Java构造时成员初始化的陷阱

让我们先来看两个类：Base和Derived类。注意其中的whenAmISet成员变量，和方法preProcess()。

情景1：（子类无构造方法）

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet = "set when declared";

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

当.java源代码转换成一个.class文件后，其转换成类似下面的等价代码：

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet;

    {whenAmISet = "set when declared";}

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

输出结果是： set when declared

情景2：（子类添加了构造方法）

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet = "set when declared";

    public Derived() {
        whenAmISet = "set in constructor";
    }

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

当.java源代码转换成一个.class文件后，其转换成类似下面的等价代码：

class Base {
    Base() {
        preProcess();
    }

    void preProcess() {
    }
}

class Derived extends Base {
    public String whenAmISet;

    public Derived() {
        whenAmISet = "set when declared";
        whenAmISet = "set in constructor";
    }

    void preProcess() {
        whenAmISet = "set in preProcess()";
    }
}

public class StaticTest {
    public static void main(String[] args) {
        Derived d = new Derived();
        System.out.println(d.whenAmISet);
    }
}

输出结果为：set in constructor

情景3：（赋值的细节）

public class Singleton {

    private static Singleton mInstance = new Singleton();  // 位置1
    public static int counter1;
    public static int counter2 = 0;

    private Singleton() {
        counter1++;
        counter2++;
    }

    public static Singleton getInstantce() {
        return mInstance;
    }

    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstantce();
        System.out.println("counter1: " + singleton.counter1);
        System.out.println("counter2: " + singleton.counter2);
    }
}

当.java源代码转换成一个.class文件后，其转换成类似下面的等价代码：

public class Singleton {

    private static Singleton mInstance;
    public static int counter1;
    public static int counter2;

    static {
        mInstance = new Singleton();
        counter2 = 0;
    }

    private Singleton() {
        counter1++;
        counter2++;
    }

    public static Singleton getInstantce() {
        return mInstance;
    }

    public static void main(String[] args) {
        Singleton singleton = Singleton.getInstantce();
        System.out.println("counter1: " + singleton.counter1);
        System.out.println("counter2: " + singleton.counter2);
    }
}

• 在Prepare阶段，mInstance、counter1、counter2的初始值为（null，0，0）；
• 执行至 mInstance = new Singleton()时，进行实例创建并调用构造方法，使counter1、counter2变量的值改变为（1，1）；
• 执行counter2 = 0时，counter2的值再次置为0，最终程序的输出结果为：counter1: 1   counter2: 0

同理，以下代码的最终输出结果为：counter1: 1  counter2: 1

public class Singleton {

    public static int counter1;
    public static int counter2 = 0;
    private static Singleton mInstance = new Singleton(); // 位置2

    private Singleton() {
        counter1++;
        counter2++;
    }

    public static Singleton getInstantce() {
        return mInstance;
    }

    public static void main(String[] args) {

        Singleton singleton = Singleton.getInstantce();
        System.out.println("counter1: " + singleton.counter1);
        System.out.println("counter2: " + singleton.counter2);
    }
}

原因分析：

1. 陈皓博客
2. Java Tutor - Visualize Java code execution to learn Java online (also visualize Python, Java, JavaScript, TypeScript, Ruby, C, and C++ code)