Gradle task that will fail if there are compile dependencies that require Java > 5.

checkJava5.gradle

task checkCompileDependenciesJava5Compatibility << {
	configurations.compile.each { dep ->
		  def failureReason
		  zipTree(dep).matching { include("**/*.class") }.visit { fileDetails ->
			  if (!fileDetails.isDirectory()) {
				  def stream = fileDetails.file.newDataInputStream()
				  def magic1 = stream.readUnsignedShort()
				  def magic2 = stream.readUnsignedShort()
				  stream.skipBytes(2); // minor version
				  def majorVersion = stream.readUnsignedShort()
				  def javaVersion = majorVersion - 44
				  if (Long.toHexString(magic1) + Long.toHexString(magic2) != "cafebabe") {
					  println "Warning: bad magic number for ${fileDetails.name} in ${dep.name}"
				  }
				  if (javaVersion > 5) {
					  // Throwing exception directly from here will show a 'cannot open ZIP' message.
					  failureReason = "Dependency has class with Java version $javaVersion, max allowed is 5: ${fileDetails.name} in ${dep.name}"
				  }
				  fileDetails.stopVisiting();
				  stream.close()
			  }
		  }
		  if (failureReason) {
			  throw new GradleException(failureReason)
		  }
	 }
}