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

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;

        }

    }

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。

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：
watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQvd2FuZ2ZlaTU4NDUyMQ==/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/SouthEast" alt="Measure验证project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        }

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

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