At runtime find all classes in a Java application that extend a base class

Question

I want to do something like this   List lt Animal gt  animals   new ArrayList lt Animal gt      for  Class c  list of all classes available to my app        if  c is Animal        animals add  new c        So  I want to look at all of the classes in my application s universe  and when I find one that descends from Animal  I want to create a new object of that type and add it to the list   This allows me to add functionality without having to update a list of things   I can avoid the following   List lt Animal gt  animals   new ArrayList lt Animal gt     animals add  new Dog      animals add  new Cat      animals add  new Donkey            With the above approach  I can simply create a new class that extends Animal and it ll get picked up automatically   UPDATE  10 16 2008 9 00 a m  Pacific Standard Time   This question has generated a lot of great responses -- thank you   From the responses and my research  I ve found that what I really want to do is just not possible under Java   There are approaches  such as ddimitrov s ServiceLoader mechanism that can work -- but they are very heavy for what I want  and I believe I simply move the problem from Java code to an external configuration file  Update 5 10 19  11 years later   There are now several libraries that can help with this according to  IvanNik s answer org reflections looks good  Also ClassGraph from  Luke Hutchison s answer looks interesting  There are several more possibilities in the answers as well   Another way to state what I want  a static function in my Animal class finds and instantiates all classes that inherit from Animal -- without any further configuration coding   If I have to configure  I might as well just instantiate them in the Animal class anyway   I understand that because a Java program is just a loose federation of  class files that that s just the way it is   Interestingly  it seems this is fairly trivial in C

User · Accepted Answer

I use org reflections   Reflections reflections   new Reflections  com mycompany        Set lt Class lt   extends MyInterface gt  gt  classes   reflections getSubTypesOf MyInterface class     Another example   public static void main String   args  throws IllegalAccessException  InstantiationException       Reflections reflections   new Reflections  java util        Set lt Class lt   extends List gt  gt  classes   reflections getSubTypesOf java util List class       for  Class lt   extends List gt  aClass   classes            System out println aClass getName             if aClass    ArrayList class                List list   aClass newInstance                list add  test                System out println list getClass   getName            list size

User · Answer

The Java way to do what you want is to use the ServiceLoader mechanism    Also many people roll their own by having a file in a well known classpath location  i e   META-INF services myplugin properties  and then using ClassLoader getResources   to enumerate all files with this name from all jars  This allows each jar to export its own providers and you can instantiate them by reflection using Class forName

User · Answer

This is a tough problem and you will need to find out this information using static analysis  its not available easily at runtime  Basically get the classpath of your app and scan through the available classes and read the  bytecode information of a class which class it inherits from  Note that a class Dog may not directly inherit from Animal but might inherit from Pet which is turn inherits from Animal so you will need to keep track of that hierarchy

User · Answer

Using OpenPojo you can do the following   String package    com mycompany   List lt Animal gt  animals   new ArrayList lt Animal gt      for PojoClass pojoClass   PojoClassFactory enumerateClassesByExtendingType package  Animal class  null      animals add  Animal  InstanceFactory getInstance pojoClass

User · Answer

Java dynamically loads classes  so your universe of classes would be only those that have already been loaded  and not yet unloaded   Perhaps you can do something with a custom class loader that could check the supertypes of each loaded class  I don t think there s an API to query the set of loaded classes

User · Answer

I solved this problem pretty elegantly using Package Level Annotations and then making that annotation have as an argument a list of classes   Find Java classes implementing an interface  Implementations just have to create a package-info java and put the magic annotation in with the list of classes they want to support

User · Answer

You could use ResolverUtil  raw source  from the Stripes Framework if you need something simple and quick without refactoring any existing code   Here s a simple example not having loaded any of the classes   package test   import java util Set  import net sourceforge stripes util ResolverUtil   public class BaseClassTest       public static void main String   args  throws Exception           ResolverUtil lt Animal gt  resolver   new ResolverUtil lt Animal gt             resolver findImplementations Animal class   test            Set lt Class lt   extends Animal gt  gt  classes   resolver getClasses             for  Class lt   extends Animal gt  clazz   classes                System out println clazz                      class Animal    class Dog extends Animal    class Cat extends Animal    class Donkey extends Animal      This also works in an application server as well since that s where it was designed to work     The code basically does the following    iterate over all the resources in the package s  you specify keep only the resources ending in  class Load those classes using ClassLoader loadClass String fullyQualifiedName  Checks if Animal class isAssignableFrom loadedClass

User · Answer

Unfortunately this isn t entirely possible as the ClassLoader won t tell you what classes are available   You can  however  get fairly close doing something like this   for  String classpathEntry   System getProperty  java class path   split System getProperty  path separator           if  classpathEntry endsWith   jar              File jar   new File classpathEntry            JarInputStream is   new JarInputStream new FileInputStream jar             JarEntry entry          while   entry   is getNextJarEntry       null                if entry getName   endsWith   class                         Class forName entry getName    and check                      for implementation of the interface                                   Edit  johnstok is correct  in the comments  that this only works for standalone Java applications  and won t work under an application server

User · Answer

Thanks all who answered this question     It seems this is indeed a tough nut to crack   I ended up giving up and creating a static array and getter in my baseclass     public abstract class Animal      private static Animal   animals  null      public static Animal   getAnimals            if  animals  null               animals   new Animal                    new Dog                    new Cat                    new Lion                                    return animals            It seems that Java just isn t set up for self-discoverability the way C  is   I suppose the problem is that since a Java app is just a collection of  class files out in a directory   jar file somewhere  the runtime doesn t know about a class until it s referenced   At that time the loader loads it -- what I m trying to do is discover it before I reference it which is not possible without going out to the file system and looking   I always like code that can discover itself instead of me having to tell it about itself  but alas this works too   Thanks again

User · Answer

Thanks all who answered this question     It seems this is indeed a tough nut to crack   I ended up giving up and creating a static array and getter in my baseclass     public abstract class Animal      private static Animal   animals  null      public static Animal   getAnimals            if  animals  null               animals   new Animal                    new Dog                    new Cat                    new Lion                                    return animals            It seems that Java just isn t set up for self-discoverability the way C  is   I suppose the problem is that since a Java app is just a collection of  class files out in a directory   jar file somewhere  the runtime doesn t know about a class until it s referenced   At that time the loader loads it -- what I m trying to do is discover it before I reference it which is not possible without going out to the file system and looking   I always like code that can discover itself instead of me having to tell it about itself  but alas this works too   Thanks again

User · Answer

Think about this from an aspect-oriented point of view  what you want to do  really  is know all the classes at runtime that HAVE extended the Animal class    I think that s a slightly more accurate description of your problem than your title  otherwise  I don t think you have a runtime question    So what I think you want is to create a constructor of your base class  Animal  which adds to your static array  I prefer ArrayLists  myself  but to each their own  the type of the current Class which is being instantiated   So  roughly   public abstract class Animal           private static ArrayList lt Class gt  instantiatedDerivedTypes      public Animal             Class derivedClass   this getClass            if   instantiatedDerivedClass contains derivedClass                 instantiatedDerivedClass Add derivedClass                     Of course  you ll need a static constructor on Animal to initialize instantiatedDerivedClass     I think this ll do what you probably want   Note that this is execution-path dependent  if you have a Dog class that derives from Animal that never gets invoked  you won t have it in your Animal Class list

User · Answer

The Java way to do what you want is to use the ServiceLoader mechanism    Also many people roll their own by having a file in a well known classpath location  i e   META-INF services myplugin properties  and then using ClassLoader getResources   to enumerate all files with this name from all jars  This allows each jar to export its own providers and you can instantiate them by reflection using Class forName

User · Answer

Unfortunately this isn t entirely possible as the ClassLoader won t tell you what classes are available   You can  however  get fairly close doing something like this   for  String classpathEntry   System getProperty  java class path   split System getProperty  path separator           if  classpathEntry endsWith   jar              File jar   new File classpathEntry            JarInputStream is   new JarInputStream new FileInputStream jar             JarEntry entry          while   entry   is getNextJarEntry       null                if entry getName   endsWith   class                         Class forName entry getName    and check                      for implementation of the interface                                   Edit  johnstok is correct  in the comments  that this only works for standalone Java applications  and won t work under an application server

User · Answer

Unfortunately this isn t entirely possible as the ClassLoader won t tell you what classes are available   You can  however  get fairly close doing something like this   for  String classpathEntry   System getProperty  java class path   split System getProperty  path separator           if  classpathEntry endsWith   jar              File jar   new File classpathEntry            JarInputStream is   new JarInputStream new FileInputStream jar             JarEntry entry          while   entry   is getNextJarEntry       null                if entry getName   endsWith   class                         Class forName entry getName    and check                      for implementation of the interface                                   Edit  johnstok is correct  in the comments  that this only works for standalone Java applications  and won t work under an application server

User · Answer

Think about this from an aspect-oriented point of view  what you want to do  really  is know all the classes at runtime that HAVE extended the Animal class    I think that s a slightly more accurate description of your problem than your title  otherwise  I don t think you have a runtime question    So what I think you want is to create a constructor of your base class  Animal  which adds to your static array  I prefer ArrayLists  myself  but to each their own  the type of the current Class which is being instantiated   So  roughly   public abstract class Animal           private static ArrayList lt Class gt  instantiatedDerivedTypes      public Animal             Class derivedClass   this getClass            if   instantiatedDerivedClass contains derivedClass                 instantiatedDerivedClass Add derivedClass                     Of course  you ll need a static constructor on Animal to initialize instantiatedDerivedClass     I think this ll do what you probably want   Note that this is execution-path dependent  if you have a Dog class that derives from Animal that never gets invoked  you won t have it in your Animal Class list

User · Answer

Java dynamically loads classes  so your universe of classes would be only those that have already been loaded  and not yet unloaded   Perhaps you can do something with a custom class loader that could check the supertypes of each loaded class  I don t think there s an API to query the set of loaded classes

User · Answer

Java dynamically loads classes  so your universe of classes would be only those that have already been loaded  and not yet unloaded   Perhaps you can do something with a custom class loader that could check the supertypes of each loaded class  I don t think there s an API to query the set of loaded classes

User · Answer

One way is to make the classes use a static initializers    I don t think these are inherited  it won t work if they are    public class Dog extends Animal   static      Animal a   new Dog         add a to the List     It requires you to add this code to all of the classes involved  But it avoids having a big ugly loop somewhere  testing every class searching for children of Animal

User · Answer

Think about this from an aspect-oriented point of view  what you want to do  really  is know all the classes at runtime that HAVE extended the Animal class    I think that s a slightly more accurate description of your problem than your title  otherwise  I don t think you have a runtime question    So what I think you want is to create a constructor of your base class  Animal  which adds to your static array  I prefer ArrayLists  myself  but to each their own  the type of the current Class which is being instantiated   So  roughly   public abstract class Animal           private static ArrayList lt Class gt  instantiatedDerivedTypes      public Animal             Class derivedClass   this getClass            if   instantiatedDerivedClass contains derivedClass                 instantiatedDerivedClass Add derivedClass                     Of course  you ll need a static constructor on Animal to initialize instantiatedDerivedClass     I think this ll do what you probably want   Note that this is execution-path dependent  if you have a Dog class that derives from Animal that never gets invoked  you won t have it in your Animal Class list

User · Answer

Think about this from an aspect-oriented point of view  what you want to do  really  is know all the classes at runtime that HAVE extended the Animal class    I think that s a slightly more accurate description of your problem than your title  otherwise  I don t think you have a runtime question    So what I think you want is to create a constructor of your base class  Animal  which adds to your static array  I prefer ArrayLists  myself  but to each their own  the type of the current Class which is being instantiated   So  roughly   public abstract class Animal           private static ArrayList lt Class gt  instantiatedDerivedTypes      public Animal             Class derivedClass   this getClass            if   instantiatedDerivedClass contains derivedClass                 instantiatedDerivedClass Add derivedClass                     Of course  you ll need a static constructor on Animal to initialize instantiatedDerivedClass     I think this ll do what you probably want   Note that this is execution-path dependent  if you have a Dog class that derives from Animal that never gets invoked  you won t have it in your Animal Class list

User · Answer

The most robust mechanism for listing all subclasses of a given class is currently ClassGraph  because it handles the widest possible array of classpath specification mechanisms  including the new JPMS module system   I am the author    List lt Class lt Animal gt  gt  animals  try  ScanResult scanResult   new ClassGraph   whitelistPackages  com zoo animals            enableClassInfo   scan          animals   scanResult          getSubclasses Animal class getName             loadClasses Animal class

User · Answer

One way is to make the classes use a static initializers    I don t think these are inherited  it won t work if they are    public class Dog extends Animal   static      Animal a   new Dog         add a to the List     It requires you to add this code to all of the classes involved  But it avoids having a big ugly loop somewhere  testing every class searching for children of Animal

User · Answer

This is a tough problem and you will need to find out this information using static analysis  its not available easily at runtime  Basically get the classpath of your app and scan through the available classes and read the  bytecode information of a class which class it inherits from  Note that a class Dog may not directly inherit from Animal but might inherit from Pet which is turn inherits from Animal so you will need to keep track of that hierarchy

User · Answer

I solved this problem pretty elegantly using Package Level Annotations and then making that annotation have as an argument a list of classes   Find Java classes implementing an interface  Implementations just have to create a package-info java and put the magic annotation in with the list of classes they want to support

User · Answer

The Java way to do what you want is to use the ServiceLoader mechanism    Also many people roll their own by having a file in a well known classpath location  i e   META-INF services myplugin properties  and then using ClassLoader getResources   to enumerate all files with this name from all jars  This allows each jar to export its own providers and you can instantiate them by reflection using Class forName

User · Answer

use this  public static Set lt Class gt  getExtendedClasses Class superClass        try               ResolverUtil resolver   new ResolverUtil            resolver findImplementations superClass  superClass getPackage   getName             return resolver getClasses                catch Exception e       Log d  Log      Err  getExtendedClasses             return null     getExtendedClasses Animals class     Edit    library for  ResolverUtil    Stripes

User · Answer

Thanks all who answered this question     It seems this is indeed a tough nut to crack   I ended up giving up and creating a static array and getter in my baseclass     public abstract class Animal      private static Animal   animals  null      public static Animal   getAnimals            if  animals  null               animals   new Animal                    new Dog                    new Cat                    new Lion                                    return animals            It seems that Java just isn t set up for self-discoverability the way C  is   I suppose the problem is that since a Java app is just a collection of  class files out in a directory   jar file somewhere  the runtime doesn t know about a class until it s referenced   At that time the loader loads it -- what I m trying to do is discover it before I reference it which is not possible without going out to the file system and looking   I always like code that can discover itself instead of me having to tell it about itself  but alas this works too   Thanks again

User · Answer

This is a tough problem and you will need to find out this information using static analysis  its not available easily at runtime  Basically get the classpath of your app and scan through the available classes and read the  bytecode information of a class which class it inherits from  Note that a class Dog may not directly inherit from Animal but might inherit from Pet which is turn inherits from Animal so you will need to keep track of that hierarchy

User · Answer

Java dynamically loads classes  so your universe of classes would be only those that have already been loaded  and not yet unloaded   Perhaps you can do something with a custom class loader that could check the supertypes of each loaded class  I don t think there s an API to query the set of loaded classes

User · Answer

You could use ResolverUtil  raw source  from the Stripes Framework if you need something simple and quick without refactoring any existing code   Here s a simple example not having loaded any of the classes   package test   import java util Set  import net sourceforge stripes util ResolverUtil   public class BaseClassTest       public static void main String   args  throws Exception           ResolverUtil lt Animal gt  resolver   new ResolverUtil lt Animal gt             resolver findImplementations Animal class   test            Set lt Class lt   extends Animal gt  gt  classes   resolver getClasses             for  Class lt   extends Animal gt  clazz   classes                System out println clazz                      class Animal    class Dog extends Animal    class Cat extends Animal    class Donkey extends Animal      This also works in an application server as well since that s where it was designed to work     The code basically does the following    iterate over all the resources in the package s  you specify keep only the resources ending in  class Load those classes using ClassLoader loadClass String fullyQualifiedName  Checks if Animal class isAssignableFrom loadedClass

User · Answer

The Java way to do what you want is to use the ServiceLoader mechanism    Also many people roll their own by having a file in a well known classpath location  i e   META-INF services myplugin properties  and then using ClassLoader getResources   to enumerate all files with this name from all jars  This allows each jar to export its own providers and you can instantiate them by reflection using Class forName

User · Answer

use this  public static Set lt Class gt  getExtendedClasses Class superClass        try               ResolverUtil resolver   new ResolverUtil            resolver findImplementations superClass  superClass getPackage   getName             return resolver getClasses                catch Exception e       Log d  Log      Err  getExtendedClasses             return null     getExtendedClasses Animals class     Edit    library for  ResolverUtil    Stripes

User · Answer

The most robust mechanism for listing all subclasses of a given class is currently ClassGraph  because it handles the widest possible array of classpath specification mechanisms  including the new JPMS module system   I am the author    List lt Class lt Animal gt  gt  animals  try  ScanResult scanResult   new ClassGraph   whitelistPackages  com zoo animals            enableClassInfo   scan          animals   scanResult          getSubclasses Animal class getName             loadClasses Animal class

User · Answer

One way is to make the classes use a static initializers    I don t think these are inherited  it won t work if they are    public class Dog extends Animal   static      Animal a   new Dog         add a to the List     It requires you to add this code to all of the classes involved  But it avoids having a big ugly loop somewhere  testing every class searching for children of Animal

User · Answer

Using OpenPojo you can do the following   String package    com mycompany   List lt Animal gt  animals   new ArrayList lt Animal gt      for PojoClass pojoClass   PojoClassFactory enumerateClassesByExtendingType package  Animal class  null      animals add  Animal  InstanceFactory getInstance pojoClass

User · Answer

This is a tough problem and you will need to find out this information using static analysis  its not available easily at runtime  Basically get the classpath of your app and scan through the available classes and read the  bytecode information of a class which class it inherits from  Note that a class Dog may not directly inherit from Animal but might inherit from Pet which is turn inherits from Animal so you will need to keep track of that hierarchy

User · Answer

Unfortunately this isn t entirely possible as the ClassLoader won t tell you what classes are available   You can  however  get fairly close doing something like this   for  String classpathEntry   System getProperty  java class path   split System getProperty  path separator           if  classpathEntry endsWith   jar              File jar   new File classpathEntry            JarInputStream is   new JarInputStream new FileInputStream jar             JarEntry entry          while   entry   is getNextJarEntry       null                if entry getName   endsWith   class                         Class forName entry getName    and check                      for implementation of the interface                                   Edit  johnstok is correct  in the comments  that this only works for standalone Java applications  and won t work under an application server

User · Answer

Thanks all who answered this question     It seems this is indeed a tough nut to crack   I ended up giving up and creating a static array and getter in my baseclass     public abstract class Animal      private static Animal   animals  null      public static Animal   getAnimals            if  animals  null               animals   new Animal                    new Dog                    new Cat                    new Lion                                    return animals            It seems that Java just isn t set up for self-discoverability the way C  is   I suppose the problem is that since a Java app is just a collection of  class files out in a directory   jar file somewhere  the runtime doesn t know about a class until it s referenced   At that time the loader loads it -- what I m trying to do is discover it before I reference it which is not possible without going out to the file system and looking   I always like code that can discover itself instead of me having to tell it about itself  but alas this works too   Thanks again

User · Answer

One way is to make the classes use a static initializers    I don t think these are inherited  it won t work if they are    public class Dog extends Animal   static      Animal a   new Dog         add a to the List     It requires you to add this code to all of the classes involved  But it avoids having a big ugly loop somewhere  testing every class searching for children of Animal

[java] At runtime, find all classes in a Java application that extend a base class

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