一、問(wèn)題來(lái)源
二、原因分析
　　1.getSupportActionBar()
　　2.setSupportActionBar()
　　3.完整過(guò)程圖
三、解決方法

問(wèn)題來(lái)源

給Activity在代碼中設(shè)置主題theme，然后Activity被銷毀重建了。先看主體代碼：

/*=====NoActionBarActivity.java=====*/
public class NoActionBarActivity extends AppCompatActivity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setTheme(R.style.No_Actionbar);
        Toolbar toolbar = (Toolbar) getView().findViewById(R.id.home_page_toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle(" ");
    }
    //省略代碼...
}
 

/*=====Style.xml=====*/
<style name="No_Actionbar" parent="Theme.AppCompat.Light">
    <item name="windowActionBar">false</item>
    <item name="windowNoTitle">true</item>
    <item name="android:windowBackground">@drawable/bg_actionmode</item>
</style>

正常運(yùn)行都沒(méi)問(wèn)題，但是當(dāng)Activity被銷毀重建時(shí)（模擬 Activity銷毀重建 方法）就出現(xiàn)crash了，報(bào)錯(cuò) java.lang.IllegalStateException: This Activity already has an action bar supplied by the window decor. Do not request Window.FEATURE_SUPPORT_ACTION_BAR and set windowActionBar to false in your theme to use a Toolbar instead.

原因分析

當(dāng)Activity設(shè)置ActionBar被銷毀重建時(shí)，主要涉及AppCompatDelegateImplV9調(diào)用getSupportActionBar()和setSupportActionBar()兩個(gè)方法的過(guò)程
1.getSupportActionBar()

/*=====AppCompatDelegateImplBase.java=====*/
@Override
public ActionBar getSupportActionBar() {
    // The Action Bar should be lazily created as hasActionBar
    // could change after onCreate
    initWindowDecorActionBar();   //***(1)***
    return mActionBar;
}

 
/*=====AppCompatDelegateImplV9.java=====*/
@Override
public void initWindowDecorActionBar() {
    ensureSubDecor();    //***(2)***
    if (!mHasActionBar || mActionBar != null) {   //***(6)***
        return;
    }

    if (mOriginalWindowCallback instanceof Activity) {
        mActionBar = new WindowDecorActionBar((Activity) mOriginalWindowCallback,
                mOverlayActionBar);
    } else if (mOriginalWindowCallback instanceof Dialog) {
        mActionBar = new WindowDecorActionBar((Dialog) mOriginalWindowCallback);
    }
    if (mActionBar != null) {
        mActionBar.setDefaultDisplayHomeAsUpEnabled(mEnableDefaultActionBarUp);
    }
}
 
private void ensureSubDecor() {
    if (!mSubDecorInstalled) {
        mSubDecor = createSubDecor();   //***(3)***

        // If a title was set before we installed the decor, propagate it now
        CharSequence title = getTitle();
        if (!TextUtils.isEmpty(title)) {
            onTitleChanged(title);
        }
       //省略代碼...
    }
}
 
private ViewGroup createSubDecor() {
    TypedArray a = mContext.obtainStyledAttributes(R.styleable.AppCompatTheme);

    if (!a.hasValue(R.styleable.AppCompatTheme_windowActionBar)) {
        a.recycle();
        throw new IllegalStateException(
                "You need to use a Theme.AppCompat theme (or descendant) with this activity.");
    }

    if (a.getBoolean(R.styleable.AppCompatTheme_windowNoTitle, false)) {  //***(4)***
        requestWindowFeature(Window.FEATURE_NO_TITLE);
    } else if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBar, false)) {  //***(5)***
        // Don't allow an action bar if there is no title.
        requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR);
    }
    if (a.getBoolean(R.styleable.AppCompatTheme_windowActionBarOverlay, false)) {
        requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR_OVERLAY);
    }
    if (a.getBoolean(R.styleable.AppCompatTheme_windowActionModeOverlay, false)) {
        requestWindowFeature(FEATURE_ACTION_MODE_OVERLAY);
    }
   //省略代碼..
}

首先，會(huì)從(1) initWindowDecorActionBar() —> (2) ensureSubDecor() —> (3) createSubDecor()；
然后，根據(jù)主題theme中設(shè)置的值給requestWindowFeature()傳不同的參數(shù)，在theme中設(shè)置了<item name="windowActionBar">false</item>時(shí)，正常情況下會(huì)走（4）requestWindowFeature(Window.FEATURE_NO_TITLE)，但銷毀重建時(shí)就會(huì)走（5）requestWindowFeature(FEATURE_SUPPORT_ACTION_BAR)。為什么會(huì)走到（5）去呢？大致原因就是銷毀重建getSupportActionBar()時(shí)，還沒(méi)有走到代碼中設(shè)置theme的地方（就是上面NoActionBarActivity.java中的setTheme方法），于是createSubDecor()獲取出來(lái)的theme就為默認(rèn)的了。代碼中No_Actionbar主題的父類是Theme.AppCompat.Light，默認(rèn)帶有Actionbar，所以走到了(5)傳入?yún)?shù)FEATURE_SUPPORT_ACTION_BAR；
關(guān)鍵就在調(diào)用requestWindowFeature()方法這里：

最后，回到initWindowDecorActionBar()中的(6)，當(dāng)mHasActionBar為true時(shí)就不會(huì)return，而是繼續(xù)往下走，于是就創(chuàng)建了mActionBar。
2.setSupportActionBar()
調(diào)用完getSupportActionBar()后會(huì)進(jìn)行setSupportActionBar()，crash異常也就是在這個(gè)里面拋出的：

/*=====AppCompatDelegateImplV9.java=====*/
@Override
public void setSupportActionBar(Toolbar toolbar) {
    if (!(mOriginalWindowCallback instanceof Activity)) {
        // Only Activities support custom Action Bars
        return;
    }

    final ActionBar ab = getSupportActionBar();  //***(1)***
    if (ab instanceof WindowDecorActionBar) {
        throw new IllegalStateException("This Activity already has an action bar supplied " +   //***(2)***
                "by the window decor. Do not request Window.FEATURE_SUPPORT_ACTION_BAR and set " +
                "windowActionBar to false in your theme to use a Toolbar instead.");
    }

    // If we reach here then we're setting a new action bar
    // First clear out the MenuInflater to make sure that it is valid for the new Action Bar
    mMenuInflater = null;

    // If we have an action bar currently, destroy it
    if (ab != null) {
        ab.onDestroy();
    }

    if (toolbar != null) {
        final ToolbarActionBar tbab = new ToolbarActionBar(toolbar,
                ((Activity) mOriginalWindowCallback).getTitle(), mAppCompatWindowCallback);
        mActionBar = tbab;
        mWindow.setCallback(tbab.getWrappedWindowCallback());
    } else {
        mActionBar = null;
        // Re-set the original window callback since we may have already set a Toolbar wrapper
        mWindow.setCallback(mAppCompatWindowCallback);
    }

    invalidateOptionsMenu();
}

由上面知道，在get銷毀重建時(shí)在getSupportActionBar()中會(huì)創(chuàng)建一個(gè)ActionBar出來(lái)，也就是在（1）出獲取的ActionBar不為null了，于是就走到了（2）中，拋出了異常。
3.完整過(guò)程圖

解決方法

1.在AndroidManifest.xml中為Activity設(shè)置theme，不在代碼中動(dòng)態(tài)設(shè)置
2.代碼動(dòng)態(tài)設(shè)置theme時(shí)，在super.onCreate()前面設(shè)置theme

/*=====NoActionBarActivity.java=====*/
public class NoActionBarActivity extends AppCompatActivity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        setTheme(R.style.No_Actionbar);   //修改setTheme()在super.onCreate()之前
        super.onCreate(savedInstanceState);
        Toolbar toolbar = (Toolbar) getView().findViewById(R.id.home_page_toolbar);
        setSupportActionBar(toolbar);
        getSupportActionBar().setTitle(" ");
    }
    //省略代碼...
}