Hot Deployment of Java Interface Implementations Using Reflection and Spring

During development a system may require a user‑provided implementation of a given interface; the user packages the implementation into a JAR, uploads it, and the system hot‑deploys the new implementation without restarting.

Define a simple interface – the example uses a Calculator interface with int calculate(int a, int b) and int add(int a, int b) methods:

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

Implementation class – two approaches are shown: an annotation‑based method ( calculate) and a reflection‑based method ( add). The class injects CalculatorCore and implements both methods.

@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 supporting CalculatorCore simply returns the sum of two integers:

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

Reflection‑based hot deployment – the uploaded JAR path is defined, then the JAR is loaded with a URLClassLoader, the implementation class is instantiated via reflection, and its method is invoked:

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);
}

Annotation‑based hot deployment – if the uploaded JAR contains Spring components, the system scans all classes, identifies those with Spring annotations, and registers them into the current Spring container dynamically:

/** Register beans from uploaded JAR into Spring */
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 three methods:

/** Read all class files from a JAR */
public static Set<String> readJarFile(String jarAddress) throws IOException { ... }

/** Determine whether a class has a Spring stereotype annotation */
public static boolean isSpringBeanClass(Class<?> cla) { ... }

/** Convert the class name to a bean name (lower‑case first letter) */
public static String transformName(String className) { ... }

Removing a JAR – when a JAR is deleted or replaced, the previously registered Spring beans must be removed from the container using the same bean names:

/** Delete beans of an uploaded JAR from Spring */
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 loop continuously attempts to hot‑deploy the JAR; if the JAR is not yet present, an exception is caught and the thread sleeps, allowing the developer to place the JAR in the designated 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);
    }
}

Source: https://blog.csdn.net/zhangzhiqiang_0912