Hot Deployment of Custom Java Interface Implementations Using Reflection and Spring

During development a requirement arose to allow users to upload a JAR that contains their own implementation of a predefined interface, and for the system to hot‑deploy and switch to that implementation without restarting.

Define a Simple Interface

The example uses a basic Calculator interface with two methods:

public interface Calculator {
    int calculate(int a, int b);
    int add(int a, int b);
}

Simple Implementation

Two implementation styles are considered: an annotation‑based approach managed by Spring and a reflection‑based approach that does not depend on Spring. The calculate method uses the annotation style, while add uses reflection.

@Service
public class CalculatorImpl implements Calculator {
    @Autowired
    CalculatorCore calculatorCore;

    /** annotation style */
    @Override
    public int calculate(int a, int b) {
        int c = calculatorCore.add(a, b);
        return c;
    }

    /** reflection style */
    @Override
    public int add(int a, int b) {
        return new CalculatorCore().add(a, b);
    }
}

The CalculatorCore bean provides the actual addition logic:

@Service
public class CalculatorCore {
    public int add(int a, int b) {
        return a + b;
    }
}

Reflection‑Based Hot Deployment

Users place their JAR at a known directory. The system loads the JAR via a URLClassLoader, obtains the implementation class by its fully‑qualified name, creates an instance via reflection, and invokes the methods.

private static String jarAddress = "E:/zzq/IDEA_WS/CalculatorTest/lib/Calculator.jar";
private static String jarPath = "file:/" + jarAddress;

public static void hotDeployWithReflect() throws Exception {
    URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{new URL(jarPath)}, Thread.currentThread().getContextClassLoader());
    Class clazz = urlClassLoader.loadClass("com.nci.cetc15.calculator.impl.CalculatorImpl");
    Calculator calculator = (Calculator) clazz.newInstance();
    int result = calculator.add(1, 2);
    System.out.println(result);
}

Annotation‑Based Hot Deployment

If the uploaded JAR contains Spring‑managed beans, the system scans all classes in the JAR, identifies those annotated with @Component, @Service, or @Repository, and registers them dynamically into the existing Spring container.

public static void hotDeployWithSpring() throws Exception {
    Set<String> classNameSet = DeployUtils.readJarFile(jarAddress);
    URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{new URL(jarPath)}, Thread.currentThread().getContextClassLoader());
    for (String className : classNameSet) {
        Class clazz = urlClassLoader.loadClass(className);
        if (DeployUtils.isSpringBeanClass(clazz)) {
            BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(clazz);
            defaultListableBeanFactory.registerBeanDefinition(DeployUtils.transformName(className), beanDefinitionBuilder.getBeanDefinition());
        }
    }
}

The utility class DeployUtils provides methods to read JAR entries, determine if a class is a Spring bean, and transform class names to bean IDs.

public static Set<String> readJarFile(String jarAddress) throws IOException {
    Set<String> classNameSet = new HashSet<>();
    JarFile jarFile = new JarFile(jarAddress);
    Enumeration<JarEntry> entries = jarFile.entries();
    while (entries.hasMoreElements()) {
        JarEntry jarEntry = entries.nextElement();
        String name = jarEntry.getName();
        if (name.endsWith(".class")) {
            String className = name.replace(".class", "").replaceAll("/", ".");
            classNameSet.add(className);
        }
    }
    return classNameSet;
}

public static boolean isSpringBeanClass(Class<?> cla) {
    if (cla == null || cla.isInterface() || Modifier.isAbstract(cla.getModifiers())) return false;
    return cla.getAnnotation(Component.class) != null ||
           cla.getAnnotation(Repository.class) != null ||
           cla.getAnnotation(Service.class) != null;
}

public static String transformName(String className) {
    String tmp = className.substring(className.lastIndexOf('.') + 1);
    return tmp.substring(0, 1).toLowerCase() + tmp.substring(1);
}

Removing Beans When Deleting a JAR

When a JAR is removed or replaced, the previously registered beans must be deregistered from the Spring container using the same bean IDs.

public static void delete() throws Exception {
    Set<String> classNameSet = DeployUtils.readJarFile(jarAddress);
    URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{new URL(jarPath)}, Thread.currentThread().getContextClassLoader());
    for (String className : classNameSet) {
        Class clazz = urlClassLoader.loadClass(className);
        if (DeployUtils.isSpringBeanClass(clazz)) {
            defaultListableBeanFactory.removeBeanDefinition(DeployUtils.transformName(className));
        }
    }
}

Testing

A test class simulates the upload process by repeatedly attempting hot deployment; if the JAR is not yet present, an exception is caught and the thread sleeps before retrying.

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
DefaultListableBeanFactory defaultListableBeanFactory = (DefaultListableBeanFactory) applicationContext.getAutowireCapableBeanFactory();
while (true) {
    try {
        hotDeployWithReflect();
        // hotDeployWithSpring();
        // delete();
    } catch (Exception e) {
        e.printStackTrace();
        Thread.sleep(1000 * 10);
    }
}

The article concludes with source attribution and promotional links.