Difference Between Library & Library Project & External Libraries

Question

In the Android Studio IDE you can add a .jar file. You can also add entire projects that are 'Library Projects'. And there are also the Android libraries (from Gradle) added as External Libraries.

.jar

Library Project

External Library

I get that .jars are just that. But then how come other libraries (Library Projects) need to get added that have entire build files of their own (like gradle, res, src etc). And further complicating my understanding, the Gradle downloaded ones are added as 'External Libraries' and those have .jar files and a res folder.

Could you explain why libraries can be added as .jar, entire projects, or as external libraries?

Answer 1

Could you explain why libraries can be added as .jar, entire projects, or as external libraries?

You have different possibilities, because you can have different cases. Often the library is built by other team and you can't decide how it is distributed.

• You can have a own library or a fork locally. In this case you have the code and you can add this library as a module

In this case just add in the module/build.gradle:

apply plugin: 'com.android.library'

and add the dependency in the main project like:

dependencies{
  compile project(':module')
}

• You can use a maven dependency. In this case someone uploaded the library in a maven repository.
  Currently it is the best solution in my opinion.

In this case just add a dependency in your project

dependencies{
  compile 'group:name:version'
}

This dependency can be a aar file, but also a jar file.
Also you can publish in a public or private maven your own libraries.

• You can add an aar file in your project

In this case define your flat repository:

repositories {
    flatDir {
        dirs 'libs'
    }
}

and add the dependency:

dependencies {
   compile(name:'nameOfYourAARFileWithoutExtension', ext:'aar')
 }

• You can add a jar file in your project

In this case usually you can put all jars in the libs folder and add the dependency:

dependencies {
    compile fileTree(dir: 'libs', include: ['*.jar'])
}