Stop Manually Deploying JARs: Embrace Dynamic Hot Deployment

During recent development a requirement arose to let users provide their own implementation of a predefined interface, package it as a JAR, upload it to the system, and have the system hot‑deploy the new implementation without restarting.

Define a simple interface

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

Implementation class (annotation and reflection modes)

The implementation uses Spring annotation for calculate and plain reflection for add:

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

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

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

The CalculatorCore bean is used to verify that, in annotation mode, the system 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;
    }
}

Reflection‑based hot deployment

Users specify the JAR file path ( jarAddress) and its URL ( jarPath). The system loads the JAR with a URLClassLoader, obtains the implementation class by its fully‑qualified name, creates an instance via reflection, and invokes the desired method.

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 components, the system scans all classes in the JAR, detects those annotated with @Component, @Repository or @Service, and registers each as a bean in the existing Spring context.

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 (DeployUtils)

Read all class files from a 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 whether a class is a Spring bean :

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;
}

Transform class name to bean name (lower‑case first letter):

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

Removing a JAR

When a JAR is deleted or replaced, the beans that were registered from it must be removed from the Spring container using the same context:

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));
        }
    }
}

Test harness

A test class simulates the user uploading the JAR. It repeatedly attempts hot deployment, sleeping ten seconds after a failure to allow the JAR to be placed in the directory.

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 therefore provides a complete, step‑by‑step guide to dynamically loading, registering, invoking, and cleaning up user‑provided JAR implementations in a Java/Spring backend.