Java Class文件學習

class 文件結構

minor version:副版本

major version:主版本號

文件的版本號 major_version.minor_version

flags:訪問標誌

1. ACC_PUBLICH 聲明為publich
2. ACC_FINAL 聲明為final
3. ACC_SUPER 當用到invokespecial指令時，需要對父類方法做特殊處理
4. ACC_INTERFACE 該class文件定義的是介面而不是類
5. ACC_ABSTRACT 聲明為abstract
6. ACC_SYNTHETIC 聲明為synthetic
7. ACC_ANNOTATION 標識註解類型
8. ACC_ENUM 標識枚舉類型

constant pool 常量池

方法調用和返回指令

1. invokevirtual 指令用於調用對象的實例方法，根據對象的實際類型進行分派
2. invokeinterface 指令用於調用介面方法，它會在運行時搜索由特定對象實現的這個介面方法，並找出適合的方法進行調用
3. invokespecial 指令用於調用一些需要特殊的實例方法，包括實例的初始化，私有方法和父類方法
4. invokestatic 指令用於調用命名類中的靜態方法
5. invokedynamic 指令用於調用可以綁定invokedynamic指令的調用點對象作為目標方法。

aaload_:從局部變數表中載入索引值為n的引用

aconst_null:將一個null值壓入到操作數棧頂

putfield:為制定類的欄位賦值

通過執行javac xxx.java javap -verbose xxx.class 查看class文件

public class AconstNullDemo {
Integer i=null;

void test{
i=1;
if(i.equals(Integer.valueOf(1))){
System.out.println("eq");
}
}
}

public class common.jvm.AconstNullDemo
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #9.#20 // java/lang/Object."":V
#2 = Fieldref #8.#21 // common/jvm/AconstNullDemo.i:Ljava/lang/Integer;
#3 = Methodref #22.#23 // java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
#4 = Methodref #22.#24 // java/lang/Integer.equals:(Ljava/lang/Object;)Z
#5 = Fieldref #25.#26 // java/lang/System.out:Ljava/io/PrintStream;
#6 = String #27 // eq
#7 = Methodref #28.#29 // java/io/PrintStream.println:(Ljava/lang/String;)V
#8 = Class #30 // common/jvm/AconstNullDemo
#9 = Class #31 // java/lang/Object
#10 = Utf8 i
#11 = Utf8 Ljava/lang/Integer;
#12 = Utf8
#13 = Utf8 V
#14 = Utf8 Code
#15 = Utf8 LineNumberTable
#16 = Utf8 test
#17 = Utf8 StackMapTable
#18 = Utf8 SourceFile
#19 = Utf8 AconstNullDemo.java
#20 = NameAndType #12:#13 // "":V
#21 = NameAndType #10:#11 // i:Ljava/lang/Integer;
#22 = Class #32 // java/lang/Integer
#23 = NameAndType #33:#34 // valueOf:(I)Ljava/lang/Integer;
#24 = NameAndType #35:#36 // equals:(Ljava/lang/Object;)Z
#25 = Class #37 // java/lang/System
#26 = NameAndType #38:#39 // out:Ljava/io/PrintStream;
#27 = Utf8 eq
#28 = Class #40 // java/io/PrintStream
#29 = NameAndType #41:#42 // println:(Ljava/lang/String;)V
#30 = Utf8 common/jvm/AconstNullDemo
#31 = Utf8 java/lang/Object
#32 = Utf8 java/lang/Integer
#33 = Utf8 valueOf
#34 = Utf8 (I)Ljava/lang/Integer;
#35 = Utf8 equals
#36 = Utf8 (Ljava/lang/Object;)Z
#37 = Utf8 java/lang/System
#38 = Utf8 out
#39 = Utf8 Ljava/io/PrintStream;
#40 = Utf8 java/io/PrintStream
#41 = Utf8 println
#42 = Utf8 (Ljava/lang/String;)V
{
java.lang.Integer i;
descriptor: Ljava/lang/Integer;
flags:

public common.jvm.AconstNullDemo;
descriptor: V
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":V
4: aload_0
//將null壓入棧頂
5: aconst_null
//將null賦值給屬性i
6: putfield #2 // Field i:Ljava/lang/Integer;
9: return
LineNumberTable:
line 6: 0
line 7: 4

void test;
descriptor: V
flags:
Code:
stack=2, locals=1, args_size=1
//載入this到操作棧
0: aload_0
//載入常量1到操作棧
1: iconst_1
//調用靜態方法
2: invokestatic #3 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
//將計算結果賦值給屬性，為什麼putfield的索引不是連續的？這是因為invokestatic操作和操作數長度為3個slot
5: putfield #2 // Field i:Ljava/lang/Integer;
//載入this到操作棧
8: aload_0
//將屬性壓入棧頂
9: getfield #2 // Field i:Ljava/lang/Integer;
//載入常量1到操作棧頂
12: iconst_1
//調用靜態方法Integer.valueOf
13: invokestatic #3 // Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
//調用虛方法equals
16: invokevirtual #4 // Method java/lang/Integer.equals:(Ljava/lang/Object;)Z
//如果ifeq的結果為0則挑戰到30行
19: ifeq 30
22: getstatic #5 // Field java/lang/System.out:Ljava/io/PrintStream;
25: ldc #6 // String eq
27: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
LineNumberTable:
line 10: 0
line 11: 8
line 12: 22
line 14: 30
StackMapTable: number_of_entries = 1
frame_type = 30 /* same */
}

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