为什么doInBackground在我的android REST调用中返回null？

Question

在AVD中，我测试了webservice http://10.0.2.2:8080/details，它工作正常。但在我的Android应用程序中，getForObject方法返回中的null。我不知道这里有什么问题。请帮助我们。下面是我的代码：

的Manifest.xml

<?xml version="1.0" encoding="utf-8"?> 
<manifest xmlns:android="http://schemas.android.com/apk/res/android" 
    package="com.example.user.myapp"> 

    <uses-permission android:name="android.permission.INTERNET" /> 

    <application 
     android:allowBackup="true" 
     android:icon="@mipmap/ic_launcher" 
     android:label="@string/app_name" 
     android:roundIcon="@mipmap/ic_launcher_round" 
     android:supportsRtl="true" 
     android:theme="@style/AppTheme"> 
     <activity android:name=".MainActivity"> 
      <intent-filter> 
       <action android:name="android.intent.action.MAIN" /> 

       <category android:name="android.intent.category.LAUNCHER" /> 
      </intent-filter> 
     </activity> 

    </application> 

</manifest> 

的build.gradle

apply plugin: 'com.android.application' 

android { 
    compileSdkVersion 25 
    buildToolsVersion "25.0.3" 
    defaultConfig { 
     applicationId "com.example.user.myapp" 
     minSdkVersion 15 
     targetSdkVersion 25 
     versionCode 1 
     versionName "1.0" 
     testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner" 
    } 
    buildTypes { 
     release { 
      minifyEnabled false 
      proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro' 
     } 
    } 

    //Added by me 
    packagingOptions { 
     exclude 'META-INF/ASL2.0' 
     exclude 'META-INF/LICENSE' 
     exclude 'META-INF/license.txt' 
     exclude 'META-INF/NOTICE' 
     exclude 'META-INF/notice.txt' 
    } 
} 

dependencies { 
    compile fileTree(dir: 'libs', include: ['*.jar']) 
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', { 
     exclude group: 'com.android.support', module: 'support-annotations' 
    }) 
    compile 'com.android.support:appcompat-v7:25.3.1' 
    compile 'com.android.support.constraint:constraint-layout:1.0.0-alpha7' 
    compile 'com.android.support:design:25.3.1' 
//****** Mes dependencies for Android RestTemplate and Spring interaction 
compile 'org.springframework.android:spring-android-rest-template:1.0.1.RELEASE' 
compile 'com.fasterxml.jackson.core:jackson-databind:2.3.2' 
//*****END 
testCompile 'junit:junit:4.12' 
} 

我的数据类

@JsonIgnoreProperties(ignoreUnknown = true) 
public class Moyennes { 
    private String id,name,age; 

    /*Getters and setters*/ 

    //Constructor 

    public Students(){ 

    } 

} 

**我的活动类**

public class MainActivity extends AppCompatActivity { 

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

    } 

    @Override 
    protected void onStart() { 
     super.onStart(); 
     new HttpRequestTask().execute(); 
    } 

    private class HttpRequestTask extends AsyncTask<Void, Void, Students> { 

     @Override 
     protected Students doInBackground(Void... params) { 

      // To allow Debugging 
      if(android.os.Debug.isDebuggerConnected()) 
       android.os.Debug.waitForDebugger(); 

      try { 
       final String url = "http://10.0.2.2:8080/details"; 
       RestTemplate restTemplate = new RestTemplate(); 
       restTemplate.getMessageConverters().add(new MappingJackson2HttpMessageConverter()); 
       Students students = restTemplate.getForObject(url, Students.class); 
       return students ; 
      } catch (Exception e) { 
       // Log.e("MainActivity", e.getMessage(), e); 
       Message.message(context,e.getMessage()); 
      } 

      return null; 
     } 

    @Override 
    protected void onPostExecute(Students student) { 
     String id = student.getId(); 
     String name = student.getName(); 
     String age = student.getAge(); 
     // some other work 
    } 
} 

Answer 1

之前回答这里：Students类是1个学生一类;不是学生名单。

该问题与应用程序无法理解web服务的返回类型（尽管是JSON）有关。应用程序期待它可以转换为一个，但从getForObject接收List。

为了解决这个问题，以下是doInBackground所做的更改：

@Override 
protected Students[] doInBackground(Integer... params) { 

    if(android.os.Debug.isDebuggerConnected()) // To allow Debugging => using breaking points below in doInBackground 
     android.os.Debug.waitForDebugger(); 

     try { 
      publishProgress(count); 
      final String url = "http://10.0.2.2:8080/details"; 

      RestTemplate restTemplate = new RestTemplate(); 
      restTemplate.getMessageConverters().add(new MappingJackson2HttpMessageConverter()); 

      ResponseEntity<Students[]> res = restTemplate.getForEntity(url, Students[].class); 
      Students[] m = res.getBody(); 

      return m; 

     } catch (Exception e) { 
      Log.e("Error : ", e.getMessage(), e); 
     } 

    return null; 
}