java基础-静态,非静态(构造)代码块,类加载

vvpale 发布于2019-08-15 13:55 / 2274人阅读

摘要：想来想去先来一题比较好输出结果分析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输出结果顺序非静态代码块被编译器拷贝到了构造块内所以称

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

Invoking Baz.testAsserts() cause Baz to be initialized
default value int x=0;
Before Baz initialize , Bar must be initialized
Bar"s static initializer again invoking Baz.testAsserts()
so x is 0 , then x+2 , x=2
go on initializing Baz , init x = 1;
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(" 
====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(" 
====fisrt time instantiate AA====" + newLine);
        
        new AA();
        System.out.println(" 
====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(" 
====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");
    }
}

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