在xml里写的layout_constraintWidth_percent=“0.4”

app:layout_constraintWidth_default="percent"
app:layout_constraintWidth_percent="0.4"

在java中可以这样写

mConstraintLayout = (ConstraintLayout) findViewById(R.id.myconstraint_layout)

ConstraintSet set = new ConstraintSet();

set.connect(子view的id, ConstraintSet.TOP, ConstraintLayout的id, ConstraintSet.TOP);
set.constrainPercentHeight(子view的id, 0.4);
set.constrainPercentWidth(子view的id, 0.4);

set.applyTo(ConstraintLayout对象实例);