Can you find all classes in a package using reflection?

Question

Is it possible to find all classes or interfaces in a given package? (Quickly looking at e.g. Package, it would seem like no.)

Answer 1

In general class loaders do not allow for scanning through all the classes on the classpath. But usually the only used class loader is UrlClassLoader from which we can retrieve the list of directories and jar files (see getURLs) and open them one by one to list available classes. This approach, called class path scanning, is implemented in Scannotation and Reflections.

Reflections reflections = new Reflections("my.package");
Set<Class<? extends Object>> classes = reflections.getSubTypesOf(Object.class);

Another approach is to use Java Pluggable Annotation Processing API to write annotation processor which will collect all annotated classes at compile time and build the index file for runtime use. This mechanism is implemented in ClassIndex library:

// package-info.java
@IndexSubclasses
package my.package;

// your code
Iterable<Class> classes = ClassIndex.getPackageClasses("my.package");

Notice that no additional setup is needed as the scanning is fully automated thanks to Java compiler automatically discovering any processors found on the classpath.

Answer 2

You need to look up every class loader entry in the class path:

    String pkg = "org/apache/commons/lang";
    ClassLoader cl = ClassLoader.getSystemClassLoader();
    URL[] urls = ((URLClassLoader) cl).getURLs();
    for (URL url : urls) {
        System.out.println(url.getFile());
        File jar = new File(url.getFile());
        // ....
    }   

If entry is directory, just look up in the right subdirectory:

if (jar.isDirectory()) {
    File subdir = new File(jar, pkg);
    if (!subdir.exists())
        continue;
    File[] files = subdir.listFiles();
    for (File file : files) {
        if (!file.isFile())
            continue;
        if (file.getName().endsWith(".class"))
            System.out.println("Found class: "
                    + file.getName().substring(0,
                            file.getName().length() - 6));
    }
}   

If the entry is the file, and it's jar, inspect the ZIP entries of it:

else {
    // try to open as ZIP
    try {
        ZipFile zip = new ZipFile(jar);
        for (Enumeration<? extends ZipEntry> entries = zip
                .entries(); entries.hasMoreElements();) {
            ZipEntry entry = entries.nextElement();
            String name = entry.getName();
            if (!name.startsWith(pkg))
                continue;
            name = name.substring(pkg.length() + 1);
            if (name.indexOf('/') < 0 && name.endsWith(".class"))
                System.out.println("Found class: "
                        + name.substring(0, name.length() - 6));
        }
    } catch (ZipException e) {
        System.out.println("Not a ZIP: " + e.getMessage());
    } catch (IOException e) {
        System.err.println(e.getMessage());
    }
}

Now once you have all class names withing package, you can try loading them with reflection and analyze if they are classes or interfaces, etc.

Answer 3

If you are merely looking to load a group of related classes, then Spring can help you.

Spring can instantiate a list or map of all classes that implement a given interface in one line of code. The list or map will contain instances of all the classes that implement that interface.

That being said, as an alternative to loading the list of classes out of the file system, instead just implement the same interface in all the classes you want to load, regardless of package and use Spring to provide you instances of all of them. That way, you can load (and instantiate) all the classes you desire regardless of what package they are in.

On the other hand, if having them all in a package is what you want, then simply have all the classes in that package implement a given interface.

Answer 4

plain java: FindAllClassesUsingPlainJavaReflectionTest.java

@Slf4j
class FindAllClassesUsingPlainJavaReflectionTest {

  private static final Function<Throwable, RuntimeException> asRuntimeException = throwable -> {
    log.error(throwable.getLocalizedMessage());
    return new RuntimeException(throwable);
  };

  private static final Function<String, Collection<Class<?>>> findAllPackageClasses = basePackageName -> {

    Locale locale = Locale.getDefault();
    Charset charset = StandardCharsets.UTF_8;
    val fileManager = ToolProvider.getSystemJavaCompiler()
                                  .getStandardFileManager(/* diagnosticListener */ null, locale, charset);

    StandardLocation location = StandardLocation.CLASS_PATH;
    JavaFileObject.Kind kind = JavaFileObject.Kind.CLASS;
    Set<JavaFileObject.Kind> kinds = Collections.singleton(kind);
    val javaFileObjects = Try.of(() -> fileManager.list(location, basePackageName, kinds, /* recurse */ true))
                             .getOrElseThrow(asRuntimeException);

    String pathToPackageAndClass = basePackageName.replace(".", File.separator);
    Function<String, String> mapToClassName = s -> {
      String prefix = Arrays.stream(s.split(pathToPackageAndClass))
                            .findFirst()
                            .orElse("");
      return s.replaceFirst(prefix, "")
              .replaceAll(File.separator, ".");
    };

    return StreamSupport.stream(javaFileObjects.spliterator(), /* parallel */ true)
                        .filter(javaFileObject -> javaFileObject.getKind().equals(kind))
                        .map(FileObject::getName)
                        .map(fileObjectName -> fileObjectName.replace(".class", ""))
                        .map(mapToClassName)
                        .map(className -> Try.of(() -> Class.forName(className))
                                             .getOrElseThrow(asRuntimeException))
                        .collect(Collectors.toList());
  };

  @Test
  @DisplayName("should get classes recursively in given package")
  void test() {
    Collection<Class<?>> classes = findAllPackageClasses.apply(getClass().getPackage().getName());
    assertThat(classes).hasSizeGreaterThan(4);
    classes.stream().map(String::valueOf).forEach(log::info);
  }
}

PS: to simplify boilerplates for handling errors, etc, I'm using here vavr and lombok libraries

other implementations could be found in my GitHub daggerok/java-reflection-find-annotated-classes-or-methods repo

Answer 5

Spring

This example is for Spring 4, but you can find the classpath scanner in earlier versions as well.

// create scanner and disable default filters (that is the 'false' argument)
final ClassPathScanningCandidateComponentProvider provider = new ClassPathScanningCandidateComponentProvider(false);
// add include filters which matches all the classes (or use your own)
provider.addIncludeFilter(new RegexPatternTypeFilter(Pattern.compile(".*")));

// get matching classes defined in the package
final Set<BeanDefinition> classes = provider.findCandidateComponents("my.package.name");

// this is how you can load the class type from BeanDefinition instance
for (BeanDefinition bean: classes) {
    Class<?> clazz = Class.forName(bean.getBeanClassName());
    // ... do your magic with the class ...
}

Google Guava

Note: In version 14, the API is still marked as @Beta, so beware in production code.

final ClassLoader loader = Thread.currentThread().getContextClassLoader();

for (final ClassPath.ClassInfo info : ClassPath.from(loader).getTopLevelClasses()) {
  if (info.getName().startsWith("my.package.")) {
    final Class<?> clazz = info.load();
    // do something with your clazz
  }
}

Answer 6

I put together a simple github project that solves this problem:

https://github.com/ddopson/java-class-enumerator

It should work for BOTH file-based classpaths AND for jar files.

If you run 'make' after checking out the project it will print this out:

 Cleaning...
rm -rf build/
 Building...
javac -d build/classes src/pro/ddopson/ClassEnumerator.java src/test/ClassIShouldFindOne.java src/test/ClassIShouldFindTwo.java src/test/subpkg/ClassIShouldFindThree.java src/test/TestClassEnumeration.java
 Making JAR Files...
jar cf build/ClassEnumerator_test.jar -C build/classes/ . 
jar cf build/ClassEnumerator.jar -C build/classes/ pro
 Running Filesystem Classpath Test...
java -classpath build/classes test.TestClassEnumeration
ClassDiscovery: Package: 'test' becomes Resource: 'file:/Users/Dopson/work/other/java-class-enumeration/build/classes/test'
ClassDiscovery: Reading Directory '/Users/Dopson/work/other/java-class-enumeration/build/classes/test'
ClassDiscovery: FileName 'ClassIShouldFindOne.class'  =>  class 'test.ClassIShouldFindOne'
ClassDiscovery: FileName 'ClassIShouldFindTwo.class'  =>  class 'test.ClassIShouldFindTwo'
ClassDiscovery: FileName 'subpkg'  =>  class 'null'
ClassDiscovery: Reading Directory '/Users/Dopson/work/other/java-class-enumeration/build/classes/test/subpkg'
ClassDiscovery: FileName 'ClassIShouldFindThree.class'  =>  class 'test.subpkg.ClassIShouldFindThree'
ClassDiscovery: FileName 'TestClassEnumeration.class'  =>  class 'test.TestClassEnumeration'
 Running JAR Classpath Test...
java -classpath build/ClassEnumerator_test.jar  test.TestClassEnumeration
ClassDiscovery: Package: 'test' becomes Resource: 'jar:file:/Users/Dopson/work/other/java-class-enumeration/build/ClassEnumerator_test.jar!/test'
ClassDiscovery: Reading JAR file: '/Users/Dopson/work/other/java-class-enumeration/build/ClassEnumerator_test.jar'
ClassDiscovery: JarEntry 'META-INF/'  =>  class 'null'
ClassDiscovery: JarEntry 'META-INF/MANIFEST.MF'  =>  class 'null'
ClassDiscovery: JarEntry 'pro/'  =>  class 'null'
ClassDiscovery: JarEntry 'pro/ddopson/'  =>  class 'null'
ClassDiscovery: JarEntry 'pro/ddopson/ClassEnumerator.class'  =>  class 'null'
ClassDiscovery: JarEntry 'test/'  =>  class 'null'
ClassDiscovery: JarEntry 'test/ClassIShouldFindOne.class'  =>  class 'test.ClassIShouldFindOne'
ClassDiscovery: JarEntry 'test/ClassIShouldFindTwo.class'  =>  class 'test.ClassIShouldFindTwo'
ClassDiscovery: JarEntry 'test/subpkg/'  =>  class 'null'
ClassDiscovery: JarEntry 'test/subpkg/ClassIShouldFindThree.class'  =>  class 'test.subpkg.ClassIShouldFindThree'
ClassDiscovery: JarEntry 'test/TestClassEnumeration.class'  =>  class 'test.TestClassEnumeration'
 Tests Passed. 

See also my other answer