Expression expr = parser.parseRaw("str");
Object value = expr.getValue(ctx);
assertEquals("wibble", value);
expr = parser.parseRaw("str");
expr.setValue(ctx, "wobble");
expr = parser.parseRaw("str");
value = expr.getValue(ctx);
assertEquals("wobble", value);
// or using assignment within the expression
expr = parser.parseRaw("str='wabble'");
value = expr.getValue(ctx);