Скачать или смотреть Resolving @ Autowired Issues in Spring Batch Custom Writers

Struggling with `@ Autowired` not working in your Spring Batch custom writer? Discover how to fix NullPointerExceptions and ensure proper dependency injection in your Java applications.
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This video is based on the question https://stackoverflow.com/q/60349672/ asked by the user 'Vickey Choudhary' ( https://stackoverflow.com/u/11863020/ ) and on the answer https://stackoverflow.com/a/62227804/ provided by the user 'Vickey Choudhary' ( https://stackoverflow.com/u/11863020/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving @ Autowired Issues in Spring Batch Custom Writers

In the world of Spring Batch development, you might encounter problems with dependency injection, particularly while using the @ Autowired annotation. A common issue is when a service you expect to be injected into a component turns out to be null. This often leads to frustrating NullPointerExceptions that halt your application. Let’s delve into this specific problem and discover how to effectively resolve it.

Understanding the Problem

You may find yourself in a situation where the CategoryService field within your custom writer class is not being injected as expected, leading to a NullPointerException when you attempt to use it. The relevant code snippet looks like this:

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During runtime, if you manually instantiate the custom writer like so:

[[See Video to Reveal this Text or Code Snippet]]

The categoryService remains uninitialized, resulting in a null value, which is why you encounter the NullPointerException.

Why Does This Happen?

The root cause of this problem lies in how Spring handles dependency injection. Here are the key points to understand:

Manual Instantiation: When you create an instance of a class manually (e.g., new Writer()), Spring is not involved in its lifecycle. This means any dependencies annotated with @ Autowired are not injected, as the Spring container has no knowledge of this instance.

Component Annotations: Annotations like @ Service, @ Repository, and @ Controller are specializations of @ Component. For auto-wiring to work, your components need to be recognized by Spring, either through these annotations or via the @ Component annotation itself.

Solution Steps

To resolve this issue and ensure your @ Autowired dependencies are correctly injected, follow these steps:

1. Use @ Component or Specialized Annotations

Make sure your custom writer class is annotated as a Spring component. You can do this by using @ Component, @ Service, or any relevant annotation based on the nature of your class. For example:

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2. Autowire the Writer

Instead of manually creating an instance of your writer class, you should let Spring manage this object. When defining the job or batch configuration, autowire the writer like this:

[[See Video to Reveal this Text or Code Snippet]]

Now, the Spring container will take care of injecting the CategoryService into your Writer instance.

3. Verify Component Scanning

Ensure that Spring's component scanning is properly set up in your configuration. This allows Spring to detect and manage the lifecycle of your beans. Typically, you might have something like:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following these steps, you can effectively resolve the @ Autowired not working issue in your Spring Batch custom writer. Properly managing the lifecycle of your components ensures that all dependencies are injected as intended, preventing any NullPointerExceptions from interrupting your application flow.

Now, you can proceed confidently with your Spring Batch implementations, knowing you have addressed the common pitfalls associated with dependency injection!