attach.gradle

configurations {
    attachSource
}


def loadSourcePaths() {
    def result = new HashMap<String, String>()
    configurations.attachSource.each {
        def depFile = file(it)
        def filename = depFile.getName()
        def depname = filename.substring(0, filename.indexOf('-'))
        result.put(depname, it) // maps based on artifact id
    }
    result
}

eclipse.classpath.file {
    withXml {
        def sourcePaths = loadSourcePaths()
        def root = it.asNode()
        root.children().each { child ->
            def attr = child.attributes()
            if ('lib'.equals(attr.get('kind'))) { // only attach to libraries
                def path = attr.get('path')
                def pathfile = file(path)
                def filename = pathfile.getName()
                def dep = filename.substring(0, filename.contains('-') ? filename.indexOf('-') : filename.length() - 1) // should match the artifactId from the gradle configuration
                if (sourcePaths.containsKey(dep)) { // ignore if we don't need to attach a custom source to this library
                    def srcpath = sourcePaths.get(dep)
                    attr.put('sourcepath', srcpath)
                }
            }
        }
    }
}

build.gradle

// This is all that's needed for the example

// with forgegradle, place this line just after any of the "apply plugin: '...'" lines
apply from: '../attach.gradle' // might want to remove the "../" if you place attach.gradle in the same folder as build.gradle


// place the rest of this at the end of the file (or merge with the repositories & dependencies blocks you already have) (these repos and dependencies are provided as an example of the attach.gradle system)
repositories {
    maven {
        name "CB Repo"
        url "http://chickenbones.net/maven"
    }
    maven {
        name "Forge Repo"
        url "http://files.minecraftforge.net/maven"
    }
}

dependencies {
    compile 'codechicken:CodeChickenCore:1.7.10-1.0.4.29:dev'
    compile 'codechicken:CodeChickenLib:1.7.10-1.1.1.105:dev'
    compile 'codechicken:ForgeMultipart:1.7.10-1.1.0.311:dev'
    compile 'codechicken:NotEnoughItems:1.7.10-1.0.3.62:dev'
    attachSource 'codechicken:CodeChickenCore:1.7.10-1.0.4.29:src'
    attachSource 'codechicken:CodeChickenLib:1.7.10-1.1.1.105:src'
    attachSource 'codechicken:ForgeMultipart:1.7.10-1.1.0.311:src'
    attachSource 'codechicken:NotEnoughItems:1.7.10-1.0.3.62:src'
}