Java 中静态代码块初始化问题测试

Java 中静态代码块初始化问题测试 原创

情况一：变量是 static final 修饰的“编译期常量”,如 public static final String a = "JD";

public class Test {

    public static void main(String[] args) {
        System.out.println(Test2.a);
    }

}

class Test2 {
    public static final String a = "JD";

    static {
        System.out.print("OK");
    }
}

参考答案

``` JD ```

情况二：变量是 static final 修饰的“非编译期常量”,如 public static final String a = new String("JD");

public class Test {

    public static void main(String[] args) {
        System.out.println(Test2.a);
    }

}

class Test2 {
    public static final String a = new String("JD");

    static {
        System.out.print("OK");
    }
}

参考答案

``` OKJD ```

情况三：static 变量域不是 final,如 public static String a = "JD";

public class Test {

    public static void main(String[] args) {
        System.out.println(Test2.a);
    }

}

class Test2 {
    public static String a = "JD";
    
    static {
        System.out.print("OK");
    }
}

参考答案

``` OKJD ```

情况四：继承是JAVA语言的一个特性，针对类的继承，虚拟机会如何进行父类和子类的初始化加载呢？

public class Test {

    public static void main(String[] args) {
        System.out.print(B.c);
    }
}

class A {
    static {
        System.out.print("A");
    }
}

class B extends A {
    static {
        System.out.print("B");
    }

    public static final String c = "C";
}

参考答案

``` C ```

public class Test {

    public static void main(String[] args) {
        System.out.print(B.c);
    }
}

class A {
    static {
        System.out.print("A");
    }
}

class B extends A {
    static {
        System.out.print("B");
    }

    public static String c = "C";
}

参考答案

``` ABC ```

public class Test {

    public static void main(String[] args) {
        System.out.print(B.c);
    }
}

class A {
    static {
        System.out.print("A");
    }
}

class B extends A {
    static {
        System.out.print("B");
    }

    public static String c = new String("C");
}

参考答案

``` ABC ```

public class Test {

    public static void main(String[] args) {
        System.out.print(B.c);
    }
}

class A {
    static {
        System.out.print("A");
    }
}

class B extends A {
    static {
        System.out.print("B");
    }

    public static final String c = new String("C");
}

参考答案

``` ABC ```

public class Test {

    public static void main(String[] args) {
        System.out.print(B.c);
    }
}

class A {
    static {
        System.out.print("A");
    }

    // 测试：只能选择其中一种一条语句
    // public static final String c = "C"; 
    // public static  String c = "C";
    // public static final String c = new String("C");
    public static String c = new String("C");
}

class B extends A {
    static {
        System.out.print("B");
    }
}

参考答案

``` public class Test {

public static void main(String[] args) {
    System.out.print(B.c);
}

}

class A {
static {
System.out.print("A");
}

// 测试：只能选择其中一种一条语句
// public static final String c = "C"; // C
// public static  String c = "C"; // AC
// public static final String c = new String("C"); // AC
public static String c = new String("C"); // AC

}

class B extends A {
static {
System.out.print("B");
}
}

</div>

<blockquote><b>总结：</b>
1、如果一个 <b>static final</b> 变量是<b>"编译期常量"</b>,就像 <i style="color:red;">public static final String a = "JD";</i>那样，那么这个值不需要对 Test2 类进行初始化就可以读取。

2、但是,如果只是将一个变量的域设置为 static 和 final 的,那<b>不足以</b>确保这种行为。例如,对 <i style="color:red;">public static final String a = new String("JD");</i>的访问将强制对 Test2 类进行初始化,因为<b>它不是</b>一个"编译期常量"。

3、如果一个 static 变量域不是 final,那么在对它进行访问时，总是要求在它被读取之前,要先进行<b>链接</b> (为这个域分配存储空间) 和<b>初始化</b> (初始化该存储空间) 就像 <i style="color:red;">public static String a = "JD";</i>。
</blockquote>

**参考链接**
* [https://blog.csdn.net/dreamzuora/article/details/80188708](https://blog.csdn.net/dreamzuora/article/details/80188708)