@Library annotation in Jenkins - Step-by-Step Execution

Process Flow - @Library annotation

Start Jenkinsfile

↓

@Library annotation reads library name

↓

Jenkins loads shared library code

↓

Library functions/classes become available

↓

Jenkinsfile calls library code

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Pipeline executes with library support

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End

The @Library annotation tells Jenkins to load shared library code before running the pipeline script.

Execution Sample

Jenkins

@Library('my-shared-lib') _
pipeline {
  agent any
  stages {
    stage('Example') {
      steps { myFunction() }
    }
  }
}

This Jenkinsfile uses @Library to load 'my-shared-lib' and calls a function from it in the pipeline.

Process Table

Step	Action	Evaluation	Result
1	Read Jenkinsfile start	Detect @Library('my-shared-lib')	Prepare to load 'my-shared-lib'
2	Load shared library	Fetch 'my-shared-lib' from configured source	Library code loaded and available
3	Parse pipeline block	Recognize stages and steps	Pipeline structure ready
4	Execute stage 'Example'	Call myFunction() from library	myFunction() runs successfully
5	Complete pipeline	No errors	Pipeline finishes successfully

💡 Pipeline ends after all stages run with library functions available

Status Tracker

Variable	Start	After Step 2	After Step 4	Final
libraryLoaded	false	true	true	true
myFunctionAvailable	false	true	true	true
pipelineStatus	not started	not started	running	completed

Key Moments - 2 Insights

Why do we need the underscore (_) after the @Library annotation?

The underscore tells Jenkins to load the library immediately and make its functions available in the script, as shown in execution_table step 2 and 4.

What happens if the library name is incorrect or missing?

Jenkins will fail to load the library, causing errors when calling library functions, stopping the pipeline before step 4.

Visual Quiz - 3 Questions

Look at the execution table, at which step is the shared library code loaded?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Check the 'Action' and 'Result' columns in execution_table row 2

According to variable_tracker, when does 'myFunctionAvailable' become true?

A After Step 2

B Start

C After Step 4

D Final

💡 Hint

Look at the 'myFunctionAvailable' row and its values after each step

If the @Library annotation is missing the underscore (_), what likely happens?

A Pipeline fails to start

B Library loads but functions are not available

C Pipeline runs normally

D Library loads twice

💡 Hint

Refer to key_moments explanation about the underscore usage

Concept Snapshot

@Library annotation loads shared Jenkins libraries.
Syntax: @Library('lib-name') _
Loads code before pipeline runs.
Makes library functions/classes available.
Use underscore (_) to apply immediately.
Without it, library code is not imported.

Full Transcript

The @Library annotation in Jenkinsfiles tells Jenkins to load shared library code before running the pipeline. The underscore (_) after the annotation is important because it instructs Jenkins to import the library functions immediately, making them available in the pipeline script. The execution flow starts by reading the Jenkinsfile, detecting the @Library annotation, loading the shared library, then running the pipeline stages that can call library functions. Variables like 'libraryLoaded' and 'myFunctionAvailable' change from false to true once the library is loaded. If the library name is wrong or the underscore is missing, the pipeline will fail or the functions won't be available. This visual trace helps beginners see how the annotation affects pipeline execution step-by-step.