Android read/write 本地文件（NullPointerException）？

Question

我正在使用 read/write oneLine 字符串到本地文件的简单代码。它运行良好，直到我将 Read/write_file_operations 放入另一个 class。现在这段代码得到了空指针异常，我认为上下文有问题，但我不知道是什么...

MainActivity.java

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.TextView;

import ru.true_code.android.jsonusingionlibrary.lib.RWfile;


public class MainActivity extends Activity {

    TextView tv1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv1 = (TextView) findViewById(R.id.textView);
    }

    //Button write string to file
    public void btnWriteFile(View view) { 
        String data = "String for write to local file";
        RWfile rw = new RWfile();
        rw.writeToFile(data);
    }

    //Button read string from file and put it to textView
    public void brnReadFile(View view) {
        RWfile rw = new RWfile();
        tv1.setText(rw.readFromFile());
    }



}

class 用于 read/write 操作：

import android.app.Activity;
import android.content.Context;
import android.util.Log;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;


/**
 * Created by User on 04.02.2015.
 */
public class RWfile extends Activity {



    public void writeToFile(String data) {
        try {
            OutputStreamWriter outputStreamWriter = new OutputStreamWriter(openFileOutput("config.txt", Context.MODE_PRIVATE));
            outputStreamWriter.write(data);
            outputStreamWriter.close();
        }
        catch (IOException e) {
            Log.e("Exception", "File write failed: " + e.toString());
        }
    }

    public String readFromFile() {

        String ret = "";

        try {
            InputStream inputStream = openFileInput("config.txt");

            if ( inputStream != null ) {
                InputStreamReader inputStreamReader = new InputStreamReader(inputStream);
                BufferedReader bufferedReader = new BufferedReader(inputStreamReader);
                String receiveString = "";
                StringBuilder stringBuilder = new StringBuilder();

                while ( (receiveString = bufferedReader.readLine()) != null ) {
                    stringBuilder.append(receiveString);
                }

                inputStream.close();
                ret = stringBuilder.toString();
            }
        }
        catch (FileNotFoundException e) {
            Log.e("login activity", "File not found: " + e.toString());
        } catch (IOException e) {
            Log.e("login activity", "Can not read file: " + e.toString());
        }

        return ret;
    }
}

异常logcat:

02-04 09:11:50.938      479-479/ru.true_code.android.jsonusingionlibrary E/AndroidRuntime﹕ FATAL EXCEPTION: main
    java.lang.IllegalStateException: Could not execute method of the activity
            at android.view.View.onClick(View.java:2144)
            at android.view.View.performClick(View.java:2485)
            at android.view.View$PerformClick.run(View.java:9080)
            at android.os.Handler.handleCallback(Handler.java:587)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:123)
            at android.app.ActivityThread.main(ActivityThread.java:3683)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.reflect.InvocationTargetException
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at android.view.View.onClick(View.java:2139)
            at android.view.View.performClick(View.java:2485)
            at android.view.View$PerformClick.run(View.java:9080)
            at android.os.Handler.handleCallback(Handler.java:587)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:123)
            at android.app.ActivityThread.main(ActivityThread.java:3683)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at android.content.ContextWrapper.openFileOutput(ContextWrapper.java:158)
            at ru.true_code.android.jsonusingionlibrary.lib.RWfile.writeToFile(RWfile.java:31)
            at ru.true_code.android.jsonusingionlibrary.MainActivity.btnWriteFile(MainActivity.java:27)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at android.view.View.onClick(View.java:2139)
            at android.view.View.performClick(View.java:2485)
            at android.view.View$PerformClick.run(View.java:9080)
            at android.os.Handler.handleCallback(Handler.java:587)
            at android.os.Handler.dispatchMessage(Handler.java:92)
            at android.os.Looper.loop(Looper.java:123)
            at android.app.ActivityThread.main(ActivityThread.java:3683)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:507)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
            at dalvik.system.NativeStart.main(Native Method)

Answer 1

IllegalStateException: Could not execute method of the activity

此异常是由于 RWfile class 中的 openFileOutput 方法调用而发生的。因为您正在尝试调用 RWfile class 方法，该方法通过创建对象来扩展 Activity class。

要修复异常，请执行以下操作：

1. 如果 RWfile 是正常的 class 那么不需要扩展 Activity

2. 在 writeToFile 方法中添加 Context 参数以从 Activity:

访问 openFileOutput

public void writeToFile(String data,Context mContext) {
OutputStreamWriter outputStreamWriter = new OutputStreamWriter(
         mContext.openFileOutput("config.txt", Context.MODE_PRIVATE));
}

对 readFromFile 方法执行相同操作，因为您还在 readFromFile 方法中调用 openFileOutput

Android read/write 本地文件（NullPointerException）？

Android read/write local file (NullPointerException)?

java

android

nullpointerexception

writefile