Question

我正在使用以下代码将json文件从我的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，直到加载了真实数据：

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());
        }

但我一直收到以下错误：

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)

这一行：

outputStreamWriter.write（线）;

Answer 1

我们有以下周期：

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

所以，显然，line最后等于null。现在，我们来看看Writer#write(String)实现。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，那么str.length()将失败并显示NullPointerException。

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

Answer 2

这是因为此时line为空看看循环：
while ((line = r.readLine()) != null) { total.append(line); }
当循环结束时，line为空。当您尝试运行以下代码时：
outputStreamWriter.write(line);您将空值传递给write方法。
java.io.Writer.write(String str)实施如下：
write(str, 0, str.length());

在null对象上调用length（）方法会导致NullPointerException。

Answer 3

改变这个：

OutputStreamWriter outputStreamWriter = new OutputStreamWriter(
                    myContext.openFileOutput("settings.json", Context.MODE_PRIVATE));
            outputStreamWriter.write(line);
            outputStreamWriter.close();

到

OutputStreamWriter outputStreamWriter = new OutputStreamWriter(
                    myContext.openFileOutput("settings.json", Context.MODE_PRIVATE));
            outputStreamWriter.write(total.toString());
            outputStreamWriter.close();

尝试将文件写入内部数据时，继续获取NullPointerException

3 个答案: