面对应用程序崩溃（初学者）

Question

我已尽力而为，但无法解决此问题，我尝试调试没有任何反应，并尝试使新项目出现相同的错误。直到下午的项目都运行良好，但随后却出现了一个错误，只是试图更新Android应用程序的UI：

E/StudioProfiler: JVMTI error: 15(JVMTI_ERROR_THREAD_NOT_ALIVE) 
    E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.nodemcu.controlnodemcu, PID: 6681
                  java.lang.IllegalStateException: Could not execute method for android:onClick
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:389)
                      at android.view.View.performClick(View.java:6294)
                      at android.view.View$PerformClick.run(View.java:24770)
                      at android.os.Handler.handleCallback(Handler.java:790)
                      at android.os.Handler.dispatchMessage(Handler.java:99)
                      at android.os.Looper.loop(Looper.java:164)
                      at android.app.ActivityThread.main(ActivityThread.java:6494)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
                   Caused by: java.lang.reflect.InvocationTargetException
                      at java.lang.reflect.Method.invoke(Native Method)
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:384)
                      at android.view.View.performClick(View.java:6294) 
                      at android.view.View$PerformClick.run(View.java:24770) 
                      at android.os.Handler.handleCallback(Handler.java:790) 
                      at android.os.Handler.dispatchMessage(Handler.java:99) 
                      at android.os.Looper.loop(Looper.java:164) 
                      at android.app.ActivityThread.main(ActivityThread.java:6494) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.lang.ref.WeakReference.get()' on a null object reference
                      at com.example.nodemcu.nodemcucontroller.MainActivity$HttpRequestTask.<init>(MainActivity.java:80)
                      at com.example.nodemcu.nodemcucontroller.MainActivity$HttpRequestTask.<init>(MainActivity.java:68)
                      at com.example.nodemcu.nodemcucontroller.MainActivity.buttonClick(MainActivity.java:63)
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:384) 
                      at android.view.View.performClick(View.java:6294) 
                      at android.view.View$PerformClick.run(View.java:24770) 
                      at android.os.Handler.handleCallback(Handler.java:790) 
                      at android.os.Handler.dispatchMessage(Handler.java:99) 
                      at android.os.Looper.loop(Looper.java:164) 
                      at android.app.ActivityThread.main(ActivityThread.java:6494) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 
Application terminated.

导入的库：

package com.example.nodemcu.nodemcucontroller;
import android.content.Context;
import android.os.AsyncTask;
import android.support.v7.app.AlertDialog.Builder;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.lang.ref.WeakReference;
import java.net.URI;
import java.net.URISyntaxException;

MainActivity.java的代码：

public class MainActivity extends AppCompatActivity {

final Context context = this;
private EditText ipAddress;
private Button ledOn, ledOff;

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

    ipAddress = (EditText) findViewById(R.id.edt_ip);
    ledOn = (Button) findViewById(R.id.btn_ledOn);
    ledOff = (Button) findViewById(R.id.btn_ledOff);

}
public void buttonClick(View view) {
    String ledStatus;

    if (ipAddress.getText().toString().equals(""))
        Toast.makeText(MainActivity.this, "Please enter the ip address...", Toast.LENGTH_SHORT).show();

    else {
        if (view == ledOn)
            ledStatus = "1";

        else
            ledStatus = "0";

        String serverAdress = ipAddress.getText().toString() + ":" + "80";
        HttpRequestTask requestTask = new HttpRequestTask(serverAdress);
        requestTask.execute(ledStatus);
    }
}

private static class HttpRequestTask extends AsyncTask<String, Void, String>{

    private String serverAdress;
    private String serverResponse = "";
    AlertDialog dialog;
    private WeakReference<MainActivity> activityReference;
    HttpRequestTask(MainActivity context){
        activityReference = new WeakReference<>(context);
    }
    private HttpRequestTask(String serverAdress) {
        this.serverAdress = serverAdress;

        dialog = new AlertDialog.Builder(activityReference.get())
                .setTitle("HTTP Response from Ip Address:")
                .setCancelable(true)
                .create();
    }

    @Override
    protected String doInBackground(String... params) {
        dialog.setMessage("Data sent , waiting response from server...");

        if (!dialog.isShowing())
            dialog.show();

        String val = params[0];
        final String url = "http://" + serverAdress + "/led/" + val;

        try {
            HttpClient client = new DefaultHttpClient();
            HttpGet getRequest = new HttpGet();
            getRequest.setURI(new URI(url));
            HttpResponse response = client.execute(getRequest);

            InputStream inputStream = null;
            inputStream = response.getEntity().getContent();
            BufferedReader bufferedReader =
                    new BufferedReader(new InputStreamReader(inputStream));

            serverResponse = bufferedReader.readLine();
            inputStream.close();

        } catch (URISyntaxException e) {
            e.printStackTrace();
            serverResponse = e.getMessage();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
            serverResponse = e.getMessage();
        } catch (IOException e) {
            e.printStackTrace();
            serverResponse = e.getMessage();
        }
        return serverResponse;
    }

    @Override
    protected void onPostExecute(String s) {
        dialog.setMessage(serverResponse);

        if (!dialog.isShowing())
            dialog.show();
    }

    @Override
    protected void onPreExecute() {
        dialog.setMessage("Sending data to server, please wait...");

        if (!dialog.isShowing())
            dialog.show();
       }
   }
}

Activity_main.xml文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >

<EditText
    android:id="@+id/edt_ip"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:ems="10"
    android:hint="@string/ip_address"
    android:inputType="text"/>

<Button
    android:id="@+id/btn_ledOff"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:onClick="buttonClick"
    android:text="@string/led_off" />

<Button
    android:id="@+id/btn_ledOn"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:onClick="buttonClick"
    android:text="@string/led_on" />

</LinearLayout>

Answer 1

完成编辑

按照崩溃日志中的这一行

java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object java.lang.ref.WeakReference.get()' on a null object reference at com.example.nodemcu.nodemcucontroller.MainActivity$HttpRequestTask.

问题是您创建警报对话框时activityReference为空。

您的two中有HttpRequestTask个构造函数。

编辑

像这样更改您的HttpRequestTask构造函数：

 private WeakReference<MainActivity> activityReference;

    private HttpRequestTask(String serverAdress, MainActivity context) {
        this.serverAdress = serverAdress;
        this.activityReference = new WeakReference<>(context);

        dialog = new AlertDialog.Builder(activityReference.get())
                .setTitle("HTTP Response from Ip Address:")
                .setCancelable(true)
                .create();
    }

Answer 2

尝试以下代码：

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

private Context context = this;
private EditText ipAddress;
private Button ledOn, ledOff;
private String ledStatus;

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

   ipAddress = (EditText) findViewById(R.id.edt_ip);
   ledOn = (Button) findViewById(R.id.btn_ledOn);
   ledOff = (Button) findViewById(R.id.btn_ledOff);

   if (ipAddress.getText().toString().equals("")) {
            Toast.makeText(MainActivity.this, "Please enter the ip address...", Toast.LENGTH_SHORT).show(); 
       } else {
         ledOn.setOnclicklistener(this);
         ledOff.setOnclicklistener(this);
    }

}

@Override
public void onClick(View view) {

    switch(view.getId()){
       case R.id.ledOn:
         ledStatus = "1";
       break;
       case R.id.ledOff:
         if (ledStatus == "1") { ledStatus = "0"; }
       break;
    }
   // do your async task here
}
}

按钮现在应该可以正常工作了。不知道您的异步任务，但是它也应该工作，这取决于它是从ledOn或ledOff按钮接收输入的，然后可以将代码放在此处，否则就可以了。尝试这种方式。