static block and non-static block(constructor block)

[toc]

想来想去,先来一题比较好

public class Foo {
    public static void main(String[] args) {
        Baz.testAsserts();
        Baz.testAsserts();
        // Will execute after Baz is initialized.
    }
}

class Bar {
    static {
        Baz.testAsserts();
        // Will execute before Baz is initialized!
    }
}


class Baz extends Bar {
    static int x = 1;

    static void testAsserts() {

        System.out.println("x is " + x);
        x=x+2;
    }

}

• 输出结果

  x is 0
  x is 1
  x is 3

• 分析

  1. Invoking Baz.testAsserts() cause Baz to be initialized
  2. default value int x=0;
  3. Before Baz initialize , Bar must be initialized
  4. Bar’s static initializer again invoking Baz.testAsserts()
  5. so x is 0 , then x+2 , x=2
  6. go on initializing Baz , init x = 1;
  7. Invoking Baz.testAsserts() x = 1 , so x is 1;

Java类初始化顺序

父类静态变量 ——>父类静态代码块——>子类静态代码块——>父类非静态变量 ——>
父类非静态代码块——>父类构造函数 ——>子类非静态变量——>子类非静态代码块——>
子类构造函数

非静态代码块 non-static block(constructor block)

class A {

    int x ;

    //block num 1
    {
        x = 1;
        System.out.println("block num 1, x is " + x);
    }

    A() {
        x = 3;
        System.out.println("constructor block x is " + x);
    }

    //block num 2
    {
        x = 2;
        System.out.println("block num 2, x is " + x);
    }

}


public class Non_staticBlock {

    public static void main(String[] args) {

        String newLine = System.getProperty("line.separator");

        System.out.println("====first time instantiate ====" + newLine);
        new A();

        System.out.println(" \n====second time instantiate ====" + newLine);
        new A();
    }

}

• 输出结果、顺序

  ====first time instantiate ====

  block num 1, x is 1
  block num 2, x is 2
  constructor x is 3

  ====second time instantiate ====

  block num 1, x is 1
  block num 2, x is 2
  constructor x is 3

• 非静态代码块被java编译器拷贝到了构造块内,所以称为”constructor block”也是可以的,所以每次 new 构造函数也都执行

• .class 文件如下, 非静态代码块被java编译器拷贝到了构造块内.

class A {
    int x = 1;

    A() {
        System.out.println("block num 1, x is " + this.x);
        this.x = 2;
        System.out.println("block num 2, x is " + this.x);
        this.x = 3;
        System.out.println("constructor x is " + this.x);
    }
}

静态代码块 static block

class AA {

    AA() {
        x = 3;
        System.out.println("constructor x is " + x);
    }

    static int x = 1;

    //block num 1
    static {
        System.out.println("static block num 1 , x is " + x);
    }

    //block num 2
    static {
        x = 2;
        System.out.println("static block num 2 , x is " + x);
    }

    static void print() {
        System.out.println("static method");
    }
}


public class StaticBlock {
        static {
        System.out.println("==== first ====");
    }

    public static void main(String[] args) {
        String newLine = System.getProperty("line.separator");
        System.out.println("====AA class init ====" + newLine);
        // class init
        AA.print();
        
        System.out.println(" \n====fisrt time instantiate AA====" + newLine);
        
        new AA();
        System.out.println(" \n====sencond time instantiate AA====" + newLine);
        new AA();
    }
}

• 输出结果、顺序

  ==== first ====
  ====AA class init ====

  static block num 1 , x is 1
  static block num 2 , x is 2
  static method
  ==== first time instantiate AA ====

  constructor x is 3

  ==== second time instantiate AA ====

  constructor x is 3

• 由于JVM在加载类时会执行静态代码块,且只会执行一次. 本例静态引用AA.print();触发类初始化
• 静态代码块先于主方法执行,本例优先打印first
• 更多内容搜索jvm类加载

• .class 文件如下

  class AA {
  static int x = 1;
  
  AA() {
      x = 3;
      System.out.println("constructor x is " + x);
  }
  
  static void print() {
      System.out.println("static method");
  }
  
  static {
      System.out.println("static block num 1 , x is " + x);
      x = 2;
      System.out.println("static block num 2 , x is " + x);
  }
  }

联合看一下

class AAA {
    int x;

    //block num 1
    {
        x = 1;
        System.out.println("non-static block num 1 x is " + x);
    }

    AAA() {
        x = 3;
        System.out.println("constructor x is " + x);

    }

    //block num 2
    {
        x = 2;
        System.out.println("non-static block num 2 x is " + x);
    }


    // The static block only gets called once,when the class itself is initialized,
    // no matter how many objects of that type you create
    static {
        System.out.println("static block");
    }


    //Gets called every time an instance of the  class is constructed.
    //the non-static block is actually copied by the Java compiler into every constructor the class has (source).
    //So it is still the constructor's job to initialize fields.
    //to understand "actually " , find the result in the .class file of A.class
    {
        System.out.println("non-static block");
    }
}


public class BlockSample {

    public static void main(String[] args) {

        String newLine = System.getProperty("line.separator");

        System.out.println("====first time instantiate AAA ====" + newLine);
        new AAA();

        System.out.println(" \n====second time instantiate AAA ====" + newLine);
        new AAA();

    }
}

• 输出结果、顺序

====first time instantiate AAA ====

static block
non-static block num 1 x is 1
non-static block num 2 x is 2
non-static block
constructor x is 3

====second time instantiate AAA ====

non-static block num 1 x is 1
non-static block num 2 x is 2
non-static block
constructor x is 3

• .class 文件

  class AAA {
  int x = 1;
  
  AAA() {
      System.out.println("non-static block num 1 x is " + this.x);
      this.x = 2;
      System.out.println("non-static block num 2 x is " + this.x);
      System.out.println("non-static block");
      this.x = 3;
      System.out.println("constructor x is " + this.x);
  }
  
  static {
      System.out.println("static block");
  }
  }