How to change already compiled class file without decompile

Question

I want to change  class file s method  I installed JD Eclipse Decompiler and opened the  class file  I added some codes and save  class file  But   class file is not changing    I don t know how to use decompiler  And if is it possible  how to change  class file without using decompiler   I am using Ubuntu   Regards  EDIT   Here is my decompiled code              package org hibernate id                       import java io Serializable            import java sql ResultSet            import java sql SQLException            import java util HashMap            import java util Properties            import org apache commons logging Log            import org apache commons logging LogFactory            import org hibernate HibernateException            import org hibernate MappingException            import org hibernate dialect Dialect            import org hibernate type Type            import org hibernate util ReflectHelper                       public final class IdentifierGeneratorFactory                 25      private static final Log log   LogFactory getLog IdentifierGeneratorFactory class                  64      private static final HashMap GENERATORS   new HashMap                   66      public static final Serializable SHORT CIRCUIT INDICATOR   new Serializable                   public String toString     return  SHORT CIRCUIT INDICATOR                       66                        70      public static final Serializable POST INSERT INDICATOR   new Serializable                   public String toString     return  POST INSERT INDICATOR                       70                                public static Serializable getGeneratedIdentity ResultSet rs  Type type                throws SQLException  HibernateException  IdentifierGenerationException                   32        if    rs next           33          throw new HibernateException  The database returned no natively generated identity value                        35        Serializable id   get rs  type                  37        if  log isDebugEnabled    log debug  Natively generated identity      id       38        return id                                       public static Serializable get ResultSet rs  Type type                throws SQLException  IdentifierGenerationException                   45        Class clazz   type getReturnedClass        46        if  clazz    Long class        47          return new Long rs getLong 1                        49        if  clazz    Integer class        50          return new Integer rs getInt 1                        52        if  clazz    Short class        53          return new Short rs getShort 1                        55        if  clazz    String class        56          return rs getString 1                                   if clazz    java math BigDecimal class                       return rs getBigDecimal 1                                    59        throw new IdentifierGenerationException  this id generator generates long  integer  short or string78                                         public static IdentifierGenerator create String strategy  Type type  Properties params  Dialect dialect                throws MappingException                             try                     92          Class clazz   getIdentifierGeneratorClass strategy  dialect       93          IdentifierGenerator idgen    IdentifierGenerator clazz newInstance        94          if  idgen instanceof Configurable    Configurable idgen  configure type  params  dialect       95          return idgen                                catch  Exception e        98          throw new MappingException  could not instantiate id generator   e                                                        public static Class getIdentifierGeneratorClass String strategy  Dialect dialect       103        Class clazz    Class GENERATORS get strategy      104        if   native  equals strategy   clazz   dialect getNativeIdentifierGeneratorClass                  try      106          if  clazz    null  clazz   ReflectHelper classForName strategy                                 catch  ClassNotFoundException e       109          throw new MappingException  could not interpret id generator strategy      strategy                      111        return clazz                                       public static Number createNumber long value  Class clazz  throws IdentifierGenerationException      115        if  clazz    Long class       116          return new Long value                      118        if  clazz    Integer class       119          return new Integer  int value                      121        if  clazz    Short class       122          return new Short  short  int value                                  125        throw new IdentifierGenerationException  this id generator generates long  integer  short                                         static                   75        GENERATORS put  uuid   UUIDHexGenerator class       GENERATORS put  hilo   TableHiLoGenerator class        GENERATORS put  assigned   Assigned class        GENERATORS put  identity   IdentityGenerator class       GENERATORS put  select   SelectGenerator class       GENERATORS put  sequence   SequenceGenerator class        GENERATORS put  seqhilo   SequenceHiLoGenerator class       GENERATORS put  increment   IncrementGenerator class      GENERATORS put  foreign   ForeignGenerator class        GENERATORS put  guid   GUIDGenerator class        GENERATORS put  uuid hex   UUIDHexGenerator class        GENERATORS put  sequence-identity   SequenceIdentityGenerator class

User · Answer

Sometime we need to compile one single file out of thousand files to fix the problem  In such a case  One can create same folder structure like class path  decompile the file into java or copy java file from source code  Make required changes  compile one particular file into class with all dependency classes in place and finally replace the class file  Finally restart the container  Once war is exploded file will not be replaced

User · Answer

You can use any decompiler to first decompile the file    I had once faced a simillar problem where I didn t have source code of the application and had to make a very small change in a file       Below is what I did     Extracted the class file from the jar Opened it in a decompiler  I use JD GUI  you can get it easily from many resources on internet  You may download it from here You can actually view all the files in a jar using JD GUI  Made changes to the file I wanted to and copied all the code from that file Created a new project in eclipse with only this class  with the same package structure as in the jar   provided the original jar as library and all other dependencies  Compiled the class  injected the  class file back to the jar from bin folder of my workspace Tested my change  celebrated it by sipping a cup of coffee

User · Answer

Use java assist Java library for manipulating the Java bytecode   class file  of an application   -  Spring   Hibernate   EJB using this for proxy implementation   -  we can bytecode manipulation to do some program analysis  -  we can use Javassist to implement a transparent cache for method return values  by intercepting all method invocations and only delegating to the super implementation on the first invocation

User · Answer

You can change the code when you decompiled it  but it has to be recompiled to a class file  the decompiler outputs java code  this has to be recompiled with the same classpath as the original jar class file

User · Answer

Use a bytecode editor  like   http   set ee jbe   Be careful because you need a very good knowledge of the Java bytecode   You can also change the class at runtime with bytecode weaving  like AspectJ

User · Answer

You can follow these steps to modify your java class    Decompile the  class file as you have done and save it as  java Create a project in Eclipse with that java file  the original JAR as library  and all its dependencies  Change the  java and compile Get the modified  class file and put it again inside the original JAR

User · Answer

I added some codes and save  class file    What you see in JD EClipse Decompiler is decompiled representation of byte code in the  class file  Even though you change the text it won t affect the byte code

User · Answer

when you decompile and change the code you have to go on the root folder of your eclipse project and check your class in bin folder wich is on the same level as src  then open you original jar with zip tool   7zip is good for that   and put the modified class in tha same package inside the jar

User · Answer

As far as I ve been able to find out  there is no simple way to do it  The easiest way is to not actually convert the class file into an executable  but to wrap an executable launcher around the class file  That is  create an executable file  perhaps an OS-based  executable scripting file  which simply invokes the Java class through the command line   If you want to actually have a program that does it  you should look into some of the automated installers out there   Here is a way I ve found    code  import java io    import java util jar     class OnlyExt implements FilenameFilter  String ext  public OnlyExt String ext   this ext       ext      Override public boolean accept File dir String name   return name endsWith ext        public class ExeCreator   public static int buffer   10240  protected void create File exefile  File   listFiles    try   byte b     new byte buffer   FileOutputStream fout   new FileOutputStream exefile   JarOutputStream out   new JarOutputStream fout  new Manifest     for  int i   0  i  lt  listFiles length  i      if  listFiles i     null     listFiles i  exists     listFiles i  isDirectory    System out println  Adding     listFiles i  getName     JarEntry addFiles   new JarEntry listFiles i  getName     addFiles setTime listFiles i  lastModified     out putNextEntry addFiles    FileInputStream fin   new FileInputStream listFiles i    while  true    int len   fin read b  0  b length   if  len  lt   0  break  out write b  0  len     fin close      out close    fout close    System out println  Jar File is created successfully       catch  Exception ex        public static void main String  args   ExeCreator exe new ExeCreator    FilenameFilter ff   new OnlyExt  class    File folder   new File    examples    File   files   folder listFiles ff   File file new File  examples exe    exe create file  files            code

[java] How to change already compiled .class file without decompile?

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