Javap tools and examples in Java-javaTutorial-php.cn

Javap tools and examples in Java

Thejavap tool is a useful way to retrieve information about a specific class or interface. Through its disassembly capabilities, the javap command (also known as Java disassembler) is given the role of providing comprehensive internal information to the user. From the two options -c or -verbose, each giving unique results, the user can find himself in the realm of bytecode and bytecode orchestration. If no options are used, javap runs the public, protected, and package fields and methods of the input class.

grammar

javap [option] [classname]

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When not using options

Example

javap class_name

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Output

C:\Users\Aashi>javap java.lang.Object 
Compiled from "Object.java"
public class java.lang.Object {
   public java.lang.Object();
   public final native java.lang.Class<?> getClass();
   public native int hashCode();
   public boolean equals(java.lang.Object);
   protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException; public java.lang.String toString();
   public final native void notify();
   public final native void notifyAll();
   public final native void wait (long) throws java.lang.InterruptedException;
   public final void wait(long, int) throws java.lang.InterruptedException;
   public final void wait() throws java.lang.InterruptedException;
   protected void finalize() throws java.lang.Throwable;
   static {};
}

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When to use options

Here are instructions for each option and how to apply it -

-help or --help or -? -

This is used to print help messages for the javap command.

Example

javap -help

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Output

C:\Users\Aashi>javap -help 
Usage: javap <options> <classes> 
where possible options include: 
-help --help -?      Print this usage message
-version             Version information
-V  -verbose         Print additional information
-1                   Print line number and local variable tables
-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                   Print internal type signatures
-sysinfo             Show system info (path, size, date, MD5 hash) of class being processed
-constants           Show final constants
-classpath <path>    Specify where to find user class files
-cp <path>           Specify where to find user class files
-bootclasspath <path> Override location of bootstrap class files

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-Version

Used to print Java version information.

Example

javap -version

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Output

C:\Users\Aashi>javap -version
1.8.0_151

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-v or -verbose -

This is used to print additional information such as stack size, number of local variables and method parameters.

Example

javap -v class_name

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Output

C:\Users\Aashi>javap -v java.lang.Object
Classfile jar:file:/C:/Program%20Files/Java/jdk1.8.0_151/jre/lib/rt.jar! /java/lang/Object.class
Last modified Sep 5, 2017; size 1497 bytes
MD5 checksum 074ebc688a81170b8740f1158648a3c7 
Compiled from "Object.java"
public class java.lang.Object 
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Integer         999999
#2 - String          #16                //@
#3 = String          #38                //nanosecond timeout value out of range
#4 = String          #42               //timeout value is negative
#5 - Utf8            ()I
#6 = Utf8            ()Ljava/lang/Object;
#7 - Utf8            ()Ljava/lang/String;
#8 = Utf8            ()V
#9 - Utf8            (I)Ljava/lang/String;
#10 - Utf8           (J)V
#11 = Utf8           (JI)V
#12 - Utf8           (Ljava/lang/Object;)Z
#13 = Utf8           (Ljava/lang/String;)V
#14 - Utf8           <clinit>
#15 = Utf8           <init>
#16 - Utf8           @
#17 - Utf8           Code
#18 = Utf8           Exceptions
#19 - Utf8           Line Number Table
#20 = Utf8           Signature
#21- Utf8            Source File
#22 = Utf8           StackMapTable
#23 Utf8             Append
#24 Utf8             Clone
#25 = Utf8           Equals
#26 Utf8             finalize
#27 = Utf8           getClass
#28 Utf8             getName
#29 - Utf8           hashCode
#30 = Utf8           java/lang/Class
#31 - Utf8           java/lang/CloneNotSupportedException
#32 = Utf8           java/lang/IllegalArgumentException
#33 - Utf8           Java/lang/Integer
#34 Utf8             java/lang/InterruptedException

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-l -

This is used to print line numbers and local variable tables.

Example

javap -l class_name

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Output

C:\Users\Aashi>javap -1 java.lang.Object 
Compiled from "Object.java” 
public class java.lang.Object { 
   public java.lang.Object();
   LineNumberTable:
   line 37: 0

   public final native java.lang.Class<?> getClass();

   public native int hashCode();

   public boolean equals(java.lang.Object);
   LineNumberTable:
   line 149: 0

   protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException;

   public java.lang.String toString();
   LineNumberTable:
   line 236: 0

   public final native void notify();

   public final native void notifyAll();

   public final native void wait(long) throws Java.lang.InterruptedException;
   public final void wait(long, int) throws java.lang.InterruptedException;
   LineNumberTable:
   line 447: 0
   line 448: 6 
   line 451: 16 
   line 452: 26 
   line 456: 36 
   line 457: 40 
   line 460: 44
   line 461: 49

   public final void wait() throws java.lang.InterruptedException;
   LineNumberTable:
   line 502:0
   line 503: 5
   protected void finalize() throws java.lang.Throwable;
}

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-public

This is used to print only public classes and members.

Example

javap -public class_name

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Output

C:\Users\Aashi>javap -public java.lang.Object 
Compiled from "Object.java" 
public class java.lang.Object {
   public java.lang.Object();
   public final native java.lang.Class<?> getClass();
   public native int hashCode();
   public boolean equals(java.lang.Object);
   public java.lang.String toString();
   public final native void notify();
   public final native void notifyAll();
   public final native void wait (long) throws java.lang.InterruptedException; public final void wait (long, int) throws java.lang.InterruptedException; public final void wait() throws java.lang.InterruptedException;
}

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-protected

This is used to print protected/public classes and members.

Example

javap -protected class_name

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Output

C:\Users\Aashi>javap -protected java.lang.Object 
Compiled from "Object.java"
public class java.lang.Object {
   public java.lang.Object();
   public final native java.lang.Class<?> getClass();
   public native int hashCode();
   public boolean equals(java.lang.Object);
   protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException; 
   public java.lang.String toString();
   public final native void notify();
   public final native void notifyAll();
   public final native void wait(long) throws java.lang.InterruptedException;
   public final void wait (long, int) throws java.lang.InterruptedException;
   public final void wait() throws java.lang.InterruptedException;
   protected void finalize() throws java.lang.Throwable;
}

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-Bag

This is used to print package/protected/public classes and members (default).

Example

javap -package class_name

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Output

C:\Users\Aashi>javap -package java.lang.Object 
Compiled from "Object.java"
   public class java.lang.Object {
   public java.lang.Object();
   public final native java.lang.Class<?> getClass();
   public native int hashCode();
   public boolean equals(java.lang.Object);
   protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException; 
   public java.lang.String toString();
   public final native void notify();
   public final native void notifyAll();
   public final native void wait (long) throws java.lang.InterruptedException;
   public final void wait (long, int) throws java.lang.InterruptedException;
   public final void wait() throws java.lang.InterruptedException;
   protected void finalize() throws java.lang.Throwable;
   static {};
}

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-c

This is used to print disassembly code.

Example

javap -c class_name

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Output

C:\Users\Aashi>javap -c java.lang.Object 
Compiled from "Object.java"
public class java.lang.Object (public java.lang.Object();
Code:
0: return
public final native java.lang.Class<?> getClass();

public native int hashCode();

public boolean equals(java.lang.Object);
Code:
0: aload e
1: aload_1
2: 1f_acmpne   9
5:iconst_1 
6: goto        10
9: iconst_0
10: ireturn

protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException;

public java.lang.String toString();
Code:
0: new             #52      //class java/lang/StringBuilder
3: dup
4: invokespecial   #74     //Method java/lang/StringBuilder."<init>": ()V
7: aload_e
8: invokevirtual   #73    // Method getClass: ()Ljava/lang/Class;
11: invokevirtual  #67    // Method java/lang/class.getName:()Ljava/lang/String;
14: invokevirtual  #76   // Method java/lang/StringBuilder.append: (Ljava/lang/String;) Ljava/lang/StringBuilder;
17: 1dc            #2    // String @
19: invokevirtual  #76   // Method java/lang/StringBuilder.append: (Ljava/lang/String;)Ljava/lang/StringBuilder;
22: aload_e
23: invokevirtual  #78   // Method hashCode: ()I
26: invokestatic   #69   // Method java/lang/Integer.toHexString: (I) Ljava/lang/String;
29: invokevirtual  #76   // Method java/lang/StringBuilder.append: (Ljava/lang/String;) Ljava/lang/StringBuilder;
32: invokevirtual  #75   // Method java/lang/StringBuilder.toString: ()Ljava/lang/String;
35: areturn
public final native void notify();

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-s -

This is used to print internal type signatures.

Example

javap -s class_name

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Output

C:\Users\Aashi>javap -s java.lang.Object 
Compiled from "Object.java" 
public class java.lang.Object { 
   public java.lang.Object(); 
   descriptor: ()V

   public final native java.lang.Class<?> getClass(); 
   descriptor: ()Ljava/lang/class;

   public native int hashCode();
   descriptor: ()I

   public boolean equals(java.lang.Object);
   descriptor: (Ljava/lang/Object;)Z

   protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException; 
   descriptor:()Ljava/lang/Object;

   public java.lang.String toString();
   descriptor: ()Ljava/lang/String;

   public final native void notify();
   descriptor: ()V

   public final native void notifyAll();
   descriptor: ()V

   public final native void wait(long) throws java.lang.InterruptedException; descriptor: (J)V

   public final void wait(long, int) throws java.lang.InterruptedException; descriptor: (JI)V

   public final void wait() throws java.lang.InterruptedException; 
   descriptor: ()V
   protected void finalize() throws java.lang.Throwable;
   descriptor: ()V
   static {}; 
   descriptor: ()V
}

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-sysinfo -

This is used to print system information (path, size, date, MD5 hash) of the class being processed.

Example

javap -sysinfo class_name

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Output

C:\Users\Aashi>javap -sysinfo java.lang.Object
Classfile jar:file:/C:/Program%20Files/Java/jdk1.8.0_151/jre/lib/rt.jar! /java/lang/Object.class 
Last modified Sep 5, 2017; size 1497 bytes
MD5 checksum 074ebc688a81170b8740f1158648a3c7 
Compiled from "Object.java"
public class java.lang.Object {
   public java.lang.Object();
   public final native java.lang.Class<?> getClass();
   public native int hashCode();
   public boolean equals(java.lang.Object);
   protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException;
   public java.lang.String toString();
   public final native void notify();
   public final native void notifyAll();
   public final native void wait (long) throws java.lang.InterruptedException;
   public final void wait (long, int) throws java.lang.InterruptedException;
   public final void wait() throws java.lang.InterruptedException;
   protected void finalize() throws java.lang.Throwable;
   static {};
}

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-Constant -

This is used to print the final constants of the class.

Example

javap -constants class_name

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Output

C:\Users\Aashi>javap -constants java.lang.Object 
Compiled from "Object.java"
public class java.lang.Object {

   public java.lang.Object();
   public final native java.lang.Class<?> getClass();
   public native int hashCode();
   public boolean equals(java.lang.Object);
   protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException; 
   public java.lang.String toString();
   public final native void notify();
   public final native void notifyAll();
   public final native void wait (long) throws java.lang.InterruptedException;
   public final void wait(long, int) throws java.lang.InterruptedException;
   public final void wait() throws java.lang.InterruptedException;
   protected void finalize() throws java.lang.Throwable;
   static {}; 
}

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in conclusion

javap tool is a multi-faceted and very powerful tool for Java coders, enabling them to diagnose and debug class documents, evaluate fields and methods, and generate clean code from the bytecode of class documents. This indispensable tool is suitable for Java developers of all skill levels, allowing them to easily expand their knowledge and troubleshoot Java programs. The javap tool is an irreplaceable possession for any aspiring programmer and it will surely help improve their programming skills quickly. Additionally, its wide range of features helps coders refine their practices, giving them a competitive edge when building Java applications.

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