JVM字节码与代码优化
一.字节码查看
将.java源文件编译成.class二进制字节码文件，运行该字节码文件
1.将class字节码文件内容输出到文本文件当中
javap -v xxx.class > xxx.txt
第一个部分：
显示生成class的java源文件的基本信息
Classfile /C:/Users/FLC/Desktop/授课内容/授课案例/Y2170/day22/jvm_project/jvm_day01/target/classes/com/wdksoft/ClassTest.class
Last modified 2020-3-11; size 585 bytes
MD5 checksum 39fa2636495e5b4bf08da6decc537381
Compiled from "ClassTest.java"
public class com.wdksoft.ClassTest
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
第二个部分：显示该类所涉及到的常量池，共有35个常量
Constant pool:
#1 = Methodref #5.#23 // java/lang/Object."<init>":()V
#2 = Fieldref #24.#25 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Methodref #26.#27 // java/io/PrintStream.println:(I)V
#4 = Class #28 // com/wdksoft/ClassTest
#5 = Class #29 // java/lang/Object
#6 = Utf8 <init>
#7 = Utf8 ()V
#8 = Utf8 Code
#9 = Utf8 LineNumberTable
#10 = Utf8 LocalVariableTable
#11 = Utf8 this
#12 = Utf8 Lcom/wdksoft/ClassTest;
#13 = Utf8 main
#14 = Utf8 ([Ljava/lang/String;)V
#15 = Utf8 args
#16 = Utf8 [Ljava/lang/String;
#17 = Utf8 a
#18 = Utf8 I
#19 = Utf8 b
#20 = Utf8 c
#21 = Utf8 SourceFile
#22 = Utf8 ClassTest.java
#23 = NameAndType #6:#7 // "<init>":()V
#24 = Class #30 // java/lang/System
#25 = NameAndType #31:#32 // out:Ljava/io/PrintStream;
#26 = Class #33 // java/io/PrintStream
#27 = NameAndType #34:#35 // println:(I)V
#28 = Utf8 com/wdksoft/ClassTest
#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 (I)V
第三个部分：显示该类的构造器，编译器自动生成一个无参构造
public com.wdksoft.ClassTest();
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/wdksoft/ClassTest;
第四部分：显示main方法的信息
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V 方法的描述，V代表返回值为void
flags: ACC_PUBLIC, ACC_STATIC 方法修饰符 public static
Code:
stack=2, locals=4, args_size=1 stack代表操作栈的大小 locals本地变量表大小 args_size代表参数个数
0: iconst_2 将数据2压入到操作栈当中，位于栈顶
1: istore_1 从操作栈当中弹出一个数据，放到本地变量表当中，下标为1，0是this，操作栈当中数据清空
2: iconst_5 将数据5压入到操作栈当中，位于栈顶
3: istore_2 从操作栈当中弹出一个数据，放到本地变量表当中，下标为2，0是this，操作栈当中数据清空
4: iload_2 将本地变量表中的下标为2的数据压入到操作站
5: iload_1 将本地变量表中的下标为1的数据压入到操作站
6: isub 将操作栈中的两个数据相减
7: istore_3 将相减的结果压入到本地本地变量表当中，下标为3
8: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
11: iload_3 将本地变量表中下标为3的数据压入到操作站
12: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
15: return 返回
LineNumberTable: 行号列表
line 5: 0
line 6: 2
line 7: 4
line 8: 8
line 9: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 args [Ljava/lang/String;
2 14 1 a I
4 12 2 b I
8 8 3 c I
二.动态字节技术
在程序运行或者编译时期，通过动态字节码技术对类新增，删除，修改类的内部结构，包括字段和方法
动态字节码技术应用场景：AOP，Lombok，动态修改class文件等等
字节码操作类库：
1.BCEL
2.ASM：轻量级的Java字节码操作框架，SpringAOP底层基于ASM字节码技术
3.CGLIB 基于ASM
4.javassist
通过javap命令查看class文件的字节码内容
1.1创建一个简单的测试类：
public class Test {
public static void main(String[] args) {
int a=2;
int b=5;
int c=b-a;
System.out.println(c);
}
}
1.2 cmd打开窗口，使用如下命令：
javap ‐v Test.class > Test.txt
1.3查看Test01.txt文件，内容如下：
Classfile /G:/学习/Y2/Test/Test01.class
Last modified 2020-3-9; size 576 bytes
MD5 checksum e263d731887a9c6376ddab948c1efa19
Compiled from "Test01.java"
public class com.my.Test.Test01
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #5.#23 // java/lang/Object."<init>":()V
#2 = Fieldref #24.#25 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Methodref #26.#27 // java/io/PrintStream.println:(I)V
#4 = Class #28 // com/wn/Test/Test01
#5 = Class #29 // java/lang/Object
#6 = Utf8 <init>
#7 = Utf8 ()V
#8 = Utf8 Code
#9 = Utf8 LineNumberTable
#10 = Utf8 LocalVariableTable
#11 = Utf8 this
#12 = Utf8 Lcom/wn/Test/Test01;
#13 = Utf8 main
#14 = Utf8 ([Ljava/lang/String;)V
#15 = Utf8 args
#16 = Utf8 [Ljava/lang/String;
#17 = Utf8 a
#18 = Utf8 I
#19 = Utf8 b
#20 = Utf8 c
#21 = Utf8 SourceFile
#22 = Utf8 Test01.java
#23 = NameAndType #6:#7 // "<init>":()V
#24 = Class #30 // java/lang/System
#25 = NameAndType #31:#32 // out:Ljava/io/PrintStream;
#26 = Class #33 // java/io/PrintStream
#27 = NameAndType #34:#35 // println:(I)V
#28 = Utf8 com/wn/Test/Test01
#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 (I)V
{
public com.my.Test.Test01();
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/wn/Test/Test01;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=4, args_size=1
0: iconst_2
1: istore_1
2: iconst_5
3: istore_2
4: iload_2
5: iload_1
6: isub
7: istore_3
8: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
11: iload_3
12: invokevirtual #3 // Method java/io/PrintStream.println:(I)V
15: return
LineNumberTable:
line 5: 0
line 6: 2
line 7: 4
line 8: 8
line 9: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 args [Ljava/lang/String;
2 14 1 a I
4 12 2 b I
8 8 3 c I
}
SourceFile: "Test01.java"
二.代码优化
{...}
for (int i = 0, i < length; i++)
{...}
