java object initialization order

存在继承关系的java对象初始化顺序

1. 执行父类静态域和静态初始化块。
2. 执行子类静态域和静态初始化块。
3. 执行父类非静态的实例域和实例初始化块。
4. 执行父类的构造器方法。
5. 执行子类非静态的实例域和实例初始化块。
6. 执行子类的构造器方法。

示例

public class ObjectInitializeOrder {

    public static void main(String[] args) throws Exception {
        new SonClass();
    }

}

class FatherClass {

    private static final Logger LOGGER = LoggerFactory.getLogger(FatherClass.class.getSimpleName());

    // 实例属性fatherInstanceVariable和instance block中的instanceBlockInitial()是相同优先级的，看代码顺序
    public int fatherInstanceVariable = fatherInstanceVariableInitial();

    public static int fatherStaticVariable = fatherStaticVariableInitial();

    {
        instanceBlockInitial();
    }

    // 这里static的fatherStaticVariable和staticBlockInitial()要看代码顺序，哪个在前就先招待哪个，优先级相同
    static {
        staticBlockInitial();
    }

    public FatherClass() {
        LOGGER.info("4. FatherClass Constructor invoked by : {}", this.getClass().getSimpleName());
    }

    public int fatherInstanceVariableInitial() {
        LOGGER.info("3. father public instance variable initialized in {} and by : {}", FatherClass.class.getSimpleName(), this.getClass().getSimpleName());
        return 0;
    }

    public static int fatherStaticVariableInitial() {
        LOGGER.info("1. father public static variable initialized in : {}", FatherClass.class.getSimpleName());
        return 1;
    }

    public void instanceBlockInitial() {
        LOGGER.info("3. father instance block initialized in {} and by : {}", FatherClass.class.getSimpleName(), this.getClass().getSimpleName());
    }

    public static void staticBlockInitial() {
        LOGGER.info("1. father static block initialized in : {}", FatherClass.class.getSimpleName());
    }

}

class SonClass extends FatherClass {

    private static final Logger LOGGER = LoggerFactory.getLogger(SonClass.class.getSimpleName());

    public int sonInstanceVariable = sonInstanceVariableInitial();

    public static int sonStaticVariable = sonStaticVariableInitial();

    {
        sonInstanceBlockInitial();
    }

    static {
        sonStaticBlockInitial();
    }

    public SonClass() {
        LOGGER.info("6. SonClass Constructor invoked by : {}", this.getClass().getSimpleName());
    }

    public int sonInstanceVariableInitial() {
        LOGGER.info("5. son public instance variable initialized in : {}", this.getClass().getSimpleName());
        return 0;
    }

    public static int sonStaticVariableInitial() {
        LOGGER.info("2. son public static variable initialized in : {}", SonClass.class.getSimpleName());
        return 1;
    }

    public void sonInstanceBlockInitial() {
        LOGGER.info("5. son instance block initialized in : {}", this.getClass().getSimpleName());
    }

    public static void sonStaticBlockInitial() {
        LOGGER.info("2. son static block initialized in : {}", SonClass.class.getSimpleName());
    }

}

运行结果

1. father public static int initialized in : FatherClass

1. father static block initialized in : FatherClass

2. son public static int initialized in : SonClass

2. son static block initialized in : SonClass

3. father public int initialized in FatherClass and by : SonClass

3. father instance block initialized in FatherClass and by : SonClass

4. FatherClass Constructor invoked by : SonClass

5. son public int initialized in : SonClass

5. son instance block initialized in : SonClass

6. SonClass Constructor invoked by : SonClass

对象初始化的几点说明

1. 类的每个基本类型数据成员（包括static成员）都会有一个默认初始值，对象的话为特殊值null。
2. 无论创建多少个对象，相同类的静态数据都只占用一份存储区域。
3. 静态域和静态块初始化只在Class对象首次加载的时候进行一次。
4. 虽然没有显示地使用static关键字，构造器实际上也是静态方法。
5. 实例初始化块是在构造器之前执行的。
6. 创建类的第一个对象，或者是类的static域和方法被访问时，会触发类加载。
7. 如果加载的类有基类，则会加载基类。
8. 基类的构造器会被自动调用。

References

1. Object initialization in Java
2. Creation of New Class Instances