How does one go about and try to find all subclasses of a given class (or all implementors of a given interface) in Java? As of now, I have a method to do this, but I find it quite inefficient (to say the least). The method is:
In Eclipse, there is a nice feature called the Type Hierarchy that manages to show this quite efficiently. How does one go about and do it programmatically?
This is not possible to do using only the built-in Java Reflections API.
A project exists that does the necessary scanning and indexing of your classpath so you can get access this information...
A Java runtime metadata analysis, in the spirit of Scannotations
Reflections scans your classpath, indexes the metadata, allows you to query it on runtime and may save and collect that information for many modules within your project.Using Reflections you can query your metadata for:
- get all subtypes of some type
- get all types annotated with some annotation
- get all types annotated with some annotation, including annotation parameters matching
- get all methods annotated with some
(disclaimer: I have not used it, but the project's description seems to be an exact fit for your needs.)
It should be noted as well that this will of course only find all those subclasses that exist on your current classpath. Presumably this is OK for what you are currently looking at, and chances are you did consider this, but if you have at any point released a non-final class into the wild (for varying levels of "wild") then it is entirely feasible that someone else has written their own subclass that you will not know about.
Thus if you happened to be wanting to see all subclasses because you want to make a change and are going to see how it affects subclasses' behaviour - then bear in mind the subclasses that you can't see. Ideally all of your non-private methods, and the class itself should be well-documented; make changes according to this documentation without changing the semantics of methods/non-private fields and your changes should be backwards-compatible, for any subclass that followed your definition of the superclass at least.
If you intend to load all subclassess of given class which are in the same package, you can do so:
public static List<Class> loadAllSubClasses(Class pClazz) throws IOException, ClassNotFoundException {
ClassLoader classLoader = pClazz.getClassLoader();
assert classLoader != null;
String packageName = pClazz.getPackage().getName();
String dirPath = packageName.replace(".", "/");
Enumeration<URL> srcList = classLoader.getResources(dirPath);
List<Class> subClassList = new ArrayList<>();
while (srcList.hasMoreElements()) {
File dirFile = new File(srcList.nextElement().getFile());
File[] files = dirFile.listFiles();
if (files != null) {
for (File file : files) {
String subClassName = packageName + '.' + file.getName().substring(0, file.getName().length() - 6);
if (! subClassName.equals(pClazz.getName())) {
subClassList.add(Class.forName(subClassName));
}
}
}
}
return subClassList;
}
I did this several years ago. The most reliable way to do this (i.e. with official Java APIs and no external dependencies) is to write a custom doclet to produce a list that can be read at runtime.
You can run it from the command line like this:
javadoc -d build -doclet com.example.ObjectListDoclet -sourcepath java/src -subpackages com.example
or run it from ant like this:
<javadoc sourcepath="${src}" packagenames="*" >
<doclet name="com.example.ObjectListDoclet" path="${build}"/>
</javadoc>
Here's the basic code:
public final class ObjectListDoclet {
public static final String TOP_CLASS_NAME = "com.example.MyClass";
/** Doclet entry point. */
public static boolean start(RootDoc root) throws Exception {
try {
ClassDoc topClassDoc = root.classNamed(TOP_CLASS_NAME);
for (ClassDoc classDoc : root.classes()) {
if (classDoc.subclassOf(topClassDoc)) {
System.out.println(classDoc);
}
}
return true;
}
catch (Exception ex) {
ex.printStackTrace();
return false;
}
}
}
For simplicity, I've removed command line argument parsing and I'm writing to System.out rather than a file.
I know I'm a few years late to this party, but I came across this question trying to solve the same problem. You can use Eclipse's internal searching programatically, if you're writing an Eclipse Plugin (and thus take advantage of their caching, etc), to find classes which implement an interface. Here's my (very rough) first cut:
protected void listImplementingClasses( String iface ) throws CoreException
{
final IJavaProject project = <get your project here>;
try
{
final IType ifaceType = project.findType( iface );
final SearchPattern ifacePattern = SearchPattern.createPattern( ifaceType, IJavaSearchConstants.IMPLEMENTORS );
final IJavaSearchScope scope = SearchEngine.createWorkspaceScope();
final SearchEngine searchEngine = new SearchEngine();
final LinkedList<SearchMatch> results = new LinkedList<SearchMatch>();
searchEngine.search( ifacePattern,
new SearchParticipant[]{ SearchEngine.getDefaultSearchParticipant() }, scope, new SearchRequestor() {
@Override
public void acceptSearchMatch( SearchMatch match ) throws CoreException
{
results.add( match );
}
}, new IProgressMonitor() {
@Override
public void beginTask( String name, int totalWork )
{
}
@Override
public void done()
{
System.out.println( results );
}
@Override
public void internalWorked( double work )
{
}
@Override
public boolean isCanceled()
{
return false;
}
@Override
public void setCanceled( boolean value )
{
}
@Override
public void setTaskName( String name )
{
}
@Override
public void subTask( String name )
{
}
@Override
public void worked( int work )
{
}
});
} catch( JavaModelException e )
{
e.printStackTrace();
}
}
The first problem I see so far is that I'm only catching classes which directly implement the interface, not all their subclasses - but a little recursion never hurt anyone.
Depending on your particular requirements, in some cases Java's service loader mechanism might achieve what you're after.
In short, it allows developers to explicitly declare that a class subclasses some other class (or implements some interface) by listing it in a file in the JAR/WAR file's META-INF/services directory. It can then be discovered using the java.util.ServiceLoader class which, when given a Class object, will generate instances of all the declared subclasses of that class (or, if the Class represents an interface, all the classes implementing that interface).
The main advantage of this approach is that there is no need to manually scan the entire classpath for subclasses - all the discovery logic is contained within the ServiceLoader class, and it only loads the classes explicitly declared in the META-INF/services directory (not every class on the classpath).
There are, however, some disadvantages:
META-INF/services directory. This is an additional burden on the developer, and can be error-prone.ServiceLoader.iterator() generates subclass instances, not their Class objects. This causes two issues:
Apparently Java 9 will be addressing some of these shortcomings (in particular, the ones regarding instantiation of subclasses).
Suppose you're interested in finding classes that implement an interface com.example.Example:
package com.example;
public interface Example {
public String getStr();
}
The class com.example.ExampleImpl implements that interface:
package com.example;
public class ExampleImpl implements Example {
public String getStr() {
return "ExampleImpl's string.";
}
}
You would declare the class ExampleImpl is an implementation of Example by creating a file META-INF/services/com.example.Example containing the text com.example.ExampleImpl.
Then, you could obtain an instance of each implementation of Example (including an instance of ExampleImpl) as follows:
ServiceLoader<Example> loader = ServiceLoader.load(Example.class)
for (Example example : loader) {
System.out.println(example.getStr());
}
// Prints "ExampleImpl's string.", plus whatever is returned
// by other declared implementations of com.example.Example.
I just write a simple demo to use the org.reflections.Reflections to get subclasses of abstract class:
If you intend to load all subclassess of given class which are in the same package, you can do so:
public static List<Class> loadAllSubClasses(Class pClazz) throws IOException, ClassNotFoundException {
ClassLoader classLoader = pClazz.getClassLoader();
assert classLoader != null;
String packageName = pClazz.getPackage().getName();
String dirPath = packageName.replace(".", "/");
Enumeration<URL> srcList = classLoader.getResources(dirPath);
List<Class> subClassList = new ArrayList<>();
while (srcList.hasMoreElements()) {
File dirFile = new File(srcList.nextElement().getFile());
File[] files = dirFile.listFiles();
if (files != null) {
for (File file : files) {
String subClassName = packageName + '.' + file.getName().substring(0, file.getName().length() - 6);
if (! subClassName.equals(pClazz.getName())) {
subClassList.add(Class.forName(subClassName));
}
}
}
}
return subClassList;
}
Scanning for classes is not easy with pure Java.
The spring framework offers a class called ClassPathScanningCandidateComponentProvider that can do what you need. The following example would find all subclasses of MyClass in the package org.example.package
ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AssignableTypeFilter(MyClass.class));
// scan in org.example.package
Set<BeanDefinition> components = provider.findCandidateComponents("org/example/package");
for (BeanDefinition component : components)
{
Class cls = Class.forName(component.getBeanClassName());
// use class cls found
}
This method has the additional benefit of using a bytecode analyzer to find the candidates which means it will not load all classes it scans.
I needed to do this as a test case, to see if new classes had been added to the code. This is what I did
final static File rootFolder = new File(SuperClass.class.getProtectionDomain().getCodeSource().getLocation().getPath());
private static ArrayList<String> files = new ArrayList<String>();
listFilesForFolder(rootFolder);
@Test(timeout = 1000)
public void testNumberOfSubclasses(){
ArrayList<String> listSubclasses = new ArrayList<>(files);
listSubclasses.removeIf(s -> !s.contains("Superclass.class"));
for(String subclass : listSubclasses){
System.out.println(subclass);
}
assertTrue("You did not create a new subclass!", listSubclasses.size() >1);
}
public static void listFilesForFolder(final File folder) {
for (final File fileEntry : folder.listFiles()) {
if (fileEntry.isDirectory()) {
listFilesForFolder(fileEntry);
} else {
files.add(fileEntry.getName().toString());
}
}
}
The reason you see a difference between your implementation and Eclipse is because you scan each time, while Eclipse (and other tools) scan only once (during project load most of the times) and create an index. Next time you ask for the data it doesn't scan again, but look at the index.
I just write a simple demo to use the org.reflections.Reflections to get subclasses of abstract class:
The reason you see a difference between your implementation and Eclipse is because you scan each time, while Eclipse (and other tools) scan only once (during project load most of the times) and create an index. Next time you ask for the data it doesn't scan again, but look at the index.
Scanning for classes is not easy with pure Java.
The spring framework offers a class called ClassPathScanningCandidateComponentProvider that can do what you need. The following example would find all subclasses of MyClass in the package org.example.package
ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AssignableTypeFilter(MyClass.class));
// scan in org.example.package
Set<BeanDefinition> components = provider.findCandidateComponents("org/example/package");
for (BeanDefinition component : components)
{
Class cls = Class.forName(component.getBeanClassName());
// use class cls found
}
This method has the additional benefit of using a bytecode analyzer to find the candidates which means it will not load all classes it scans.
Add them to a static map inside (this.getClass().getName()) the parent classes constructor (or create a default one) but this will get updated in runtime. If lazy initialization is an option you can try this approach.
I'm using a reflection lib, which scans your classpath for all subclasses: https://github.com/ronmamo/reflections
This is how it would be done:
Reflections reflections = new Reflections("my.project");
Set<Class<? extends SomeType>> subTypes = reflections.getSubTypesOf(SomeType.class);
I know I'm a few years late to this party, but I came across this question trying to solve the same problem. You can use Eclipse's internal searching programatically, if you're writing an Eclipse Plugin (and thus take advantage of their caching, etc), to find classes which implement an interface. Here's my (very rough) first cut:
protected void listImplementingClasses( String iface ) throws CoreException
{
final IJavaProject project = <get your project here>;
try
{
final IType ifaceType = project.findType( iface );
final SearchPattern ifacePattern = SearchPattern.createPattern( ifaceType, IJavaSearchConstants.IMPLEMENTORS );
final IJavaSearchScope scope = SearchEngine.createWorkspaceScope();
final SearchEngine searchEngine = new SearchEngine();
final LinkedList<SearchMatch> results = new LinkedList<SearchMatch>();
searchEngine.search( ifacePattern,
new SearchParticipant[]{ SearchEngine.getDefaultSearchParticipant() }, scope, new SearchRequestor() {
@Override
public void acceptSearchMatch( SearchMatch match ) throws CoreException
{
results.add( match );
}
}, new IProgressMonitor() {
@Override
public void beginTask( String name, int totalWork )
{
}
@Override
public void done()
{
System.out.println( results );
}
@Override
public void internalWorked( double work )
{
}
@Override
public boolean isCanceled()
{
return false;
}
@Override
public void setCanceled( boolean value )
{
}
@Override
public void setTaskName( String name )
{
}
@Override
public void subTask( String name )
{
}
@Override
public void worked( int work )
{
}
});
} catch( JavaModelException e )
{
e.printStackTrace();
}
}
The first problem I see so far is that I'm only catching classes which directly implement the interface, not all their subclasses - but a little recursion never hurt anyone.
You can use org.reflections library and then, create an object of Reflections class. Using this object, you can get list of all subclasses of given class. https://www.javadoc.io/doc/org.reflections/reflections/0.9.10/org/reflections/Reflections.html
Reflections reflections = new Reflections("my.project.prefix");
System.out.println(reflections.getSubTypesOf(A.class)));
Scanning for classes is not easy with pure Java.
The spring framework offers a class called ClassPathScanningCandidateComponentProvider that can do what you need. The following example would find all subclasses of MyClass in the package org.example.package
ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AssignableTypeFilter(MyClass.class));
// scan in org.example.package
Set<BeanDefinition> components = provider.findCandidateComponents("org/example/package");
for (BeanDefinition component : components)
{
Class cls = Class.forName(component.getBeanClassName());
// use class cls found
}
This method has the additional benefit of using a bytecode analyzer to find the candidates which means it will not load all classes it scans.
I needed to do this as a test case, to see if new classes had been added to the code. This is what I did
final static File rootFolder = new File(SuperClass.class.getProtectionDomain().getCodeSource().getLocation().getPath());
private static ArrayList<String> files = new ArrayList<String>();
listFilesForFolder(rootFolder);
@Test(timeout = 1000)
public void testNumberOfSubclasses(){
ArrayList<String> listSubclasses = new ArrayList<>(files);
listSubclasses.removeIf(s -> !s.contains("Superclass.class"));
for(String subclass : listSubclasses){
System.out.println(subclass);
}
assertTrue("You did not create a new subclass!", listSubclasses.size() >1);
}
public static void listFilesForFolder(final File folder) {
for (final File fileEntry : folder.listFiles()) {
if (fileEntry.isDirectory()) {
listFilesForFolder(fileEntry);
} else {
files.add(fileEntry.getName().toString());
}
}
}
You can use org.reflections library and then, create an object of Reflections class. Using this object, you can get list of all subclasses of given class. https://www.javadoc.io/doc/org.reflections/reflections/0.9.10/org/reflections/Reflections.html
Reflections reflections = new Reflections("my.project.prefix");
System.out.println(reflections.getSubTypesOf(A.class)));
It should be noted as well that this will of course only find all those subclasses that exist on your current classpath. Presumably this is OK for what you are currently looking at, and chances are you did consider this, but if you have at any point released a non-final class into the wild (for varying levels of "wild") then it is entirely feasible that someone else has written their own subclass that you will not know about.
Thus if you happened to be wanting to see all subclasses because you want to make a change and are going to see how it affects subclasses' behaviour - then bear in mind the subclasses that you can't see. Ideally all of your non-private methods, and the class itself should be well-documented; make changes according to this documentation without changing the semantics of methods/non-private fields and your changes should be backwards-compatible, for any subclass that followed your definition of the superclass at least.
Don't forget that the generated Javadoc for a class will include a list of known subclasses (and for interfaces, known implementing classes).
Add them to a static map inside (this.getClass().getName()) the parent classes constructor (or create a default one) but this will get updated in runtime. If lazy initialization is an option you can try this approach.
Don't forget that the generated Javadoc for a class will include a list of known subclasses (and for interfaces, known implementing classes).
It should be noted as well that this will of course only find all those subclasses that exist on your current classpath. Presumably this is OK for what you are currently looking at, and chances are you did consider this, but if you have at any point released a non-final class into the wild (for varying levels of "wild") then it is entirely feasible that someone else has written their own subclass that you will not know about.
Thus if you happened to be wanting to see all subclasses because you want to make a change and are going to see how it affects subclasses' behaviour - then bear in mind the subclasses that you can't see. Ideally all of your non-private methods, and the class itself should be well-documented; make changes according to this documentation without changing the semantics of methods/non-private fields and your changes should be backwards-compatible, for any subclass that followed your definition of the superclass at least.
I'm using a reflection lib, which scans your classpath for all subclasses: https://github.com/ronmamo/reflections
This is how it would be done:
Reflections reflections = new Reflections("my.project");
Set<Class<? extends SomeType>> subTypes = reflections.getSubTypesOf(SomeType.class);
It should be noted as well that this will of course only find all those subclasses that exist on your current classpath. Presumably this is OK for what you are currently looking at, and chances are you did consider this, but if you have at any point released a non-final class into the wild (for varying levels of "wild") then it is entirely feasible that someone else has written their own subclass that you will not know about.
Thus if you happened to be wanting to see all subclasses because you want to make a change and are going to see how it affects subclasses' behaviour - then bear in mind the subclasses that you can't see. Ideally all of your non-private methods, and the class itself should be well-documented; make changes according to this documentation without changing the semantics of methods/non-private fields and your changes should be backwards-compatible, for any subclass that followed your definition of the superclass at least.
The reason you see a difference between your implementation and Eclipse is because you scan each time, while Eclipse (and other tools) scan only once (during project load most of the times) and create an index. Next time you ask for the data it doesn't scan again, but look at the index.
I did this several years ago. The most reliable way to do this (i.e. with official Java APIs and no external dependencies) is to write a custom doclet to produce a list that can be read at runtime.
You can run it from the command line like this:
javadoc -d build -doclet com.example.ObjectListDoclet -sourcepath java/src -subpackages com.example
or run it from ant like this:
<javadoc sourcepath="${src}" packagenames="*" >
<doclet name="com.example.ObjectListDoclet" path="${build}"/>
</javadoc>
Here's the basic code:
public final class ObjectListDoclet {
public static final String TOP_CLASS_NAME = "com.example.MyClass";
/** Doclet entry point. */
public static boolean start(RootDoc root) throws Exception {
try {
ClassDoc topClassDoc = root.classNamed(TOP_CLASS_NAME);
for (ClassDoc classDoc : root.classes()) {
if (classDoc.subclassOf(topClassDoc)) {
System.out.println(classDoc);
}
}
return true;
}
catch (Exception ex) {
ex.printStackTrace();
return false;
}
}
}
For simplicity, I've removed command line argument parsing and I'm writing to System.out rather than a file.
Depending on your particular requirements, in some cases Java's service loader mechanism might achieve what you're after.
In short, it allows developers to explicitly declare that a class subclasses some other class (or implements some interface) by listing it in a file in the JAR/WAR file's META-INF/services directory. It can then be discovered using the java.util.ServiceLoader class which, when given a Class object, will generate instances of all the declared subclasses of that class (or, if the Class represents an interface, all the classes implementing that interface).
The main advantage of this approach is that there is no need to manually scan the entire classpath for subclasses - all the discovery logic is contained within the ServiceLoader class, and it only loads the classes explicitly declared in the META-INF/services directory (not every class on the classpath).
There are, however, some disadvantages:
META-INF/services directory. This is an additional burden on the developer, and can be error-prone.ServiceLoader.iterator() generates subclass instances, not their Class objects. This causes two issues:
Apparently Java 9 will be addressing some of these shortcomings (in particular, the ones regarding instantiation of subclasses).
Suppose you're interested in finding classes that implement an interface com.example.Example:
package com.example;
public interface Example {
public String getStr();
}
The class com.example.ExampleImpl implements that interface:
package com.example;
public class ExampleImpl implements Example {
public String getStr() {
return "ExampleImpl's string.";
}
}
You would declare the class ExampleImpl is an implementation of Example by creating a file META-INF/services/com.example.Example containing the text com.example.ExampleImpl.
Then, you could obtain an instance of each implementation of Example (including an instance of ExampleImpl) as follows:
ServiceLoader<Example> loader = ServiceLoader.load(Example.class)
for (Example example : loader) {
System.out.println(example.getStr());
}
// Prints "ExampleImpl's string.", plus whatever is returned
// by other declared implementations of com.example.Example.
Don't forget that the generated Javadoc for a class will include a list of known subclasses (and for interfaces, known implementing classes).
The reason you see a difference between your implementation and Eclipse is because you scan each time, while Eclipse (and other tools) scan only once (during project load most of the times) and create an index. Next time you ask for the data it doesn't scan again, but look at the index.
Scanning for classes is not easy with pure Java.
The spring framework offers a class called ClassPathScanningCandidateComponentProvider that can do what you need. The following example would find all subclasses of MyClass in the package org.example.package
ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
provider.addIncludeFilter(new AssignableTypeFilter(MyClass.class));
// scan in org.example.package
Set<BeanDefinition> components = provider.findCandidateComponents("org/example/package");
for (BeanDefinition component : components)
{
Class cls = Class.forName(component.getBeanClassName());
// use class cls found
}
This method has the additional benefit of using a bytecode analyzer to find the candidates which means it will not load all classes it scans.
This is not possible to do using only the built-in Java Reflections API.
A project exists that does the necessary scanning and indexing of your classpath so you can get access this information...
A Java runtime metadata analysis, in the spirit of Scannotations
Reflections scans your classpath, indexes the metadata, allows you to query it on runtime and may save and collect that information for many modules within your project.Using Reflections you can query your metadata for:
- get all subtypes of some type
- get all types annotated with some annotation
- get all types annotated with some annotation, including annotation parameters matching
- get all methods annotated with some
(disclaimer: I have not used it, but the project's description seems to be an exact fit for your needs.)
Keeping in mind the limitations mentioned in the other answers, you can also use openpojo's PojoClassFactory (available on Maven) in the following manner:
for(PojoClass pojoClass : PojoClassFactory.enumerateClassesByExtendingType(packageRoot, Superclass.class, null)) {
System.out.println(pojoClass.getClazz());
}
Where packageRoot is the root String of the packages you wish to search in (e.g. "com.mycompany" or even just "com"), and Superclass is your supertype (this works on interfaces as well).
Try ClassGraph. (Disclaimer, I am the author). ClassGraph supports scanning for subclasses of a given class, either at runtime or at build time, but also much more. 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 selected packages, and you can query this 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.
This is not possible to do using only the built-in Java Reflections API.
A project exists that does the necessary scanning and indexing of your classpath so you can get access this information...
A Java runtime metadata analysis, in the spirit of Scannotations
Reflections scans your classpath, indexes the metadata, allows you to query it on runtime and may save and collect that information for many modules within your project.Using Reflections you can query your metadata for:
- get all subtypes of some type
- get all types annotated with some annotation
- get all types annotated with some annotation, including annotation parameters matching
- get all methods annotated with some
(disclaimer: I have not used it, but the project's description seems to be an exact fit for your needs.)
Don't forget that the generated Javadoc for a class will include a list of known subclasses (and for interfaces, known implementing classes).
Try ClassGraph. (Disclaimer, I am the author). ClassGraph supports scanning for subclasses of a given class, either at runtime or at build time, but also much more. 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 selected packages, and you can query this 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.
Source: Stackoverflow.com