Java类加载策略之双亲委派机制全面分析讲解

protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException{
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }
                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);
                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

import java.lang.reflect.Method;
import java.nio.file.Files;
import java.nio.file.Paths;
public class CustomClassLoader extends ClassLoader {
    // 自定义类加载器必须提供一个加载类文件的位置
    private String classesPath;
    public CustomClassLoader(String classesPath, ClassLoader parent) {
        super(parent);
        this.classesPath = classesPath;
    }
    @Override
    protected Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
        //首先,检查已加载的类
        Class<?> loadedClass = findLoadedClass(name);
        if (loadedClass == null) {
            // 如果已加载类中没有该类, 尝试用自定义的方法加载
            try {
                loadedClass = findClassInPath(name);
            } catch (ClassNotFoundException e) {
                // 如果自定义加载方法找不到类,则委托给父类加载器
                loadedClass = super.loadClass(name, resolve);
            }
        }
        if (resolve) {
            resolveClass(loadedClass);
        }
        return loadedClass;
    }
    private Class<?> findClassInPath(String className) throws ClassNotFoundException {
        try {
            String filePath = className.replace('.', '/') + ".class";
            byte[] classBytes = Files.readAllBytes(Paths.get(classesPath, filePath));
            return defineClass(className, classBytes, 0, classBytes.length);
        } catch (Exception e) {
            throw new ClassNotFoundException("Class not found in classes path: " + className, e);
        }
    }
    public static void main(String[] args) throws Exception {
        String pathToClasses = "/path/to/your/classes";
        String className = "com.example.SampleClass";
        String methodName = "sampleMethod";
        // 创建自定义类加载器实例，将类的加载权交给它
        CustomClassLoader customClassLoader = new CustomClassLoader(pathToClasses, CustomClassLoader.class.getClassLoader());
        // 使用自定义类加载器加载类
        Class<?> customClass = customClassLoader.loadClass(className);
        // 创建类的实例并调用方法
        Object obj = customClass.newInstance();
        Method method = customClass.getDeclaredMethod(methodName);
        method.setAccessible(true);
        method.invoke(obj);
    }
}