xr.js - A Nashorn JavaScript function to navigate inside the structure of any Java object. Designed for nudge4j. https://lorenzoongithub.github.io/nudge4j/

xr.js

function xr(oj, name) {

	var arrayLength = java.lang.reflect.Array.getLength;
	var isStaticField = function(field) {
		return java.lang.reflect.Modifier.isStatic(field.getModifiers()); 
	}
	
	/** 
	 * Returns true if the input appears to represent a Class, false otherwise.
	 * 
	 * isShortCutForClass(java.lang.String)                  --> true
	 * isShortCutForClass(java.lang.String.class)            --> true
	 * isShortCutForClass(Java.type(java.lang.String.class)) --> true
	 * 
	 * isShortCutForClass('java.lang.String')                --> false
	 * isShortCutForClass(java.lang.StringX)                 --> false
	 * 
	**/
	function isShortCutForClass(x) {
	    if (Java.isJavaFunction(x)) {
	        return ((''+x).indexOf('[JavaClass ')===0); 
	    } 
	    if (Java.isJavaObject(x)) {
	        if (x.getClass == undefined) return false;
	        return (x.getClass() == java.lang.Class.class);
	    }
	    return false; 
	}

	/**
	 * If it is a Java Object (e.g HashMap, Thread, StringBuilder) returns its class.
	 * If it is a Nashorn shortcut for a JavaClass (e.g. java.util.HashMap) returns the represented class
	 * All other cases it returns null.
	 */
	function getClassFor(x) {
	    if (Java.isJavaFunction(x)) {
	        var str = ''+x; 
	        if (str.indexOf('[JavaClass ')===0) {
	            return java.lang.Class.forName( str.substring(11, str.length - 1));
	        }
	    } 
	    if (Java.isJavaObject(x)) {
	        if (x.getClass == undefined) return null;
	        var cz = x.getClass();
	        return (cz != java.lang.Class.class) ? cz : x;
	    }
	    return null;    
	}
	
	
    if (oj === null) return 'Error: null';

    var cz = getClassFor(oj); 
    if (cz === null) {
    	return "Error: Not a Java Object nor a Class";
    }
 
 //
 // class is an array.
 //
    if (cz.isArray()) {
        if (cz == oj)         return cz.getComponentType().getSimpleName() + '[]';   // e.g. $(java.lang.Class.forName('int[]');
        if (name == null)     return cz.getComponentType().getSimpleName() + '[' + arrayLength(oj) + ']';
        if (name == 'length') return arrayLength(oj);
        var index;
        try { 
        	index = java.lang.Integer.parseInt(''+name,10)
        } catch (e) {
        	return 'Error: Invalid index';
        }
        return (index >= 0 && index < arrayLength(oj)) ? java.lang.reflect.Array.get(oj, index) : 'Error: Out of bounds';
    }
    
    
    var isClass = isShortCutForClass(oj)
    
//
// listing the fields.
//
    if (name == undefined) {
        var fs = {};
        while (true) {
            var fields = cz.getDeclaredFields();
            for (var i = 0; i < fields.length; i++) {
                if ( isClass &&  isStaticField(fields[i])) fs[fields[i].getName()] = 1; 
                if (!isClass && !isStaticField(fields[i])) fs[fields[i].getName()] = 1; 
            }
            cz = cz.getSuperclass();
            if (cz === null) break;
        }
        var array = [];
        for ( var i in fs) {
            array.push(i);
        }
        array.sort();
        return array.join('\n');
    }

//
// getting a field.
//
    while (true) {
        var field;
        try {
            field = cz.getDeclaredField(name);
        } catch (eee) {
            cz = cz.getSuperclass();
            if (cz === null) {
                return 'Error: not a valid field';
            }
            continue;
        }

        if (isStaticField(field) == false && isClass) {
            return 'Error: Attempt to access a non static field';
        }

        field.setAccessible(true);
        var v = field.get(oj);
        field.setAccessible(false);
        return v;
    }
}

you can use it in Nashorn/Console jjs or in nudge4j. Just Type:

// load the x-rays
load('https://gist.githubusercontent.com/lorenzoongithub/7273902ac628b2761f46de2d0c2a31cd/raw/a80b9369f3b63517da5eba4d962d3632cc26882c/xr.js');

// use the x-rays :-)
xr(java.lang.Thread.currentThread());