JVM字节码

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前面通过tomcat本身的参数以及JVM的参数对tomcat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,那么也是不行的,所以,对于程序本身的优化也很重要;

对于程序本身的优化,可以借鉴很多前辈们的经验,但是有时候,在从源码角度方面分析的话,不好鉴别出那个效率高,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class文件,在通过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:

package com.wn.Test;

public class Test01 {
    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 Test01.class > Test01.txt

  1.3查看Test01.txt文件,内容如下:

Classfile /E:/北大青鸟/Y2/jmeter/案例/tomcat/target/classes/com/wn/Test/Test01.class
  Last modified 2020-3-9; size 576 bytes
  MD5 checksum e263d731887a9c6376ddab948c1efa19
  Compiled from "Test01.java"
public class com.wn.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.wn.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"

    内容大致分为四个部分:

      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:显示了该类中涉及到常量池,共35个;

      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;

2.常量池

  1.2 官方文档

    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

    3.1.1 官网

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

      FieldTypeterm      Type      Interpretation     

       B           byte      signed byte        

       C           char      Unicode character code point in the Basic Multilingual Plane, encoded with UTF-16

       D           double     double-precision floating-point value
       F           float      single-precision floating-point value
       I            int      integer
       J           long      long integer
       LClassName;     reference     an instance of class ClassName
       S           short      signed short
       Z           boolean     true or false
       [           reference    one array dimension

  3.2 方法描述符

    3.2.1 官网

      https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.3.3

    3.2.2 实例

      The method descriptor for the method:     

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

      is: 

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

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: "Test01.java"

  4.1 图解

    技术图片

    技术图片

     技术图片

5.研究i++与++i的不同

  都知道,i++表示,先返回在+1;++i表示,先+1在返回;

    测试代码:

package com.wn.Test;

public class Test02 {
    public static void main(String[] args) {
        new Test02().method01();
        new Test02().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);
    }
}

  5.1 查看class字节码

javap -v Test02.class > Test02.txt
Classfile /E:/北大青鸟/Y2/jmeter/案例/tomcat/target/classes/com/wn/Test/Test02.class
  Last modified 2020-3-9; size 792 bytes
  MD5 checksum cfa985a5953fb3c7a88de6d9fa95bdac
  Compiled from "Test02.java"
public class com.wn.Test.Test02
  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/wn/Test/Test02
   #3 = Methodref          #2.#27         // com/wn/Test/Test02."<init>":()V
   #4 = Methodref          #2.#29         // com/wn/Test/Test02.method01:()V
   #5 = Methodref          #2.#30         // com/wn/Test/Test02.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/wn/Test/Test02;
  #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               Test02.java
  #27 = NameAndType        #9:#10         // "<init>":()V
  #28 = Utf8               com/wn/Test/Test02
  #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.wn.Test.Test02();
    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/Test02;

  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/wn/Test/Test02
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: invokevirtual #4                  // Method method01:()V
        10: new           #2                  // class com/wn/Test/Test02
        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/wn/Test/Test02;
            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/wn/Test/Test02;
            2      13     1     i   I
            7       8     2     a   I
}
SourceFile: "Test02.java"

  5.2 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/wn/Test/Test02;
            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/wn/Test/Test02;
            2      13     1     i   I
            7       8     2     a   I

  5.3 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

  6.1 常见的字符串拼接方式

    +号拼接,str+"123"

    StringBuilder拼接

    StringBuffer拼接

  StringBuffer是保证线程安全的,下列比较低,我们更多的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,那个效率高?

  6.2 测试代码

package com.wn.Test;

public class Test03 {
    public static void main(String[] args) {
        new Test03().m1();
        new Test03().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);
    }
}

  6.3 查看class字节码

javap -v Test03.class > Test03.txt
Classfile /E:/北大青鸟/Y2/jmeter/案例/tomcat/target/classes/com/wn/Test/Test03.class
  Last modified 2020-3-9; size 1111 bytes
  MD5 checksum 2f52301c461e860c7985882234cbdd5d
  Compiled from "Test03.java"
public class com.wn.Test.Test03
  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/wn/Test/Test03
   #3 = Methodref          #2.#36         // com/wn/Test/Test03."<init>":()V
   #4 = Methodref          #2.#38         // com/wn/Test/Test03.m1:()V
   #5 = Methodref          #2.#39         // com/wn/Test/Test03.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/wn/Test/Test03;
  #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               Test03.java
  #36 = NameAndType        #15:#16        // "<init>":()V
  #37 = Utf8               com/wn/Test/Test03
  #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.wn.Test.Test03();
    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/Test03;

  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/wn/Test/Test03
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: invokevirtual #4                  // Method m1:()V
        10: new           #2                  // class com/wn/Test/Test03
        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 9: 0
        line 10: 3
        line 11: 6
        line 12: 25
        line 13: 32
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      33     0  this   Lcom/wn/Test/Test03;
            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 15: 0
        line 16: 3
        line 17: 6
        line 18: 14
        line 19: 20
        line 20: 26
        line 21: 32
        line 22: 40
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            0      41     0  this   Lcom/wn/Test/Test03;
            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: "Test03.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02

package com.wn.Test;

public class Test04 {
    public static void main(String[] args) {
        new Test04().m1();
        new Test04().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());
    }
}

    m1()和m2哪个方法的效率高?

  6.4 查看class字节码02

Classfile /E:/北大青鸟/Y2/jmeter/案例/tomcat/target/classes/com/wn/Test/Test04.class
  Last modified 2020-3-9; size 1189 bytes
  MD5 checksum 40930ac74643e9d0a1d5942e3767043e
  Compiled from "Test04.java"
public class com.wn.Test.Test04
  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/wn/Test/Test04
   #3 = Methodref          #2.#39         // com/wn/Test/Test04."<init>":()V
   #4 = Methodref          #2.#41         // com/wn/Test/Test04.m1:()V
   #5 = Methodref          #2.#42         // com/wn/Test/Test04.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/wn/Test/Test04;
  #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               Test04.java
  #39 = NameAndType        #15:#16        // "<init>":()V
  #40 = Utf8               com/wn/Test/Test04
  #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.wn.Test.Test04();
    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/Test04;

  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/wn/Test/Test04
         3: dup
         4: invokespecial #3                  // Method "<init>":()V
         7: invokevirtual #4                  // Method m1:()V
        10: new           #2                  // class com/wn/Test/Test04
        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
         3: iconst_0
         4: istore_2
         5: iload_2
         6: iconst_5
         7: if_icmpge     35
        10: new           #7                  // class java/lang/StringBuilder
        13: dup
        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
        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 9: 0
        line 10: 3
        line 11: 10
        line 10: 29
        line 13: 35
        line 14: 42
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
            5      30     2     i   I
            0      43     0  this   Lcom/wn/Test/Test04;
            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 16: 0
        line 17: 8
        line 18: 15
        line 17: 21
        line 20: 27
        line 21: 37
      LocalVariableTable:
        Start  Length  Slot  Name   Signature
           10      17     2     i   I
            0      38     0  this   Lcom/wn/Test/Test04;
            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: "Test04.java"

    可以看出,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的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。让程序执行下列更高;

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