Saul cast his spear at him to strike him. By this Jonathan knew that his father was determined to put David to death.
And David sware moreover, and said, Your father certainly knows that you and I are good friends; and so he said, Don't let Jonathan know this, lest he is grieved: but truly as the Lord liveth, and as thy soul liveth, there is but a step between me and death ( 1 Samuel 20:3 ).