Scanning Java annotations at runtime

Question

What is the best way of searching the whole classpath for an annotated class   I m doing a library and I want to allow the users to annotate their classes  so when the Web application starts I need to scan the whole classpath for certain annotation   Do you know a library or a Java facility to do this   Edit  I m thinking about something like the new functionality for Java EE 5 Web Services or EJB s  You annotate your class with  WebService or  EJB and the system finds these classes while loading so they are accessible remotely

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The Classloader API doesn t have an  enumerate  method  because class loading is an  on-demand  activity -- you usually have thousands of classes in your classpath  only a fraction of which will ever be needed  the rt jar alone is 48MB nowadays     So  even if you could enumerate all classes  this would be very time- and memory-consuming    The simple approach is to list the concerned classes in a setup file  xml or whatever suits your fancy   if you want to do this automatically  restrict yourself to one JAR or one class directory

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Google Reflections seems to be much faster than Spring  Found this feature request that adresses this difference  http   www opensaga org jira browse OS-738  This is a reason to use Reflections as startup time of my application is really important during development  Reflections seems also to be very easy to use for my use case  find all implementers of an interface

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Google Reflections seems to be much faster than Spring  Found this feature request that adresses this difference  http   www opensaga org jira browse OS-738  This is a reason to use Reflections as startup time of my application is really important during development  Reflections seems also to be very easy to use for my use case  find all implementers of an interface

User · Answer

If you re looking for an alternative to reflections I d like to recommend Panda Utilities - AnnotationsScanner  It s a Guava-free  Guava has  3MB  Panda Utilities has  200kb  scanner based on the reflections library source code    It s also dedicated for future-based searches  If you d like to scan multiple times included sources or even provide an API  which allows someone scanning current classpath  AnnotationsScannerProcess caches all fetched ClassFiles  so it s really fast   Simple example of AnnotationsScanner usage   AnnotationsScanner scanner   AnnotationsScanner createScanner            includeSources ExampleApplication class           build     AnnotationsScannerProcess process   scanner createWorker            addDefaultProjectFilters  net dzikoysk            fetch     Set lt Class lt   gt  gt  classes   process createSelector            selectTypesAnnotatedWith AnnotationTest class

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Is it too late to answer   I would say  its better to go by Libraries like ClassPathScanningCandidateComponentProvider or like Scannotations  But even after somebody wants to try some hands on it with classLoader  I have written some on my own to print the annotations from classes in a package    public class ElementScanner    public void scanElements        try         Get the package name from configuration file     String packageName   readConfig           Load the classLoader which loads this class      ClassLoader classLoader   getClass   getClassLoader           Change the package structure to directory structure     String packagePath    packageName replace                URL urls   classLoader getResource packagePath          Get all the class files in the specified URL Path      File folder   new File urls getPath         File   classes   folder listFiles         int size   classes length      List lt Class lt   gt  gt  classList   new ArrayList lt Class lt   gt  gt          for int i 0 i lt size i             int index   classes i  getName   indexOf               String className   classes i  getName   substring 0  index           String classNamePath   packageName     className          Class lt   gt  repoClass          repoClass   Class forName classNamePath           Annotation   annotations   repoClass getAnnotations            for int j  0 j lt annotations length j                 System out println  Annotation in class   repoClass getName      is   annotations j  annotationType   getName                       classList add repoClass               catch  ClassNotFoundException e            e printStackTrace                    Unmarshall the configuration file     return     public String readConfig        try          URL url   getClass   getClassLoader   getResource  WEB-INF config xml            JAXBContext jContext   JAXBContext newInstance RepositoryConfig class            Unmarshaller um    jContext createUnmarshaller             RepositoryConfig rc    RepositoryConfig  um unmarshal new File url getFile               return rc getRepository   getPackageName             catch Exception e               e printStackTrace                  return null         And in config File  you put the package name and unmarshall it to a class

User · Answer

Spring has something called a AnnotatedTypeScanner class   This class internally uses   ClassPathScanningCandidateComponentProvider   This class has the code for actual scanning of the classpath resources  It does this by using the class metadata available at runtime   One can simply extend this class or use the same class for scanning  Below is the constructor definition              Creates a new   link AnnotatedTypeScanner  for the given annotation types                  param considerInterfaces whether to consider interfaces as well          param annotationTypes the annotations to scan for              public AnnotatedTypeScanner boolean considerInterfaces  Class lt   extends Annotation gt     annotationTypes             this annotationTypess   Arrays asList annotationTypes           this considerInterfaces   considerInterfaces

User · Answer

The Classloader API doesn t have an  enumerate  method  because class loading is an  on-demand  activity -- you usually have thousands of classes in your classpath  only a fraction of which will ever be needed  the rt jar alone is 48MB nowadays     So  even if you could enumerate all classes  this would be very time- and memory-consuming    The simple approach is to list the concerned classes in a setup file  xml or whatever suits your fancy   if you want to do this automatically  restrict yourself to one JAR or one class directory

User · Answer

You can find classes with any given annotation with ClassGraph  as well as searching for other criteria of interest  e g  classes that implement a given interface   Disclaimer  I am the author of ClassGraph   ClassGraph can build an abstract representation of the entire class graph  all classes  annotations  methods  method parameters  and fields  in memory  for all classes on the classpath  or for classes in whitelisted packages  and you can query that class graph however you want  ClassGraph supports more classpath specification mechanisms and classloaders than any other scanner  and also works seamlessly with the new JPMS module system  so if you base your code on ClassGraph  your code will be maximally portable  See the API here

User · Answer

There s a wonderful comment by zapp that sinks in all those answers   new Reflections  my package   getTypesAnnotatedWith MyAnnotation class

User · Answer

If you re looking for an alternative to reflections I d like to recommend Panda Utilities - AnnotationsScanner  It s a Guava-free  Guava has  3MB  Panda Utilities has  200kb  scanner based on the reflections library source code    It s also dedicated for future-based searches  If you d like to scan multiple times included sources or even provide an API  which allows someone scanning current classpath  AnnotationsScannerProcess caches all fetched ClassFiles  so it s really fast   Simple example of AnnotationsScanner usage   AnnotationsScanner scanner   AnnotationsScanner createScanner            includeSources ExampleApplication class           build     AnnotationsScannerProcess process   scanner createWorker            addDefaultProjectFilters  net dzikoysk            fetch     Set lt Class lt   gt  gt  classes   process createSelector            selectTypesAnnotatedWith AnnotationTest class

User · Answer

Use org springframework context annotation ClassPathScanningCandidateComponentProvider   API     A component provider that scans the classpath from a base package  It then applies exclude and include filters to the resulting classes to find candidates     ClassPathScanningCandidateComponentProvider scanner   new ClassPathScanningCandidateComponentProvider  lt DO YOU WANT TO USE DEFALT FILTER gt     scanner addIncludeFilter new AnnotationTypeFilter  lt TYPE YOUR ANNOTATION HERE gt  class     for  BeanDefinition bd   scanner findCandidateComponents  lt TYPE YOUR BASE PACKAGE HERE gt        System out println bd getBeanClassName

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You can use Java Pluggable Annotation Processing API to write annotation processor which will be executed during the compilation process and will collect all annotated classes and build the index file for runtime use   This is the fastest way possible to do annotated class discovery because you don t need to scan your classpath at runtime  which is usually very slow operation  Also this approach works with any classloader and not only with URLClassLoaders usually supported by runtime scanners   The above mechanism is already implemented in ClassIndex library   To use it annotate your custom annotation with  IndexAnnotated meta-annotation  This will create at compile time an index file  META-INF annotations com test YourCustomAnnotation listing all annotated classes  You can acccess the index at runtime by executing   ClassIndex getAnnotated com test YourCustomAnnotation class

User · Answer

If you want a really light weight  no dependencies  simple API  15 kb jar file  and very fast solution  take a look at annotation-detector found at https   github com rmuller infomas-asl   Disclaimer  I am the author

User · Answer

And another solution is Google reflections   Quick review     Spring solution is the way to go if you re using Spring  Otherwise it s a big dependency  Using ASM directly is a bit cumbersome  Using Java Assist directly is clunky too  Annovention is super lightweight and convenient  No maven integration yet  Google reflections pulls in Google collections  Indexes everything and then is super fast

User · Answer

Use org springframework context annotation ClassPathScanningCandidateComponentProvider   API     A component provider that scans the classpath from a base package  It then applies exclude and include filters to the resulting classes to find candidates     ClassPathScanningCandidateComponentProvider scanner   new ClassPathScanningCandidateComponentProvider  lt DO YOU WANT TO USE DEFALT FILTER gt     scanner addIncludeFilter new AnnotationTypeFilter  lt TYPE YOUR ANNOTATION HERE gt  class     for  BeanDefinition bd   scanner findCandidateComponents  lt TYPE YOUR BASE PACKAGE HERE gt        System out println bd getBeanClassName

User · Answer

The Classloader API doesn t have an  enumerate  method  because class loading is an  on-demand  activity -- you usually have thousands of classes in your classpath  only a fraction of which will ever be needed  the rt jar alone is 48MB nowadays     So  even if you could enumerate all classes  this would be very time- and memory-consuming    The simple approach is to list the concerned classes in a setup file  xml or whatever suits your fancy   if you want to do this automatically  restrict yourself to one JAR or one class directory

User · Answer

With Spring you can also just write the following using AnnotationUtils class  i e    Class lt   gt  clazz   AnnotationUtils findAnnotationDeclaringClass Target class  null     For more details and all different methods check official docs   https   docs spring io spring docs current javadoc-api org springframework core annotation AnnotationUtils html

User · Answer

You can use Java Pluggable Annotation Processing API to write annotation processor which will be executed during the compilation process and will collect all annotated classes and build the index file for runtime use   This is the fastest way possible to do annotated class discovery because you don t need to scan your classpath at runtime  which is usually very slow operation  Also this approach works with any classloader and not only with URLClassLoaders usually supported by runtime scanners   The above mechanism is already implemented in ClassIndex library   To use it annotate your custom annotation with  IndexAnnotated meta-annotation  This will create at compile time an index file  META-INF annotations com test YourCustomAnnotation listing all annotated classes  You can acccess the index at runtime by executing   ClassIndex getAnnotated com test YourCustomAnnotation class

User · Answer

With Spring you can also just write the following using AnnotationUtils class  i e    Class lt   gt  clazz   AnnotationUtils findAnnotationDeclaringClass Target class  null     For more details and all different methods check official docs   https   docs spring io spring docs current javadoc-api org springframework core annotation AnnotationUtils html

User · Answer

If you want a really light weight  no dependencies  simple API  15 kb jar file  and very fast solution  take a look at annotation-detector found at https   github com rmuller infomas-asl   Disclaimer  I am the author

User · Answer

You can find classes with any given annotation with ClassGraph  as well as searching for other criteria of interest  e g  classes that implement a given interface   Disclaimer  I am the author of ClassGraph   ClassGraph can build an abstract representation of the entire class graph  all classes  annotations  methods  method parameters  and fields  in memory  for all classes on the classpath  or for classes in whitelisted packages  and you can query that class graph however you want  ClassGraph supports more classpath specification mechanisms and classloaders than any other scanner  and also works seamlessly with the new JPMS module system  so if you base your code on ClassGraph  your code will be maximally portable  See the API here

User · Answer

Google Reflection if you want to discover interfaces as well    Spring ClassPathScanningCandidateComponentProvider is not discovering interfaces

User · Answer

The Classloader API doesn t have an  enumerate  method  because class loading is an  on-demand  activity -- you usually have thousands of classes in your classpath  only a fraction of which will ever be needed  the rt jar alone is 48MB nowadays     So  even if you could enumerate all classes  this would be very time- and memory-consuming    The simple approach is to list the concerned classes in a setup file  xml or whatever suits your fancy   if you want to do this automatically  restrict yourself to one JAR or one class directory

User · Answer

Google Reflection if you want to discover interfaces as well    Spring ClassPathScanningCandidateComponentProvider is not discovering interfaces

User · Answer

There s a wonderful comment by zapp that sinks in all those answers   new Reflections  my package   getTypesAnnotatedWith MyAnnotation class

User · Answer

Spring has something called a AnnotatedTypeScanner class   This class internally uses   ClassPathScanningCandidateComponentProvider   This class has the code for actual scanning of the classpath resources  It does this by using the class metadata available at runtime   One can simply extend this class or use the same class for scanning  Below is the constructor definition              Creates a new   link AnnotatedTypeScanner  for the given annotation types                  param considerInterfaces whether to consider interfaces as well          param annotationTypes the annotations to scan for              public AnnotatedTypeScanner boolean considerInterfaces  Class lt   extends Annotation gt     annotationTypes             this annotationTypess   Arrays asList annotationTypes           this considerInterfaces   considerInterfaces

User · Answer

And another solution is Google reflections   Quick review     Spring solution is the way to go if you re using Spring  Otherwise it s a big dependency  Using ASM directly is a bit cumbersome  Using Java Assist directly is clunky too  Annovention is super lightweight and convenient  No maven integration yet  Google reflections pulls in Google collections  Indexes everything and then is super fast

User · Answer

Is it too late to answer   I would say  its better to go by Libraries like ClassPathScanningCandidateComponentProvider or like Scannotations  But even after somebody wants to try some hands on it with classLoader  I have written some on my own to print the annotations from classes in a package    public class ElementScanner    public void scanElements        try         Get the package name from configuration file     String packageName   readConfig           Load the classLoader which loads this class      ClassLoader classLoader   getClass   getClassLoader           Change the package structure to directory structure     String packagePath    packageName replace                URL urls   classLoader getResource packagePath          Get all the class files in the specified URL Path      File folder   new File urls getPath         File   classes   folder listFiles         int size   classes length      List lt Class lt   gt  gt  classList   new ArrayList lt Class lt   gt  gt          for int i 0 i lt size i             int index   classes i  getName   indexOf               String className   classes i  getName   substring 0  index           String classNamePath   packageName     className          Class lt   gt  repoClass          repoClass   Class forName classNamePath           Annotation   annotations   repoClass getAnnotations            for int j  0 j lt annotations length j                 System out println  Annotation in class   repoClass getName      is   annotations j  annotationType   getName                       classList add repoClass               catch  ClassNotFoundException e            e printStackTrace                    Unmarshall the configuration file     return     public String readConfig        try          URL url   getClass   getClassLoader   getResource  WEB-INF config xml            JAXBContext jContext   JAXBContext newInstance RepositoryConfig class            Unmarshaller um    jContext createUnmarshaller             RepositoryConfig rc    RepositoryConfig  um unmarshal new File url getFile               return rc getRepository   getPackageName             catch Exception e               e printStackTrace                  return null         And in config File  you put the package name and unmarshall it to a class

[java] Scanning Java annotations at runtime

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