in this method. Of course it could be improved by using some templating engine (like Velocity),īut for the purposes of this example, that is enough. annotationprocessor public abstract class DeployableService Suppose we have a service that accepts some messages: I will show an example based on a problem that occurred recently in my project at work. Will result in inconsistency that can be difficult to debug.Īn example could be older servers that do not support annotations and need XML descriptors for Java code. So whenever you modify the A code, you have to modify the B code and failing to do so It often happens that some parts of the code depend on each other. Is the result of this procedure dependent on another part of the code and may result in inconsistencies in the project if not done? This is a very strong premise for automating code.Įven if you assume that you may not have to repeat the procedure in the future, you usually have to.Īnd then you thank yourself for automating something in time.ģ. Is the procedure always the same and only the inputs change? Sometimes it’s hard to estimate it, in that case I use 3.Ģ. You have to guess (or maybe you know) how many times in the future you will have to repeat this task. You may ask: but what is X? Is it 3, 5, 10? Will it take me less time to automate it than to do it X times? The decision as to whether to automate or generate something can be made on the basis of the following questions:ġ. Sometimes it is a short fragment that is not worth automating. When to use?Īlthough code generation is enjoyable (at least for me) be careful not to overdo it. If you want to go straight to the code, you can find it on my GitHub. In this post I will show you how to create some custom annotations and generate code based on them. That’s why I decided to share my experience with you :) Have you ever written code that you felt could be generated without any problems?
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