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.
javap [option] [classname]
javap class_name
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 {}; }
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.
javap -help
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
-Version
Used to print Java version information.
javap -version
C:\Users\Aashi>javap -version 1.8.0_151
-v or -verbose -
This is used to print additional information such as stack size, number of local variables and method parameters.
javap -v class_name
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
-l -
This is used to print line numbers and local variable tables.
javap -l class_name
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; }
-public
This is used to print only public classes and members.
javap -public class_name
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; }
-protected
This is used to print protected/public classes and members.
javap -protected class_name
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; }
-Bag
This is used to print package/protected/public classes and members (default).
javap -package class_name
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 {}; }
-c
This is used to print disassembly code.
javap -c class_name
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();
-s -
This is used to print internal type signatures.
javap -s class_name
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 }
-sysinfo -
This is used to print system information (path, size, date, MD5 hash) of the class being processed.
javap -sysinfo class_name
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 {}; }
-Constant -
This is used to print the final constants of the class.
javap -constants class_name
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 {}; }
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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