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Understanding javap tool
The javap command disassembles a class file. The javap command displays information about the fields, constructors and methods present in a class file.
Syntax to use javap tool
Let’s see how to use javap tool or command.
Example to use javap tool
Output:
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 {}; } Another example to use javap tool for your class
Let’s use the javap command for our java file.
FileName: Simple.java
Now let’s use the javap tool to disassemble the class file.
Output:
Compiled from "Simple.java" class Simple { Simple(); public static void main(java.lang.String[]); } javap -c command
You can use the javap -c command to see disassembled code. The code that reflects the java bytecode.
Output:
Compiled from "Simple.java" class Simple { Simple(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return public static void main(java.lang.String[]); Code: 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3 // String hello java 5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return } Options of javap tool
The important options of javap tool are as follows.
| Option | Description |
|---|---|
| -help | prints the help message. |
| -l | prints line number and local variable |
| -c | disassembles the code |
| -s | prints internal type signature |
| -sysinfo | shows system info (path, size, date, MD5 hash) |
| -constants | shows static final constants |
| -version | shows version information |
Let’s see how one can use these options with the help of an example. For the following file (ABC.java) we will use the above-mentioned options.
FileName: ABC.java
Command: javap -c ABC
Output:
Compiled from "ABC.java" public class ABC { public ABC(); Code: 0: aload_0 1: invokespecial #1 // Method java/lang/Object."":()V 4: return public static void main(java.lang.String[]); Code: 0: bipush 7 2: newarray int 4: dup 5: iconst_0 6: bipush 6 8: iastore 9: dup 10: iconst_1 11: bipush 7 13: iastore 14: dup 15: iconst_2 16: bipush 8 18: iastore 19: dup 20: iconst_3 21: bipush 6 23: iastore 24: dup 25: iconst_4 26: bipush 8 28: iastore 29: dup 30: iconst_5 31: iconst_0 32: iastore 33: dup 34: bipush 6 36: iconst_4 37: iastore 38: astore_1 39: aload_1 40: arraylength 41: istore_2 42: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream; 45: iload_2 46: invokedynamic #13, 0 // InvokeDynamic #0:makeConcatWithConstants:(I)Ljava/lang/String; 51: invokevirtual #17 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 54: getstatic #7 // Field java/lang/System.out:Ljava/io/PrintStream; 57: aload_1 58: bipush 8 60: iaload 61: invokedynamic #23, 0 // InvokeDynamic #1:makeConcatWithConstants:(I)Ljava/lang/String; 66: invokevirtual #17 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 69: return } Command: javap -l ABC
Output:
Compiled from "ABC.java" public class ABC { public ABC(); LineNumberTable: line 1: 0 public static void main(java.lang.String[]); LineNumberTable: line 6: 0 line 9: 39 line 12: 42 line 14: 54 line 16: 69 } Command: javap -s ABC
Output:
Compiled from "ABC.java" public class ABC { public ABC(); descriptor: ()V public static void main(java.lang.String[]); descriptor: ([Ljava/lang/String;)V } Command: javap -sysinfo ABC
Output:
Classfile /C:/Users/Nikhil Kumar/Documents/ABC.class Last modified Sep 11, 2021; size 970 bytes SHA-256 checksum 576adf03386399a4691e0ce5b6c5aa5d964b082a1a61299bac5632942e413312 Compiled from "ABC.java" public class ABC { public ABC(); public static void main(java.lang.String[]); } Command: javap -constants ABC
Output:
Compiled from "ABC.java" public class ABC { public ABC(); public static void main(java.lang.String[]); } Command: javap -version ABC
Output:
14 Compiled from "ABC.java" public class ABC { public ABC(); public static void main(java.lang.String[]); } Next Topiccreating-javap-tool