JVM字节码
前面我们通过tomcat本身的参数以及jvm的参数对tomcat做了优化，其实要想将应用程序跑的更快、效率更高，除了对tomcat容器以及jvm优化外，应用程序代码本身如果写的效率不高的，那么也是不行的，所以，对于程序本身的优化也就很重要了。
对于程序本身的优化，可以借鉴很多前辈们的经验，但是有些时候，在从源码角度方面分析的话，不好鉴别出哪个效率高，如对字符串拼接的操作，是直接“+”号拼接效率高还是使用StringBuilder效率高？
这个时候，就需要通过查看编译好的class文件中字节码，就可以找到答案。
我们都知道，java编写应用，需要先通过javac命令编译成class文件，再通过jvm执行，jvm执行时是需要将class文件中的字节码载入到jvm进行运行的。
1、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 Test1 {
public static void main(String[] args) {
int a = 2;
int b = 5;
int c = b-a;
System.out.println(c);
}
}
执行成功target目录下便会生成class文件：
1.2 cmd 使用命令
javap -v Test1.class > Test1.txt
执行成功相应目录下生成Test1.txt文件：
1.3 查看Test1.txt文件 内容如下
Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test1.class
Last modified 2020-3-10; size 563 bytes
MD5 checksum 227f9972c01499bef690d9371d0b0e14
Compiled from "Test1.java"
public class com.zn.Test1
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/zn/Test1
#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/zn/Test1;
#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 Test1.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/zn/Test1
#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.zn.Test1();
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/zn/Test1;
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: "Test1.java"
内容大致分为4个部分：
第一部分：显示了生成这个class的java源文件、版本信息、生成时间等。
第二部分：显示了该类中所涉及到常量池，共35个常量。
第三部分：显示该类的构造器，编译器自动插入的。
第四部分：显示了main方的信息。（这个是需要我们重点关注的）
2、常量池
官网文档：
https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4-140
Constant Type Value 说明
CONSTANT_Class 7 类或接口的符号引用
CONSTANT_Fieldref 9 字段的符号引用
CONSTANT_Methodref 10 类中方法的符号引用
CONSTANT_InterfaceMethodref 11 接口中方法的符号引用
CONSTANT_String 8 字符串类型常量
CONSTANT_Integer 3 整形常量
CONSTANT_Float 4 浮点型常量
CONSTANT_Long 5 长整型常量
CONSTANT_Double 6 双精度浮点型常量
CONSTANT_NameAndType 12 字段或方法的符号引用
CONSTANT_Utf8 1 UTF-8编码的字符串
CONSTANT_MethodHandle 15 表示方法句柄
CONSTANT_MethodType 16 标志方法类型
CONSTANT_InvokeDynamic 18 表示一个动态方法调用点
3、描述符
3.1 字段描述符
官网：https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.3.2
3.2 方法描述符
官网：https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.3.3
示例：
4、解读方法字节码
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V //方法描述，V表示该方法的返回值为void
flags: ACC_PUBLIC, ACC_STATIC //方法修饰符，public，static的个数
Code: //stack=2，操作栈的大小为2，locals=4，本地变量表大小，args_size=1，参数的个数
stack=2, locals=4, args_size=1
0: iconst_2 //将数字2值压入擦作栈，位于栈的最上面
1: istore_1 //从操作栈中弹出一个元素（数字2），放入到本地变量表中，位于下标为1的位置（下标为0的是this）
2: iconst_5 //将数据5值压入操作栈，位于栈的最上面
3: istore_2 //操作栈中国弹出一个元素（5），放到本地变量表中，位于下标为2的位置
4: iload_2 //将本地变量表中下标为2的位置元素压入操作栈（5）
5: iload_1 //将本地变量表中下标为1的位置元素压入操作栈（2）
6: isub //操作栈中的2个数字相减
7: istore_3
8: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 通过#2号找到对应的常量，即可找到对应的引用
11: iload_3
12: invokevirtual #3 // Method java/io/PrintStream.println:(I)V 通过#3号找到对应的常量，即可找到对应的引用，进行方法调用
15: return
LineNumberTable:
line 5: 0
line 6: 2
line 7: 4
line 8: 8
line 9: 15
LocalVariableTable: //本地变量表
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: "Test1.java"
4.1 图解
5、研究 i++ 与 ++i 的不同
i++表示，先返回再+1，++i表示，先+1再返回5.1 编写测试代码
package com.zn;
public class Test2 {
public static void main(String[] args) {
new Test2().method01();
new Test2().method02();
}
public void method01(){
int i=1;
int a=i++;
System.out.println(a);
}
public void method02(){
int i=1;
int a=++i;
System.out.println(a);
}
}
执行成功target目录下便会生成class文件：
5.2 cmd 使用命令
javap -v Test2.class > Test2.txt
执行成功相应目录下生成Test2.txt文件：
5.3 查看class字节码
Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test2.class
Last modified 2020-3-10; size 779 bytes
MD5 checksum f811b140c108be80c28048c85c5f9963
Compiled from "Test2.java"
public class com.zn.Test2
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #8.#27 // java/lang/Object."<init>":()V
#2 = Class #28 // com/zn/Test2
#3 = Methodref #2.#27 // com/zn/Test2."<init>":()V
#4 = Methodref #2.#29 // com/zn/Test2.method01:()V
#5 = Methodref #2.#30 // com/zn/Test2.method02:()V
#6 = Fieldref #31.#32 // java/lang/System.out:Ljava/io/PrintStream;
#7 = Methodref #33.#34 // java/io/PrintStream.println:(I)V
#8 = Class #35 // java/lang/Object
#9 = Utf8 <init>
#10 = Utf8 ()V
#11 = Utf8 Code
#12 = Utf8 LineNumberTable
#13 = Utf8 LocalVariableTable
#14 = Utf8 this
#15 = Utf8 Lcom/zn/Test2;
#16 = Utf8 main
#17 = Utf8 ([Ljava/lang/String;)V
#18 = Utf8 args
#19 = Utf8 [Ljava/lang/String;
#20 = Utf8 method01
#21 = Utf8 i
#22 = Utf8 I
#23 = Utf8 a
#24 = Utf8 method02
#25 = Utf8 SourceFile
#26 = Utf8 Test2.java
#27 = NameAndType #9:#10 // "<init>":()V
#28 = Utf8 com/zn/Test2
#29 = NameAndType #20:#10 // method01:()V
#30 = NameAndType #24:#10 // method02:()V
#31 = Class #36 // java/lang/System
#32 = NameAndType #37:#38 // out:Ljava/io/PrintStream;
#33 = Class #39 // java/io/PrintStream
#34 = NameAndType #40:#41 // println:(I)V
#35 = Utf8 java/lang/Object
#36 = Utf8 java/lang/System
#37 = Utf8 out
#38 = Utf8 Ljava/io/PrintStream;
#39 = Utf8 java/io/PrintStream
#40 = Utf8 println
#41 = Utf8 (I)V
{
public com.zn.Test2();
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/zn/Test2;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: new #2 // class com/zn/Test2
3: dup
4: invokespecial #3 // Method "<init>":()V
7: invokevirtual #4 // Method method01:()V
10: new #2 // class com/zn/Test2
13: dup
14: invokespecial #3 // Method "<init>":()V
17: invokevirtual #5 // Method method02:()V
20: return
LineNumberTable:
line 5: 0
line 6: 10
line 7: 20
LocalVariableTable:
Start Length Slot Name Signature
0 21 0 args [Ljava/lang/String;
public void method01();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: iconst_1
1: istore_1
2: iload_1
3: iinc 1, 1
6: istore_2
7: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
10: iload_2
11: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
14: return
LineNumberTable:
line 9: 0
line 10: 2
line 11: 7
line 12: 14
LocalVariableTable:
Start Length Slot Name Signature
0 15 0 this Lcom/zn/Test2;
2 13 1 i I
7 8 2 a I
public void method02();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: iconst_1
1: istore_1
2: iinc 1, 1
5: iload_1
6: istore_2
7: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
10: iload_2
11: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
14: return
LineNumberTable:
line 14: 0
line 15: 2
line 16: 7
line 17: 14
LocalVariableTable:
Start Length Slot Name Signature
0 15 0 this Lcom/zn/Test2;
2 13 1 i I
7 8 2 a I
}
SourceFile: "Test2.java"
5.4 对比
i++：
public void method01();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: iconst_1 //将数字1压入到操作栈
1: istore_1 //将数字1从操作栈弹出，压入到本地变量表中，下标为1
2: iload_1 //将本地变量表中获取下标为1的数据，压入到操作栈中
3: iinc 1, 1 //将本地变量中的1，在+1
6: istore_2 //将数字1从操作栈弹出，压入到本地变量表中，下标为2
7: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
10: iload_2 //从本地变量表中获取下标为2的数据，压入到操作栈中
11: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
14: return
LineNumberTable:
line 9: 0
line 10: 2
line 11: 7
line 12: 14
LocalVariableTable:
Start Length Slot Name Signature
0 15 0 this Lcom/zn/Test2;
2 13 1 i I
7 8 2 a I
++i：
public void method02();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: iconst_1 //将数字1压入到操作栈中
1: istore_1 //将数字1从操作栈弹出，压入到本地变量表中，下标为1
2: iinc 1, 1 //将本地变量中的1，在+1
5: iload_1 //从本地变量表中获取下标为1的数据，压入到操作栈中
6: istore_2 //将数据2从操作栈弹出，压入到本地变量表中，下标为2
7: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
10: iload_2 //将本地变量表中获取下标为2的数据，压入到操作栈中
11: invokevirtual #7 // Method java/io/PrintStream.println:(I)V
14: return
LineNumberTable:
line 14: 0
line 15: 2
line 16: 7
line 17: 14
LocalVariableTable:
Start Length Slot Name Signature
0 15 0 this Lcom/zn/Test2;
2 13 1 i I
7 8 2 a I
区别：
i++
只是在本地变量中对数字做了相加，并没有将数据压入到操作栈
将前面拿到的数字1，再次从操作栈中拿到，压入到本地变量中
++i
将本地变量中的数字做了相加，并且将数据压入到操作栈
将操作栈中的数据，再次压入到本地变量中
小结：可以通过查看字节码的方式对代码的底层做研究，探究其原理。
6、字符串拼接
字符串的拼接在开发过程中使用是非常频繁的，常用的方式有三种：
+号拼接： str+"456"
StringBuilder拼接
StringBuffer拼接
StringBuffer是保证线程安全的，效率是比较低的，我们更多的是使用场景是不会涉及到
线程安全的问题的，所以更多的时候会选择StringBuilder，效率会高一些。
下面探究StringBuilder和“+”号拼接，哪个效率高：
6.1 编写测试代码
package com.zn;
public class Test3 {
public static void main(String[] args) {
new Test3().m1();
new Test3().m2();
}
public void m1() {
String s1 = "123";
String s2 = "456";
String s3 = s1 + s2;
System.out.println(s3);
}
public void m2() {
String s1 = "123";
String s2 = "456";
StringBuilder sb = new StringBuilder();
sb.append(s1);
sb.append(s2);
String s3 = sb.toString();
System.out.println(s3);
}
}
执行成功target目录下便会生成class文件：
6.2 cmd 使用命令
javap -v Test3.class > Test3.txt
执行成功相应目录下生成Test3.txt文件：
6.3 查看class字节码
Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test3.class
Last modified 2020-3-10; size 1098 bytes
MD5 checksum aedb0fe86c340564b9e93a5281e3069a
Compiled from "Test3.java"
public class com.zn.Test3
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #14.#36 // java/lang/Object."<init>":()V
#2 = Class #37 // com/zn/Test3
#3 = Methodref #2.#36 // com/zn/Test3."<init>":()V
#4 = Methodref #2.#38 // com/zn/Test3.m1:()V
#5 = Methodref #2.#39 // com/zn/Test3.m2:()V
#6 = String #40 // 123
#7 = String #41 // 456
#8 = Class #42 // java/lang/StringBuilder
#9 = Methodref #8.#36 // java/lang/StringBuilder."<init>":()V
#10 = Methodref #8.#43 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#11 = Methodref #8.#44 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#12 = Fieldref #45.#46 // java/lang/System.out:Ljava/io/PrintStream;
#13 = Methodref #47.#48 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = Class #49 // java/lang/Object
#15 = Utf8 <init>
#16 = Utf8 ()V
#17 = Utf8 Code
#18 = Utf8 LineNumberTable
#19 = Utf8 LocalVariableTable
#20 = Utf8 this
#21 = Utf8 Lcom/zn/Test3;
#22 = Utf8 main
#23 = Utf8 ([Ljava/lang/String;)V
#24 = Utf8 args
#25 = Utf8 [Ljava/lang/String;
#26 = Utf8 m1
#27 = Utf8 s1
#28 = Utf8 Ljava/lang/String;
#29 = Utf8 s2
#30 = Utf8 s3
#31 = Utf8 m2
#32 = Utf8 sb
#33 = Utf8 Ljava/lang/StringBuilder;
#34 = Utf8 SourceFile
#35 = Utf8 Test3.java
#36 = NameAndType #15:#16 // "<init>":()V
#37 = Utf8 com/zn/Test3
#38 = NameAndType #26:#16 // m1:()V
#39 = NameAndType #31:#16 // m2:()V
#40 = Utf8 123
#41 = Utf8 456
#42 = Utf8 java/lang/StringBuilder
#43 = NameAndType #50:#51 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#44 = NameAndType #52:#53 // toString:()Ljava/lang/String;
#45 = Class #54 // java/lang/System
#46 = NameAndType #55:#56 // out:Ljava/io/PrintStream;
#47 = Class #57 // java/io/PrintStream
#48 = NameAndType #58:#59 // println:(Ljava/lang/String;)V
#49 = Utf8 java/lang/Object
#50 = Utf8 append
#51 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#52 = Utf8 toString
#53 = Utf8 ()Ljava/lang/String;
#54 = Utf8 java/lang/System
#55 = Utf8 out
#56 = Utf8 Ljava/io/PrintStream;
#57 = Utf8 java/io/PrintStream
#58 = Utf8 println
#59 = Utf8 (Ljava/lang/String;)V
{
public com.zn.Test3();
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/zn/Test3;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: new #2 // class com/zn/Test3
3: dup
4: invokespecial #3 // Method "<init>":()V
7: invokevirtual #4 // Method m1:()V
10: new #2 // class com/zn/Test3
13: dup
14: invokespecial #3 // Method "<init>":()V
17: invokevirtual #5 // Method m2:()V
20: return
LineNumberTable:
line 5: 0
line 6: 10
line 7: 20
LocalVariableTable:
Start Length Slot Name Signature
0 21 0 args [Ljava/lang/String;
public void m1();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=4, args_size=1
0: ldc #6 // String 123
2: astore_1
3: ldc #7 // String 456
5: astore_2
6: new #8 // class java/lang/StringBuilder
9: dup
10: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
13: aload_1
14: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
17: aload_2
18: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
21: invokevirtual #11 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
24: astore_3
25: getstatic #12 // Field java/lang/System.out:Ljava/io/PrintStream;
28: aload_3
29: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
32: return
LineNumberTable:
line 10: 0
line 11: 3
line 12: 6
line 13: 25
line 14: 32
LocalVariableTable:
Start Length Slot Name Signature
0 33 0 this Lcom/zn/Test3;
3 30 1 s1 Ljava/lang/String;
6 27 2 s2 Ljava/lang/String;
25 8 3 s3 Ljava/lang/String;
public void m2();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=5, args_size=1
0: ldc #6 // String 123
2: astore_1
3: ldc #7 // String 456
5: astore_2
6: new #8 // class java/lang/StringBuilder
9: dup
10: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
13: astore_3
14: aload_3
15: aload_1
16: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
19: pop
20: aload_3
21: aload_2
22: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
25: pop
26: aload_3
27: invokevirtual #11 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
30: astore 4
32: getstatic #12 // Field java/lang/System.out:Ljava/io/PrintStream;
35: aload 4
37: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
40: return
LineNumberTable:
line 17: 0
line 18: 3
line 19: 6
line 20: 14
line 21: 20
line 22: 26
line 23: 32
line 24: 40
LocalVariableTable:
Start Length Slot Name Signature
0 41 0 this Lcom/zn/Test3;
3 38 1 s1 Ljava/lang/String;
6 35 2 s2 Ljava/lang/String;
14 27 3 sb Ljava/lang/StringBuilder;
32 9 4 s3 Ljava/lang/String;
}
SourceFile: "Test3.java"
从字节码中可以看出，m1()方法源码中是使用+好拼接，但是字节码中也被编译成了StringBuilder方式；
所以可以得出结论，字符串拼接，+号和StringBuilder是相等的，效率一样；
6.4 测试代码2
package com.zn;
public class Test4 {
public static void main(String[] args) {
new Test4().m1();
new Test4().m2();
}
public void m1() {
String str = "";
for (int i = 0; i < 5; i++) {
str = str + i;
}
System.out.println(str);
}
public void m2() {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 5; i++) {
sb.append(i);
}
System.out.println(sb.toString());
}
}
6.5 生成txt查看字节码
javap -v Test4.class > Test4.txt
Classfile /E:/accp/Y2/进阶内容/JVM/jvmTest/JvmTest/JVM_Project1/target/classes/com/zn/Test4.class
Last modified 2020-3-10; size 1176 bytes
MD5 checksum dcc28dd685e81c8add6f9c826dd910a0
Compiled from "Test4.java"
public class com.zn.Test4
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #14.#39 // java/lang/Object."<init>":()V
#2 = Class #40 // com/zn/Test4
#3 = Methodref #2.#39 // com/zn/Test4."<init>":()V
#4 = Methodref #2.#41 // com/zn/Test4.m1:()V
#5 = Methodref #2.#42 // com/zn/Test4.m2:()V
#6 = String #43 //
#7 = Class #44 // java/lang/StringBuilder
#8 = Methodref #7.#39 // java/lang/StringBuilder."<init>":()V
#9 = Methodref #7.#45 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#10 = Methodref #7.#46 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#11 = Methodref #7.#47 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#12 = Fieldref #48.#49 // java/lang/System.out:Ljava/io/PrintStream;
#13 = Methodref #50.#51 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = Class #52 // java/lang/Object
#15 = Utf8 <init>
#16 = Utf8 ()V
#17 = Utf8 Code
#18 = Utf8 LineNumberTable
#19 = Utf8 LocalVariableTable
#20 = Utf8 this
#21 = Utf8 Lcom/zn/Test4;
#22 = Utf8 main
#23 = Utf8 ([Ljava/lang/String;)V
#24 = Utf8 args
#25 = Utf8 [Ljava/lang/String;
#26 = Utf8 m1
#27 = Utf8 i
#28 = Utf8 I
#29 = Utf8 str
#30 = Utf8 Ljava/lang/String;
#31 = Utf8 StackMapTable
#32 = Class #53 // java/lang/String
#33 = Utf8 m2
#34 = Utf8 sb
#35 = Utf8 Ljava/lang/StringBuilder;
#36 = Class #44 // java/lang/StringBuilder
#37 = Utf8 SourceFile
#38 = Utf8 Test4.java
#39 = NameAndType #15:#16 // "<init>":()V
#40 = Utf8 com/zn/Test4
#41 = NameAndType #26:#16 // m1:()V
#42 = NameAndType #33:#16 // m2:()V
#43 = Utf8
#44 = Utf8 java/lang/StringBuilder
#45 = NameAndType #54:#55 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#46 = NameAndType #54:#56 // append:(I)Ljava/lang/StringBuilder;
#47 = NameAndType #57:#58 // toString:()Ljava/lang/String;
#48 = Class #59 // java/lang/System
#49 = NameAndType #60:#61 // out:Ljava/io/PrintStream;
#50 = Class #62 // java/io/PrintStream
#51 = NameAndType #63:#64 // println:(Ljava/lang/String;)V
#52 = Utf8 java/lang/Object
#53 = Utf8 java/lang/String
#54 = Utf8 append
#55 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#56 = Utf8 (I)Ljava/lang/StringBuilder;
#57 = Utf8 toString
#58 = Utf8 ()Ljava/lang/String;
#59 = Utf8 java/lang/System
#60 = Utf8 out
#61 = Utf8 Ljava/io/PrintStream;
#62 = Utf8 java/io/PrintStream
#63 = Utf8 println
#64 = Utf8 (Ljava/lang/String;)V
{
public com.zn.Test4();
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/zn/Test4;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: new #2 // class com/zn/Test4
3: dup
4: invokespecial #3 // Method "<init>":()V
7: invokevirtual #4 // Method m1:()V
10: new #2 // class com/zn/Test4
13: dup
14: invokespecial #3 // Method "<init>":()V
17: invokevirtual #5 // Method m2:()V
20: return
LineNumberTable:
line 5: 0
line 6: 10
line 7: 20
LocalVariableTable:
Start Length Slot Name Signature
0 21 0 args [Ljava/lang/String;
public void m1();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: ldc #6 // String
2: astore_1 // 将空字符串压入到本地变量表中的下标为1的位置
3: iconst_0 // 将数字0压入操作栈顶
4: istore_2 // 将栈顶数字0压入到本地变量表中的下标为2的位置
5: iload_2 // 将本地变量中下标为2的数字0压入操作栈顶
6: iconst_5 // 将数字5压入操作栈顶
7: if_icmpge 35 //比较栈顶两int型数值大小，当结果大于等于0时跳 转到35
10: new #7 // class java/lang/StringBuilder
13: dup //复制栈顶数值并将复制值压入栈顶(数字5)
14: invokespecial #8 // Method java/lang/StringBuilder."<init>":()V
17: aload_1
18: invokevirtual #9 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
21: iload_2 //将本地变量中下标为2的数字0压入操作栈顶
22: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
25: invokevirtual #11 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
28: astore_1
29: iinc 2, 1
32: goto 5
35: getstatic #12 // Field java/lang/System.out:Ljava/io/PrintStream;
38: aload_1
39: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
42: return
LineNumberTable:
line 10: 0
line 11: 3
line 12: 10
line 11: 29
line 14: 35
line 15: 42
LocalVariableTable:
Start Length Slot Name Signature
5 30 2 i I
0 43 0 this Lcom/zn/Test4;
3 40 1 str Ljava/lang/String;
StackMapTable: number_of_entries = 2
frame_type = 253 /* append */
offset_delta = 5
locals = [ class java/lang/String, int ]
frame_type = 250 /* chop */
offset_delta = 29
public void m2();
descriptor: ()V
flags: ACC_PUBLIC
Code:
stack=2, locals=3, args_size=1
0: new #7 // class java/lang/StringBuilder
3: dup
4: invokespecial #8 // Method java/lang/StringBuilder."<init>":()V
7: astore_1
8: iconst_0
9: istore_2
10: iload_2
11: iconst_5
12: if_icmpge 27
15: aload_1
16: iload_2
17: invokevirtual #10 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
20: pop
21: iinc 2, 1
24: goto 10
27: getstatic #12 // Field java/lang/System.out:Ljava/io/PrintStream;
30: aload_1
31: invokevirtual #11 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
34: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
37: return
LineNumberTable:
line 18: 0
line 19: 8
line 20: 15
line 19: 21
line 22: 27
line 23: 37
LocalVariableTable:
Start Length Slot Name Signature
10 17 2 i I
0 38 0 this Lcom/zn/Test4;
8 30 1 sb Ljava/lang/StringBuilder;
StackMapTable: number_of_entries = 2
frame_type = 253 /* append */
offset_delta = 10
locals = [ class java/lang/StringBuilder, int ]
frame_type = 250 /* chop */
offset_delta = 16
}
SourceFile: "Test4.java"
可以看到，m1()方法中的循环体内，每一次循环都会创建StringBuilder对象，效率低于m2()方法。
7、小结
使用字节码的方式可以很好查看代码底层的执行，从而可以看出哪些实现效率高，哪些实现效率低。可以更好的对我们的代码做优化。让程序执行效率更高。
来源：https://www.cnblogs.com/Zzzzn/p/12458051.html