View在onCreate的时候，没有渲染组件，所以获取到的宽度和高度为0，

需要添加一个观察者，在layout渲染后再去取宽高。代码如下：

 

private Button btn_icon;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_image_button);        btn_icon = findViewById(R.id.btn_icon);        ViewTreeObserver vto = btn_icon.getViewTreeObserver();        vto.addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {            @Override            public void onGlobalLayout() {                btn_icon.getViewTreeObserver().removeGlobalOnLayoutListener(this);                LinearLayout.LayoutParams lp = (LinearLayout.LayoutParams) btn_icon.getLayoutParams();                Context ctx = btn_icon.getContext();                int marginLen = (DisplayUtil.getScreenWidth(ctx) - btn_icon.getWidth()) / 2;                lp.setMargins(marginLen, 20, marginLen, 20);                btn_icon.setLayoutParams(lp);            }        });    }

　　

 

参考：https://blog.csdn.net/zheng5229875/article/details/39314159