1. 双亲委派

// ClassLoader中的方法
protected Class<?> loadClass(String name, boolean resolve)throws ClassNotFoundException{synchronized (getClassLoadingLock(name)) {// First, check if the class has already been loaded// 首先，检查是否已经被加载过Class<?> c = findLoadedClass(name);// null，表示没有加载过if (c == null) {long t0 = System.nanoTime();try {if (parent != null) {// 递归调用父加载器的loadClassc = parent.loadClass(name, false);} else {// 如果没有了parentClassloader，说明到了BootstrapClassLoader这一层c = findBootstrapClassOrNull(name);}} catch (ClassNotFoundException e) {// ClassNotFoundException thrown if class not found// from the non-null parent class loader// class没有找到就会抛出ClassNotFoundException(从非空父加载器)}// 如果为空，调用每一层的findClass去找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// 每次都做一些记录PerfCounter.getParentDelegationTime().addTime(t1 - t0);PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);PerfCounter.getFindClasses().increment();}}if (resolve) {resolveClass(c);}return c;}}

2.打破双亲委派

/*可见想要打破双亲委派，就是不要调用此loadClass,可以自定一个实现,重写其逻辑。或者自定义classLoader，加载类，然后重写findClass(重写find机制-缓存等)返回自定义classLoader加载的类。
*/
public class MyClassLoader extends URLClassLoader {public MyClassLoader(URL[] urls, ClassLoader parent) {super(urls, parent);}@Overridepublic Class<?> loadClass(String name) throws ClassNotFoundException {if (name.startsWith("com.mysql")) {System.out.println("进来了，打破双亲委派啦");// 如果类名以指定的包名开头，自己加载return findClass(name);} else {// 否则委托给父类加载器加载return super.loadClass(name);}}@Overrideprotected Class<?> findClass(String name) throws ClassNotFoundException {File file = new File("D:\\Application\\MavenRepo\\com\\mysql\\mysql-connector-j\\8.0.33\\mysql-connector-j-8.0.33.jar");try (JarFile jarFile = new JarFile(file)) {JarEntry entry = jarFile.getJarEntry(name.replace(".", "/") + ".class");if (entry != null) {try (InputStream is = jarFile.getInputStream(entry)) {ByteArrayOutputStream baos = new ByteArrayOutputStream();int len;byte[] buffer = new byte[4096];while ((len = is.read(buffer)) != -1) {baos.write(buffer, 0, len);}byte[] data = baos.toByteArray();return defineClass(name, data, 0, data.length);}}} catch (IOException e) {throw new ClassNotFoundException("Error loading class " + name, e);}throw new ClassNotFoundException(name);}
}public class Test{public static void main(String[] args) throws Exception {File file = new File("D:\\Application\\MavenRepo\\com\\mysql\\mysql-connector-j\\8.0.33\\mysql-connector-j-8.0.33.jar");URL url = file.toURI().toURL();MyClassLoader loader = new MyClassLoader(new URL[]{url}, Thread.currentThread().getContextClassLoader());System.out.println("thread.currentThread.contextClassLoader: " + Thread.currentThread().getContextClassLoader());Class<?> driverClass = loader.loadClass("com.mysql.cj.jdbc.Driver");System.out.println("classLoader: " + driverClass.getClassLoader());Connection connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test","root", "root");System.out.println(connection);PreparedStatement pdst = connection.prepareStatement("select * from users;");ResultSet rs = pdst.executeQuery();while (rs.next()) {int userid = rs.getInt("userid");String username = rs.getString("username");String usersex = rs.getString("usersex");System.out.println("userid=" + userid + ", username=" + username + ",usersex=" + usersex);}rs.close();connection.close();}
}/*thread.currentThread.contextClassLoader: jdk.internal.loader.ClassLoaders$AppClassLoader@7b1d7fff进来了，打破双亲委派啦进来了，打破双亲委派啦classLoader: cn.gbase.admin.plugin.controller.MyClassLoader@31dc339bcom.mysql.cj.jdbc.ConnectionImpl@37e4d7bbuserid=1, username=张三, usersex=男userid=2, username=李四, usersex=男userid=8, username=袁婷, usersex=女
*/