java - 尝试将文件写入内部数据时不断出现 NullPointerException

Question

我使用以下代码将 json 文件从 Assets 文件夹复制到数据文件夹，以便在首次启动时使用此数据，直到加载实际数据:

AssetManager am = myContext.getAssets();
        InputStream is = null;
        try {
            is = am.open("settings.json");
        } catch (IOException e) {
            e.printStackTrace();
        }
        BufferedReader r = new BufferedReader(new InputStreamReader(is));
        StringBuilder total = new StringBuilder();
        String line = "";
        try {
            while ((line = r.readLine()) != null) {
                total.append(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        try {
            OutputStreamWriter outputStreamWriter = new OutputStreamWriter(
                    myContext.openFileOutput("settings.json", Context.MODE_PRIVATE));
            outputStreamWriter.write(line);
            outputStreamWriter.close();
        } catch (IOException e) {
            Log.e("Test", "File write failed: " + e.toString());
        }

但我不断收到以下错误:

06-08 14:07:24.576: E/AndroidRuntime(10509): Caused by: java.lang.NullPointerException
06-08 14:07:24.576: E/AndroidRuntime(10509):    at     java.io.Writer.write(Writer.java:141)
06-08 14:07:24.576: E/AndroidRuntime(10509):    at     com.test.app.MainActivity.onCreate(MainActivity.java:370)
06-08 14:07:24.576: E/AndroidRuntime(10509):    at     android.app.Activity.performCreate(Activity.java:5243)
06-08 14:07:24.576: E/AndroidRuntime(10509):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)

这是哪一行:

outputStreamWriter.write(line);

Answer 1

我们有以下循环:

while ((line = r.readLine()) != null) {
    total.append(line);
}

因此，显然，line 等于其末尾的 null。现在，我们来看看Writer#write(String)执行。它调用 write() 函数的 3 参数版本，如下所示:

public void write(String str, int offset, int count) throws IOException {
    if ((offset | count) < 0 || offset > str.length() - count) {
        throw new StringIndexOutOfBoundsException(str, offset, count);
    }
    char[] buf = new char[count];
    str.getChars(offset, offset + count, buf, 0);
    synchronized (lock) {
        write(buf, 0, buf.length);
    }
}

如果 str 为 null，则 str.length() 将失败并出现 NullPointerException。

我认为您需要的是outputStreamWriter.write(total.toString());，而不是outputStreamWriter.write(line);。