ObjectMapping.readValue（）不执行任何操作

Question

我正在尝试使用Jackson从JSON中读取值，但objectMapper.readValue(URL,class) 什么都不做，所以我得到null作为回报。我查看并尝试了许多不同的解决方案，但没有一个有效。我觉得我错过了一个非常小的细节或东西，但我找不到它。请帮忙。

import android.os.AsyncTask;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import com.fasterxml.jackson.databind.ObjectMapper;

import java.io.IOException;
import java.net.URL;
import java.util.Locale;

public class MainActivity extends AppCompatActivity {

private int mIssueNumber = 1;
private TextView mMessageTextView;

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

    mMessageTextView = (TextView)findViewById(R.id.message_text);
    Button incrementButton = (Button)findViewById(R.id.button);

    incrementButton.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View v) {
            mIssueNumber++;
            String urlString = String.format(Locale.US, "http://xkcd.com/%d/info.0.json", mIssueNumber);
            new GetComicTask().execute(urlString);
        }
    });
}

public class GetComicTask extends AsyncTask<String, Void, Comic> {

    @Override
    protected Comic doInBackground(String... urlStrings) {
        Comic comic = null;
        ObjectMapper om = new ObjectMapper();
        try {
            comic = om.readValue(new URL(urlStrings[0]), Comic.class);
        } catch (IOException e) {
            e.printStackTrace();
        }
        return comic;
    }

    @Override
    protected void onPostExecute(Comic comic) {
        super.onPostExecute(comic);
        if(comic != null){
            mMessageTextView.setText(comic.toString());
        }else{
            mMessageTextView.setText("It is null");
        }

    }
}

}

我的漫画课程：

public class Comic {
private int num;
private int month;
private int day;
private int year;
private String link;
private String news;
private String transcript;
private String safe_title;
private String alt;
private String img;
private String title;

public Comic() {
}

public int getNum() {
    return num;
}

public void setNum(int num) {
    this.num = num;
}

public int getMonth() {
    return month;
}

public void setMonth(int month) {
    this.month = month;
}

public int getDay() {
    return day;
}

public void setDay(int day) {
    this.day = day;
}

public int getYear() {
    return year;
}

public void setYear(int year) {
    this.year = year;
}

public String getLink() {
    return link;
}

public void setLink(String link) {
    this.link = link;
}

public String getNews() {
    return news;
}

public void setNews(String news) {
    this.news = news;
}

public String getTranscript() {
    return transcript;
}

public void setTranscript(String transcript) {
    this.transcript = transcript;
}

public String getSafe_title() {
    return safe_title;
}

public void setSafe_title(String safe_title) {
    this.safe_title = safe_title;
}

public String getAlt() {
    return alt;
}

public void setAlt(String alt) {
    this.alt = alt;
}

public String getImg() {
    return img;
}

public void setImg(String img) {
    this.img = img;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

@Override
public String toString() {
    return "Comic{" +
            "num=" + num +
            ", month=" + month +
            ", day=" + day +
            ", year=" + year +
            ", link='" + link + '\'' +
            ", news='" + news + '\'' +
            ", transcript='" + transcript + '\'' +
            ", safe_title='" + safe_title + '\'' +
            ", alt='" + alt + '\'' +
            ", img='" + img + '\'' +
            ", title='" + title + '\'' +
            '}';
}

}

我的Gradle：

apply plugin: 'com.android.application'

android {
compileSdkVersion 25
buildToolsVersion "25.0.3"
packagingOptions {
    exclude 'META-INF/DEPENDENCIES'
    exclude 'META-INF/NOTICE'
    exclude 'META-INF/LICENSE'
    exclude 'META-INF/LICENSE.txt'
    exclude 'META-INF/NOTICE.txt'
}
defaultConfig {
    applicationId "com.mycompany.JsonButton"
    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'
    }
}
}

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:1.0.2'
compile 'com.fasterxml.jackson.core:jackson-core:2.8.9'
compile 'com.fasterxml.jackson.core:jackson-annotations:2.8.9'
compile 'com.fasterxml.jackson.core:jackson-databind:2.8.9'
testCompile 'junit:junit:4.12'

}

我将<uses-permission android:name="android.permission.INTERNET" />添加到我的清单中。