java - 使用静态对象的第二个非静态引用时出现 NullPointer 异常

Question

我在一个类中有一个静态对象，其中该类是一个编写器，然后该类的实例需要引用其中一个静态对象。为了避免代码重复(我必须多次编写相同的代码，每次唯一的区别在于使用哪个静态对象)。 我的解决方案是有一个名为 writer1 的静态写入器和一个名为 writer7 的静态写入器，然后有一个名为 otherWriter 的非静态写入器，它在 writer1 或 writer7 的构造函数中指向另一个写入器。
但是，当我访问 otherWriter 时，我不断收到 NullPointer 异常。下面的错误和代码 - 有什么想法吗？ 抱歉，代码并不整洁 - 它很糟糕，因为在这个阶段只是摆弄。 谢谢

错误:

java.lang.NullPointerException
    at popl.PoplFormative.generalWrite(PoplFormative.java:108)
    at popl.PoplFormative.run(PoplFormative.java:51)

package popl;

import java.util.Arrays;

public class PoplFormative extends Thread {

    int currentIndex = 0;
    int lastIndexWritten = -1;

    static int data[];
    int writerId;

    static PoplFormative writer1;
    static PoplFormative writer7;

    PoplFormative otherWriter;

    public static void main(String[] args) {
        data = new int[10];
        writer1 = new PoplFormative(1);
        writer7 = new PoplFormative(7);
        writer1.start();
        writer7.start();
        try {
            Thread.sleep(600);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        System.out.println(Arrays.toString(data));
    }

    public PoplFormative(int writer) {
        this.writerId = writer;
        if (writerId == 1) {
            this.otherWriter = PoplFormative.writer7;
        }
        else if (writerId == 7) {
            this.otherWriter = PoplFormative.writer1;
        }
        else {
            System.out.println("Big nasty error occurred");
            while (true) {
            }
        }
    }

    public void run() {
        try {
            generalWrite();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            System.out.println("Exception Thrown");
            e.printStackTrace();
        }
    }

    public void generalWrite() throws InterruptedException {
        while (currentIndex < 5) {
            //System.out.println(writerId + " writer has currentIndex; " + currentIndex);
            if (data[Math.max(lastIndexWritten,0)] == writerId || (lastIndexWritten == -1 && writerId == 7)) {
                write();
                synchronized (this) {
                    notify();
                }
                synchronized (otherWriter) {
                    otherWriter.wait();
                }
            }
            else {
                synchronized (otherWriter) {
                    otherWriter.wait();
                    write();
                }
                synchronized (this) {
                    notify();
                }
            }
            lastIndexWritten += 1;
            currentIndex += 1;
        }
        System.out.println(writerId + " has completed");
    }

    public void write() {
        System.out.println("writer: " + writerId + " currentIndex: " + currentIndex + " lastIndex: " + lastIndexWritten);
        data[currentIndex] = writerId;
    }

}

Answer 1

在您的 main 中，你有

writer1 = new PoplFormative(1);

调用构造函数

public PoplFormative(int writer) {
    this.writerId = writer;
    if (writerId == 1) {
        this.otherWriter = PoplFormative.writer7;
    }
    else if (writerId == 7) {
        this.otherWriter = PoplFormative.writer1;
    }
    else {
        System.out.println("Big nasty error occurred");
        while (true) {
        }
    }
}

在这种情况下，将执行此代码

if (writerId == 1) {
    this.otherWriter = PoplFormative.writer7;
}

但是PoplFormative.writer7是 null ，因为它还没有初始化，所以 otherWriter也变成null 。

如果您需要这些循环引用，请考虑使用 setter。