2 JVM 바이트 코드
바람둥이의 JVM 매개 변수를 최적화 바람둥이의 매개 변수에 의해 우리 자신의 앞에, 사실, 우리는 응용 프로그램이 컨테이너와 JVM을 최적화 바람둥이뿐만 아니라, 더 빨리 더 효율적으로 실행하려면, 응용 프로그램 코드 자체는 효율은 기록되어 있지 않은 경우 고, 그것은 충분히 따라서도 매우 중요한 프로그램 자체를 최적화 할 수 없습니다.
자체 프로그램을 최적화하기 위해, 당신은 전임자의 경험을 많이 배울 수 있지만, 때로는보기의 소스 지점에서 분석의 관점에서, 이러한 문자열 연결의 작동으로하는 높은 효율을 식별하는 데 좋지 않아 직접 "+"기호 접합 효율이다 높거나 높은 StringBuilder의 효율?
이 시간, 우리는 당신이 답을 찾을 수 있습니다, 컴파일 된 클래스 파일 바이트 코드를 볼 필요가있다.
쓰기 응용 프로그램을 자바로, 필요가 JVM에 의해 실행 한 후 클래스 파일로 javac의 명령 컴파일을 통과하고 있음을 우리 모두는 알고는 JVM을 실행 시간 클래스의 바이트 코드 파일에 대한 필요성이 JVM에로드되어 실행되고있다.
은 javap 내용 2.1, 뷰 클래스 파일 바이트 코드 명령
첫째, 단순한 모양에서 Test1 클래스 코드 :
명령은 javap의 내용에 의해 확인 바이트 코드의 클래스 파일 :
참조있는 test1.txt 파일로 다음과 같습니다 :
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"
: 요약 4 개 부분으로 대별
파트 I :이 쇼 결과 클래스 자바 소스 파일, 버전 정보, 생성 시간. 파트 II는 : 쇼는 클래스 상수 풀, 35 개 상수의 총을하고있었습니다.
파트 III : 클래스 생성자를 표시, 컴파일러가 자동으로 삽입.
파트 IV : 주요 당사자들에 대한 정보를 표시합니다. (이것은 우리의 관심을 집중할 필요가있다)
2.2 상수 풀
문서 공식 웹 사이트 :
https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4-140
2.3 설명
2.3.1 필드 설명
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:
보기 Test3.class 바이트 코드
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"
디 바이트 코드로부터 알 수있는 바와 같이, M1 () 메소드를 사용하여 소스 코드 번호 + 접합하는 것이지만, 또한, 바이트 코드로 컴파일
하여 StringBuilder 방법.
따라서, 결론 문자열 연결, + 및 StringBuilder에 동일한 효율 동일 할 수있다. 다음으로, 우리는 경우를 보면 :
이는 고효율 M1 () 및 m2 () 메소드?
연구 바이트 코드의 방법으로 아직도있다.
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"
각 사이클은 모두 StringBuilder 객체를 생성되며, 루프 본체 M1 () 메소드를 볼 수 있고,보다 효율적인
m2 () 메소드.
2.7 요약
바이트 코드는 낮은 효율을 달성 고효율이 달성된다는 것을 알 수있다 기본 코드 실행, 볼 수있는 좋은 방법이 될 수있다. 우리는 더 잘 할 우리의 코드를 최적화 할 수 있습니다. 프로그램이보다 효율적으로 실행할 수 있습니다.