javap is a Java class file decomposer, which can be decompiled (that is, decompiled files compiled by javac), and can also view the bytecode generated by the java compiler. Used to decompose class files.
test class
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 command parameters
C:\>javap -help
usage: javap <options> <classes>
Among them, possible options include:
-help --help -? print this usage message
-version version information
-v -verbose output additional information
-l output line numbers and local variable table
-public show only public classes and members
-protected show protected/public classes and members
-package show package/protected/public classes
and members (default)
-p -private show all classes and members
-c disassemble the code
-s output internal type signature
-sysinfo shows the class's
System info (path, size, date, MD5 hash)
-constants show static final constants
-classpath <path> specifies where to look for user class files
-bootclasspath <path> Override the location of the boot class file
javap -version
1.7.0_71
show java version
javap -p
C:\Users\user\Desktop>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();
}
Displays all members of the accessible modifier scope "private" of the class
javap -public
C:\Users\user\Desktop>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[]);
}
show the public members of the class
javap -protected
C:\Users\user\Desktop>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[]);
}
Displays all members of the class that are accessible by modifier scope > protected
javap-l
C:\Users\user\Desktop>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
}
Output line numbers and local variable table? I don't understand
javap -package
C:\Users\user\Desktop>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[]);
}
Similar to javap -public
javap -v/-p -v
C:\Users\user\Desktop>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."<init>":()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."<init>":()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 <init>
#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 // "<init>":()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."<init>":()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: verse 1, 1
16: istore_1
17: here 2, 1
20: iload_2
21: istore_2
22: here 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."<init>":()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."<init>":()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
}
The command description is: output additional information
class file path, last modification time, file size, etc. full path of
class , source (java) file, etc.
constant pool
constant definition, value
construction method
program call and execution logic (the content involved is compared More)
In short, the javap -v command is a very powerful command!
javap -c
C:\Users\user\Desktop>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."<init>":()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: verse 1, 1
16: istore_1
17: here 2, 1
20: iload_2
21: istore_2
22: here 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."<init>":()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."<init>":()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
}
In fact, the output of javap -c already exists in javap -v. We will introduce the output content of the javap -c command in detail.
0: iconst_0 The front 0: indicates the order of execution, icont_0 puts the value 0 on the top of the stack, 0 in _0 Represents the value pushed on the stack, such as: icont_5, that is, push 5 to the top of the stack
1: istore_1 Put the value of the top of the stack into variable 1, _1 represents the sequence of variables, in this case: m, such as: istore_2 is the variable n assignment
6: iload_3 puts the value of variable 3, i.e. i, on the top of the stack. Different from icont, the value of iload operation is the value that has already been defined, and icont is the push operation when it is defined.
13: iinc 1, 1 What is the final result of adding 1 to the value of variable 1 in
the demo at the beginning of the article?
m = 0
n = 10
why?
Let's focus on these lines:
12: iload_1 -- put the value of variable 1 on the stack item
13: iinc 1, 1 -- increment variable 1 by 1 to become 2, the top value of the stack is still 1. where the first 1 represents a variable and the second 1 represents an increment of
16: istore_1 -- assign the top value of the stack to variable 1, variable 1 is still 1
17: iinc 2, 1 -- increment variable 2 by 1 to become 2 , the top value of the stack is still 1. The first 2 represents the variable, and the second 1 represents an increment of
20: iload_2 -- put the value of variable 2 on the top of the stack
21: istore_2 -- assign the top value of the stack to variable 2, and the value of variable 2 is 2.
Remember a point:
int m = 0, n = 0;
m = m++; -- will first assign the m value (ie 0) to m and then ++
n = ++n; -- n first ++ and then assign the value to n
At this time, m = 0, n = 1;
this place is a bit confusing, you need to read it a few more times, straighten out the logic and relation!
Here you can refer to http://blog.csdn.net/junsure2012/article/details/7099222, which is very detailed and easy to understand!
javap -s/-p -s
C:\Users\user\Desktop>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
}
output internal type signature
javap -sysinfo/-p -sysinfo
C:\Users\user\Desktop>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[]);
}
Display
system (path, size, date, MD5 hash)
javap -constants/-p -constants
C:\Users\user\Desktop>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[]);
}
show static final constants
javap -classpath/-bootclasspath
skip first