分析java.lang.NullPointerException thrown in RelativeLayout measure()

1. 典型的再现环境
   
   模型: Sony Ericsson
   
   Android version: 2.3.4
   
   StackTrace:

   E/AndroidRuntime( 3579): FATAL EXCEPTION: main
   E/AndroidRuntime( 3579): java.lang.NullPointerException
   E/AndroidRuntime( 3579): 	at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:431)
   E/AndroidRuntime( 3579): 	at android.view.View.measure(View.java:8462)
   E/AndroidRuntime( 3579): 	at com.example.measureverify.MainActivity$MenuListAdapter.getView(MainActivity.java:85)
   E/AndroidRuntime( 3579): 	at android.widget.AbsListView.obtainView(AbsListView.java:1519)
   E/AndroidRuntime( 3579): 	at android.widget.ListView.measureHeightOfChildren(ListView.java:1220)
   E/AndroidRuntime( 3579): 	at android.widget.ListView.onMeasure(ListView.java:1131)
   E/AndroidRuntime( 3579): 	at android.view.View.measure(View.java:8462)
   E/AndroidRuntime( 3579): 	at android.widget.RelativeLayout.measureChildHorizontal(RelativeLayout.java:581)
   E/AndroidRuntime( 3579): 	at android.widget.RelativeLayout.onMeasure(RelativeLayout.java:365)
   E/AndroidRuntime( 3579): 	at android.view.View.measure(View.java:8462)
   E/AndroidRuntime( 3579): 	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3231)
   E/AndroidRuntime( 3579): 	at android.widget.FrameLayout.onMeasure(FrameLayout.java:254)
   E/AndroidRuntime( 3579): 	at android.view.View.measure(View.java:8462)
   E/AndroidRuntime( 3579): 	at android.widget.LinearLayout.measureVertical(LinearLayout.java:535)
   E/AndroidRuntime( 3579): 	at android.widget.LinearLayout.onMeasure(LinearLayout.java:313)
   E/AndroidRuntime( 3579): 	at android.view.View.measure(View.java:8462)
   E/AndroidRuntime( 3579): 	at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:3231)
   E/AndroidRuntime( 3579): 	at android.widget.FrameLayout.onMeasure(FrameLayout.java:254)
   E/AndroidRuntime( 3579): 	at android.view.View.measure(View.java:8462)
   E/AndroidRuntime( 3579): 	at android.view.ViewRoot.performTraversals(ViewRoot.java:861)
   E/AndroidRuntime( 3579): 	at android.view.ViewRoot.handleMessage(ViewRoot.java:1882)
   E/AndroidRuntime( 3579): 	at android.os.Handler.dispatchMessage(Handler.java:99)
   E/AndroidRuntime( 3579): 	at android.os.Looper.loop(Looper.java:130)
   E/AndroidRuntime( 3579): 	at android.app.ActivityThread.main(ActivityThread.java:3701)
   E/AndroidRuntime( 3579): 	at java.lang.reflect.Method.invokeNative(Native Method)
   E/AndroidRuntime( 3579): 	at java.lang.reflect.Method.invoke(Method.java:507)
   E/AndroidRuntime( 3579): 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
   E/AndroidRuntime( 3579): 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:624)
   E/AndroidRuntime( 3579): 	at dalvik.system.NativeStart.main(Native Method)
   W/ActivityManager(  237):   Force finishing activity com.example.measureverify/.MainActivity
   

   验证代码：

       public class MenuListAdapter extends ArrayAdapter<ListItem> {
           private Activity context;
   
           public MenuListAdapter(Activity context) {
               super(context, 0);
               this.context = context;
           }
   
           public View getView(int position, View convertView, ViewGroup parent) {
               if (convertView == null) {
                   if (unsafetyInflate) {
                       // risk inflate case
                       convertView = LayoutInflater.from(context).inflate(
                               R.layout.custom_infowindow, null);
                   } else {
                       // safety inflate case(### Solution 1 ###)
                       // The second parameter "mListView" provides a set of
                       // LayoutParams values for root of the returned hierarchy
                       convertView = LayoutInflater.from(context).inflate(
                               R.layout.custom_infowindow, mListView, false);
                   }
   
                   // ### Solution 2 ###
                   if (MainActivity.this.setLayoutParamsProgrammatically) {
                       // NOTE: the layout params set here should be of the
                       // {ParentView}.LayoutParams
                       convertView
                               .setLayoutParams(new ListView.LayoutParams(
                                       ListView.LayoutParams.WRAP_CONTENT,
                                       ListView.LayoutParams.WRAP_CONTENT));
                   }
                   Log.d(TAG, "case 1 parent:" + convertView.getParent() + " layoutParams:"
                           + convertView.getLayoutParams());
                   final int width = context.getWindow().getDecorView().getWidth();
                   final int height = context.getWindow().getDecorView().getHeight();
                   convertView.measure(width, height);
                   MainActivity.this.mLayout = convertView;
               }
               final ListItem item = (ListItem) getItem(position);
               TextView title = (TextView) convertView.findViewById(R.id.title);
   
               title.setText("title " + Math.random() + item.prop_1);
               TextView snippet = (TextView) convertView.findViewById(R.id.snippet);
               snippet.setText("snippet " + Math.random() + item.prop_2);
               return convertView;
           }
   
       }

2. 相关Android2.3.6 Framework层代码
   
   View.java中有一成员代表layout參数（子View对父View的请求，或者一种期望值，终于由整个View层次树决定）

       /**
   1623     * The layout parameters associated with this view and used by the parent
   1624     * {@link android.view.ViewGroup} to determine how this view should be
   1625     * laid out.
   1626     * {@hide}
   1627     */
   1628    protected ViewGroup.LayoutParams mLayoutParams;

   以及获取该成员的函数：

   4973    /**
   4974     * Get the LayoutParams associated with this view. All views should have
   4975     * layout parameters. These supply parameters to the <i>parent</i> of this
   4976     * view specifying how it should be arranged. There are many subclasses of
   4977     * ViewGroup.LayoutParams, and these correspond to the different subclasses
   4978     * of ViewGroup that are responsible for arranging their children.
   4979     * @return The LayoutParams associated with this view
   4980     */
   4981    @ViewDebug.ExportedProperty(deepExport = true, prefix = "layout_")
   4982    public ViewGroup.LayoutParams getLayoutParams() {
   4983        return mLayoutParams;
   4984    }

   ViewGroup类中有一个方法用于将detached的View attach到父View：

   2352     * This method should be called only for view which were detached from their parent.
   2353     *
   2354     * @param child the child to attach
   2355     * @param index the index at which the child should be attached
   2356     * @param params the layout parameters of the child
   2357     *
   2358     * @see #removeDetachedView(View, boolean)
   2359     * @see #detachAllViewsFromParent()
   2360     * @see #detachViewFromParent(View)
   2361     * @see #detachViewFromParent(int)
   2362     */
   2363    protected void attachViewToParent(View child, int index, LayoutParams params) {
   2364        child.mLayoutParams = params; // 编者注：此处子View的mLayoutParams被设置
   2365
   2366        if (index < 0) {
   2367            index = mChildrenCount;
   2368        }
   2369
   2370        addInArray(child, index);
   2371
   2372        child.mParent = this;
   2373        child.mPrivateFlags = (child.mPrivateFlags & ~DIRTY_MASK & ~DRAWING_CACHE_VALID) | DRAWN;
   2374
   2375        if (child.hasFocus()) {
   2376            requestChildFocus(child, child.findFocus());
   2377        }
   2378    }

   在ListView/GridView中都用调用此方法来设置好子View。
   
   ViewGroup类中同一时候还有另外一个更主流的方法（整个Layout被从xml中铺陈开attach到Window并变得有活力的过程中该方法会被调用到）

    private void addViewInner(View child, int index, LayoutParams params,
   1973            boolean preventRequestLayout) {
   1974
   1975        if (child.getParent() != null) {
   1976            throw new IllegalStateException("The specified child already has a parent. " +
   1977                    "You must call removeView() on the child's parent first.");
   1978        }
   1979
   1980        if (!checkLayoutParams(params)) {
   1981            params = generateLayoutParams(params);
   1982        }
   1983
   1984        if (preventRequestLayout) {
   1985            child.mLayoutParams = params;  // 编者注：直接不理会子View请求/意愿的case，直接由父View分配，强迫子View接受
   1986        } else {
   1987            child.setLayoutParams(params);  // 编者注：温和一刀的做法。很体谅的设置给子View。究竟惬意不惬意，取决于子View自身
   1988        }
   1989
   1990        if (index < 0) {
   1991            index = mChildrenCount;
   1992        }
   1993
   1994        addInArray(child, index);
   1995
   1996        // tell our children
   1997        if (preventRequestLayout) {
   1998            child.assignParent(this);
   1999        } else {
   2000            child.mParent = this;
   2001        }
   2002
   2003        if (child.hasFocus()) {
   2004            requestChildFocus(child, child.findFocus());
   2005        }
   2006
   2007        AttachInfo ai = mAttachInfo;
   2008        if (ai != null) {
   2009            boolean lastKeepOn = ai.mKeepScreenOn;
   2010            ai.mKeepScreenOn = false;
   2011            child.dispatchAttachedToWindow(mAttachInfo, (mViewFlags&VISIBILITY_MASK));
   2012            if (ai.mKeepScreenOn) {
   2013                needGlobalAttributesUpdate(true);
   2014            }
   2015            ai.mKeepScreenOn = lastKeepOn;
   2016        }
   2017
   2018        if (mOnHierarchyChangeListener != null) {
   2019            mOnHierarchyChangeListener.onChildViewAdded(this, child);
   2020        }
   2021
   2022        if ((child.mViewFlags & DUPLICATE_PARENT_STATE) == DUPLICATE_PARENT_STATE) {
   2023            mGroupFlags |= FLAG_NOTIFY_CHILDREN_ON_DRAWABLE_STATE_CHANGE;
   2024        }
   2025    }
   2026
   
   由以上代码片断综合可知，在一个View只从XML中Inflate出来未被attach到View层次树里边去的时候，View.mLayoutParams成员为空。

3. RelativeLayout为什么会NullPointerException在onMeasure
   回到文章开头的StackTrace，同一时候參照RelativeLayout的onMeasure():
   426        if (isWrapContentWidth) {
   427            // Width already has left padding in it since it was calculated by looking at
   428            // the right of each child view
   429            width += mPaddingRight;
   430
   431            if (mLayoutParams.width >= 0) { // 编者注：该处为onMeasure方法中第一次使用到mLayoutParams的地方
   432                width = Math.max(width, mLayoutParams.width);
   433            }
   434
   435            width = Math.max(width, getSuggestedMinimumWidth());
   436            width = resolveSize(width, widthMeasureSpec);
   437
   438            if (offsetHorizontalAxis) {
   439                for (int i = 0; i < count; i++) {
   440                    View child = getChildAt(i);
   441                    if (child.getVisibility() != GONE) {
   442                        LayoutParams params = (LayoutParams) child.getLayoutParams();
   不难看出，在RelativeLayout被add/attach到父View之前mLayoutParams成员为空。调用measure方法将导致上图标注处代码抛出空指针异常。

4. 解决方式有两种
   a) 在measure之前显式设置LayoutParams（代表着对父View的Layout请求。必须是父View的内部LayoutParams类型）
   b) 自己主动设置LayoutParams的inflate方式
   请见代码：
        public View getView(int position, View convertView, ViewGroup parent) {
               if (convertView == null) {
                   if (unsafetyInflate) {
                       // risk inflate case
                       convertView = LayoutInflater.from(context).inflate(
                               R.layout.custom_infowindow, null);
                   } else {
                       // safety inflate case(### Solution 1 ###)
                       // The second parameter "mListView" provides a set of
                       // LayoutParams values for root of the returned hierarchy
                       convertView = LayoutInflater.from(context).inflate(
                               R.layout.custom_infowindow, mListView, false);
                   }
   
                   // ### Solution 2 ###
                   if (MainActivity.this.setLayoutParamsProgrammatically) {
                       // NOTE: the layout params set here should be of the
                       // {ParentView}.LayoutParams
                       convertView
                               .setLayoutParams(new ListView.LayoutParams(
                                       ListView.LayoutParams.WRAP_CONTENT,
                                       ListView.LayoutParams.WRAP_CONTENT));
                   }
                   Log.d(TAG, "case 1 parent:" + convertView.getParent() + " layoutParams:"
                           + convertView.getLayoutParams());
                   final int width = context.getWindow().getDecorView().getWidth();
                   final int height = context.getWindow().getDecorView().getHeight();
                   convertView.measure(width, height);
                   MainActivity.this.mLayout = convertView;
               }
               final ListItem item = (ListItem) getItem(position);
               TextView title = (TextView) convertView.findViewById(R.id.title);
   
               title.setText("title " + Math.random() + item.prop_1);
               TextView snippet = (TextView) convertView.findViewById(R.id.snippet);
               snippet.setText("snippet " + Math.random() + item.prop_2);
               return convertView;
           }
   完整project代码截图：
   watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvd2FuZ2ZlaTU4NDUyMQ==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast" alt="Measure验证project截图">
   

5. Android各个平台上这一问题的可复现性
   在查看多个版本号Android源代码后发现，Android 4.4_r1中已经针对该NullPointerException做了防范：
   539            if (mLayoutParams != null && mLayoutParams.width >= 0) {
   540                width = Math.max(width, mLayoutParams.width);
   541            }
   542
   543            width = Math.max(width, getSuggestedMinimumWidth());
   544            width = resolveSize(width, widthMeasureSpec);
   545
   546            if (offsetHorizontalAxis) {
   547                for (int i = 0; i < count; i++) {
   548                    View child = getChildAt(i);
   549                    if (child.getVisibility() != GONE) {
   550                        LayoutParams params = (LayoutParams) child.getLayoutParams();
   551                        final int[] rules = params.getRules(layoutDirection);
   552                        if (rules[CENTER_IN_PARENT] != 0 || rules[CENTER_HORIZONTAL] != 0) {
   553                            centerHorizontal(child, params, width);
   554                        } else if (rules[ALIGN_PARENT_RIGHT] != 0) {
   555                            final int childWidth = child.getMeasuredWidth();
   556                            params.mLeft = width - mPaddingRight - childWidth;
   557                            params.mRight = params.mLeft + childWidth;
   558                        }
   559                    }
   560                }
   561            }
   562        }
   

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