白话设计模式-模板方法模式

简单描述什么是模板方法模式：

　　封装不变的流程，公开一个或多个可变的具体逻辑埋点、引用、钩子等叫什么都可以

例如：

　　JDK中的ClassLoader类中就定义了类加载的流程，但存在多个ClassLoader的具体实现，通过重写findClass方法各自分工分别从不同的目录或网络地址加载class，而不需要在每个实现类里重新定义流程，或者直接调用父类方法

　　ClassLoader中的方法定义：

　　/**
     * Finds the class with the specified <a href="#name">binary name</a>.
     * This method should be overridden by class loader implementations that
     * follow the delegation model for loading classes, and will be invoked by
     * the {@link #loadClass <tt>loadClass</tt>} method after checking the
     * parent class loader for the requested class.  The default implementation
     * throws a <tt>ClassNotFoundException</tt>.
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     *
     * @since  1.2
     */
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }

　　在URLClassLoader中的具体实现

　　/**
     * Finds and loads the class with the specified name from the URL search
     * path. Any URLs referring to JAR files are loaded and opened as needed
     * until the class is found.
     *
     * @param name the name of the class
     * @return the resulting class
     * @exception ClassNotFoundException if the class could not be found,
     *            or if the loader is closed.
     * @exception NullPointerException if {@code name} is {@code null}.
     */
    protected Class<?> findClass(final String name)
        throws ClassNotFoundException
    {
        final Class<?> result;
        try {
            result = AccessController.doPrivileged(
                new PrivilegedExceptionAction<Class<?>>() {
                    public Class<?> run() throws ClassNotFoundException {
                        String path = name.replace('.', '/').concat(".class");
                        Resource res = ucp.getResource(path, false);
                        if (res != null) {
                            try {
                                return defineClass(name, res);
                            } catch (IOException e) {
                                throw new ClassNotFoundException(name, e);
                            }
                        } else {
                            return null;
                        }
                    }
                }, acc);
        } catch (java.security.PrivilegedActionException pae) {
            throw (ClassNotFoundException) pae.getException();
        }
        if (result == null) {
            throw new ClassNotFoundException(name);
        }
        return result;
    }

JUC下的各种锁实现也是类似。