javap is the Java Disassembler tool which can be used to open a .class file in a readable format. javap is located in the /bin folder of the JDK installation directory. The Java Decomplier (javap) displays information about the package, protected and public fields, and methods of the classes passed to it. The javap tool prints its output to stdout. In this article lets learn how to open a .class file in java with Example using javap.
Syntax for Javap
javap <<option>> <<.class file1>> <<.class file2>>
Javap Command Example in Java
lets use the Java Decomplier (Javap tool) to decompile our Demo class.
Demo.java
package com.javainterviewpoint;
public class Demo
{
public void disp()
{
System.out.println("Welcome to JavaInterviewPoint!!!");
}
public static void main(String args[])
{
Demo d1 = new Demo();
d1.disp();
}
}
Output :
upon executing javap command in cmd
javap com.javainterviewpoint.Demo
The below output will be produced
C:\JIP>javap com.javainterviewpoint.Demo
Compiled from "Demo.java"
public class com.javainterviewpoint.Demo {
public com.javainterviewpoint.Demo();
public void disp();
public static void main(java.lang.String[]);
}
In the above code we have not used any options along with javap command in cmd, the most popular option is ‘-c’, this option prints instructions that comprise the Java bytecodes for each of the methods in the class.
C:\JIP>javap -c com.javainterviewpoint.Demo
Compiled from "Demo.java"
public class com.javainterviewpoint.Demo {
public com.javainterviewpoint.Demo();
Code:
0: aload_0
1: invokespecial #1 // Method java/lang/Object."":()V
4: return
public void disp();
Code:
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #3 // String Welcome to JavaInterviewPoint!!!
5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
public static void main(java.lang.String[]);
Code:
0: new #5 // class com/javainterviewpoint/Demo
3: dup
4: invokespecial #6 // Method "":()V
7: astore_1
8: aload_1
9: invokevirtual #7 // Method disp:()V
12: return
}
How to use Javap command in Eclipse
Running javap command in eclipse can be done using External Tool Configuration only. Goto “Run –> External Tools –> External Tools Configurations” as shown in below image.
Enter the below information in the new Configuration window
- Click on Program and click on New
- Give the name for the configuration, here we have given ‘javap -verbose’
- Select the Location of javap.exe, by browsing the file system. Usually javap.exe will be location in the <<jdk installation>>/bin directory
- Select the Working Directory, the location where the class file resides (Demo Class). Click on browse workspace under bin select the package under where Demo class resides (Here com/javainterviewpoint)
- In the Arguments block give -verbose Demo , where -verbose is the option which you want to execute along with javap and Demo is our class name.
- Click on Apply and Finally Run.
We will get the below out displayed
Classfile /C:/Jackson/EclipseKepler/Workspace/MyJava/bin/com/javainterviewpoint/Demo.class
Last modified Feb 5, 2016; size 696 bytes
MD5 checksum ed681f968119b07f83eadbed004dcbee
Compiled from "Demo.java"
public class com.javainterviewpoint.Demo
SourceFile: "Demo.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Class #2 // com/javainterviewpoint/Demo
#2 = Utf8 com/javainterviewpoint/Demo
#3 = Class #4 // java/lang/Object
#4 = Utf8 java/lang/Object
#5 = Utf8
#6 = Utf8 ()V
#7 = Utf8 Code
#8 = Methodref #3.#9 // java/lang/Object."":()V
#9 = NameAndType #5:#6 // "":()V
#10 = Utf8 LineNumberTable
#11 = Utf8 LocalVariableTable
#12 = Utf8 this
#13 = Utf8 Lcom/javainterviewpoint/Demo;
#14 = Utf8 disp
#15 = Fieldref #16.#18 // java/lang/System.out:Ljava/io/PrintStream;
#16 = Class #17 // java/lang/System
#17 = Utf8 java/lang/System
#18 = NameAndType #19:#20 // out:Ljava/io/PrintStream;
#19 = Utf8 out
#20 = Utf8 Ljava/io/PrintStream;
#21 = String #22 // Welcome to JavaInterviewPoint!!!
#22 = Utf8 Welcome to JavaInterviewPoint!!!
#23 = Methodref #24.#26 // java/io/PrintStream.println:(Ljava/lang/String;)V
#24 = Class #25 // java/io/PrintStream
#25 = Utf8 java/io/PrintStream
#26 = NameAndType #27:#28 // println:(Ljava/lang/String;)V
#27 = Utf8 println
#28 = Utf8 (Ljava/lang/String;)V
#29 = Utf8 main
#30 = Utf8 ([Ljava/lang/String;)V
#31 = Methodref #1.#9 // com/javainterviewpoint/Demo."":()V
#32 = Methodref #1.#33 // com/javainterviewpoint/Demo.disp:()V
#33 = NameAndType #14:#6 // disp:()V
#34 = Utf8 args
#35 = Utf8 [Ljava/lang/String;
#36 = Utf8 d1
#37 = Utf8 SourceFile
#38 = Utf8 Demo.java
{
public com.javainterviewpoint.Demo();
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #8 // Method java/lang/Object."":()V
4: return
LineNumberTable:
line 3: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/javainterviewpoint/Demo;
public void disp();
flags: ACC_PUBLIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #15 // Field java/lang/System.out:Ljava/io/PrintStream;
3: ldc #21 // String Welcome to JavaInterviewPoint!!!
5: invokevirtual #23 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
8: return
LineNumberTable:
line 7: 0
line 8: 8
LocalVariableTable:
Start Length Slot Name Signature
0 9 0 this Lcom/javainterviewpoint/Demo;
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=2, args_size=1
0: new #1 // class com/javainterviewpoint/Demo
3: dup
4: invokespecial #31 // Method "":()V
7: astore_1
8: aload_1
9: invokevirtual #32 // Method disp:()V
12: return
LineNumberTable:
line 11: 0
line 12: 8
line 13: 12
LocalVariableTable:
Start Length Slot Name Signature
0 13 0 args [Ljava/lang/String;
8 5 1 d1 Lcom/javainterviewpoint/Demo;
}
you can also add different options in the Arguments block for different output. The complete list of the option which can be used along with Java Disassembler is present here.
Happy Learning !!:)
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