javap使用
javap
通过反编译生成的字节码文件，我们可以深入的了解java代码的工作机制。但是，自己分析类文件结构太麻烦了！除了使用第三方的jclasslib工具之外，oracle官方也提供了工具：javap。
javap是jdk自带的反解析工具。它的作用就是根据class字节码文件，反解析出当前类对应的code区 （字节码指令）、局部变量表、异常表和代码行偏移量映射表、常量池等信息。
通过局部变量表，我们可以查看局部变量的作用域范围、所在槽位等信息，甚至可以看到槽位复用等信息。
javac -g操作
解析字节码文件得到的信息中，有些信息（如局部变量表、指令和代码行偏移量映射表、常量池中方法的参数名称等等）需要在使用javac编译成class文件时，指定参数才能输出。
比如，你直接javac xx.java，就不会在生成对应的局部变量表等信息，如果你使用javac -g xx.java就可以生成所有相关信息了。如果你使用的eclipse或IDEA，则默认情况下， eclipse、IDEA在编译时会帮你生成局部变量表、指令和代码行偏移量映射表等信息的。
javap的用法
一般常用的是-v -l -c三个选项。
- javap -l 会输出行号和本地变量表信息。
- javap -c 会对当前class字节码进行反编译生成汇编代码。
- javap -v classxx除了包含一c内容外，还会输出行号、局部变量表信息、常量池等信息。
public class JavapTest {
private int num;
boolean flag;
protected char gender;
public String info;
public static final int COUNTS = 1;
static {
String url = "www.atguigu.com";
}
{
info = "java";
}
public JavapTest() {
}
private JavapTest(boolean flag) {
this.flag = flag;
}
private void methodPrivate() {
}
int getNum(int i) {
return num + i;
}
protected char showGender() {
return gender;
}
public void showInfo() {
int i = 10;
System.out.println(info + i);
}
}
与方法、字段相关
javap -version
与字节码无关
11.0.2
Compiled from "JavapTest.java"
public class JavapTest {
boolean flag;
protected char gender;
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
int getNum(int);
protected char showGender();
public void showInfo();
static {};
}
javap -public
显示公共信息
Compiled from "JavapTest.java"
public class JavapTest {
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
public void showInfo();
}
javap -protected
显示受保护的/公共类和成员
Compiled from "JavapTest.java"
public class JavapTest {
protected char gender;
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
protected char showGender();
public void showInfo();
}
javap -p / -private
显示所有类和成员（大于private的）
Compiled from "JavapTest.java"
public class JavapTest {
private int num;
boolean flag;
protected char gender;
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
private JavapTest(boolean);
private void methodPrivate();
int getNum(int);
protected char showGender();
public void showInfo();
static {};
}
javap -package
显示程序包/受保护的/公共类和成员（默认），非私有的信息
Compiled from "JavapTest.java"
public class JavapTest {
boolean flag;
protected char gender;
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
int getNum(int);
protected char showGender();
public void showInfo();
static {};
}
javap -sysinfo
Classfile /Users/xxxx/IdeaProjects/jvm/out/production/jvm/JavapTest.class
Last modified 2021年3月8日; size 1322 bytes
MD5 checksum b54262c057411610fa71d6556609caa8
Compiled from "JavapTest.java"
public class JavapTest {
boolean flag;
protected char gender;
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
int getNum(int);
protected char showGender();
public void showInfo();
static {};
}
javap -constants
与前面命令的主要区别，常量值会显示出来COUNTS = 1;
Compiled from "JavapTest.java"
public class JavapTest {
boolean flag;
protected char gender;
public java.lang.String info;
public static final int COUNTS = 1;
public JavapTest();
int getNum(int);
protected char showGender();
public void showInfo();
static {};
}
与细节相关
javap -s
输出内部类型签名（描述符）
Compiled from "JavapTest.java"
public class JavapTest {
boolean flag;
descriptor: Z
protected char gender;
descriptor: C
public java.lang.String info;
descriptor: Ljava/lang/String;
public static final int COUNTS;
descriptor: I
public JavapTest();
descriptor: ()V
int getNum(int);
descriptor: (I)I
protected char showGender();
descriptor: ()C
public void showInfo();
descriptor: ()V
static {};
descriptor: ()V
}
javap -l
输出行号和本地变量表
Compiled from "JavapTest.java"
public class JavapTest {
boolean flag;
protected char gender;
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
LineNumberTable:
line 17: 0
line 14: 4
line 19: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this LJavapTest;
int getNum(int);
LineNumberTable:
line 30: 0
LocalVariableTable:
Start Length Slot Name Signature
0 7 0 this LJavapTest;
0 7 1 i I
protected char showGender();
LineNumberTable:
line 34: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LJavapTest;
public void showInfo();
LineNumberTable:
line 38: 0
line 39: 3
line 40: 30
LocalVariableTable:
Start Length Slot Name Signature
0 31 0 this LJavapTest;
3 28 1 i I
static {};
LineNumberTable:
line 10: 0
line 11: 3
LocalVariableTable:
Start Length Slot Name Signature
}
javap -c
对代码进行反汇编
Compiled from "JavapTest.java"
public class JavapTest {
boolean flag;
protected char gender;
public java.lang.String info;
public static final int COUNTS;
public JavapTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: return
int getNum(int);
Code:
0: aload_0
1: getfield #5 // Field num:I
4: iload_1
5: iadd
6: ireturn
protected char showGender();
Code:
0: aload_0
1: getfield #6 // Field gender:C
4: ireturn
public void showInfo();
Code:
0: bipush 10
2: istore_1
3: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
6: new #8 // class java/lang/StringBuilder
9: dup
10: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
13: aload_0
14: getfield #3 // Field info:Ljava/lang/String;
17: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: iload_1
21: invokevirtual #11 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
24: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
static {};
Code:
0: ldc #14 // String www.atguigu.com
2: astore_0
3: return
javap -v / -verbose
输出附加信息（包括行号、本地变量表、反汇编等详细信息），-v是最全的
Classfile /Users/xxx/IdeaProjects/jvm/out/production/jvm/JavapTest.class
Last modified 2021年3月8日; size 1322 bytes
MD5 checksum b54262c057411610fa71d6556609caa8
Compiled from "JavapTest.java"
public class JavapTest
minor version: 0
major version: 52
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #15 // JavapTest
super_class: #16 // java/lang/Object
interfaces: 0, fields: 5, methods: 7, attributes: 1
Constant pool:
#1 = Methodref #16.#46 // java/lang/Object."<init>":()V
#2 = String #47 // java
#3 = Fieldref #15.#48 // JavapTest.info:Ljava/lang/String;
#4 = Fieldref #15.#49 // JavapTest.flag:Z
#5 = Fieldref #15.#50 // JavapTest.num:I
#6 = Fieldref #15.#51 // JavapTest.gender:C
#7 = Fieldref #52.#53 // java/lang/System.out:Ljava/io/PrintStream;
#8 = Class #54 // java/lang/StringBuilder
#9 = Methodref #8.#46 // java/lang/StringBuilder."<init>":()V
#10 = Methodref #8.#55 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#11 = Methodref #8.#56 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#12 = Methodref #8.#57 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#13 = Methodref #58.#59 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = String #60 // www.atguigu.com
#15 = Class #61 // JavapTest
#16 = Class #62 // java/lang/Object
#17 = Utf8 num
#18 = Utf8 I
#19 = Utf8 flag
#20 = Utf8 Z
#21 = Utf8 gender
#22 = Utf8 C
#23 = Utf8 info
#24 = Utf8 Ljava/lang/String;
#25 = Utf8 COUNTS
#26 = Utf8 ConstantValue
#27 = Integer 1
#28 = Utf8 <init>
#29 = Utf8 ()V
#30 = Utf8 Code
#31 = Utf8 LineNumberTable
#32 = Utf8 LocalVariableTable
#33 = Utf8 this
#34 = Utf8 LJavapTest;
#35 = Utf8 (Z)V
#36 = Utf8 methodPrivate
#37 = Utf8 getNum
#38 = Utf8 (I)I
#39 = Utf8 i
#40 = Utf8 showGender
#41 = Utf8 ()C
#42 = Utf8 showInfo
#43 = Utf8 <clinit>
#44 = Utf8 SourceFile
#45 = Utf8 JavapTest.java
#46 = NameAndType #28:#29 // "<init>":()V
#47 = Utf8 java
#48 = NameAndType #23:#24 // info:Ljava/lang/String;
#49 = NameAndType #19:#20 // flag:Z
#50 = NameAndType #17:#18 // num:I
#51 = NameAndType #21:#22 // gender:C
#52 = Class #63 // java/lang/System
#53 = NameAndType #64:#65 // out:Ljava/io/PrintStream;
#54 = Utf8 java/lang/StringBuilder
#55 = NameAndType #66:#67 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#56 = NameAndType #66:#68 // append:(I)Ljava/lang/StringBuilder;
#57 = NameAndType #69:#70 // toString:()Ljava/lang/String;
#58 = Class #71 // java/io/PrintStream
#59 = NameAndType #72:#73 // println:(Ljava/lang/String;)V
#60 = Utf8 www.atguigu.com
#61 = Utf8 JavapTest
#62 = Utf8 java/lang/Object
#63 = Utf8 java/lang/System
#64 = Utf8 out
#65 = Utf8 Ljava/io/PrintStream;
#66 = Utf8 append
#67 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#68 = Utf8 (I)Ljava/lang/StringBuilder;
#69 = Utf8 toString
#70 = Utf8 ()Ljava/lang/String;
#71 = Utf8 java/io/PrintStream
#72 = Utf8 println
#73 = Utf8 (Ljava/lang/String;)V
{
boolean flag;
descriptor: Z
flags: (0x0000)
protected char gender;
descriptor: C
flags: (0x0004) ACC_PROTECTED
public java.lang.String info;
descriptor: Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
public static final int COUNTS;
descriptor: I
flags: (0x0019) ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: int 1
public JavapTest();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: return
LineNumberTable:
line 17: 0
line 14: 4
line 19: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this LJavapTest;
int getNum(int);
descriptor: (I)I
flags: (0x0000)
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: getfield #5 // Field num:I
4: iload_1
5: iadd
6: ireturn
LineNumberTable:
line 30: 0
LocalVariableTable:
Start Length Slot Name Signature
0 7 0 this LJavapTest;
0 7 1 i I
protected char showGender();
descriptor: ()C
flags: (0x0004) ACC_PROTECTED
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #6 // Field gender:C
4: ireturn
LineNumberTable:
line 34: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LJavapTest;
public void showInfo();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=3, locals=2, args_size=1
0: bipush 10
2: istore_1
3: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
6: new #8 // class java/lang/StringBuilder
9: dup
10: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
13: aload_0
14: getfield #3 // Field info:Ljava/lang/String;
17: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: iload_1
21: invokevirtual #11 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
24: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
LineNumberTable:
line 38: 0
line 39: 3
line 40: 30
LocalVariableTable:
Start Length Slot Name Signature
0 31 0 this LJavapTest;
3 28 1 i I
static {};
descriptor: ()V
flags: (0x0008) ACC_STATIC
Code:
stack=1, locals=1, args_size=0
0: ldc #14 // String www.atguigu.com
2: astore_0
3: return
LineNumberTable:
line 10: 0
line 11: 3
LocalVariableTable:
Start Length Slot Name Signature
}
SourceFile: "JavapTest.java"
组合使用
-v 不包含私有信息，所以可以使用组合方式获取更全的信息 javap -v -p JavapTest.class
Classfile /Users/tonghao/IdeaProjects/jvm/out/production/jvm/JavapTest.class
Last modified 2021年3月8日; size 1322 bytes
MD5 checksum b54262c057411610fa71d6556609caa8
Compiled from "JavapTest.java"
public class JavapTest
minor version: 0
major version: 52
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #15 // JavapTest
super_class: #16 // java/lang/Object
interfaces: 0, fields: 5, methods: 7, attributes: 1
Constant pool:
#1 = Methodref #16.#46 // java/lang/Object."<init>":()V
#2 = String #47 // java
#3 = Fieldref #15.#48 // JavapTest.info:Ljava/lang/String;
#4 = Fieldref #15.#49 // JavapTest.flag:Z
#5 = Fieldref #15.#50 // JavapTest.num:I
#6 = Fieldref #15.#51 // JavapTest.gender:C
#7 = Fieldref #52.#53 // java/lang/System.out:Ljava/io/PrintStream;
#8 = Class #54 // java/lang/StringBuilder
#9 = Methodref #8.#46 // java/lang/StringBuilder."<init>":()V
#10 = Methodref #8.#55 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#11 = Methodref #8.#56 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#12 = Methodref #8.#57 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#13 = Methodref #58.#59 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = String #60 // www.atguigu.com
#15 = Class #61 // JavapTest
#16 = Class #62 // java/lang/Object
#17 = Utf8 num
#18 = Utf8 I
#19 = Utf8 flag
#20 = Utf8 Z
#21 = Utf8 gender
#22 = Utf8 C
#23 = Utf8 info
#24 = Utf8 Ljava/lang/String;
#25 = Utf8 COUNTS
#26 = Utf8 ConstantValue
#27 = Integer 1
#28 = Utf8 <init>
#29 = Utf8 ()V
#30 = Utf8 Code
#31 = Utf8 LineNumberTable
#32 = Utf8 LocalVariableTable
#33 = Utf8 this
#34 = Utf8 LJavapTest;
#35 = Utf8 (Z)V
#36 = Utf8 methodPrivate
#37 = Utf8 getNum
#38 = Utf8 (I)I
#39 = Utf8 i
#40 = Utf8 showGender
#41 = Utf8 ()C
#42 = Utf8 showInfo
#43 = Utf8 <clinit>
#44 = Utf8 SourceFile
#45 = Utf8 JavapTest.java
#46 = NameAndType #28:#29 // "<init>":()V
#47 = Utf8 java
#48 = NameAndType #23:#24 // info:Ljava/lang/String;
#49 = NameAndType #19:#20 // flag:Z
#50 = NameAndType #17:#18 // num:I
#51 = NameAndType #21:#22 // gender:C
#52 = Class #63 // java/lang/System
#53 = NameAndType #64:#65 // out:Ljava/io/PrintStream;
#54 = Utf8 java/lang/StringBuilder
#55 = NameAndType #66:#67 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#56 = NameAndType #66:#68 // append:(I)Ljava/lang/StringBuilder;
#57 = NameAndType #69:#70 // toString:()Ljava/lang/String;
#58 = Class #71 // java/io/PrintStream
#59 = NameAndType #72:#73 // println:(Ljava/lang/String;)V
#60 = Utf8 www.atguigu.com
#61 = Utf8 JavapTest
#62 = Utf8 java/lang/Object
#63 = Utf8 java/lang/System
#64 = Utf8 out
#65 = Utf8 Ljava/io/PrintStream;
#66 = Utf8 append
#67 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#68 = Utf8 (I)Ljava/lang/StringBuilder;
#69 = Utf8 toString
#70 = Utf8 ()Ljava/lang/String;
#71 = Utf8 java/io/PrintStream
#72 = Utf8 println
#73 = Utf8 (Ljava/lang/String;)V
{
private int num;
descriptor: I
flags: (0x0002) ACC_PRIVATE
boolean flag;
descriptor: Z
flags: (0x0000)
protected char gender;
descriptor: C
flags: (0x0004) ACC_PROTECTED
public java.lang.String info;
descriptor: Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
public static final int COUNTS;
descriptor: I
flags: (0x0019) ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: int 1
public JavapTest();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: return
LineNumberTable:
line 17: 0
line 14: 4
line 19: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this LJavapTest;
private JavapTest(boolean);
descriptor: (Z)V
flags: (0x0002) ACC_PRIVATE
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: aload_0
11: iload_1
12: putfield #4 // Field flag:Z
15: return
LineNumberTable:
line 21: 0
line 14: 4
line 22: 10
line 23: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this LJavapTest;
0 16 1 flag Z
private void methodPrivate();
descriptor: ()V
flags: (0x0002) ACC_PRIVATE
Code:
stack=0, locals=1, args_size=1
0: return
LineNumberTable:
line 27: 0
LocalVariableTable:
Start Length Slot Name Signature
0 1 0 this LJavapTest;
int getNum(int);
descriptor: (I)I
flags: (0x0000)
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: getfield #5 // Field num:I
4: iload_1
5: iadd
6: ireturn
LineNumberTable:
line 30: 0
LocalVariableTable:
Start Length Slot Name Signature
0 7 0 this LJavapTest;
0 7 1 i I
protected char showGender();
descriptor: ()C
flags: (0x0004) ACC_PROTECTED
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #6 // Field gender:C
4: ireturn
LineNumberTable:
line 34: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LJavapTest;
public void showInfo();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=3, locals=2, args_size=1
0: bipush 10
2: istore_1
3: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
6: new #8 // class java/lang/StringBuilder
9: dup
10: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
13: aload_0
14: getfield #3 // Field info:Ljava/lang/String;
17: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: iload_1
21: invokevirtual #11 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
24: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
LineNumberTable:
line 38: 0
line 39: 3
line 40: 30
LocalVariableTable:
Start Length Slot Name Signature
0 31 0 this LJavapTest;
3 28 1 i I
static {};
descriptor: ()V
flags: (0x0008) ACC_STATIC
Code:
stack=1, locals=1, args_size=0
0: ldc #14 // String www.atguigu.com
2: astore_0
3: return
LineNumberTable:
line 10: 0
line 11: 3
LocalVariableTable:
Start Length Slot Name Signature
}
SourceFile: "JavapTest.java"
结构解读
Classfile /Users/tonghao/IdeaProjects/jvm/out/production/jvm/JavapTest.class // 字节码文件所属的路径
Last modified 2021年3月8日; size 1322 bytes // 最后修改时间，字节码文件的大小
MD5 checksum b54262c057411610fa71d6556609caa8 // MD5散列值
Compiled from "JavapTest.java" // 源文件的名称
public class JavapTest
minor version: 0 // 副版本
major version: 52 // 主版本
flags: (0x0021) ACC_PUBLIC, ACC_SUPER // 访问标识
this_class: #15 // JavapTest
super_class: #16 // java/lang/Object
interfaces: 0, fields: 5, methods: 7, attributes: 1
Constant pool: // 常量池
#1 = Methodref #16.#46 // java/lang/Object."<init>":()V
#2 = String #47 // java
#3 = Fieldref #15.#48 // JavapTest.info:Ljava/lang/String;
#4 = Fieldref #15.#49 // JavapTest.flag:Z
#5 = Fieldref #15.#50 // JavapTest.num:I
#6 = Fieldref #15.#51 // JavapTest.gender:C
#7 = Fieldref #52.#53 // java/lang/System.out:Ljava/io/PrintStream;
#8 = Class #54 // java/lang/StringBuilder
#9 = Methodref #8.#46 // java/lang/StringBuilder."<init>":()V
#10 = Methodref #8.#55 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#11 = Methodref #8.#56 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#12 = Methodref #8.#57 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#13 = Methodref #58.#59 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = String #60 // www.atguigu.com
#15 = Class #61 // JavapTest
#16 = Class #62 // java/lang/Object
#17 = Utf8 num
#18 = Utf8 I
#19 = Utf8 flag
#20 = Utf8 Z
#21 = Utf8 gender
#22 = Utf8 C
#23 = Utf8 info
#24 = Utf8 Ljava/lang/String;
#25 = Utf8 COUNTS
#26 = Utf8 ConstantValue
#27 = Integer 1
#28 = Utf8 <init>
#29 = Utf8 ()V
#30 = Utf8 Code
#31 = Utf8 LineNumberTable
#32 = Utf8 LocalVariableTable
#33 = Utf8 this
#34 = Utf8 LJavapTest;
#35 = Utf8 (Z)V
#36 = Utf8 methodPrivate
#37 = Utf8 getNum
#38 = Utf8 (I)I
#39 = Utf8 i
#40 = Utf8 showGender
#41 = Utf8 ()C
#42 = Utf8 showInfo
#43 = Utf8 <clinit>
#44 = Utf8 SourceFile
#45 = Utf8 JavapTest.java
#46 = NameAndType #28:#29 // "<init>":()V
#47 = Utf8 java
#48 = NameAndType #23:#24 // info:Ljava/lang/String;
#49 = NameAndType #19:#20 // flag:Z
#50 = NameAndType #17:#18 // num:I
#51 = NameAndType #21:#22 // gender:C
#52 = Class #63 // java/lang/System
#53 = NameAndType #64:#65 // out:Ljava/io/PrintStream;
#54 = Utf8 java/lang/StringBuilder
#55 = NameAndType #66:#67 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#56 = NameAndType #66:#68 // append:(I)Ljava/lang/StringBuilder;
#57 = NameAndType #69:#70 // toString:()Ljava/lang/String;
#58 = Class #71 // java/io/PrintStream
#59 = NameAndType #72:#73 // println:(Ljava/lang/String;)V
#60 = Utf8 www.atguigu.com
#61 = Utf8 JavapTest
#62 = Utf8 java/lang/Object
#63 = Utf8 java/lang/System
#64 = Utf8 out
#65 = Utf8 Ljava/io/PrintStream;
#66 = Utf8 append
#67 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#68 = Utf8 (I)Ljava/lang/StringBuilder;
#69 = Utf8 toString
#70 = Utf8 ()Ljava/lang/String;
#71 = Utf8 java/io/PrintStream
#72 = Utf8 println
#73 = Utf8 (Ljava/lang/String;)V
############################################ 字段表集合的信息 ################################################
{
private int num; // 字段名
descriptor: I // 字段描述符：字段的类型
flags: (0x0002) ACC_PRIVATE // 访问标识
boolean flag;
descriptor: Z
flags: (0x0000)
protected char gender;
descriptor: C
flags: (0x0004) ACC_PROTECTED
public java.lang.String info;
descriptor: Ljava/lang/String;
flags: (0x0001) ACC_PUBLIC
public static final int COUNTS;
descriptor: I
flags: (0x0019) ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: int 1 // 常量字段的属性ConstantValue
############################################ 方法表的信息 ################################################
public JavapTest(); // 构造器1的信息
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: return
LineNumberTable:
line 17: 0
line 14: 4
line 19: 10
LocalVariableTable:
Start Length Slot Name Signature
0 11 0 this LJavapTest;
private JavapTest(boolean); // 构造器2的信息
descriptor: (Z)V
flags: (0x0002) ACC_PRIVATE
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: aload_0
5: ldc #2 // String java
7: putfield #3 // Field info:Ljava/lang/String;
10: aload_0
11: iload_1
12: putfield #4 // Field flag:Z
15: return
LineNumberTable:
line 21: 0
line 14: 4
line 22: 10
line 23: 15
LocalVariableTable:
Start Length Slot Name Signature
0 16 0 this LJavapTest;
0 16 1 flag Z
private void methodPrivate();
descriptor: ()V
flags: (0x0002) ACC_PRIVATE
Code:
stack=0, locals=1, args_size=1
0: return
LineNumberTable:
line 27: 0
LocalVariableTable:
Start Length Slot Name Signature
0 1 0 this LJavapTest;
int getNum(int);
descriptor: (I)I
flags: (0x0000)
Code:
stack=2, locals=2, args_size=2
0: aload_0
1: getfield #5 // Field num:I
4: iload_1
5: iadd
6: ireturn
LineNumberTable:
line 30: 0
LocalVariableTable:
Start Length Slot Name Signature
0 7 0 this LJavapTest;
0 7 1 i I
protected char showGender();
descriptor: ()C
flags: (0x0004) ACC_PROTECTED
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: getfield #6 // Field gender:C
4: ireturn
LineNumberTable:
line 34: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this LJavapTest;
public void showInfo();
descriptor: ()V // 方法描述符：方法的形参列表、返回值类型
flags: (0x0001) ACC_PUBLIC // 方法的访问标识
Code: // 方法的Code属性
stack=3, locals=2, args_size=1 // stack：操作数栈的最大深度 locals：局部变量表的长度 args_size：方法接受参数的个数
// 偏移量 操作码 操作数
0: bipush 10
2: istore_1
3: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream;
6: new #8 // class java/lang/StringBuilder
9: dup
10: invokespecial #9 // Method java/lang/StringBuilder."<init>":()V
13: aload_0
14: getfield #3 // Field info:Ljava/lang/String;
17: invokevirtual #10 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
20: iload_1
21: invokevirtual #11 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
24: invokevirtual #12 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
27: invokevirtual #13 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
30: return
// 行号表：指明字节码指令的偏移量和Java源程序中代码行号的一一对应关系
LineNumberTable:
line 38: 0
line 39: 3
line 40: 30
// 局部变量表：描述内部局部变量的相关信息
LocalVariableTable:
Start Length Slot Name Signature
0 31 0 this LJavapTest;
3 28 1 i I
static {};
descriptor: ()V
flags: (0x0008) ACC_STATIC
Code:
stack=1, locals=1, args_size=0
0: ldc #14 // String www.atguigu.com
2: astore_0
3: return
LineNumberTable:
line 10: 0
line 11: 3
LocalVariableTable:
Start Length Slot Name Signature
}
SourceFile: "JavapTest.java" // 附加属性：指明当前字节码文件对应的源程序文件名
小结
通过javap命令可以查看一个java类反汇编得到的Class文件版本号、常量池、访问标识、变量表、指令代码行号表等等信息。不显示类索引、父类索引、接口索引集合、<clinit>()、<init>()等结构
通过对前面例子代码反汇编文件的简单分析，可以发现，一个方法的执行通常会涉及下面几块内存的操作:
- java栈中:局部变量表、操作数栈。
- java堆。通过对象的地址引用去操作。
- 常量池。
- 其他如帧数据区、方法区的剩余部分等情况，测试中没有显示出来，这里说明一下。
平常，我们比较关注的是java类中每个方法的反汇编中的指令操作过程，这些指令都是顺序执行的，可以参考官方文档查看每个指令的含义，很简单：https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-6.html
浙公网安备 33010602011771号