java - Android fatal error "Unable to instantiate Activity"

Question

我正在开发的应用程序中遇到 fatal error 。日志文件和代码如下。据我所知，它甚至没有进入主要 Activity 。

日志文件:

Shutting down VM
threadid=1: thread exiting with uncaught exception (group=0x401db760)
FATAL EXCEPTION: main
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{A3.cal/A3.cal.A3Activity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1739)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1831)
    at android.app.ActivityThread.access$500(ActivityThread.java:122)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1024)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:132)
    at android.app.ActivityThread.main(ActivityThread.java:4123)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:491)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
    at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
    at android.content.ContextWrapper.getResources(ContextWrapper.java:81)
    at android.view.View.<init>(View.java:2366)
    at android.view.View.<init>(View.java:2411)
    at android.widget.TextView.<init>(TextView.java:371)
    at android.widget.Button.<init>(Button.java:108)
    at android.widget.Button.<init>(Button.java:104)
    at android.widget.Button.<init>(Button.java:100)
    at A3.cal.A3Activity.<init>(A3Activity.java:32)
    at java.lang.Class.newInstanceImpl(Native Method)
    at java.lang.Class.newInstance(Class.java:1301)
    at android.app.Instrumentation.newActivity(Instrumentation.java:1022)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1730)
    ... 11 more

代码:

package A3.cal;

import java.util.Calendar;
import android.app.Activity;
import android.graphics.Color;
import android.os.Bundle;
import android.view.Display;
import android.view.Gravity;
import android.view.View;
import android.view.ViewGroup.LayoutParams;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.PopupWindow;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.TimePicker;

public class A3Activity extends Activity {
    public int currentMonth = 0;
    private TextView[] dayTableTV = new TextView[6*7]; 
    private Integer[] dayTable = null;
    public Integer currentYear = 0;
    public int firstDayOfWeek = 0;
    public boolean leapYear = false;
    private PopupWindow createEventPopup = null;
    boolean click = false;
    private Button closeAddEventButton = new Button(this);
    private Button submitAddEventButton = new Button(this);

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        makeTable();

        Button addEvent = (Button) findViewById(R.id.addEvent);
        addEvent.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                if(click == false){
                    addEventPopup();
                    click = true;
                }else{
                    createEventPopup.dismiss();
                    click = false;
                }
            }
        });

        Button nextMonthButton = (Button) findViewById(R.id.nextMonthButton);
        nextMonthButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
               changeMonthNext(getMonth(currentMonth, 1));
            }
        });

        Button previousMonthButton = (Button) findViewById(R.id.previousMonthButton);
        previousMonthButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
               changeMonthBack(getMonth(currentMonth, -1));
            }
        });

        closeAddEventButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                createEventPopup.dismiss();
            }
        });

        submitAddEventButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
               submitEvent();
            }

        });

    }

此后还有更多代码，但为了保存字符，我不会发布它。如果您需要，请在评论中告诉我。

Answer 1

在其他地方初始化按钮，例如 onCreate .

---

问题是this当 A3Activity 时，( Button )状态实例在实例变量初始值设定项中实例化( closeAddEventButton 和 submitAddEventButton )。

实例变量初始值设定项在构造函数之前运行，但对象在其构造函数执行之前并未完全初始化。在此之前引用的对象将处于不确定状态。

(状态可以确定；我只是说实例可能尚未完全可用。)

Button 中的某些内容初始化流程使用 A3Activity 中的未初始化值并抛出 NPE 。 ( JLS 12.5: Creation of New Class Instances 详细介绍了实例初始化过程。)

请注意，初始化程序在构造函数之前按文本顺序运行 ( JLS 12.4.2 )，这是一个简单的演示:

public class ThisHere {
    public String aww;
    public String notYet = this.aww;

    GedankenExperiment wat = new GedankenExperiment(this);

    // public class GedankenExperiment {
    //     public GedankenExperiment(thatThere) {
    //         thatThere.aww = ???
    //     }
    // }

    public ThisHere() {
        this.aww = "Hi there";
    }

    public static void main(String[] args) {
        ThisHere thisHere = new ThisHere();
        System.out.println("aww=" + thisHere.aww);
        // > aww=Hi there (initialized in ctor)
        System.out.println("notYet=" + thisHere.notYet);
        // > notYet=null (initialized before ctor)
    }
}

---

仅供引用，明显的迹象是以下日志行:

at A3.cal.A3Activity.<init>(A3Activity.java:32)

这是我真正看到的唯一日志行，因为我的第一步是寻找对 Android 和 Java 包之外的任何内容的引用。 <init>部分意味着它不在方法中，因此它与实例的初始值设定项有关。

堆栈跟踪包括Button初始值设定项。再加上 this 的使用在他们的构造函数中意味着这是唯一真正的可能性，因此无需太多挖掘即可快速周转。