java踩坑記-static final 順序

這個坑是關於單例模式 getInstance時候的static final順序，而且問提十分隱蔽。在實際的項目，在QA環境下由於判斷條件為false，無法發現此問題;而到生產上之后，判斷條件為true，拋了空指針異常。

情況1:判斷條件為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

情況2:判斷條件為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

出現以上情況的原因，是因為在Bpp.getInstace()時，在Bpp中new了自己，而在new的過程中又需要用到Cpp的Instance，但是Cpp的static塊在Bpp的new之后，也就是在Bpp new自己的時候，Cpp還是null，所以會拋出NPE；但是在情況1中，由於判斷條件為false，並沒有跑到使用CPP的地方，所以不會拋出NPE。

正確做法：把new Bpp放到所有static塊的最后

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;
    }
}

輸出結果：

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

Process finished with exit code 0