JVM 类加载器（2）类的初始化

作者: zidea | 来源:发表于2019-04-09 07:33 被阅读17次

ClassLoad
类的加载
每天一个知识点(3)--JVM-类加载子系统-类加载器
我的秋招之路-面经篇
jvm系列之类加载器及热加载的简易实现
JVM类加载学习三-类加载器
【深入理解Java虚拟机】类的加载器
java类加载机制、类加载器、自定义类加载器
类加载机制
tomcat类加载器

core-java.jpg

对于静态字段来说，只有直接定义了该字段的类才会被初始化，当一个类在初始化时，要求其父类全部都已经初始化完毕了。

class MParent{
    public static final String mString = "hello jvm";

    static {
        System.out.println("call MParent static block");
    }

}
public class ZiClientA {
    public static void main(String[] args) {
        System.out.println(MParent.mString);
    }
}

常量在编译阶段存入到调用这个常量的方法所在的类的常量池中。从上面代码来看就是 ZiClientA 会把 MParent 中的 mString 这个常量值保存到自己的常量池，而不是去引用 MParent 类的常量 mString

 public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
    flags: (0x0009) ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #4                  // String hello jvm
         5: invokevirtual #5                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
         8: return
      LineNumberTable:
        line 14: 0
        line 15: 8
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0       9     0  args   [Ljava/lang/String;
}

java_bean.jpg

3: ldc           #4                  // String hello jvm

助记符中 ldc 是将 ZiClientA 自己常量池 #4(看下面 ZiClient.class 常量池表）
如何读懂这个常量表参见
读懂 java 字节码（1）
读懂 java 字节码（2）
读懂 java 字节码（3）

 #1 = Methodref          #7.#21         // java/lang/Object."<init>":()V
   #2 = Fieldref           #22.#23        // java/lang/System.out:Ljava/io/PrintStream;
   #3 = Class              #24            // com/zidea/jvm/demo/MParent
   #4 = String             #25            // hello jvm
   #5 = Methodref          #26.#27        // java/io/PrintStream.println:(Ljava/lang/String;)V
   #6 = Class              #28            // com/zidea/jvm/demo/ZiClientA
   #7 = Class              #29            // java/lang/Object
   #8 = Utf8               <init>
   #9 = Utf8               ()V
  #10 = Utf8               Code
  #11 = Utf8               LineNumberTable
  #12 = Utf8               LocalVariableTable
  #13 = Utf8               this
  #14 = Utf8               Lcom/zidea/jvm/demo/ZiClientA;
  #15 = Utf8               main
  #16 = Utf8               ([Ljava/lang/String;)V
  #17 = Utf8               args
  #18 = Utf8               [Ljava/lang/String;
  #19 = Utf8               SourceFile
  #20 = Utf8               ZiClientA.java
  #21 = NameAndType        #8:#9          // "<init>":()V
  #22 = Class              #30            // java/lang/System
  #23 = NameAndType        #31:#32        // out:Ljava/io/PrintStream;
  #24 = Utf8               com/zidea/jvm/demo/MParent
  #25 = Utf8               hello jvm
  #26 = Class              #33            // java/io/PrintStream
  #27 = NameAndType        #34:#35        // println:(Ljava/lang/String;)V
  #28 = Utf8               com/zidea/jvm/demo/ZiClientA
  #29 = Utf8               java/lang/Object
  #30 = Utf8               java/lang/System
  #31 = Utf8               out
  #32 = Utf8               Ljava/io/PrintStream;
  #33 = Utf8               java/io/PrintStream
  #34 = Utf8               println
  #35 = Utf8               (Ljava/lang/String;)V

助记符

ldc 表示将 int、float 或是 String 类型的常量值推送至栈顶
bipush 表示将单字节（-128 ~ 127）的常量值推送至栈顶
sipush 表示将一个短整型常量值推送至栈顶
iconst_1 表示将 int 类型（iconst_m1 - iconst_5）推送至栈顶

静态变量

public static final short P_SHORT = 1000;

 stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: sipush        1000
         6: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
         9: return

public static final int P_MINUS_COUTER = -1;

Code:
      stack=2, locals=1, args_size=1
         0: getstatic     #2                  // Field java/lang/System.out:Ljava/io/PrintStream;
         3: iconst_m1
         4: invokevirtual #4                  // Method java/io/PrintStream.println:(I)V
         7: return

在 ICONST 中源码查看一下，从源码中可以看出 iconst_1 将几个特殊整形的数值加载到栈顶，((i >= -1) && (i <= 5)) 对这几个特殊字符进行了处理。

 public ICONST(final int i) {
        super(com.sun.org.apache.bcel.internal.Const.ICONST_0, (short) 1);
        if ((i >= -1) && (i <= 5)) {
            super.setOpcode((short) (com.sun.org.apache.bcel.internal.Const.ICONST_0 + i)); // Even works for i == -1
        } else {
            throw new ClassGenException("ICONST can be used only for value between -1 and 5: " + i);
        }
        value = i;
    }

i263315.jpg