Java字节码例子解析
程序员文章站
2022-05-12 14:21:59
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举个简单的例子：
public class Hello {
public static void main(String[] args) {
String string1 = "ab";
String string2 = "c";
String string3 = string1 + "c";
System.out.println(string1 == string3);
}
}
过程大致分析如图：
第一步 将线程栈中的string1、string2引用分别指向了常量池ab、c的地址。
第二步 轮到了string3 = string1 + "c"，首先会初始化StringBuilder到堆中，然后调append将string1字符串拼接、然后调append再拼接"c"。
第三步 StringBuilder指向常量池的"abc"地址，然后通过toString返回值.
然后执行如下指令：
$ javac Hello.java //编译
$ javap -verbose Hello //反编译，结果如下：Classfile /C:/Users/lisam/Desktop/新建文件夹/Hello.class
Last modified 2018-9-17; size 706 bytes
MD5 checksum 4d2164bd48f0690cad84271e27b237a5
Compiled from "Hello.java"
public class Hello
SourceFile: "Hello.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
//方法常量池
Constant pool:
#1 = Methodref #11.#24 // java/lang/Object."<init>":()V
#2 = String #25 // ab
#3 = String #26 // c
#4 = Class #27 // java/lang/StringBuilder
#5 = Methodref #4.#24 // java/lang/StringBuilder."<init>":()V
#6 = Methodref #4.#28 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#7 = Methodref #4.#29 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#8 = Fieldref #30.#31 // java/lang/System.out:Ljava/io/PrintStream;
#9 = Methodref #32.#33 // java/io/PrintStream.println:(Z)V
#10 = Class #34 // Hello
#11 = Class #35 // java/lang/Object
#12 = Utf8 <init>
#13 = Utf8 ()V
#14 = Utf8 Code
#15 = Utf8 LineNumberTable
#16 = Utf8 main
#17 = Utf8 ([Ljava/lang/String;)V
#18 = Utf8 StackMapTable
#19 = Class #36 // "[Ljava/lang/String;"
#20 = Class #37 // java/lang/String
#21 = Class #38 // java/io/PrintStream
#22 = Utf8 SourceFile
#23 = Utf8 Hello.java
#24 = NameAndType #12:#13 // "<init>":()V
#25 = Utf8 ab
#26 = Utf8 c
#27 = Utf8 java/lang/StringBuilder
#28 = NameAndType #39:#40 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#29 = NameAndType #41:#42 // toString:()Ljava/lang/String;
#30 = Class #43 // java/lang/System
#31 = NameAndType #44:#45 // out:Ljava/io/PrintStream;
#32 = Class #38 // java/io/PrintStream
#33 = NameAndType #46:#47 // println:(Z)V
#34 = Utf8 Hello
#35 = Utf8 java/lang/Object
#36 = Utf8 [Ljava/lang/String;
#37 = Utf8 java/lang/String
#38 = Utf8 java/io/PrintStream
#39 = Utf8 append
#40 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#41 = Utf8 toString
#42 = Utf8 ()Ljava/lang/String;
#43 = Utf8 java/lang/System
#44 = Utf8 out
#45 = Utf8 Ljava/io/PrintStream;
#46 = Utf8 println
#47 = Utf8 (Z)V
{
public Hello();
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 1: 0
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=4, args_size=1
//将字符串"ab"压入常量池
0: ldc #2 // String ab
//存储到局部变量表
2: astore_1
//将字符串"c"压入常量池
3: ldc #3 // String c
//存储到局部变量表
5: astore_2
//String3的拼接过程中，会先new
6: new #4 // class java/lang/StringBuilder
9: dup
//然后invokespecial调用初始化StringBuilder构造器
10: invokespecial #5 // Method java/lang/StringBuilder."<init>":()V
//把局部变量表中n=1的引用(即步骤2: astore_1)的值"ab"装在到操作数栈中。
13: aload_1
//然后invokevirtual来调用方法append string1
14: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
//将字符串"c"压入常量池
17: ldc #3 // String c
//然后append "c"
19: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
//调用toString返回值
22: invokevirtual #7 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
//然后将string3的值"abc"存储到局部变量表
25: astore_3
//获取类的静态域，这里拿到了静态System.out，并将值压入栈顶
26: getstatic #8 // Field java/lang/System.out:Ljava/io/PrintStream;
//把局部变量表中n=1的引用(即步骤2: astore_1)的值"ab"装在到操作数栈中。
29: aload_1
//把局部变量表中n=3的引用(即步骤25: astore_3)的值"abc"装在到操作数栈中。
30: aload_3
//比较上面栈顶两引用型数值，当结果不相等时跳转
31: if_acmpne 38
//将int为1压入栈顶
34: iconst_1
35: goto 39
38: iconst_0
//调用静态类中System.out.println
39: invokevirtual #9 // Method java/io/PrintStream.println:(Z)V
//返回空
42: return
LineNumberTable:
line 3: 0
line 4: 3
line 5: 6
line 6: 26
line 7: 42
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 38
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String ]
stack = [ class java/io/PrintStream ]
frame_type = 255 /* full_frame */
offset_delta = 0
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String, class java/lang/String ]
stack = [ class java/io/PrintStream, int ]
}