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

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

    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;
        }

    }

    /**
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;

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    }

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    }

 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。

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。

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：

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        }