Releasing Java libraries to the Central Repository

As an open source developer I want to share my work and make utilizing the libraries as easy as possible. It is often not sufficient to share the sources to GitHub, because users may not be willed to build the binaries by themselves or they have to keep track of dependencies.

.NET has NuGet as a package repository and Java offers the Central Repository to publish open source libraries. The release process to NuGet is a very simple one: just register an account and push your package. It was a little harder to release my small libraries to the Central Repository without going all in on Maven.

Here is a small description of the necessary steps for manually releasing a library to the Central Repository using the Sonatype OSS Repository Hosting (OSSRH).

Prerequisites for Deployment

Initial Setup

The initial repository for a new project has to be approved, which requires human interaction (Explanation). Basically you only have to register in the Sonatype JIRA and issue a new Project ticket. You have to choose a Group ID, describe your project and wait for your request to be approved.

Deployment Requirements

It is not sufficient to simply upload a JAR to the OSSRH. There is a list of Requirements for publishing to the Central Repository, that will be checked when uploading an artifact. The Requirements are:

  • Supply Javadoc and Sources
  • Sign Files with GPG/PGP
  • Sufficient Metadata

Create and Upload Signing Key

The packages need to be signed with PGP/GPG and come with an .asc file containing the signature. The first step in the signature process is to create a Signarure key, which can be done with the following command (or a UI of course!):

gpg --gen-key

The Central Repository then requires you to upload the signature key to the keyserver. A kind warning first: If you publish your key to a public keyserver, your associated mail address is going to be visible to everyone. Keyservers are possibly harvested by spammers for mail addresses, so you either need to have trust in your spam filters or you better create a mail address specific to the signature part.

The upload to a Keyserver can be done with the following command (or a UI of course!):

gpg --keyserver --send-keys <your KEYID>

For a more detailed writeup I suggest to read the Sonatype documentation on Working with PGP Signatures.

Project Object Model (POM)

The next step is to create the Project Object Model (POM) for your project. A POM file is used by Maven to build a Java project and it also contains the projects metadata (Project Name, Authors, Licenses, Dependencies, ...). The full list of required tags can be found at the Central Repository guide on Requirements.

Here is the POM file for my PgBulkInsert project:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns=""


    <!-- Basic Project Information -->
    <description>PgBulkInsert is a Java 1.8 library for Bulk Inserts with PostgreSQL.</description>

    <!-- Define the License -->
            <name>MIT License</name>
    <!-- Developers -->
            <name>Philipp Wagner</name>

    <!-- Dependencies -->


Manual Staging Bundle Creation

The Central Repository documentation has a page on Manual Staging Bundle Creation and Deployment, which describes how to bundle a Java library for deploying it to the Central Repository. I have turned the described steps into a small Batch script, that automatically creates the bundle to upload.

The script basically looks up and bundles two files:

  1. The JAR file artifacts/VERSION/FILENAME-VERSION.jar, where artifact directory, filename and version are set in the Batch script.
  2. The POM file at artifacts/VERSION/pom.xml, which has to be created in the artifacts directory.

Make sure, these two files are present in the artifacts directory. And in order to successfully deploy the bundle to the Central Repository, you have to make sure, that the version of the Bundle filename and version in the POM file match.

The Central Repository deployment process requires to also bundle the javadoc and sources for the library. I don't have included them in my bundles and have included empty JAR files for both. If your library should include javadoc and sources, you have to modify the script accordingly.

:: Copyright (c) Philipp Wagner. All rights reserved.
:: Licensed under the MIT license. See LICENSE file in the project root for full license information.

@echo off

echo ---------------------------------------------------
echo - Bundling Artifacts for Manual Repository Upload -
echo ---------------------------------------------------

:: Define the Executables, so we don't have to rely on pathes:
set JAR_EXECUTABLE="C:\Program Files\Java\jdk1.8.0_71\bin\jar.exe"
set GPG_EXECUTABLE="C:\Program Files (x86)\GNU\GnuPG\pub\gpg.exe"

:: Logs to be used:
set STDOUT=stdout.log
set STDERR=stderr.log

:: Version to build the bundle for:
set VERSION=0.4

:: Set the Target Bundle file:
set TARGET_BUNDLE=bundle\pgbulkinsert-bundle-%VERSION%.jar

:: Define the Artifacts to be signed. Simply use an absolute path here:
set ARTIFACTS_DIR=artifacts

:: Set the Filename for artifacts to use:
set FILENAME=pgbulkinsert-%VERSION%


set POM_FILE=pom.xml


:: Ask for GPG Signing Passphrase:

set /p PASSPHRASE="Signing Passphrase: "


    :: Create Fake:

    :: Sign JAR and POM Files:
    echo %PASSPHRASE%|%GPG_EXECUTABLE% --batch --yes --passphrase-fd 0  -b -a -s "%BASE_DIR%\%JAR_FILE%"  
    echo %PASSPHRASE%|%GPG_EXECUTABLE% --batch --yes --passphrase-fd 0  -b -a -s "%BASE_DIR%\%POM_FILE%"
    echo %PASSPHRASE%|%GPG_EXECUTABLE% --batch --yes --passphrase-fd 0  -b -a -s "%BASE_DIR%\%JAR_SOURCES_FILE%"
    echo %PASSPHRASE%|%GPG_EXECUTABLE% --batch --yes --passphrase-fd 0  -b -a -s "%BASE_DIR%\%JAR_JAVADOC_FILE%"

    :: Create the Bundle File for manual upload:



When you have run the script, you can find the created bundle at bundle\FILENAME-%VERSION%.jar, which can be configured with the variable TARGET_BUNDLE.

Artifact Bundle Upload

Once the bundle has been created, it can be uploaded manually by using the Nexus Repository Manager.

  1. Switch to the menu item Staging Upload and choose Upload Mode: Artifact Bundle, then upload your Bundle.
  2. Switch to Staging Repositories and select the Repository created by the Bundle upload. 2.1. Under the Tab Activity you can see if all Rules for a Release have passed.
  3. If all rules have been passed, you can click on the Release button to release your Bundle to the Central Repository.
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