对于自定义字体在工具栏，你可以覆盖textView字体的风格，然后每个textView在你的应用程序也工具栏标题字体自动改变 我在android studio 3.1.3中进行了测试

在风格上这样做:

<style name="defaultTextViewStyle" parent="android:Widget.TextView">
        <item name="android:fontFamily">@font/your_custom_font</item>
</style>

然后在你的主题中使用这个:

<item name="android:textViewStyle">@style/defaultTextViewStyle</item>

要在工具栏中使用自定义标题，您可以添加一个自定义标题，如:

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.Toolbar xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/toolbar"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    android:elevation="5dp"
    app:contentInsetLeft="0dp"
    app:contentInsetStart="0dp"
    app:popupTheme="@style/ThemeOverlay.AppCompat.Light"
    app:theme="@style/ThemeOverlay.AppCompat.Dark">


        <LinearLayout
            android:id="@+id/lnrTitle"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center_horizontal"
            android:orientation="vertical">

            <TextView
                android:id="@+id/txvHeader"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center_horizontal|center"
                android:gravity="center"
                android:ellipsize="end"
                android:maxLines="1"
                android:text="Header"
                android:textColor="@color/white"
                android:textSize="18sp" />


        </LinearLayout>


</android.support.v7.widget.Toolbar>

Java代码:

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

setSupportActionBar(toolbar);

if (getSupportActionBar() == null)
    return;

getSupportActionBar().setTitle("Title");

getSupportActionBar().setDisplayHomeAsUpEnabled(true);

布局:

<android.support.v7.widget.Toolbar
    android:id="@+id/toolbar_top"
    android:layout_height="wrap_content"
    android:layout_width="match_parent"
    android:minHeight="?attr/actionBarSize"
    android:background="@color/action_bar_bkgnd"
    app:theme="@style/ToolBarTheme" >

     <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Toolbar Title"
        android:layout_gravity="center"
        android:gravity="center"
        android:id="@+id/toolbar_title" />
</android.support.v7.widget.Toolbar>

代码:

    Toolbar mToolbar = parent.findViewById(R.id.toolbar_top);
    TextView mToolbarCustomTitle = parent.findViewById(R.id.toolbar_title);

    //setup width of custom title to match in parent toolbar
    mToolbar.postDelayed(new Runnable()
    {
        @Override
        public void run ()
        {
            int maxWidth = mToolbar.getWidth();
            int titleWidth = mToolbarCustomTitle.getWidth();
            int iconWidth = maxWidth - titleWidth;

            if (iconWidth > 0)
            {
                //icons (drawer, menu) are on left and right side
                int width = maxWidth - iconWidth * 2;
                mToolbarCustomTitle.setMinimumWidth(width);
                mToolbarCustomTitle.getLayoutParams().width = width;
            }
        }
    }, 0);

对于自定义字体在工具栏，你可以覆盖textView字体的风格，然后每个textView在你的应用程序也工具栏标题字体自动改变 我在android studio 3.1.3中进行了测试

在风格上这样做:

<style name="defaultTextViewStyle" parent="android:Widget.TextView">
        <item name="android:fontFamily">@font/your_custom_font</item>
</style>

然后在你的主题中使用这个:

<item name="android:textViewStyle">@style/defaultTextViewStyle</item>

定义以下类:

public class CenteredToolbar extends Toolbar {

    private TextView centeredTitleTextView;

    public CenteredToolbar(Context context) {
        super(context);
    }

    public CenteredToolbar(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public CenteredToolbar(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public void setTitle(@StringRes int resId) {
        String s = getResources().getString(resId);
        setTitle(s);
    }

    @Override
    public void setTitle(CharSequence title) {
        getCenteredTitleTextView().setText(title);
    }

    @Override
    public CharSequence getTitle() {
        return getCenteredTitleTextView().getText().toString();
    }

    public void setTypeface(Typeface font) {
        getCenteredTitleTextView().setTypeface(font);
    }

    private TextView getCenteredTitleTextView() {
        if (centeredTitleTextView == null) {
            centeredTitleTextView = new TextView(getContext());
            centeredTitleTextView.setTypeface(...);
            centeredTitleTextView.setSingleLine();
            centeredTitleTextView.setEllipsize(TextUtils.TruncateAt.END);
            centeredTitleTextView.setGravity(Gravity.CENTER);
            centeredTitleTextView.setTextAppearance(getContext(), R.style.TextAppearance_AppCompat_Widget_ActionBar_Title);

            Toolbar.LayoutParams lp = new Toolbar.LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
            lp.gravity = Gravity.CENTER;
            centeredTitleTextView.setLayoutParams(lp);

            addView(centeredTitleTextView);
        }
        return centeredTitleTextView;
    }
}

.．.然后使用它来代替常规工具栏，就像这样:

<RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@color/colorAccent">

        <your.packagename.here.CenteredToolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:background="?attr/colorPrimary"
            android:minHeight="?attr/actionBarSize"
            android:theme="?attr/actionBarTheme"
            app:title="@string/reset_password_page_title"/>

        <!-- Other views -->

</RelativeLayout>

在Activity中你仍然需要这两行代码(和标准工具栏一样):

Toolbar toolbar = (Toolbar) findViewByid(R.id.toolbar); // note that your activity doesn't need to know that it is actually a custom Toolbar
setSupportActionBar(binding.toolbar);

就是这样!您不需要隐藏标准的左对齐标题，不需要重复相同的XML代码，等等，只需使用CenteredToolbar，如果它是默认的工具栏。您还可以通过编程方式设置您的自定义字体，因为您现在可以直接访问TextView。希望这能有所帮助。

private void makeTitleCenter(String title, Toolbar toolbar) {
    if (title != null && !TextUtils.isEmpty(title.trim())) {
        final String tag = " ";
        if (getSupportActionBar() != null) {
            getSupportActionBar().setTitle(tag);
        }
        TextView titleTv = null;
        View leftBtn = null;
        for (int i = 0; i < toolbar.getChildCount(); i++) {
            View view = toolbar.getChildAt(i);
            CharSequence text = null;
            if (view instanceof TextView && (text = ((TextView) view).getText()) != null && text.equals(tag)) {
                titleTv = (TextView) view;
            } else if (view instanceof ImageButton) {
                leftBtn = view;
            }
        }
        if (titleTv != null) {
            final TextView fTitleTv = titleTv;
            final View fLeftBtn = leftBtn;
            fTitleTv.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
                @Override
                public void onGlobalLayout() {
                    fTitleTv.getViewTreeObserver().removeOnGlobalLayoutListener(this);
                    int leftWidgetWidth = fLeftBtn != null ? fLeftBtn.getWidth() : 0;
                    fTitleTv.setPadding(DimenUtil.getResources().getDisplayMetrics().widthPixels / 2 - leftWidgetWidth - fTitleTv.getWidth() / 2, 0, 0, 0);
                    fTitleTv.requestLayout();
                }
            });
        }
    }
}