java - 理解上下文和上下文包装器时出现问题

Question

我无法理解这些类(class)。我一直在尝试在内部存储的新目录中创建一个新文件，在其中放入一些文本，然后将其读出。如果没有 ContextWrapper，这似乎不起作用。所以我尝试了这个:

public class DownloadActivity extends Activity {
  ...
 class Download extends AsyncTask<String, Void, String>{

 ....
 private void searchAndSave(String s) throws IOException {
....
   ContextWrapper cw = new ContextWrapper(getBaseContext());
   File folder = cw.getDir("folder", Context.MODE_PRIVATE);
   File fileInFolder = new File(folder, "fileInFolder");

   String string = "Hello world!";
   FileOutputStream outputStream = openFileOutput("fileInFolder",
                                Context.MODE_PRIVATE);
   outputStream.write(string.getBytes());
   outputStream.close();

   File fl = new File(cw.getDir("folder", Context.MODE_PRIVATE)+"/fileInFolder");
   FileInputStream fin = new FileInputStream(fl);

   BufferedReader reader = new BufferedReader(
                                new   InputStreamReader(fin));
   StringBuilder sb = new StringBuilder();
   String line = null;
   while ((line = reader.readLine()) != null) {
       sb.append(line).append("\n");
   }
   String result = sb.toString();
   reader.close();
   fin.close();
   }
}

如果没有 ContextWrapper，则无法创建文件。我已经读了很多书，但我仍然有一些问题需要理解，Context 和 Contextwrapper 实际做什么以及为什么我需要它们来创建文件。另外，在我的代码中，创建 FileInputStream 不起作用。当程序达到

FileInputStream fin = new FileInputStream(fl);

我总是收到错误:

05-21 11:18:25.721: W/System.err(7344): java.io.FileNotFoundException: 
/data/data/com.example.dice/app_folder/fileInFolder: open failed: 
ENOENT (No such file or directory)

我真的很希望能得到一些帮助来理解和解决这个问题。

更新:我做了一个更备用的 Activity ，也许现在重建整个事情更容易了。 (我应该在新答案中完成此操作，还是可以编辑我的第一篇文章？)尽管我在尝试创建文件时没有收到错误消息，但似乎不起作用。在这里，我尝试读取“Hello world”字符串，但收到 FileNotFoundException (EISDIR)。只是为了让你知道:)

public class FolderActivity extends Activity {
public final static String TAG = "FolderActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_folder);

    Button button = (Button) findViewById(R.id.button_folder);

    button.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            try {
                folder();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}
    private void folder() throws IOException{
        Log.d(TAG, "folder");
        ContextWrapper cw = new ContextWrapper(getBaseContext());
        Log.d(TAG, "cw = new ContextWrapper");
        File folder = cw.getDir("folder", Context.MODE_PRIVATE);
        Log.d(TAG, "folder = cw.getDir");
        File fileInFolder = new File(folder, "fileInFolder");
        Log.d(TAG, "fileInFolder = new File");
        /*Log.d(TAG,
                "fileInFolder.getAbsolutePath()"
                        + fileInFolder.getAbsolutePath());*/

        String string = "Hello world!";
// Aksioms suggestion
        if (!fileInFolder.exists() && !fileInFolder.mkdirs()) {
            Log.e("file", "Couldn't create file " + fileInFolder);
        } else { Log.d(TAG, "file created"); }
        FileOutputStream outputStream = openFileOutput("fileInFolder",
                Context.MODE_PRIVATE);
        outputStream.write(string.getBytes());
        outputStream.close();
        Log.d(TAG,
                "fileInFolder.getAbsolutePath()"
                        + fileInFolder.getAbsolutePath());
        String s = fileInFolder.getAbsolutePath();
        try {
            getStringFromFile(s);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

public static String getStringFromFile (String filePath) throws Exception {
    File fl = new File(filePath);
    FileInputStream fin = new FileInputStream(fl);
    String ret = convertStreamToString(fin);
    //Make sure you close all streams.
    fin.close();        
    return ret;
}

public static String convertStreamToString(InputStream is) throws Exception {
    BufferedReader reader = new BufferedReader(new InputStreamReader(is));
    StringBuilder sb = new StringBuilder();
    String line = null;
    while ((line = reader.readLine()) != null) {
      sb.append(line).append("\n");
    }
    reader.close();
    return sb.toString();
}
}

Answer 1

您正在尝试打开一个不存在的文件。确保检查并创建该文件(如果不存在):

if (!fl.exists() && !fl.mkdirs()) { 
                Log.e("file", "Couldn't create file " + fl);
}

编辑:

是的，如果未创建文件夹，getDir 会创建该文件夹。

File folder = cw.getDir("folder", Context.MODE_PRIVATE);

但问题就在这里

 File fileInFolder = new File(folder, "fileInFolder");
   String string = "Hello world!";
   FileOutputStream outputStream = openFileOutput("fileInFolder",
                                Context.MODE_PRIVATE);

在这里，您尝试打开一个不存在的文件，您刚刚构建了一个名为 fileInFolder 的新文件，但实际上您还没有该文件夹。

尝试使用我在 openFileOutput("fileInFolder", Context.MODE_PRIVATE); 之前首先编写的代码:

if (!fileInFolder.exists() && !fileInFolder.mkdirs()) { 
                    Log.e("file", "Couldn't create file " + fileInFolder);
    }

试试这个并告诉我效果如何。

编辑2:

好吧，我发现问题是如此明显。错误是我们将 fileInFolder 创建为目录，并且您不能在那里写入任何内容 :D

我们应该做的是:

删除我用于创建 fileInFolder 的代码。

if (!fileInFolder .exists() && !fileInFolder .mkdirs()) {
    Log.e("file", "Couldn't create file " + fileInFolder );
} else {
    Log.d(TAG, "file created");
}

我们不需要创建它，因为我们将把它用作文件。所以我在你的代码中所做的更改是这样的:

FileOutputStream outputStream = new FileOutputStream(fileInFolder);
    BufferedWriter out = new BufferedWriter(new OutputStreamWriter(outputStream));
    out.write(string);
    out.close();

在 String string = "Hello world!"; 行之间添加此内容还有第一个Log.d...这是写入文件的正确方法。

整个代码:

public class MainActivity extends Activity {
public final static String TAG = "FolderActivity";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    Button button = (Button) findViewById(R.id.button_folder);

    button.setOnClickListener(new OnClickListener() {

        @Override
        public void onClick(View v) {
            try {
                folder();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    });
}

private void folder() throws IOException {
    Log.d(TAG, "folder");
    ContextWrapper cw = new ContextWrapper(getBaseContext());
    Log.d(TAG, "cw = new ContextWrapper");
    File folder = cw.getDir("folder", Context.MODE_PRIVATE);
    Log.d(TAG, "folder = cw.getDir");
    File fileInFolder = new File(folder, "fileInFolder");
    Log.d(TAG, "fileInFolder = new File");
    /*
     * Log.d(TAG, "fileInFolder.getAbsolutePath()" +
     * fileInFolder.getAbsolutePath());
     */

    String string = "Hello world!";

    FileOutputStream outputStream = new FileOutputStream(fileInFolder);
    BufferedWriter out = new BufferedWriter(new OutputStreamWriter(outputStream));
    out.write(string);
    out.close();
    Log.d(TAG,
            "fileInFolder.getAbsolutePath()"
                    + fileInFolder.getAbsolutePath());
    String s = fileInFolder.getAbsolutePath();
    try {
        getStringFromFile(s);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

public static String getStringFromFile(String filePath) throws Exception {
    File fl = new File(filePath);
    FileInputStream fin = new FileInputStream(fl);
    String ret = convertStreamToString(fin);
    // Make sure you close all streams.
    fin.close();
    return ret;
}

public static String convertStreamToString(InputStream is) throws Exception {
    BufferedReader reader = new BufferedReader(new InputStreamReader(is));
    StringBuilder sb = new StringBuilder();
    String line = null;
    while ((line = reader.readLine()) != null) {
        sb.append(line).append("\n");
    }
    reader.close();
    return sb.toString();
}}

检查您的 list 中是否具有此权限:

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

我希望现在一切都清楚了。