说说javap命令

javap定义
javap是 Java class文件分解器，可以反编译（即对javac编译的文件进行反编译），也可以查看java编译器生成的字节码。用于分解class文件。
测试类
public class JavapTest {

private static final int _P_1 = 1;
public static final int _P_2 = 2;

public static void main(String[] args) {
    int m = 0, n = 0;
    for (int i = 0; i < 10; i++) {
        m = m++;
        n = ++n;
    }
    System.out.println("m = " + m);
    System.out.println("n = " + n);
}

}
javap命令参数
C:>javap -help
用法: javap
其中, 可能的选项包括:
-help --help -? 输出此用法消息
-version 版本信息
-v -verbose 输出附加信息
-l 输出行号和本地变量表
-public 仅显示公共类和成员
-protected 显示受保护的/公共类和成员
-package 显示程序包/受保护的/公共类
和成员 (默认)
-p -private 显示所有类和成员
-c 对代码进行反汇编
-s 输出内部类型签名
-sysinfo 显示正在处理的类的
系统信息 (路径, 大小, 日期, MD5 散列)
-constants 显示静态最终常量
-classpath 指定查找用户类文件的位置
-bootclasspath 覆盖引导类文件的位置

javap -version
C:>javap -version
1.7.0_71

显示java版本

javap -p
C:UsersuserDesktop>javap -p JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
private static final int _P_1;
public static final int _P_2;
public com.method.handler.JavapTest();
public static void main(java.lang.String[]);
private void say();
}

显示类所有可访问修饰符范围》private的成员

javap -public
C:UsersuserDesktop>javap -public JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
public static final int _P_2;
public com.method.handler.JavapTest();
public static void main(java.lang.String[]);
}

显示类的public成员

javap -protected
C:UsersuserDesktop>javap -protected JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
public static final int _P_2;
public com.method.handler.JavapTest();
public static void main(java.lang.String[]);
}

显示类所有可访问修饰符范围》protected的成员

javap -l
C:UsersuserDesktop>javap -p -l JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
private static final int _P_1;

public static final int _P_2;

public com.method.handler.JavapTest();
LineNumberTable:
line 6: 0

public static void main(java.lang.String[]);
LineNumberTable:
line 12: 0
line 13: 4
line 14: 12
line 15: 17
line 13: 22
line 17: 28
line 18: 53
line 19: 78

private void say();
LineNumberTable:
line 23: 0
line 24: 8
}

输出行号和本地变量表？看的不是很明白

javap -package
C:UsersuserDesktop>javap -package JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
public static final int _P_2;
public com.method.handler.JavapTest();
public static void main(java.lang.String[]);
}

与javap -public作用类似

javap -v/-p -v
C:UsersuserDesktop>javap -p -v JavapTest.class
Classfile /C:/Users/frinder_liu/Desktop/JavapTest.class
Last modified 2016-4-27; size 911 bytes
MD5 checksum e903be7495f5c462d6459a792e063628
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest
SourceFile: “JavapTest.java”
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #13.#30 // java/lang/Object.""?)V
#2 = Fieldref #31.#32 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Class #33 // java/lang/StringBuilder
#4 = Methodref #3.#30 // java/lang/StringBuilder.""?)V
#5 = String #34 // m =
#6 = Methodref #3.#35 // java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilde
r;
#7 = Methodref #3.#36 // java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
#8 = Methodref #3.#37 // java/lang/StringBuilder.toString:()Ljava/lang/String;
#9 = Methodref #38.#39 // java/io/PrintStream.println:(Ljava/lang/String;)V
#10 = String #40 // n =
#11 = String #41 // hello world…
#12 = Class #42 // com/method/handler/JavapTest
#13 = Class #43 // java/lang/Object
#14 = Utf8 _P_1
#15 = Utf8 I
#16 = Utf8 ConstantValue
#17 = Integer 1
#18 = Utf8 _P_2
#19 = Integer 2
#20 = Utf8
#21 = Utf8 ()V
#22 = Utf8 Code
#23 = Utf8 LineNumberTable
#24 = Utf8 main
#25 = Utf8 ([Ljava/lang/String;)V
#26 = Utf8 StackMapTable
#27 = Utf8 say
#28 = Utf8 SourceFile
#29 = Utf8 JavapTest.java
#30 = NameAndType #20:#21 // “”?)V
#31 = Class #44 // java/lang/System
#32 = NameAndType #45:#46 // out:Ljava/io/PrintStream;
#33 = Utf8 java/lang/StringBuilder
#34 = Utf8 m =
#35 = NameAndType #47:#48 // append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
#36 = NameAndType #47:#49 // append:(I)Ljava/lang/StringBuilder;
#37 = NameAndType #50:#51 // toString:()Ljava/lang/String;
#38 = Class #52 // java/io/PrintStream
#39 = NameAndType #53:#54 // println:(Ljava/lang/String;)V
#40 = Utf8 n =
#41 = Utf8 hello world…
#42 = Utf8 com/method/handler/JavapTest
#43 = Utf8 java/lang/Object
#44 = Utf8 java/lang/System
#45 = Utf8 out
#46 = Utf8 Ljava/io/PrintStream;
#47 = Utf8 append
#48 = Utf8 (Ljava/lang/String;)Ljava/lang/StringBuilder;
#49 = Utf8 (I)Ljava/lang/StringBuilder;
#50 = Utf8 toString
#51 = Utf8 ()Ljava/lang/String;
#52 = Utf8 java/io/PrintStream
#53 = Utf8 println
#54 = Utf8 (Ljava/lang/String;)V
{
private static final int _P_1;
flags: ACC_PRIVATE, ACC_STATIC, ACC_FINAL
ConstantValue: int 1

public static final int _P_2;
flags: ACC_PUBLIC, ACC_STATIC, ACC_FINAL
ConstantValue: int 2

public com.method.handler.JavapTest();
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object.""?)V
4: return
LineNumberTable:
line 6: 0

public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=4, args_size=1
0: iconst_0
1: istore_1
2: iconst_0
3: istore_2
4: iconst_0
5: istore_3
6: iload_3
7: bipush 10
9: if_icmpge 28
12: iload_1
13: iinc 1, 1
16: istore_1
17: iinc 2, 1
20: iload_2
21: istore_2
22: iinc 3, 1
25: goto 6
28: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
31: new #3 // class java/lang/StringBuilder
34: dup
35: invokespecial #4 // Method java/lang/StringBuilder.""?)V
38: ldc #5 // String m =
40: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/St
ringBuilder;
43: iload_1
44: invokevirtual #7 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
47: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
50: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
53: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
56: new #3 // class java/lang/StringBuilder
59: dup
60: invokespecial #4 // Method java/lang/StringBuilder.""?)V
63: ldc #10 // String n =
65: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/St
ringBuilder;
68: iload_2
69: invokevirtual #7 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
72: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
75: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
78: return
LineNumberTable:
line 12: 0
line 13: 4
line 14: 12
line 15: 17
line 13: 22
line 17: 28
line 18: 53
line 19: 78
StackMapTable: number_of_entries = 2
frame_type = 254 /* append /
offset_delta = 6
locals = [ int, int, int ]
frame_type = 250 / chop */
offset_delta = 21

private void say();
flags: ACC_PRIVATE
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #11 // String hello world…
5: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 23: 0
line 24: 8
}

命令说明是：输出附加信息
class文件的路径、最后修改时间、文件大小等
类的全路径、源（java）文件等
常量池
常量定义、值
构造方法
程序调用及执行逻辑（这个涉及的内容就比较多了）

总之，javap -v命令是很强大的一个命令！

javap -c
C:UsersuserDesktop>javap -c JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
public static final int _P_2;

public com.method.handler.JavapTest();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object.""?)V
4: return

public static void main(java.lang.String[]);
Code:
0: iconst_0
1: istore_1
2: iconst_0
3: istore_2
4: iconst_0
5: istore_3
6: iload_3
7: bipush 10
9: if_icmpge 28
12: iload_1
13: iinc 1, 1
16: istore_1
17: iinc 2, 1
20: iload_2
21: istore_2
22: iinc 3, 1
25: goto 6
28: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
31: new #3 // class java/lang/StringBuilder
34: dup
35: invokespecial #4 // Method java/lang/StringBuilder.""?)V
38: ldc #5 // String m =
40: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/Stri
ngBuilder;
43: iload_1
44: invokevirtual #7 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
47: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
50: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
53: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
56: new #3 // class java/lang/StringBuilder
59: dup
60: invokespecial #4 // Method java/lang/StringBuilder.""?)V
63: ldc #10 // String n =
65: invokevirtual #6 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/Stri
ngBuilder;
68: iload_2
69: invokevirtual #7 // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
72: invokevirtual #8 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
75: invokevirtual #9 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
78: return
}

其实，javap -c 输出内部javap -v中已经有了，我们详细介绍下javap -c命令的输出内容
0: iconst_0 前面0:表示执行的顺序，iconst_0把值 0 放入栈顶，_0中的0代表压栈的值，如：iconst_5，即把5压入栈顶
1: istore_1 将栈顶的值放入变量1 中，_1代表变量的序列，本例中为：m，如：istore_2即为变量n赋值
6: iload_3 将变量3 即 i 的的值放入栈顶，与iconst不同的是，iload操作的值是已经定义好存在的值，iconst是定义时的压栈操作
13: iinc 1, 1 将变量1 的值加1
文章开头的demo中最终的结果是什么呢？

m = 0
n = 10

为什么呢？
我们来关注下这几行：
12: iload_1 – 将变量1 值放入栈项
13: iinc 1, 1 – 将变量1 增加1 变为2，栈顶值仍然是1。其中第一个1 表示变量，第二个1 表示增量
16: istore_1 – 将栈顶值赋值给变量1，变量1 仍然为1
17: iinc 2, 1 – 将变量2 增加1 变为2，栈顶值仍然是1。其中第一个2 表示变量，第二个1 表示增量
20: iload_2 – 将变量2 值放入栈顶
21: istore_2 – 将栈顶值赋值给变量2，变量2 值为2

记住一个点：
int m = 0, n = 0;
m = m++; – 会先将m值（即0）赋值给m后再++
n = ++n; – n先++后再把值赋值给n
此时，m = 0, n = 1;
这个地方有点绕，需要多看几次，理顺逻辑与关系！

这里可以参考下http://blog.csdn.net/junsure2012/article/details/7099222，讲的很详细也易懂！

javap -s/-p -s
C:UsersuserDesktop>javap -p -s JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
private static final int _P_1;
Signature: I
public static final int _P_2;
Signature: I
public com.method.handler.JavapTest();
Signature: ()V

public static void main(java.lang.String[]);
Signature: ([Ljava/lang/String;)V

private void say();
Signature: ()V
}

输出内部类型签名

javap -sysinfo/-p -sysinfo
C:UsersuserDesktop>javap -sysinfo JavapTest.class
Classfile /C:/Users/user/Desktop/JavapTest.class
Last modified 2016-4-27; size 911 bytes
MD5 checksum 3f6dfcf7121785760b234224c5d135fd
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
public static final int _P_2;
public com.method.handler.JavapTest();
public static void main(java.lang.String[]);
}

显示正在处理的类的
系统信息 (路径, 大小, 日期, MD5 散列)

javap -constants/-p -constants
C:UsersuserDesktop>javap -constants JavapTest.class
Compiled from “JavapTest.java”
public class com.method.handler.JavapTest {
public static final int _P_2 = 2;
public com.method.handler.JavapTest();
public static void main(java.lang.String[]);
}

显示静态最终常量

javap -classpath/-bootclasspath
先跳过
原文：https://www.cnblogs.com/frinder6/p/5440173.html