JVM HotSpot架构
JVM HotSpot架构思维导图
JVM HotSpot之类加载子系统
一个Java文件从编码完成到最终执行，一般主要包括两个过程
-
编译，即把我们写好的java文件，通过javac命令编译成字节码，也就是我们常说的.class文件。
-
运行，则是把编译生成的.class文件交给Java虚拟机(JVM)执行。
而我们所说的类加载过程即是指JVM虚拟机把.class文件中类信息加载进内存，并进行解析生成对应的class对象的过程。
举个通俗点的例子来说，JVM在执行某段代码时，遇到了class A， 然而此时内存中并没有class A的相关信息，于是JVM就会到相应的class文件中去寻找class A的类信息，并加载进内存中，这就是我们所说的类加载过程。
由此可见，JVM不是一开始就把所有的类都加载进内存中，而是只有第一次遇到某个需要运行的类时才会加载，且只加载一次。
** 下面以HelloWorld.java为类剖析类加载子系统原理**
package com.example.demo;
public class HelloWorld {
private static final int a = 1;
private static int b = 2;
public static void main(String[] args) {
int c = 3;
test1();
new HelloWorld().test2();
}
public static void test1(){
System.out.println(a);
}
public void test2(){
int f = 5;
System.out.println(f);
}
}
windows下生成查看class字节码过程，打开cmd命令行工具:
编译:cd E:\study\sourcecode\demo\target\classes\com\example\demo: javac HelloWorld.java
查看字节码cd E:\study\sourcecode\demo\target\classes\com\example\demo: javap HelloWorld.class > HelloWorld.txt 将生成的字节码输出到HelloWorld.txt
Classfile /E:/study/sourcecode/demo/target/classes/com/example/demo/HelloWorld.class
Last modified 2021-10-1; size 892 bytes
MD5 checksum 725ca55458c6d045ad2c25c4b7b8477b
Compiled from "HelloWorld.java"
public class com.example.demo.HelloWorld
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #9.#34 // java/lang/Object."<init>":()V
#2 = Methodref #3.#35 // com/example/demo/HelloWorld.test1:()V
#3 = Class #36 // com/example/demo/HelloWorld
#4 = Methodref #3.#34 // com/example/demo/HelloWorld."<init>":()V
#5 = Methodref #3.#37 // com/example/demo/HelloWorld.test2:()V
#6 = Fieldref #38.#39 // java/lang/System.out:Ljava/io/PrintStream;
#7 = Methodref #40.#41 // java/io/PrintStream.println:(I)V
#8 = Fieldref #3.#42 // com/example/demo/HelloWorld.b:I
#9 = Class #43 // java/lang/Object
#10 = Utf8 a
#11 = Utf8 I
#12 = Utf8 ConstantValue
#13 = Integer 1
#14 = Utf8 b
#15 = Utf8 <init>
#16 = Utf8 ()V
#17 = Utf8 Code
#18 = Utf8 LineNumberTable
#19 = Utf8 LocalVariableTable
#20 = Utf8 this
#21 = Utf8 Lcom/example/demo/HelloWorld;
#22 = Utf8 main
#23 = Utf8 ([Ljava/lang/String;)V
#24 = Utf8 args
#25 = Utf8 [Ljava/lang/String;
#26 = Utf8 c
#27 = Utf8 MethodParameters
#28 = Utf8 test1
#29 = Utf8 test2
#30 = Utf8 f
#31 = Utf8 <clinit>
#32 = Utf8 SourceFile
#33 = Utf8 HelloWorld.java
#34 = NameAndType #15:#16 // "<init>":()V
#35 = NameAndType #28:#16 // test1:()V
#36 = Utf8 com/example/demo/HelloWorld
#37 = NameAndType #29:#16 // test2:()V
#38 = Class #44 // java/lang/System
#39 = NameAndType #45:#46 // out:Ljava/io/PrintStream;
#40 = Class #47 // java/io/PrintStream
#41 = NameAndType #48:#49 // println:(I)V
#42 = NameAndType #14:#11 // b:I
#43 = Utf8 java/lang/Object
#44 = Utf8 java/lang/System
#45 = Utf8 out
#46 = Utf8 Ljava/io/PrintStream;
#47 = Utf8 java/io/PrintStream
#48 = Utf8 println
#49 = Utf8 (I)V
{
public com.example.demo.HelloWorld();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/example/demo/HelloWorld;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: iconst_3
1: istore_1
2: invokestatic #2 // Method test1:()V
5: new #3 // class com/example/demo/HelloWorld
8: dup
9: invokespecial #4 // Method "<init>":()V
12: invokevirtual #5 // Method test2:()V
15: return
LineNumberTable:
line 7: 0
line 8: 2
line 9: 5
line 10: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 args [Ljava/lang/String;
2 14 1 c I
MethodParameters:
Name Flags
args
public static void test1();
descriptor: ()V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=0, args_size=0
0: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
3: iconst_1
4: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
7: return
LineNumberTable:
line 13: 0
line 14: 7
public void test2();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=2, args_size=1
0: iconst_5
1: istore_1
2: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
5: iload_1
6: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
9: return
LineNumberTable:
line 17: 0
line 18: 2
line 19: 9
LocalVariableTable:
Start Length Slot Name Signature
0 10 0 this Lcom/example/demo/HelloWorld;
2 8 1 f I
static {};
descriptor: ()V
flags: ACC_STATIC
Code:
stack=1, locals=0, args_size=0
0: iconst_2
1: putstatic #8 // Field b:I
4: return
LineNumberTable:
line 5: 0
}
SourceFile: "HelloWorld.java"
未完待续