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DPE University Lab: Learn how to handle cases where the outputs of one goal overwrite the inputs of a previous goal, causing build cache misses
https://github.com/gradle/dpeuni-maven-build-cache-deep-dive-outputs-overwrite-inputs

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DPE University Lab: Learn how to handle cases where the outputs of one goal overwrite the inputs of a previous goal, causing build cache misses

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# DPE University Training



[](https://dpeuniversity.gradle.com/)

Checkout all the **free** Maven, Gradle, and DPE courses at the [DPE University][dpe-university]!



## Maven Build Cache Deep Dive - Lab 05: Outputs Overwrite Inputs



This is a hands-on exercise to go along with the [Maven Build Cache Deep Dive][course-url] training module. In this exercise you will go over the following:



- How to deal with cache misses caused by outputs that overwrite inputs of a previously run goal.



## Prerequisites



- Finished going through the _Troubleshooting Build Cache Misses_ section in Maven Build Cache Deep Dive.

- Java 11+



Steps

-----



1. If you have not completed the previous labs, authenticate Maven with the Develocity server.



> [!NOTE]

> As part of taking this **free** course, you have access to a training instance of Develocity located at:

> ```

>  https://dpeuniversity-develocity.gradle.com/

>  ```

> [Sign in][develocity-url] to this Develocity server using the same account you use for the DPE University.

>

> This server is configured so users can only access the Build Scan® and Build Cache entries they publish.



Run the following command and follow the instructions in your terminal:



 ```shell

 ./mvnw com.gradle:develocity-maven-extension:provision-access-key

 ```

> [!NOTE]

> For more ways to authenticate, see the [authentication guide](https://docs.gradle.com/enterprise/maven-extension/#authenticating_with_gradle_enterprise) to see how to provide credentials.



2. In this example the `data` directory represents content used by the build that _may_ be in another repository, a external application, or other resources needed by the build.



    This directory has already been added as an input, see the `pom.xml`.



3. Run the build **multiple times**:



    ```shell

    ./mvnw clean install

    ```



4. Notice how the test goal was not cached (the tests run on every build). Open the Build Scan to see more details.

   

    Open `HelloWorldTest.java` and look at the `writeToResourcesDir` test method, this test writes to a directory that is used as an input. This changed input will cause a cache miss on the next build.



5. Fix the test.



    While it is possible to configure Develocity to [ignore a whole file](https://docs.gradle.com/enterprise/maven-extension/#ignoring_arbitrary_files), when possible it's best to fix the cause of the problem. In this case, update the test to write to a different location (a temp directory or the `target/` dir).



    ```diff

    - // resolve a file in the data dir (data/timing.txt)

    - Path timingFile = Paths.get("data/timing.txt");

    + // resolve a file relative to the `target` directory

    + Path timingFile = Paths.get("./target/test-output/timing.txt");

    + Files.createDirectories(timingFile.getParent());

    ```

6. Run the build **multiple times**:



    ```shell

    ./mvnw clean install

    ```



   After the first run the test goal will have a cache hit!



## Solution Reference



To see the solution to the lab, check out the [`solution`](https://github.com/gradle/outputs-overwrite-inputs-maven-build-cache-lab/commit/solution) branch of this project.



## More Free Labs & Courses



Be sure to check out our other **free** [courses][dpe-university] and [labs](https://github.com/gradle?q=dpe-university)!



**Related courses:**

- [Maven - Build Cache Deep Dive][course-url]

- [Maven - Maintaining an Optimized Build Cache](https://dpeuniversity.gradle.com/c/42cf9d626302011526c4a0536b26af929b5bef58)

- [Develocity - How to Use Build Scans](https://dpeuniversity.gradle.com/c/0b0b3e4a8d21709ff39074e9962eee6ca4276dc1)



**Related labs:**

- [Lab 01 - Using the local build cache](https://github.com/gradle/getting-started-maven-build-cache-lab)

- [Lab 02 - Missing Inputs With Build Caching](https://github.com/gradle/missing-inputs-maven-build-cache-lab)

- [Lab 03 - Add Build Cache Support to any Maven Plugin](https://github.com/gradle/caching-any-plugin-maven-build-cache-lab)

- [Lab 04 - Handling Cache Misses with Normalization](https://github.com/gradle/cache-misses-maven-build-cache-lab)

- [Lab 05 - Outputs Overwrite Inputs](https://github.com/gradle/outputs-overwrite-inputs-maven-build-cache-lab)

- [Lab 06 - Maintaining an Optimized Build Cache](https://github.com/gradle/maintaining-optimized-cache-maven-build-cache-lab)



[course-url]: https://dpeuniversity.gradle.com/c/47262fea1e74b719afb590d8cb3f8280bf2af732

[dpe-university]: https://dpeuniversity.gradle.com/

[develocity-url]: https://dpeuniversity-develocity.gradle.com/