Find Java classes implementing an interface

Question

Some time ago  I came across a piece of code  that used some piece of standard Java functionality to locate the classes that implemented a given interface  I know the functions were hidden in some non-logical place  but they could be used for other classes as the package name implied  Back then I did not need it  so I forgot about it  but now I do  and I can t seem to find the functions again  Where can these functions be found   Edit  I m not looking for any IDE functions or anything  but rather something that can be executed within the Java application

User · Accepted Answer

Awhile ago  I put together a package for doing what you want  and more   I needed it for a utility I was writing   It uses the ASM library  You can use reflection  but ASM turned out to perform better   I put my package in an open source library I have on my web site  The library is here  http   software clapper org javautil   You want to start with the with ClassFinder class   The utility I wrote it for is an RSS reader that I still use every day  so the code does tend to get exercised  I use ClassFinder to support a plug-in API in the RSS reader  on startup  it looks in a couple directory trees for jars and class files containing classes that implement a certain interface  It s a lot faster than you might expect   The library is BSD-licensed  so you can safely bundle it with your code  Source is available   If that s useful to you  help yourself   Update  If you re using Scala  you might find this library to be more Scala-friendly

User · Answer

If you were asking from the perspective of working this out with a running program then you need to look to the java lang   package  If you get a Class object  you can use the isAssignableFrom method to check if it is an interface of another Class   There isn t a simple built in way of searching for these  tools like Eclipse build an index of this information   If you don t have a specific list of Class objects to test you can look to the ClassLoader object  use the getPackages   method and build your own package hierarchy iterator   Just a warning though that these methods and classes can be quite slow

User · Answer

You could also use the Extensible Component Scanner  extcos  http   sf net projects extcos  and search all classes implementing an interface like so   Set lt Class lt   extends MyInterface gt  gt  classes   new HashSet lt Class lt   extends MyInterface gt  gt      ComponentScanner scanner   new ComponentScanner    scanner getClasses new ComponentQuery          Override     protected void query             select            from  my package1    my package2            andStore thoseImplementing MyInterface class  into classes            returning none                 This works for classes on the file system  within jars and even for those on the JBoss virtual file system  It s further designed to work within standalone applications as well as within any web or application container

User · Answer

I really like the reflections library for doing this   It provides a lot of different types of scanners  getTypesAnnotatedWith  getSubTypesOf  etc   and it is dead simple to write or extend your own

User · Answer

If you were asking from the perspective of working this out with a running program then you need to look to the java lang   package  If you get a Class object  you can use the isAssignableFrom method to check if it is an interface of another Class   There isn t a simple built in way of searching for these  tools like Eclipse build an index of this information   If you don t have a specific list of Class objects to test you can look to the ClassLoader object  use the getPackages   method and build your own package hierarchy iterator   Just a warning though that these methods and classes can be quite slow

User · Answer

I really like the reflections library for doing this   It provides a lot of different types of scanners  getTypesAnnotatedWith  getSubTypesOf  etc   and it is dead simple to write or extend your own

User · Answer

Spring can do this for you     BeanDefinitionRegistry bdr   new SimpleBeanDefinitionRegistry    ClassPathBeanDefinitionScanner s   new ClassPathBeanDefinitionScanner bdr    TypeFilter tf   new AssignableTypeFilter CLASS YOU WANT class   s addIncludeFilter tf   s scan  package you want1    package you want2           String   beans   bdr getBeanDefinitionNames      N B  The TypeFilter is important if you want the correct results  You can also use exclusion filters here instead   The Scanner can be found in the spring-context jar  the registry in spring-beans  the type filter is in spring-core

User · Answer

Spring can do this for you     BeanDefinitionRegistry bdr   new SimpleBeanDefinitionRegistry    ClassPathBeanDefinitionScanner s   new ClassPathBeanDefinitionScanner bdr    TypeFilter tf   new AssignableTypeFilter CLASS YOU WANT class   s addIncludeFilter tf   s scan  package you want1    package you want2           String   beans   bdr getBeanDefinitionNames      N B  The TypeFilter is important if you want the correct results  You can also use exclusion filters here instead   The Scanner can be found in the spring-context jar  the registry in spring-beans  the type filter is in spring-core

User · Answer

In full generality  this functionality is impossible  The Java ClassLoader mechanism guarantees only the ability to ask for a class with a specific name  including pacakge   and the ClassLoader can supply a class  or it can state that it does not know that class   Classes can be  and frequently are  loaded from remote servers  and they can even be constructed on the fly  it is not difficult at all to write a ClassLoader that returns a valid class that implements a given interface for any name you ask from it  a List of the classes that implement that interface would then be infinite in length   In practice  the most common case is an URLClassLoader that looks for classes in a list of filesystem directories and JAR files  So what you need is to get the URLClassLoader  then iterate through those directories and archives  and for each class file you find in them  request the corresponding Class object and look through the return of its getInterfaces   method

User · Answer

You could also use the Extensible Component Scanner  extcos  http   sf net projects extcos  and search all classes implementing an interface like so   Set lt Class lt   extends MyInterface gt  gt  classes   new HashSet lt Class lt   extends MyInterface gt  gt      ComponentScanner scanner   new ComponentScanner    scanner getClasses new ComponentQuery          Override     protected void query             select            from  my package1    my package2            andStore thoseImplementing MyInterface class  into classes            returning none                 This works for classes on the file system  within jars and even for those on the JBoss virtual file system  It s further designed to work within standalone applications as well as within any web or application container

User · Answer

In full generality  this functionality is impossible  The Java ClassLoader mechanism guarantees only the ability to ask for a class with a specific name  including pacakge   and the ClassLoader can supply a class  or it can state that it does not know that class   Classes can be  and frequently are  loaded from remote servers  and they can even be constructed on the fly  it is not difficult at all to write a ClassLoader that returns a valid class that implements a given interface for any name you ask from it  a List of the classes that implement that interface would then be infinite in length   In practice  the most common case is an URLClassLoader that looks for classes in a list of filesystem directories and JAR files  So what you need is to get the URLClassLoader  then iterate through those directories and archives  and for each class file you find in them  request the corresponding Class object and look through the return of its getInterfaces   method

User · Answer

In full generality  this functionality is impossible  The Java ClassLoader mechanism guarantees only the ability to ask for a class with a specific name  including pacakge   and the ClassLoader can supply a class  or it can state that it does not know that class   Classes can be  and frequently are  loaded from remote servers  and they can even be constructed on the fly  it is not difficult at all to write a ClassLoader that returns a valid class that implements a given interface for any name you ask from it  a List of the classes that implement that interface would then be infinite in length   In practice  the most common case is an URLClassLoader that looks for classes in a list of filesystem directories and JAR files  So what you need is to get the URLClassLoader  then iterate through those directories and archives  and for each class file you find in them  request the corresponding Class object and look through the return of its getInterfaces   method

User · Answer

The code you are talking about sounds like ServiceLoader  which was introduced in Java 6 to support a feature that has been defined since Java 1 3 or earlier  For performance reasons  this is the recommended approach to find interface implementations at runtime  if you need support for this in an older version of Java  I hope that you ll find my implementation helpful   There are a couple of implementations of this in earlier versions of Java  but in the Sun packages  not in the core API  I think there are some classes internal to ImageIO that do this   As the code is simple  I d recommend providing your own implementation rather than relying on non-standard Sun code which is subject to change

User · Answer

Obviously  Class isAssignableFrom   tells you whether an individual class implements the given interface  So then the problem is getting the list of classes to test   As far as I m aware  there s no direct way from Java to ask the class loader for  the list of classes that you could potentially load   So you ll have to do this yourself by iterating through the visible jars  calling Class forName   to load the class  then testing it   However  it s a little easier if you just want to know classes implementing the given interface from those that have actually been loaded    via the Java Instrumentation framework  you can call Instrumentation getAllLoadedClasses   via reflection  you can query the ClassLoader classes field of a given ClassLoader    If you use the instrumentation technique  then  as explained in the link  what happens is that your  agent  class is called essentially when the JVM starts up  and passed an Instrumentation object  At that point  you probably want to  save it for later  in a static field  and then have your main application code call it later on to get the list of loaded classes

User · Answer

Obviously  Class isAssignableFrom   tells you whether an individual class implements the given interface  So then the problem is getting the list of classes to test   As far as I m aware  there s no direct way from Java to ask the class loader for  the list of classes that you could potentially load   So you ll have to do this yourself by iterating through the visible jars  calling Class forName   to load the class  then testing it   However  it s a little easier if you just want to know classes implementing the given interface from those that have actually been loaded    via the Java Instrumentation framework  you can call Instrumentation getAllLoadedClasses   via reflection  you can query the ClassLoader classes field of a given ClassLoader    If you use the instrumentation technique  then  as explained in the link  what happens is that your  agent  class is called essentially when the JVM starts up  and passed an Instrumentation object  At that point  you probably want to  save it for later  in a static field  and then have your main application code call it later on to get the list of loaded classes

User · Answer

Obviously  Class isAssignableFrom   tells you whether an individual class implements the given interface  So then the problem is getting the list of classes to test   As far as I m aware  there s no direct way from Java to ask the class loader for  the list of classes that you could potentially load   So you ll have to do this yourself by iterating through the visible jars  calling Class forName   to load the class  then testing it   However  it s a little easier if you just want to know classes implementing the given interface from those that have actually been loaded    via the Java Instrumentation framework  you can call Instrumentation getAllLoadedClasses   via reflection  you can query the ClassLoader classes field of a given ClassLoader    If you use the instrumentation technique  then  as explained in the link  what happens is that your  agent  class is called essentially when the JVM starts up  and passed an Instrumentation object  At that point  you probably want to  save it for later  in a static field  and then have your main application code call it later on to get the list of loaded classes

User · Answer

In full generality  this functionality is impossible  The Java ClassLoader mechanism guarantees only the ability to ask for a class with a specific name  including pacakge   and the ClassLoader can supply a class  or it can state that it does not know that class   Classes can be  and frequently are  loaded from remote servers  and they can even be constructed on the fly  it is not difficult at all to write a ClassLoader that returns a valid class that implements a given interface for any name you ask from it  a List of the classes that implement that interface would then be infinite in length   In practice  the most common case is an URLClassLoader that looks for classes in a list of filesystem directories and JAR files  So what you need is to get the URLClassLoader  then iterate through those directories and archives  and for each class file you find in them  request the corresponding Class object and look through the return of its getInterfaces   method

User · Answer

The code you are talking about sounds like ServiceLoader  which was introduced in Java 6 to support a feature that has been defined since Java 1 3 or earlier  For performance reasons  this is the recommended approach to find interface implementations at runtime  if you need support for this in an older version of Java  I hope that you ll find my implementation helpful   There are a couple of implementations of this in earlier versions of Java  but in the Sun packages  not in the core API  I think there are some classes internal to ImageIO that do this   As the code is simple  I d recommend providing your own implementation rather than relying on non-standard Sun code which is subject to change

User · Answer

If you were asking from the perspective of working this out with a running program then you need to look to the java lang   package  If you get a Class object  you can use the isAssignableFrom method to check if it is an interface of another Class   There isn t a simple built in way of searching for these  tools like Eclipse build an index of this information   If you don t have a specific list of Class objects to test you can look to the ClassLoader object  use the getPackages   method and build your own package hierarchy iterator   Just a warning though that these methods and classes can be quite slow

User · Answer

If you were asking from the perspective of working this out with a running program then you need to look to the java lang   package  If you get a Class object  you can use the isAssignableFrom method to check if it is an interface of another Class   There isn t a simple built in way of searching for these  tools like Eclipse build an index of this information   If you don t have a specific list of Class objects to test you can look to the ClassLoader object  use the getPackages   method and build your own package hierarchy iterator   Just a warning though that these methods and classes can be quite slow

User · Answer

Obviously  Class isAssignableFrom   tells you whether an individual class implements the given interface  So then the problem is getting the list of classes to test   As far as I m aware  there s no direct way from Java to ask the class loader for  the list of classes that you could potentially load   So you ll have to do this yourself by iterating through the visible jars  calling Class forName   to load the class  then testing it   However  it s a little easier if you just want to know classes implementing the given interface from those that have actually been loaded    via the Java Instrumentation framework  you can call Instrumentation getAllLoadedClasses   via reflection  you can query the ClassLoader classes field of a given ClassLoader    If you use the instrumentation technique  then  as explained in the link  what happens is that your  agent  class is called essentially when the JVM starts up  and passed an Instrumentation object  At that point  you probably want to  save it for later  in a static field  and then have your main application code call it later on to get the list of loaded classes

User · Answer

Package Level Annotations  I know this question has already been answered a long time ago but another solution to this problem is to use Package Level Annotations   While its pretty hard to go find all the classes in the JVM its actually pretty easy to browse the package hierarchy    Package   ps   Package getPackages    for  Package p   ps      MyAno a   p getAnnotation MyAno class       Recursively descend     Then just make your annotation have an argument of an array of Class  Then in your package-info java for a particular package put the MyAno   I ll add more details  code  if people are interested but most probably get the idea   MetaInf Service Loader  To add to  erickson answer you can also use the service loader approach  Kohsuke has an awesome way of generating the the required META-INF stuff you need for the service loader approach   http   weblogs java net blog kohsuke archive 2009 03 my project of t html

User · Answer

Package Level Annotations  I know this question has already been answered a long time ago but another solution to this problem is to use Package Level Annotations   While its pretty hard to go find all the classes in the JVM its actually pretty easy to browse the package hierarchy    Package   ps   Package getPackages    for  Package p   ps      MyAno a   p getAnnotation MyAno class       Recursively descend     Then just make your annotation have an argument of an array of Class  Then in your package-info java for a particular package put the MyAno   I ll add more details  code  if people are interested but most probably get the idea   MetaInf Service Loader  To add to  erickson answer you can also use the service loader approach  Kohsuke has an awesome way of generating the the required META-INF stuff you need for the service loader approach   http   weblogs java net blog kohsuke archive 2009 03 my project of t html

User · Answer

The code you are talking about sounds like ServiceLoader  which was introduced in Java 6 to support a feature that has been defined since Java 1 3 or earlier  For performance reasons  this is the recommended approach to find interface implementations at runtime  if you need support for this in an older version of Java  I hope that you ll find my implementation helpful   There are a couple of implementations of this in earlier versions of Java  but in the Sun packages  not in the core API  I think there are some classes internal to ImageIO that do this   As the code is simple  I d recommend providing your own implementation rather than relying on non-standard Sun code which is subject to change

User · Answer

The code you are talking about sounds like ServiceLoader  which was introduced in Java 6 to support a feature that has been defined since Java 1 3 or earlier  For performance reasons  this is the recommended approach to find interface implementations at runtime  if you need support for this in an older version of Java  I hope that you ll find my implementation helpful   There are a couple of implementations of this in earlier versions of Java  but in the Sun packages  not in the core API  I think there are some classes internal to ImageIO that do this   As the code is simple  I d recommend providing your own implementation rather than relying on non-standard Sun code which is subject to change

[java] Find Java classes implementing an interface

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