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How to download jars from Maven Central

| java | maven |

We know how to download Java libraries with it’s dependencies (transitive included) via Maven pom.xml, Ant/Ivy build.xml script, Gradle build.gradle script etc. But what if we need to download them without these scripts.

There are several ways to do this. Assume that we’d like to download spark-core library (groupId=com.sparkjava, artifactId=spark-core, version=2.1) with all dependencies from Maven Central into lib folder.

Use Maven3 dependency plugin

Here is there variants for lib download:

# Download library with all dependencies
# Specify repoUrl (it's optional)
mvn dependency:get -DrepoUrl=http://download.java.net/maven/2/ -DgroupId=com.sparkjava -DartifactId=spark-core -Dversion=2.1

# OR use default repoUrl
mvn dependency:get -DgroupId=com.sparkjava -DartifactId=spark-core -Dversion=2.1

# OR use parameter artifact as groupId:artifactId:version
mvn dependency:get -Dartifact=com.sparkjava:spark-core:2.1

Now we need to copy just downloaded artifacts in our working directory:

# Copy jars from local maven repo
mvn dependency:copy-dependencies -f $HOME/.m2/repository/com/sparkjava/spark-core/2.1/spark-core-2.1.pom -DoutputDirectory=$(pwd)/lib
# the previous command doesn't copy spark-core-x.x.jar, that's why we should copy it manually
cp $HOME/.m2/repository/com/sparkjava/spark-core/2.1/spark-core-2.1.jar $(pwd)/lib

Use standalone Ivy

We can use Ivy as standalone jar to download Maven dependencies without creating Ant build file:

# 1. Download the latest ivy jar (currently it's v.2.4.0)
curl -L -O http://search.maven.org/remotecontent?filepath=org/apache/ivy/ivy/2.4.0/ivy-2.4.0.jar

# 2. Run ivy.jar to retrieve all dependencies
java -jar ivy-2.4.0.jar -dependency com.sparkjava spark-core 2.1 -retrieve "lib/[artifact]-[revision](-[classifier]).[ext]"

As you can see Ivy downloads approach is much simpler. The only cons (or pros, it depends) that ivy.jar should be additionally downloaded.

Calling Ivy from Groovy or Java

Here I’ve decided to store Evgeny’s Goldin code snippet as a reference for myself. Programmatic artifacts downloads is not a common operation. It’s alway nice to know the general concept how it can be done. Especially when Ivy documentation is not very informative.

// Groovy snippet of calling Ivy
import org.apache.ivy.Ivy
import org.apache.ivy.core.module.descriptor.DefaultDependencyDescriptor
import org.apache.ivy.core.module.descriptor.DefaultModuleDescriptor
import org.apache.ivy.core.module.id.ModuleRevisionId
import org.apache.ivy.core.resolve.ResolveOptions
import org.apache.ivy.core.settings.IvySettings
import org.apache.ivy.plugins.resolver.URLResolver
import org.apache.ivy.core.report.ResolveReport
import org.apache.ivy.plugins.parser.xml.XmlModuleDescriptorWriter


public File resolveArtifact(String groupId, String artifactId, String version) {
        //creates clear ivy settings
        IvySettings ivySettings = new IvySettings();
        //url resolver for configuration of maven repo
        URLResolver resolver = new URLResolver();
        resolver.setM2compatible(true);
        resolver.setName('central');
        //you can specify the url resolution pattern strategy
        resolver.addArtifactPattern(
            'http://repo1.maven.org/maven2/[organisation]/[module]/[revision]/[artifact](-[revision]).[ext]');
        //adding maven repo resolver
        ivySettings.addResolver(resolver);
        //set to the default resolver
        ivySettings.setDefaultResolver(resolver.getName());
        //creates an Ivy instance with settings
        Ivy ivy = Ivy.newInstance(ivySettings);

        File ivyfile = File.createTempFile('ivy', '.xml');
        ivyfile.deleteOnExit();

        String[] dep = [groupId, artifactId, version]

        DefaultModuleDescriptor md =
                DefaultModuleDescriptor.newDefaultInstance(ModuleRevisionId.newInstance(dep[0],
                dep[1] + '-caller', 'working'));

        DefaultDependencyDescriptor dd = new DefaultDependencyDescriptor(md,
                ModuleRevisionId.newInstance(dep[0], dep[1], dep[2]), false, false, true);
        md.addDependency(dd);

        //creates an ivy configuration file
        XmlModuleDescriptorWriter.write(md, ivyfile);

        String[] confs = ['default'];
        ResolveOptions resolveOptions = new ResolveOptions().setConfs(confs);

        //init resolve report
        ResolveReport report = ivy.resolve(ivyfile.toURL(), resolveOptions);

        //so you can get the jar library
        File jarArtifactFile = report.getAllArtifactsReports()[0].getLocalFile();

        return jarArtifactFile;
}

resolveArtifact( 'log4j', 'log4j', '1.2.16' )

References