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Extending with Shared Libraries

As Pipeline is adopted for more and more projects in an organization, common patterns are likely to emerge. Oftentimes it is useful to share parts of Pipelines between various projects to reduce redundancies and keep code "DRY" [1].

Pipeline has support for creating "Shared Libraries" which can be defined in external source control repositories and loaded into existing Pipelines.

Defining Shared Libraries

An Shared Library is defined with a name, a source code retrieval method such as by SCM, and optionally a default version. The name should be a short identifier as it will be used in scripts.

The version could be anything understood by that SCM; for example, branches, tags, and commit hashes all work for Git. You may also declare whether scripts need to explicitly request that library (detailed below), or if it is present by default. Furthermore, if you specify a version in Jenkins configuration, you can block scripts from selecting a different version.

The best way to specify the SCM is using an SCM plugin which has been specifically updated to support a new API for checking out an arbitrary named version (Modern SCM option). As of this writing, the latest versions of the Git and Subversion plugins support this mode; others should follow.

If your SCM plugin has not been integrated, you may select Legacy SCM and pick anything offered. In this case, you need to include ${library.yourLibName.version} somewhere in the configuration of the SCM, so that during checkout the plugin will expand this variable to select the desired version. For example, for Subversion, you can set the Repository URL to https://svnserver/project/${library.yourLibName.version} and then use versions such as trunk or branches/dev or tags/1.0.

Directory structure

The directory structure of a Shared Library repository is as follows:

(root)
+- src                     # Groovy source files
|   +- org
|       +- foo
|           +- Bar.groovy  # for org.foo.Bar class
+- vars
|   +- foo.groovy          # for global 'foo' variable
|   +- foo.txt             # help for 'foo' variable
+- resources               # resource files (external libraries only)
|   +- org
|       +- foo
|           +- bar.json    # static helper data for org.foo.Bar

The src directory should look like standard Java source directory structure. This directory is added to the classpath when executing Pipelines.

The vars directory hosts scripts that define global variables accessible from Pipeline. The basename of each *.groovy file should be a Groovy (~ Java) identifier, conventionally camelCased. The matching *.txt, if present, can contain documentation, processed through the system’s configured markup formatter (so may really be HTML, Markdown, etc., though the txt extension is required).

The Groovy source files in these directories get the same “CPS transformation” as in Scripted Pipeline.

A resources directory allows the libraryResource step to be used from an external library to load associated non-Groovy files. Currently this feature is not supported for internal libraries.

Other directories under the root are reserved for future enhancements.

Global Shared Libraries

There are several places where Shared Libraries can be defined, depending on the use-case. Manage Jenkins » Configure System » Global Pipeline Libraries as many libraries as necessary can be configured.

Add a Global Pipeline Library

Since these libraries will be globally usable, any Pipeline in the system can utilize functionality implemented in these libraries.

These libraries are considered "trusted:" they can run any methods in Java, Groovy, Jenkins internal APIs, Jenkins plugins, or third-party libraries. This allows you to define libraries which encapsulate individually unsafe APIs in a higher-level wrapper safe for use from any Pipeline. Beware that anyone able to push commits to this SCM repository could obtain unlimited access to Jenkins. You need the Overall/RunScripts permission to configure these libraries (normally this will be granted to Jenkins administrators).

Folder-level Shared Libraries

Any Folder created can have Shared Libraries associated with it. This mechanism allows scoping of specific libraries to all the Pipelines inside of the folder or subfolder.

Folder-based libraries are not considered "trusted:" they run in the Groovy sandbox just like typical Pipelines.

Automatic Shared Libraries

Other plugins may add ways of defining libraries on the fly. For example, the GitHub Organization Folder plugin allows a script to use an untrusted library such as github.com/someorg/somerepo without any additional configuration. In this case, the specified GitHub repository would be loaded, from the master branch, using an anonymous checkout.

Using libraries

Shared Libraries marked Load implicitly allows Pipelines to immediately use classes or global variables defined by any such libraries. To access other shared libraries, the Jenkinsfile needs to use the @Library annotation, specifying the library’s name:

Configuring a Global Pipeline Library
@Library('my-shared-library') _
/* Using a version specifier, such as branch, tag, etc */
@Library('my-shared-library@1.0') _
/* Accessing multiple libraries with one statement */
@Library(['my-shared-library', 'otherlib@abc1234']) _

The annotation can be anywhere in the script where an annotation is permitted by Groovy. When referring to class libraries (with src/ directories), conventionally the annotation goes on an import statement:

@Library('somelib')
import com.mycorp.pipeline.somelib.UsefulClass

For Shared Libraries which only define Global Variables (vars/), or a Jenkinsfile which only needs a Global Variable, the annotation pattern @Library('my-shared-library') _ may be useful for keeping code concise. In essence, instead of annotating an unnecessary import statement, the symbol _ is annotated.

It is not recommended to import a global variable/function, since this will force the compiler to interpret fields and methods as static even if they were intended to be instance. The Groovy compiler in this case can produce confusing error messages.

Libraries are resolved and loaded during compilation of the script, before it starts executing. This allows the Groovy compiler to understand the meaning of symbols used in static type checking, and permits them to be used in type declarations in the script, for example:

@Library('somelib')
import com.mycorp.pipeline.somelib.Helper

int useSomeLib(Helper helper) {
    helper.prepare()
    return helper.count()
}

echo useSomeLib(new Helper('some text'))

Global Variables however, are resolved at runtime.

Loading libraries dynamically

As of version 2.7 of the Pipeline: Shared Groovy Libraries plugin, there is a new option for loading (non-implicit) libraries in a script: a library step that loads a library dynamically, at any time during the build.

If you are only interested in using global variables/functions (from the vars/ directory), the syntax is quite simple:

library 'my-shared-library'

Thereafter, any global variables from that library will be accessible to the script.

Using classes from the src/ directory is also possible, but trickier. Whereas the @Library annotation prepares the “classpath” of the script prior to compilation, by the time a library step is encountered the script has already been compiled. Therefore you cannot import or otherwise “statically” refer to types from the library.

However you may use library classes dynamically (without type checking), accessing them by fully-qualified name from the return value of the library step. static methods can be invoked using a Java-like syntax:

library('my-shared-library').com.mycorp.pipeline.Utils.someStaticMethod()

You can also access static fields, and call constructors as if they were static methods named new:

def useSomeLib(helper) { // dynamic: cannot declare as Helper
    helper.prepare()
    return helper.count()
}

def lib = library('my-shared-library').com.mycorp.pipeline // preselect the package

echo useSomeLib(lib.Helper.new(lib.Constants.SOME_TEXT))

Library versions

The "Default version" for a configured Shared Library is used when "Load implicitly" is checked, or if a Pipeline references the library only by name, for example @Library('my-shared-library') _. If a "Default version" is not defined, the Pipeline must specify a version, for example @Library('my-shared-library@master') _.

If "Allow default version to be overridden" is enabled in the Shared Library’s configuration, a @Library annotation may also override a default version defined for the library. This also allows a library with "Load implicitly" to be loaded from a different version if necessary.

When using the library step you may also specify a version:

library 'my-shared-library@master'

Since this is a regular step, that version could be computed rather than a constant as with the annotation; for example:

library "my-shared-library@$BRANCH_NAME"

would load a library using the same SCM branch as the multibranch Jenkinsfile. As another example, you could pick a library by parameter:

properties([parameters([string(name: 'LIB_VERSION', defaultValue: 'master')])])
library "my-shared-library@${params.LIB_VERSION}"

Note that the library step may not be used to override the version of an implicitly loaded library. It is already loaded by the time the script starts, and a library of a given name may not be loaded twice.

Retrieval Method

The best way to specify the SCM is using an SCM plugin which has been specifically updated to support a new API for checking out an arbitrary named version (Modern SCM option). As of this writing, the latest versions of the Git and Subversion plugins support this mode.

Configuring a 'Modern SCM' for a Pipeline Library

Legacy SCM

SCM plugins which have not yet been updated to support the newer features required by Shared Libraries, may still be used via the Legacy SCM option. In this case, include ${library.yourlibrarynamehere.version} wherever a branch/tag/ref may be configured for that particular SCM plugin. This ensures that during checkout of the library’s source code, the SCM plugin will expand this variable to checkout the appropriate version of the library.

Configuring a 'Legacy SCM' for a Pipeline Library

Dynamic retrieval

If you only specify a library name (optionally with version after @) in the library step, Jenkins will look for a preconfigured library of that name. (Or in the case of a github.com/owner/repo automatic library it will load that.)

But you may also specify the retrieval method dynamically, in which case there is no need for the library to have been predefined in Jenkins. Here is an example:

library identifier: 'custom-lib@master', retriever: modernSCM(
  [$class: 'GitSCMSource',
   remote: 'git@git.mycorp.com:my-jenkins-utils.git',
   credentialsId: 'my-private-key'])

It is best to refer to Pipeline Syntax for the precise syntax for your SCM.

Note that the library version must be specified in these cases.

Writing libraries

At the base level, any valid Groovy code is okay for use. Different data structures, utility methods, etc, such as:

// src/org/foo/Point.groovy
package org.foo;

// point in 3D space
class Point {
  float x,y,z;
}

Accessing steps

Library classes cannot directly call steps such as sh or git. They can however implement methods, outside of the scope of an enclosing class, which in turn invoke Pipeline steps, for example:

// src/org/foo/Zot.groovy
package org.foo;

def checkOutFrom(repo) {
  git url: "git@github.com:jenkinsci/${repo}"
}

Which can then be called from a Scripted Pipeline:

def z = new org.foo.Zot()
z.checkOutFrom(repo)

This approach has limitations; for example, it prevents the declaration of a superclass.

Alternately, a set of steps can be passed explicitly to a library class, in a constructor, or just one method:

package org.foo
class Utilities implements Serializable {
  def steps
  Utilities(steps) {this.steps = steps}
  def mvn(args) {
    steps.sh "${steps.tool 'Maven'}/bin/mvn -o ${args}"
  }
}

When saving state on classes, such as above, the class must implement the Serializable interface. This ensures that a Pipeline using the class, as seen in the example below, can properly suspend and resume in Jenkins.

@Library('utils') import org.foo.Utilities
def utils = new Utilities(steps)
node {
  utils.mvn 'clean package'
}

If the library needs to access global variables, such as env, those should be explicitly passed into the library classes, or methods, in a similar manner.

Instead of passing numerous variables from the Scripted Pipeline into a library,

package org.foo
class Utilities {
  static def mvn(script, args) {
    script.sh "${script.tool 'Maven'}/bin/mvn -s ${script.env.HOME}/jenkins.xml -o ${args}"
  }
}

The above example shows the script being passed in to one static method, invoked from a Scripted Pipeline as follows:

@Library('utils') import static org.foo.Utilities.*
node {
  mvn this, 'clean package'
}

Defining global variables

Internally, scripts in the vars directory are instantiated on-demand as singletons. This allows multiple methods or properties to be defined in a single .groovy file which interact with each other, for example:

// vars/acme.groovy
def setName(value) {
    name = value
}
def getName() {
    name
}
def caution(message) {
    echo "Hello, ${name}! CAUTION: ${message}"
}

In the above, name is not referring to a field (even if you write it as this.name!), but to an entry created on demand in a Script.binding. To be clear about what data you intend to store and of what type, you can instead provide an explicit class declaration (the class name should match the file basename):

// vars/acme.groovy
class acme implements Serializable {
    private String name
    def setName(value) {
        name = value
    }
    def getName() {
        name
    }
    def caution(message) {
        echo "Hello, ${name}! CAUTION: ${message}"
    }
}

The Pipeline can then invoke these methods which will be defined on the acme object:

acme.name = 'Alice'
echo acme.name /* prints: 'Alice' */
acme.caution 'The queen is angry!' /* prints: 'Hello, Alice. CAUTION: The queen is angry!' */

A variable defined in a shared library will only show up in Global Variables Reference (under Pipeline Syntax) after Jenkins loads and uses that library as part of a successful Pipeline run.

Defining steps

Shared Libraries can also define global variables which behave similarly to built-in steps, such as sh or git. Global variables defined in Shared Libraries must be named with all lower-case or "camelCased" in order to be loaded properly by Pipeline. [2]

For example, to define sayHello, the file vars/sayHello.groovy should be created and should implement a call method. The call method allows the global variable to be invoked in a manner similar to a step:

// vars/sayHello.groovy
def call(String name = 'human') {
    // Any valid steps can be called from this code, just like in other
    // Scripted Pipeline
    echo "Hello, ${name}."
}

The Pipeline would then be able to reference and invoke this variable:

sayHello 'Joe'
sayHello() /* invoke with default arguments */

If called with a block, the call method will receive a Closure. The type should be defined explicitly to clarify the intent of the step, for example:

// vars/windows.groovy
def call(Closure body) {
    node('windows') {
        body()
    }
}

The Pipeline can then use this variable like any built-in step which accepts a block:

windows {
    bat "cmd /?"
}

Defining a more structured DSL

If you have a lot of Pipelines that are mostly similar, the global variable mechanism provides a handy tool to build a higher-level DSL that captures the similarity. For example, all Jenkins plugins are built and tested in the same way, so we might write a step named buildPlugin:

// vars/buildPlugin.groovy
def call(body) {
    // evaluate the body block, and collect configuration into the object
    def config = [:]
    body.resolveStrategy = Closure.DELEGATE_FIRST
    body.delegate = config
    body()

    // now build, based on the configuration provided
    node {
        git url: "https://github.com/jenkinsci/${config.name}-plugin.git"
        sh "mvn install"
        mail to: "...", subject: "${config.name} plugin build", body: "..."
    }
}

Assuming the script has either been loaded as a Global Shared Library or as a Folder-level Shared Library the resulting Jenkinsfile will be dramatically simpler:

Jenkinsfile (Scripted Pipeline)
buildPlugin {
    name = 'git'
}

Using third-party libraries

It is possible to use third-party Java libraries, typically found in Maven Central, from trusted library code using the @Grab annotation. Refer to the Grape documentation for details, but simply put:

@Grab('org.apache.commons:commons-math3:3.4.1')
import org.apache.commons.math3.primes.Primes
void parallelize(int count) {
  if (!Primes.isPrime(count)) {
    error "${count} was not prime"
  }
  // …
}

Third-party libraries are cached by default in ~/.groovy/grapes/ on the Jenkins master.

Loading resources

External libraries may load adjunct files from a resources/ directory using the libraryResource step. The argument is a relative pathname, akin to Java resource loading:

def request = libraryResource 'com/mycorp/pipeline/somelib/request.json'

The file is loaded as a string, suitable for passing to certain APIs or saving to a workspace using writeFile.

It is advisable to use an unique package structure so you do not accidentally conflict with another library.

Pretesting library changes

If you notice a mistake in a build using an untrusted library, simply click the Replay link to try editing one or more of its source files, and see if the resulting build behaves as expected. Once you are satisfied with the result, follow the diff link from the build’s status page, and apply the diff to the library repository and commit.

(Even if the version requested for the library was a branch, rather than a fixed version like a tag, replayed builds will use the exact same revision as the original build: library sources will not be checked out again.)

Replay is not currently supported for trusted libraries. Modifying resource files is also not currently supported during Replay.