JaCoCo Runtime Execution File Behavior | Generated by AI

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When you run WebSphere Liberty Profile with the JaCoCo agent in exec mode, the JaCoCo agent does not modify the original .exec file during runtime. Here’s what happens:

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How JaCoCo Works in exec Mode

• Purpose: The JaCoCo agent collects code coverage data while your application runs.
• Output: It writes coverage data to the .exec file (e.g., jacoco.exec).
• File Behavior:
  • If the file does not exist, JaCoCo creates it.
  • If the file already exists, JaCoCo appends new coverage data to it (it does not overwrite or modify existing data unless you explicitly reset or merge the file).
  • The original .exec file is not altered in a way that would break or change its structure—it only grows with new data.

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Will the .exec File Be Modified?

• Yes, but only by appending data: The file will be updated with new coverage information each time you run your application with the JaCoCo agent.
• No structural changes: The format of the .exec file remains the same; it’s a binary file containing coverage data.

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Best Practices

• Merge or Reset: If you want to start fresh, delete or rename the .exec file before running your application again.
• Merge Multiple Runs: Use the jacoco:merge goal (in Maven) or the org.jacoco.agent.rt.RT class to merge multiple .exec files if needed.

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Example Liberty Configuration

In your server.env or jvm.options for Liberty, you might see:

-javaagent:/path/to/jacocoagent.jar=destfile=/path/to/jacoco.exec,append=true

• append=true ensures new data is added to the existing file.

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