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How to Solve the package does not exist Error with Annotation Processing in Java

This guide provides a detailed guide on how to resolve the `package does not exist` error when generating Java classes with custom annotation processors in a Spring Boot and Maven project. --- This video is based on the question https://stackoverflow.com/q/69237882/ asked by the user 'Darlyn' ( https://stackoverflow.com/u/4048191/ ) and on the answer https://stackoverflow.com/a/69237946/ provided by the user 'rzwitserloot' ( https://stackoverflow.com/u/768644/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions. Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Package does not exists, generated java file from annoation processor Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l... The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license. If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com. --- Understanding the package does not exist Error in Java Annotation Processing When working on Java projects, especially within Spring Boot and Maven, you may encounter various challenges. One particularly perplexing issue arises when you attempt to use an annotation processor to generate Java classes. You may see an error message like: package org.hamcrest does not exist. This issue can be frustrating, especially if you're not well-versed in the inner workings of annotation processors. But don’t worry, we’re here to help you troubleshoot and solve this problem effectively. The Problem To illustrate, consider you have a custom annotation processor that is supposed to generate a class based on an annotation in your existing code. For example, you have: [[See Video to Reveal this Text or Code Snippet]] In an ideal scenario, this should lead to the generation of: [[See Video to Reveal this Text or Code Snippet]] However, instead of the expected output, you are faced with the error: package org.hamcrest does not exist. Let’s delve into why this error may occur and how to fix it. Possible Causes of the Error 1. Classpath Issues: Your first intuition might lead you to think that the classpath is not set up correctly. However, this isn't the root cause of the issue in most cases. 2. Annotations and Compilation Rounds: An annotation processor simply creates a text output file without parsing it immediately during the generation phase. This means the file is treated as a text file rather than a source file, and consequently not all compilation rules are applied. As a result, the annotation processor doesn’t have access to classes it needs to reference, such as org.hamcrest. This phase can create confusion regarding error visibility. Understanding Compilation Rounds When your annotation processor generates files, the compile process recognizes that new source files are now available. The Java compiler uses a system known as "rounds" to manage this compilation. Here’s how it works: First Round: The annotation processor creates new files, but errors related to missing classes are deferred until later. Subsequent Rounds: The Java compiler proceeds to compile the newly generated files in a follow-up round, which may lead to clearer errors based on the context of the generated files. However, it's crucial to understand that the classpath used for each compilation round remains the same throughout the process. What Likely Happened The most probable cause for your issue is that the necessary dependencies, particularly hamcrest, might not be present in your classpath. Often, developers tend to include only specific jars (like JUnit) without including their transitive dependencies, which may lead to missing classes at runtime. In your specific case, org.hamcrest is required and it's likely not included. Steps to Fix the Issue To address this error, follow these steps: Verify Dependencies: Ensure that hamcrest (and any other necessary dependencies) is included in your pom.xml file. You can add the hamcrest dependency like this: [[See Video to Reveal this Text or Code Snippet]] Test the Generated Class Manually: Before relying on your annotation processor, create the expected output file (AMatcher.java) manually without the processor. Compile it to see if you still face the same package does not exist error. If you do, then this confirms it’s not an annotation processing issue, but rather a dependency issue. Compile with Annotation Processors Enabled: Make sure your Maven build is configured to use annotation processors. You can enable annotation processing in your IDE or by properly configuring the Maven Compiler Plugi