# Chapter 6. Java Quickstart

To build a Java project, you use the Java Plugin. This plugin adds some tasks to your project, along with some configuration properties, which will compile and test your Java source code, and bundle it into a JAR file. We have in-depth coverage with many examples about the Java plugin, dependency management and multi-project builds in later chapters. In this chapter we want to give you an initial idea of how to build a Java project.

## 6.1. A basic Java project

Let's look at a simple example. To use the Java plugin, add the following to your build file:

Example 6.1. Java plugin

`build.gradle`

    usePlugin 'java'

Note: The code for this example can be found at `samples/java/quickstart`

This is all you need to define a Java project. This will apply the Java plugin to your project, which adds a number of tasks to your project.

### What tasks are available?

You can use `gradle -t` to list the tasks of a project. This will let you see the tasks that the Java plugin has added to your project.

Executing `gradle libs` will compile, test and jar your code. Gradle looks for your production source code under `src/main/java` and your test source code under `src/test/java`. In addition, any files under `src/main/resources` will be included in the JAR file as resources, and any files under `src/test/resources` will be included in the classpath used to run the tests. All output files will end up under the `build` directory, with the JAR file ending up in the `build/libs` directory.

### 6.1.1. External dependencies

Usually, a Java project will have some dependencies on external JAR files. To reference these JAR files in the project, you need to tell Gradle where to find them. In Gradle, artifacts such as JAR files, are located in a repository. A repository can be used for fetching the dependencies of a project, or for publishing the artifacts of a project, or both. For this example, we will use the public Maven repository:

Example 6.2. Adding Maven repository

`build.gradle`

    repositories {
        mavenCentral()
    }

Let's add some dependencies. Here, we will declare that our production classes have a compile-time dependency on commons collections, and that our test classes have a compile-time dependency on junit:

Example 6.3. Adding dependencies

`build.gradle`

    dependencies {
        compile group: 'commons-collections', name: 'commons-collections', version: '3.2'
        testCompile group: 'junit', name: 'junit', version: '4.+'
    }

You can find out more in Chapter 25, Dependency Management.

### 6.1.2. Customising the project

The Java plugin adds a number of properties to your project. These properties have default values which are usually sufficient to get started. It's easy to change these values if they don't suit. Let's look at this for our sample. Here we will specify the version number for our Java project, along with the Java version our source is written in. We also add some attributes to the JAR manifest.

Example 6.4. Customization of MANIFEST.MF

`build.gradle`

    sourceCompatibility = 1.5
    version = '1.0'
    manifest.mainAttributes(
        'Implementation-Title': 'Gradle Quickstart',
        'Implementation-Version': version
    )

### What properties are available?

You can use `gradle -r` to list the properties of a project. This will allow you to see the properties added by the Java plugin, and their default values.

The tasks which the Java plugin adds are regular tasks, exactly the same as if they were declared in the build file. This means you can use any of the mechanisms shown in earlier chapters to customise these tasks. For example, you can set the properties of a task, add behaviour to a task, change the dependencies of a task, or replace a task entirely. In our sample, we will configure the `test` task, which is of type `Test` , to add a system property when the tests are executed:

Example 6.5. Adding system property

`build.gradle`

    test {
        options.systemProperties['property'] = 'value'
    }

### 6.1.3. Publishing the JAR file

Usually the JAR file needs to be published somewhere. To do this, you need to tell Gradle where to publish the JAR file. In Gradle, artifacts such as JAR files are published to repositories. In our sample, we will publish to a local directory. You can also publish to a remote location, or multiple location.

Example 6.6. Publishing the JAR file

`build.gradle`

    uploadArchives {
        repositories {
           flatDir(dirs: file('repos'))
        }
    }

To publish the JAR file, run `gradle uploadArchives`.

### 6.1.4. Creating Eclipse project

To import your project into Eclipse, simply run `gradle eclipse`. More on eclipse task can be found in Section 16.13, “Eclipse”.

### 6.1.5. Summary

Here's the complete build file for our sample:

Example 6.7. Java example - complete build file

`build.gradle`

    usePlugin 'java'

    sourceCompatibility = 1.5
    version = '1.0'
    manifest.mainAttributes(
        'Implementation-Title': 'Gradle Quickstart',
        'Implementation-Version': version
    )

    repositories {
        mavenCentral()
    }

    dependencies {
        compile group: 'commons-collections', name: 'commons-collections', version: '3.2'
        testCompile group: 'junit', name: 'junit', version: '4.+'
    }

    test {
        options.systemProperties['property'] = 'value'
    }

    uploadArchives {
        repositories {
           flatDir(dirs: file('repos'))
        }
    }

## 6.2. Multi-project Java build

Now let's look at a typical multi-project build. Below is the layout for the project:

Example 6.8. Multi-project build - hierarchical layout

Build layout

    multiproject/
      api/
      services/
        webservice/
      shared/

Note: The code for this example can be found at `samples/java/multiproject`

Here we have three projects. Project `api` produces a JAR file which is shipped to the client to provide them a Java client for your XML webservice. Project `webservice` is a webapp which returns XML. Project `shared` contains code used both by `api` and `webservice`.

### 6.2.1. Defining a multi-project build

To define a multi-project build, you need to create a settings file. The settings file lives in the root directory of the source tree, and specifies which projects to include in the build. It must be called `settings.gradle`. For this example, we are using a simple hierarchical layout. Here is the corresponding settings file:

Example 6.9. Multi-project build - settings.gradle file

`settings.gradle`

    include "shared", "api", "services:webservice"

You can find out more about the settings file in Chapter 28, Multi-project Builds.

### 6.2.2. Common configuration

For most multi-project builds, there is some configuration which is common to all projects. In our sample, we will define this common configuration in the root project, using a technique called configuration injection. Here, the root project is like a container and the `subprojects` method iterates over the elements of this container - the projects in this instance - and injects the specified configuration. This way we can easily define the manifest content for all archives, and some common dependencies:

Example 6.10. Multi-project build - common configuration

`build.gradle`

    subprojects {
        usePlugin 'java'
        usePlugin 'eclipse'

        repositories {
           mavenCentral()
        }

        dependencies {
            testCompile 'junit:junit:4.4'
        }

        group = 'org.gradle'
        version = '1.0'
        manifest.mainAttributes(provider: 'gradle')
    }

Notice that our sample applies the Java plugin to each subproject. This means the tasks and configuration properties we have seen in the previous section are available in each subproject. So, you can compile, test, and JAR all the projects by running `gradle libs` from the root project directory.

### 6.2.3. Dependencies between projects

You can add dependencies between projects in the same build, so that, for example, the JAR file of one project is used to compile another project. In the `api` build file we will add a dependency on the JAR produced by the `shared` project. Due to this dependency, Gradle will ensure that project `shared` always gets built before project `api`.

Example 6.11. Multi-project build - dependencies between projects

`api/build.gradle`

    dependencies {
        compile project(':shared')
    }

See Section 28.7.1, “Disable the build of dependency projects.” for how to disable this functionality.

### 6.2.4. Creating a distribution

We also add a distribution, that gets shipped to the client:

Example 6.12. Multi-project build - distribution file

`api/build.gradle`

    task dist(type: Zip) {
        dependsOn configurations.runtime.buildDependencies
        files configurations.runtime
        fileSet dir: 'src/dist'
    }

## 6.3. Summary

In this chapter, you have seen how to do some of the things you commonly need to build a Java based project. This chapter is not exhaustive, and there are many other things you can do with Java projects in Gradle. These are dealt with in later chapters. Also, a lot of the behaviour you have seen in this chapter is configurable. For example, you can change where Gradle looks Java source files, or add extra tasks, or you can change what any task actually does. Again, you will see how this works in later chapters.

You can find out more about the Java plugin in Chapter 16, The Java Plugin, and you can find more sample Java projects in the `samples/java` directory in the Gradle distribution.