性能优化-JVM字节码

cwl_java

发布于 2020-02-13 12:46:28

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发布于 2020-02-13 12:46:28

文章被收录于专栏：cwl_Javacwl_Java

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：

通过javap命令查看class文件中的字节码内容：

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： https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4-140

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

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

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：

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++：

++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。

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：

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：

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。