How to Implement Hot Deployment of Custom Java Interfaces with Spring and Reflection

During recent development a requirement arose to allow users to upload a JAR that implements a given interface, and the system must hot‑deploy the new implementation.

Simple interface definition

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

Implementation example

The implementation can be written in two ways: using Spring’s annotation‑based injection (the calculate method) or using plain reflection (the add method).

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

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

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

The CalculatorCore class provides the actual addition logic:

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

Reflection‑based hot deployment

When a JAR is uploaded, its path is stored in jarAddress and jarPath. The class is loaded with a URLClassLoader, instantiated via reflection, and its methods are invoked.

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 components, the system scans all classes, registers those annotated with @Component, @Service, or @Repository into the current 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 all class names from a JAR, determine whether a class is a Spring bean, and convert class names to bean IDs.

public static Set<String> readJarFile(String jarAddress) throws IOException { … }
public static boolean isSpringBeanClass(Class<?> cla) { … }
public static String transformName(String className) { … }

Removing beans when a JAR is deleted

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

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));
        }
    }
}

A simple test loop repeatedly attempts hot deployment until the JAR appears, then calls the appropriate deployment method.

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);
    }
}