Hot‑Deploying User‑Defined Interfaces in Spring Boot: Reflection vs Annotation

During system development a requirement arose to expose an interface that users can implement, package as a JAR, upload to the system, and have the system hot‑deploy the new implementation and switch to it at runtime.

1 Define a Simple Interface

We use a basic calculator example:

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

2 A Simple Implementation

Two implementation approaches are shown: one managed by Spring annotations and one using pure reflection.

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

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

    /** Reflection‑based */
    @Override
    public int add(int a, int b) {
        return new CalculatorCore().add(a, b);
    }
}

The CalculatorCore bean is injected to verify that the annotation mode can fully construct the bean dependency graph and register it in the current Spring container.

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

3 Reflection‑Based Hot Deployment

Users upload a JAR to a predefined directory. The JAR path and address are defined as:

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

The system loads the JAR with a thread‑context class loader, obtains the implementation class by its fully‑qualified name, creates an instance via reflection, and invokes the method:

/**
 * Hot‑load Calculator implementation (reflection mode)
 */
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);
}

4 Annotation‑Based Hot Deployment

If the uploaded JAR contains Spring‑managed beans, the system scans all classes, identifies those annotated with Spring stereotypes, and registers them dynamically in the current Spring container.

/**
 * Dynamically register beans from the uploaded JAR, including dependencies
 */
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());
        }
    }
}

Utility methods used in the process:

public static Set<String> readJarFile(String jarAddress) throws IOException {
    Set<String> classNameSet = new HashSet<>();
    JarFile jarFile = new JarFile(jarAddress);
    Enumeration<JarEntry> entries = jarFile.entries(); // iterate the JAR
    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) return false;
    if (cla.isInterface()) return false;
    if (Modifier.isAbstract(cla.getModifiers())) return false;
    if (cla.getAnnotation(Component.class) != null) return true;
    if (cla.getAnnotation(Repository.class) != null) return true;
    if (cla.getAnnotation(Service.class) != null) return true;
    return false;
}

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

When a JAR is removed, the previously registered beans must also be deregistered to keep the Spring context consistent.

/**
 * Remove beans registered from the JAR when it is deleted
 */
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));
        }
    }
}

5 Test

A test class simulates the user uploading a JAR. It repeatedly attempts hot deployment, catching exceptions and waiting 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);
    }
}