java踩坑记-static final 顺序

这个坑是关于单例模式 getInstance时候的static final顺序，而且问提十分隐蔽。在实际的项目，在QA环境下由于判断条件为false，无法发现此问题;而到生产上之后，判断条件为true，抛了空指针异常。

情况1:判断条件为false

package com.paypal.dinyu;

/**
 * Created by dinyu on 26/07/2017.
 */
public class Bpp {
    private static final Bpp INSTANCE = new Bpp();
    private static final Cpp CPP = Cpp.getInstance();

    private Bpp() {
        init();
    }

    public static Bpp getInstance() {
        return INSTANCE;
    }

    private void init() {
        if (false) {
            System.out.println("CPP.toString: " + CPP.toString());
        }
    }

    public static void main(String[] args) {
        Bpp.getInstance();
    }
}

class Cpp {
    private static final Cpp INSTANCE = new Cpp();

    private Cpp() {

    }

    public static Cpp getInstance() {
        return INSTANCE;
    }
}

Output:

Process finished with exit code 0

情况2:判断条件为true

package com.paypal.dinyu;

/**
 * Created by dinyu on 26/07/2017.
 */
public class Bpp {
    private static final Bpp INSTANCE = new Bpp();
    private static final Cpp CPP = Cpp.getInstance();

    private Bpp() {
        init();
    }

    public static Bpp getInstance() {
        return INSTANCE;
    }

    private void init() {
        if (true) {
            System.out.println("CPP.toString: " + CPP.toString());
        }
    }

    public static void main(String[] args) {
        Bpp.getInstance();
    }
}

class Cpp {
    private static final Cpp INSTANCE = new Cpp();

    private Cpp() {

    }

    public static Cpp getInstance() {
        return INSTANCE;
    }
}

Output:

Exception in thread "main" java.lang.ExceptionInInitializerError
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:123)
Caused by: java.lang.NullPointerException
at com.paypal.dinyu.Bpp.init(Bpp.java:20)
at com.paypal.dinyu.Bpp.<init>(Bpp.java:11)
at com.paypal.dinyu.Bpp.<clinit>(Bpp.java:7)
... 3 more

Process finished with exit code 1

出现以上情况的原因，是因为在Bpp.getInstace()时，在Bpp中new了自己，而在new的过程中又需要用到Cpp的Instance，但是Cpp的static块在Bpp的new之后，也就是在Bpp new自己的时候，Cpp还是null，所以会抛出NPE；但是在情况1中，由于判断条件为false，并没有跑到使用CPP的地方，所以不会抛出NPE。

正确做法：把new Bpp放到所有static块的最后

package com.paypal.dinyu;

/**
 * Created by dinyu on 26/07/2017.
 */
public class Bpp {
    private static final Cpp CPP = Cpp.getInstance();
    private static final Bpp INSTANCE = new Bpp();

    private Bpp() {
        init();
    }

    public static Bpp getInstance() {
        return INSTANCE;
    }

    private void init() {
        if (true) {
            System.out.println("CPP.toString: " + CPP.toString());
        }
    }

    public static void main(String[] args) {
        Bpp.getInstance();
    }
}

class Cpp {
    private static final Cpp INSTANCE = new Cpp();

    private Cpp() {

    }

    public static Cpp getInstance() {
        return INSTANCE;
    }
}

输出结果：

CPP.toString: com.paypal.dinyu.Cpp@63947c6b

Process finished with exit code 0