How to Hot‑Deploy Custom Java Interfaces with Spring Boot and Reflection

During recent system development a requirement arose to allow users to provide their own implementation of a given interface, package it as a JAR, upload it, and have the system hot‑deploy and switch to the new implementation.

1 Define Simple Interface

<code>public interface Calculator {
    int calculate(int a, int b);
    int add(int a, int b);
}</code>

2 Simple Implementation (Annotation and Reflection)

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

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

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

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

3 Reflection‑Based Hot Deployment

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

/** Hot load Calculator implementation via reflection */
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);
}</code>

4 Annotation‑Based Hot Deployment

<code>/** Register beans from JAR into 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());
        }
    }
}

/** Read all class files from JAR */
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;
}

/** Determine if a class has a Spring annotation */
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;
}

/** Convert class name to bean name (lowercase first letter) */
public static String transformName(String className) {
    String tmpstr = className.substring(className.lastIndexOf(".") + 1);
    return tmpstr.substring(0, 1).toLowerCase() + tmpstr.substring(1);
}

/** Delete beans when JAR is removed */
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));
        }
    }
}
</code>

5 Test

<code>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);
    }
}</code>