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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:

```bash 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:

```bash 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.

```java 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.[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