Currently, it is not possible to build DE (from scratch) under Java 8, because the build system's version of gradle is too old.

Running ./gradlew setupDecompWorkspace results in:

FAILURE: Build failed with an exception.

* Where:
Build file '/Users/wlhlm/Devel/MCMods/Draconic-Evolution/build.gradle' line: 36

* What went wrong:
A problem occurred evaluating root project 'Draconic-Evolution'.
> [Ljava/util/HashMap$Entry;

HashMap$Entry has been removed in Java 8.

Line 36 in build.gradle:

project.ext.config = new ConfigSlurper().parse prop

More info in the gradle forums

Possible Solutions:

• Update to ForgeGradle 2.0
• Remove ConfigSlurper from build.gradle (for example through merging build.gradle and build.properties thus removing the need to parse config files).