java原生

public class AccessingAllClassesInPackage {
    public Set<Class> findAllClassesUsingClassLoader(String packageName) {
        InputStream stream = ClassLoader.getSystemClassLoader().getResourceAsStream(packageName.replaceAll("[.]", "/"));
        BufferedReader reader = new BufferedReader(new InputStreamReader(stream));
        return reader.lines()
          .filter(line -> line.endsWith(".class"))
          .map(line -> getClass(line, packageName))
          .collect(Collectors.toSet());
    }
    private Class getClass(String className, String packageName) {
        try {
            return Class.forName(packageName + "." + className.substring(0, className.lastIndexOf('.')));
        } catch (ClassNotFoundException e) {
            // handle the exception
        }
        return null;
    }
}

guava

ImmutableSet<ClassPath.ClassInfo> classes = ClassPath.from(ClassLoader.getSystemClassLoader()).getTopLevelClasses();
for (ClassPath.ClassInfo classInfo : classes) {
    Class<?> clss = classInfo.load();
}

Reflections

Reflections reflections = new Reflections(packageName, new SubTypesScanner(false));
Set<Class> classes = reflections.getSubTypesOf(Object.class)
    .stream()
    .collect(Collectors.toSet());

spring

import org.springframework.core.io.support.PathMatchingResourcePatternResolver;
import org.springframework.core.io.support.ResourcePatternResolver;
import org.springframework.core.type.ClassMetadata;
import org.springframework.core.type.classreading.CachingMetadataReaderFactory;
import org.springframework.core.type.classreading.MetadataReaderFactory;
import org.springframework.util.ClassUtils;

private static final ResourcePatternResolver RESOURCE_PATTERN_RESOLVER = new PathMatchingResourcePatternResolver();
private static final MetadataReaderFactory METADATA_READER_FACTORY = new CachingMetadataReaderFactory();

private Set<Class<?>> scanClasses(String packagePatterns, Class<?> assignableType) throws IOException {
    Set<Class<?>> classes = new HashSet<>();
    String[] packagePatternArray = tokenizeToStringArray(packagePatterns, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);
    for (String packagePattern : packagePatternArray) {
        Resource[] resources = RESOURCE_PATTERN_RESOLVER.getResources(ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX
            + ClassUtils.convertClassNameToResourcePath(packagePattern) + "/**/*.class");
        for (Resource resource : resources) {
            try {
                ClassMetadata classMetadata = METADATA_READER_FACTORY.getMetadataReader(resource).getClassMetadata();
                Class<?> clazz = Resources.classForName(classMetadata.getClassName());
                if (assignableType == null || assignableType.isAssignableFrom(clazz)) {
                    classes.add(clazz);
                }
            } catch (Throwable e) {
                LOGGER.warn(() -> "Cannot load the '" + resource + "'. Cause by " + e.toString());
            }
        }
    }
    return classes;
}

mybatis

import org.apache.ibatis.io.ResolverUtil;

ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
resolverUtil.find(new ResolverUtil.IsA(TypeHandler.class), packageName);
Set<Class<? extends Class<?>>> handlerSet = resolverUtil.getClasses();

ServiceLoader

ServiceLoader<IMyServiceLoader> serviceLoader = ServiceLoader.load(IMyServiceLoader.class);
for (IMyServiceLoader myServiceLoader : serviceLoader){
    System.out.println(myServiceLoader);
}