空指针异常 - 尝试获取空数组的长度（readDirectory()）

Question

我有如下代码。我的问题是在files.length处出现了NullPointerException的异常。

for(int i=0; i < files.length; i++){

出现这个问题的原因是我在某个地方遇到了“fail readDirectory() errno=13”的错误。

File[] files = f.listFiles();

但是当路径正确时，为什么会出现readDirectory失败的情况？

package com.example.androidexplorer;

import java.io.File;
import java.util.ArrayList;
import java.util.List;

import android.os.Bundle;
import android.os.Environment;
import android.app.AlertDialog;
import android.app.ListActivity;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.TextView;

public class MainActivity extends ListActivity {

    private List<String> item = null;
    private List<String> path = null;
    private String root;
    private TextView myPath;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        myPath = (TextView)findViewById(R.id.path);

        root = Environment.getExternalStorageDirectory().getPath();
        getDir(root);
    }

    private void getDir(String dirPath)
    {
        myPath.setText("Location: " + dirPath);
        item = new ArrayList<String>();
        path = new ArrayList<String>();
        File f = new File(dirPath);

Log.v("Path: ", dirPath);

        Log.v("BEFORE", "Before Reading Fail...");
        File[] files = f.listFiles();
        Log.v("AFTER", "After Reading Fail...");


        if(!dirPath.equals(root))
        {
            item.add(root);
            path.add(root);
            item.add("../");
            path.add(f.getParent());    
        }

        Log.v("CRASH", "1 Line before crash");
        for(int i=0; i < files.length; i++){
            Log.v("AFTER CRASH", "1 Line after crash");
            File file = files[i];

            if(!file.isHidden() && file.canRead()){
                path.add(file.getPath());
                if(file.isDirectory()){
                    item.add(file.getName() + "/");
                }else{
                    item.add(file.getName());
                }
            }   
        }

        ArrayAdapter<String> fileList =
                new ArrayAdapter<String>(this, R.layout.row, item);
        setListAdapter(fileList);   
    }

    @Override
    protected void onListItemClick(ListView l, View v, int position, long id) {
        // TODO Auto-generated method stub
        File file = new File(path.get(position));

        if (file.isDirectory())
        {
            if(file.canRead()){
                getDir(path.get(position));
            }else{
                new AlertDialog.Builder(this)
                    .setIcon(R.drawable.ic_launcher)
                    .setTitle("[" + file.getName() + "] folder can't be read!")
                    .setPositiveButton("OK", null).show();  
            }   
        }else {
            new AlertDialog.Builder(this)
                    .setIcon(R.drawable.ic_launcher)
                    .setTitle("[" + file.getName() + "]")
                    .setPositiveButton("OK", null).show();

          }
    }

}

日志记录：

> 04-23 15:35:34.084: D/ResourcesManager(20672): creating new
> AssetManager and set to
> /data/app/com.example.androidexplorer-1/base.apk 04-23 15:35:34.104:
> I/art(20672): Created application space
> /data/dalvik-cache/arm/data@[e-mail][e-mail]@classes.dex.art
> at 0x76eb0000~0x76f15ff8 04-23 15:35:34.104: I/art(20672): Loaded art
> file:
> /data/dalvik-cache/arm/data@[e-mail][e-mail]@classes.dex.art
> 04-23 15:35:34.194: V/BitmapFactory(20672):
> DecodeImagePath(decodeResourceStream3) :
> res/drawable-xxhdpi-v4/ic_ab_back_holo_dark_am.png 04-23 15:35:34.204:
> V/BitmapFactory(20672): DecodeImagePath(decodeResourceStream3) :
> res/drawable-xxhdpi-v4/sym_def_app_icon.png 04-23 15:35:34.234:
> D/AbsListView(20672): Get MotionRecognitionManager 04-23 15:35:34.244:
> V/BitmapFactory(20672): DecodeImagePath(decodeResourceStream3) :
> res/drawable-xhdpi-v4/ic_launcher.png 04-23 15:35:34.254:
> V/Path:(20672): /storage/emulated/0 04-23 15:35:34.254:
> V/BEFORE(20672): Before Reading Fail... 04-23 15:35:34.254:
> E/File(20672): fail readDirectory() errno=13 04-23 15:35:34.254:
> V/AFTER(20672): After Reading Fail... 04-23 15:35:34.254:
> V/CRASH(20672): 1 Line before crash 04-23 15:35:34.254:
> D/AndroidRuntime(20672): Shutting down VM 04-23 15:35:34.254:
> E/AndroidRuntime(20672): FATAL EXCEPTION: main 04-23 15:35:34.254:
> E/AndroidRuntime(20672): Process: com.example.androidexplorer, PID:
> 20672 04-23 15:35:34.254: E/AndroidRuntime(20672):
> java.lang.RuntimeException: Unable to start activity
> ComponentInfo{com.example.androidexplorer/com.example.androidexplorer.MainActivity}:
> java.lang.NullPointerException: Attempt to get length of null array
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2658)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2725)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> android.app.ActivityThread.access$900(ActivityThread.java:172) 04-23
> 15:35:34.254: E/AndroidRuntime(20672):  at
> android.app.ActivityThread$H.handleMessage(ActivityThread.java:1422)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> android.os.Handler.dispatchMessage(Handler.java:102) 04-23
> 15:35:34.254: E/AndroidRuntime(20672):  at
> android.os.Looper.loop(Looper.java:145) 04-23 15:35:34.254:
> E/AndroidRuntime(20672):    at
> android.app.ActivityThread.main(ActivityThread.java:5834) 04-23
> 15:35:34.254: E/AndroidRuntime(20672):  at
> java.lang.reflect.Method.invoke(Native Method) 04-23 15:35:34.254:
> E/AndroidRuntime(20672):    at
> java.lang.reflect.Method.invoke(Method.java:372) 04-23 15:35:34.254:
> E/AndroidRuntime(20672):    at
> com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1388)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1183) 04-23
> 15:35:34.254: E/AndroidRuntime(20672): Caused by:
> java.lang.NullPointerException: Attempt to get length of null array
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> com.example.androidexplorer.MainActivity.getDir(MainActivity.java:57)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> com.example.androidexplorer.MainActivity.onCreate(MainActivity.java:31)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> android.app.Activity.performCreate(Activity.java:6221) 04-23
> 15:35:34.254: E/AndroidRuntime(20672):  at
> android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    at
> android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2611)
> 04-23 15:35:34.254: E/AndroidRuntime(20672):    ... 10 more

更新

错误出现在我的清单文件中的权限设置。一开始我这样设置：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.spicysoftware.infoid"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="15"
        android:targetSdkVersion="15" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme"
         android:name="android.permission.READ_EXTERNAL_STORAGE">
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".sysinfo"></activity>
        <activity android:name=".daten"></activity>
        <activity android:name=".storage"></activity>
        <activity android:name="org.achartengine.GraphicalActivity"> </activity>
    </application>

</manifest>

并更新：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.spicysoftware.infoid"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="15"
        android:targetSdkVersion="15" />

    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">

        <activity
            android:name=".MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity android:name=".sysinfo"></activity>
        <activity android:name=".daten"></activity>
        <activity android:name=".storage"></activity>
        <activity android:name="org.achartengine.GraphicalActivity"> </activity>
    </application>

</manifest>

- MSeiz5

Answer 1

f.listFiles()

如果路径不存在，将返回 null。

请检查您的路径。

更新

在 Android 上，可能需要权限才能读取某些文件。 也许您需要将其添加到您的清单文件中：

 <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

更多信息：

http://developer.android.com/reference/android/Manifest.permission.html

- Chris K.

---

谢谢！我的问题是我把权限设置在了错误的位置...请查看我的更新答案！ - MSeiz5

Answer 2

你只需要进入你的清单文件，在备份行下面粘贴它即可。

android:requestLegacyExternalStorage="true"

完成 :)

- Muhammad Ahmad

Answer 3

我已经检查了条件

文件夹路径 = 新文件(getFilesDir() + "/" + getResources().getString(R.string.app_name));

    if (folderPath.exists()) {}

对于我的代码来说，它对我有效，因为当这个文件尝试读取数据时，存储中不存在目录，所以会出现错误。

现在已经正常工作了。

- Chirag Thummar