从字符串获取 JSON URL 链接.xml到 HTTP 请求

Question

我正在尝试翻译我的应用程序，该应用程序从URL获取JSON数据。我的目的是添加从字符串.xml中获取 URL。这是一段代码。当我尝试使用

String jsonURL = getApplicationContext().getString(R.string.URL);

但是该应用程序将被强制关闭。

主要活动.class

public class MainActivity extends AppCompatActivity {
String jsonURL = getApplicationContext().getString(R.string.URL);
@SuppressLint("")
private void fetchJSON(){
new AsyncTask<Void, Void, String>(){
protected String doInBackground(Void[] params) {
String response="";
HashMap<String, String> map=new HashMap<>();
try {
HttpRequest req = new HttpRequest(jsonURL);
response = req.prepare(HttpRequest.Method.POST).withData(map).sendAndReadString();
} 
}.execute();
}

字符串.xml

<string name="URL">http://example.com/data.php</string>

onCreate(( 方法

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

日志猫：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.my.app, PID: 10378
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.my.app/com.my.app.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference
at android.content.ContextWrapper.getApplicationContext(ContextWrapper.java:118)
at com.my.app.MainActivity.<init>(MainActivity.java:42)
at java.lang.Class.newInstance(Native Method)
at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:43)
at android.app.Instrumentation.newActivity(Instrumentation.java:1243)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3182)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
at android.os.Handler.dispatchMessage(Handler.java:107) 
at android.os.Looper.loop(Looper.java:214) 
at android.app.ActivityThread.main(ActivityThread.java:7356) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 
I/Process: Sending signal. PID: 10378 SIG: 9
Process 10378 terminated.

Answer 1

使用以下代码。

public class MainActivity extends AppCompatActivity {
String jsonURL;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fetchJSON();
}
@SuppressLint("")
private void fetchJSON(){
jsonURL = getString(R.string.URL);
new AsyncTask<Void, Void, String>(){
protected String doInBackground(Void[] params) {
String response="";
HashMap<String, String> map=new HashMap<>();
try {
HttpRequest req = new HttpRequest(jsonURL);
response = req.prepare(HttpRequest.Method.POST).withData(map).sendAndReadString();
} 
}.execute();
}
}