汉堡图标不显示在工具栏中

<!-- res/menu/main_menu.xml -->
<menu xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:id="@+id/action_menu"
        android:icon="@drawable/ic_hamburger"
        android:title="Menu"
        app:showAsAction="ifRoom"/>
</menu>

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main_menu, menu);
    return true;
}

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
    <!-- 其他属性 -->
    <item name="android:iconTint">@color/icon_color</item>
</style>

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    Toolbar toolbar = findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
}