CKJM-ext
An extended build of CKJM, the Chidamber and Kemerer Java metrics tool: more metrics, and machine-readable output.
CKJM computes the Chidamber and Kemerer object-oriented metrics — coupling, cohesion, depth of inheritance and the rest — by reading Java bytecode rather than source. This is Diomidis Spinellis' tool (external link, opens in a new tab) with two things added: further metric visitors beyond the original six, and output handlers that emit JSON and XML instead of only the plain text the original prints.
That second part is the point of the fork. The metrics were being computed for datasets of tens of thousands of jars, and a format something else can read without a parser of its own matters more at that scale than it does on one project.
A Maven build over Apache BCEL. It has not changed since 2012 and is here as a record of what the ecosystem studies were measured with, among them Generating the Blueprints of the Java Ecosystem.