2、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。

2.1、通过javap命令查看class文件的字节码内容

首先，看一个简单的Test1类的代码：

性能优化-JVM字节码_jvm

通过javap命令查看class文件中的字节码内容：

性能优化-JVM字节码_jvm_02

查看Test1.txt文件，内容如下：

Classfile /F:/code/itcast‐jvm/itcast‐jvm‐ test/target/classes/cn/itcast/jvm/Test1.class
Last modified 2018‐9‐27; size 577 bytes
MD5 checksum 4214859db3543c0c783ec8a216a4795f Compiled from "Test1.java"
public class cn.itcast.jvm.Test1 minor version: 0
major version: 52
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
#28 = Utf8  cn/itcast/jvm/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 cn.itcast.jvm.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  Lcn/itcast/jvm/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  6:  0
line  7:  2
line  8:  4
line  9:  8
line 10: 15
  LocalVariableTable:
  Start
  0 Length
  16  Slot
  0 Name
  args  Signature
  [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）

2.2、常量池

官网文档：

​​

​​

​​

性能优化-JVM字节码_字符串_03

2.3、描述符

2.3.1、字段描述符

官网：https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.3.2

性能优化-JVM字节码_字节码_04

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：

​The method descriptor for the method:​

Object m(int i, double d, Thread t) {...}

​is:​

(IDLjava/lang/Thread;)Ljava/lang/Object;

2.4、解读方法字节码

public static void main(java.lang.String[]);

            descriptor:([Ljava/lang/String;)V    //方法描述，V表示该方法的放回值为void
            flags:ACC_PUBLIC,ACC_STATIC    // 方法修饰符，public、static的
            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值压入操作栈，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    //将数字5值压入操作栈，位于栈的最上面
        3:istore_2    //从操作栈中弹出一个元素(5)，放入到本地变量表中，位于第下标为2个位置
        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 // 将相减的结果压入到本地本地变量表中，位于下标为3的位置
        // 通过#2号找到对应的常量，即可找到对应的引用
        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的位置元素压入操作栈（3）
        // 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
        12:invokevirtual #3    // Method java/io/PrintStream.println:(I)V
                    15:return //返回
            LineNumberTable:    //行号的列表

            line  6:0
            line  7:2
            line  8:4
            line  9:8
            line 10:15 LocalVariableTable: // 本地变量表
            Start
        0 Length
        16 Slot
        0 Name
            args Signature
        [Ljava/lang/String;
        2 14 1
            a I
        4 12 2
            b I
        8 8 3
            c I
        }

2.4.1、图解

性能优化-JVM字节码_java_05

性能优化-JVM字节码_ico_06

性能优化-JVM字节码_java_07

2.5、研究 i++ 与 ++i 的不同

我们都知道，i++表示，先返回再+1，++i表示，先+1再返回。它的底层是怎么样的呢? 我们一起探究下。

编写测试代码：

性能优化-JVM字节码_jvm_08

2.5.1、查看class字节码

Classfile /F:/code/itcast‐jvm/itcast‐jvm‐ test/target/classes/cn/itcast/jvm/Test2.class
MD5 checksum 901660fc11c43b6daadd0942150960ed Compiled from "Test2.java"
public class cn.itcast.jvm.Test2 minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref  #8.#27  // java/lang/Object."<init>": ()V
#2 = Class  #28 // cn/itcast/jvm/Test2
#3 = Methodref  #2.#27  // cn/itcast/jvm/Test2."
<init>":()V
#4 = Methodref  #2.#29  // cn/itcast/jvm/Test2.method1: ()V
#5 = Methodref  #2.#30  // cn/itcast/jvm/Test2.method2: ()V
#6 = Fieldref #31.#32 // java/lang/System.out:Ljava/io/PrintStream;
#7 = Methodref  #33.#34 // java/io/PrintStream.println: (I)V
#26 = Utf8  Test2.java
#27 = NameAndType #9:#10  // "<init>":()V
#28 = Utf8  cn/itcast/jvm/Test2
#29 = NameAndType #20:#10 // method1:()V
#30 = NameAndType #24:#10 // method2:()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 cn.itcast.jvm.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  Lcn/itcast/jvm/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 cn/itcast/jvm/Test2
3: dup

4: invokespecial #3 // Method "<init>":()V
7: invokevirtual #4 // Method method1:()V
10: new #2  // class cn/itcast/jvm/Test2
13: dup
14: invokespecial #3  // Method "<init>":()V
17: invokevirtual #5  // Method method2:()V
20: return LineNumberTable:
line 6: 0
line 7: 10
line 8: 20 LocalVariableTable:
Start Length  Slot  Name  Signature
0 21  0 args  [Ljava/lang/String;

public void method1(); 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 11: 0
line 12: 2
line 13: 7
line 14: 14 LocalVariableTable:
Start Length  Slot  Name  Signature
0 15  0 this  Lcn/itcast/jvm/Test2; 2 13  1   i I
7 8 2 a I
public void method2(); 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 17: 0
line 18: 2
line 19: 7
line 20: 14 LocalVariableTable:
Start Length  Slot  Name  Signature
0 15  0 this  Lcn/itcast/jvm/Test2; 2 13  1   i I
7 8 2 a I
}
SourceFile: "Test2.java"

2.5.2、对比

​i++：​

性能优化-JVM字节码_ico_09

​++i：​

性能优化-JVM字节码_ico_10

​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。​

2.6、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。

那么，问题来了，StringBuilder和“+”号拼接，哪个效率高呢？接下来我们通过字节码的 方式进行探究。

首先，编写个示例：

性能优化-JVM字节码_字节码_11

查看Test3.class的字节码

Classfile /F:/code/itcast‐jvm/itcast‐jvm‐ test/target/classes/cn/itcast/jvm/Test3.class
MD5 checksum b3f7629e7e37768b9b5581be01df40d6 Compiled from "Test3.java"
public class cn.itcast.jvm.Test3 minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref  #14.#36 // java/lang/Object."<init>": ()V
#2 = Class  #37 // cn/itcast/jvm/Test3
#3 = Methodref  #2.#36  // cn/itcast/jvm/Test3."
<init>":()V
#4 = Methodref  #2.#38  // cn/itcast/jvm/Test3.m1:()V
#5 = Methodref  #2.#39  // cn/itcast/jvm/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  Lcn/itcast/jvm/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  cn/itcast/jvm/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 cn.itcast.jvm.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  Lcn/itcast/jvm/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 cn/itcast/jvm/Test3
3: dup
4: invokespecial #3 // Method "<init>":()V
7: invokevirtual #4 // Method m1:()V
10: new #2  // class cn/itcast/jvm/Test3
13: dup
14: invokespecial #3  // Method "<init>":()V
17: invokevirtual #5  // Method m2:()V
20: return LineNumberTable:
line 6: 0
line 7: 10
line 8: 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 11: 0
line 12: 3
line 13: 6
line 14: 25
line 15: 32 LocalVariableTable:
Start Length  Slot  Name  Signature
0 33  0 this  Lcn/itcast/jvm/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 18: 0
line 19: 3

line 20: 6

LocalVariableTable

}
SourceFile: "Test3.java"

从解字节码中可以看出，m1()方法源码中是使用+号拼接，但是在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。 接下来，我们再看一个案例：

性能优化-JVM字节码_ico_12

m1() 与 m2() 哪个方法的效率高？

依然是通过字节码的方式进行探究。

Classfile /F:/code/itcast‐jvm/itcast‐jvm‐ test/target/classes/cn/itcast/jvm/Test4.class
MD5 checksum f87a55446b8b6cd88b6e54bd5edcc9dc Compiled from "Test4.java"
public class cn.itcast.jvm.Test4 minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref  #14.#39 // java/lang/Object."<init>": ()V
#2 = Class  #40 // cn/itcast/jvm/Test4
#3 = Methodref  #2.#39  // cn/itcast/jvm/Test4."
<init>":()V
#4 = Methodref  #2.#41  // cn/itcast/jvm/Test4.m1:()V
#5 = Methodref  #2.#42  // cn/itcast/jvm/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  Lcn/itcast/jvm/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  cn/itcast/jvm/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 cn.itcast.jvm.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  Lcn/itcast/jvm/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 cn/itcast/jvm/Test4
3: dup
4: invokespecial #3 // Method "<init>":()V
7: invokevirtual #4 // Method m1:()V
10: new #2  // class cn/itcast/jvm/Test4
13: dup
14: invokespecial #3  // Method "<init>":()V
17: invokevirtual #5  // Method m2:()V
20: return LineNumberTable:
line 6: 0
line 7: 10

line 8: 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  11: 0
line  12: 3
line  13: 10
line  12: 29
line  15: 35
line  16: 42
LocalVariableTable:
Start Length  Slot  Name  Signature
5 30  2 i I
0 43  0 this  Lcn/itcast/jvm/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 19: 0
line 20: 8
line 21: 15
line 20: 21
line 23: 27
line 24: 37 LocalVariableTable:
Start Length  Slot  Name  Signature 10    17    2   i I
0 38  0 this  Lcn/itcast/jvm/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。

2.7、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。