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How to decompile Java

I need an easy way to decompile Java. I want to learn about the transformations and modifications that the javac compiler does in my code. What would be good products and what would be good / bad ones.

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5 answers

Quick and dirty, use javapthat comes with the JDK.

javap -c test
Compiled from "test.java"
public class test extends java.lang.Object{
public test();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return

public static void main(java.lang.String[])   throws java.lang.Exception;
  Code:
   0:   ldc #2; //String CR_ABC_DEF_GHI
   2:   astore_1
   3:   ldc #3; //String TYPE=Eligibility Only|CLIENT_ID=62|EMPLOYER_ID=0|LOCATION_ID=0|PLAN_ID=all|FREQUENCY=52|FILE=null|FILETYPE=C|PROCESSTOLAUNCH=10|PROCESSTOLAUNCHSTATUS=100|SCHEDULE_ID=29837d692f4c43e786f1097fab7c360c
   5:   astore_2
   6:   ldc #4; //String \|
   8:   astore_3
   9:   ldc #5; //String FILE_NAME_TEMPLATE
   11:  astore  4
   13:  ldc #6; //String FILE
   15:  astore  5
   17:  new #7; //class java/lang/StringBuilder
   20:  dup
   21:  invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   24:  aload_3
   25:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   28:  aload   4
   30:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   33:  ldc #10; //String =
   35:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   38:  ldc #11; //String [^
   40:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   43:  aload_3
   44:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   47:  ldc #12; //String ]*
   49:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   52:  aload_3
   53:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   56:  invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   59:  astore  6
   61:  new #7; //class java/lang/StringBuilder
   64:  dup
   65:  invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   68:  aload_3
   69:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   72:  aload   5
   74:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   77:  ldc #10; //String =
   79:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   82:  ldc #11; //String [^
   84:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   87:  aload_3
   88:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   91:  ldc #12; //String ]*
   93:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   96:  aload_3
   97:  invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   100: invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   103: astore  7
   105: new #7; //class java/lang/StringBuilder
   108: dup
   109: invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   112: aload_3
   113: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   116: aload   4
   118: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   121: ldc #10; //String =
   123: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   126: aload_1
   127: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   130: aload_3
   131: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   134: invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   137: astore  8
   139: aload_2
   140: aload   6
   142: aload   8
   144: invokevirtual   #14; //Method java/lang/String.replaceAll:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
   147: aload   7
   149: aload   8
   151: invokevirtual   #14; //Method java/lang/String.replaceAll:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
   154: astore  9
   156: getstatic   #15; //Field java/lang/System.out:Ljava/io/PrintStream;
   159: new #7; //class java/lang/StringBuilder
   162: dup
   163: invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   166: ldc #16; //String before = 
   168: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   171: aload_2
   172: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   175: invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   178: invokevirtual   #17; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   181: getstatic   #15; //Field java/lang/System.out:Ljava/io/PrintStream;
   184: new #7; //class java/lang/StringBuilder
   187: dup
   188: invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   191: ldc #18; //String pattern1 = 
   193: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   196: aload   6
   198: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   201: invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   204: invokevirtual   #17; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   207: getstatic   #15; //Field java/lang/System.out:Ljava/io/PrintStream;
   210: new #7; //class java/lang/StringBuilder
   213: dup
   214: invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   217: ldc #19; //String pattern2 = 
   219: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   222: aload   7
   224: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   227: invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   230: invokevirtual   #17; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   233: getstatic   #15; //Field java/lang/System.out:Ljava/io/PrintStream;
   236: new #7; //class java/lang/StringBuilder
   239: dup
   240: invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   243: ldc #20; //String replace = 
   245: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   248: aload   8
   250: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   253: invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   256: invokevirtual   #17; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   259: getstatic   #15; //Field java/lang/System.out:Ljava/io/PrintStream;
   262: new #7; //class java/lang/StringBuilder
   265: dup
   266: invokespecial   #8; //Method java/lang/StringBuilder."<init>":()V
   269: ldc #21; //String after = 
   271: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   274: aload   9
   276: invokevirtual   #9; //Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
   279: invokevirtual   #13; //Method java/lang/StringBuilder.toString:()Ljava/lang/String;
   282: invokevirtual   #17; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   285: return

}
+5
source

The best one is probably jad:

http://www.varaneckas.com/jad

If you use Eclipse, you can use the JadClipse plugin for a more ergonomic interface:

http://jadclipse.sourceforge.net/wiki/index.php/Main_Page

+2
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.

+1
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Another jcom decompiler - AndroChef Java Decompiler is available: http://www.neshkov.com/ac_decompiler.html

+1
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I used JAD and can bring it usefulness.

Just like a head, although any decompiled code is likely to be free from any comments, and if the code is messy, then decompilation will not actually lead to you. It will still look like nonsense.

0
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Source: https://habr.com/ru/post/1778991/

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