接口中可以定义内部类举例

接口中可以定义内部类比如盐的计算：

添加用户场景:

    //添加一个用户，需要userid、username、userpass三个参数
    @RequestMapping("add_user")
    public String add_user(
    @RequestParam("userid") String userid,
    @RequestParam("username") String username,
    @RequestParam("userpass") String userpass){
        
        String hashAlgorithmName = "MD5";//选用MD5加密方式
        Object credentials = userpass;//加密对象是密码
        Object salt = ByteSource.Util.bytes(userid);//获得盐的计算结果
        int hashIterations = 5000;//加密次数
        
        //开始对密码加密
        Object md5Pass = new SimpleHash(hashAlgorithmName, credentials, salt, hashIterations);
        ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
        User adduser = new User(userid,userpass = md5Pass.toString(),username);
        int addresult = userService.add_user(adduser);
        return "redirect:/list.jsp";

Object salt = ByteSource.Util.bytes(userid);//获得盐的计算结果

Bytesource是接口，Util是内部类，bytes是内部类方法。

源码如下：

package org.apache.shiro.util;

import java.io.File;
import java.io.InputStream;

/**
 * A {@code ByteSource} wraps a byte array and provides additional encoding operations.  Most users will find the
 * {@link Util} inner class sufficient to construct ByteSource instances.
 *
 * @since 1.0
 */
public interface ByteSource {

    /**
     * Returns the wrapped byte array.
     *
     * @return the wrapped byte array.
     */
    byte[] getBytes();

    /**
     * Returns the <a href="http://en.wikipedia.org/wiki/Hexadecimal">Hex</a>-formatted String representation of the
     * underlying wrapped byte array.
     *
     * @return the <a href="http://en.wikipedia.org/wiki/Hexadecimal">Hex</a>-formatted String representation of the
     *         underlying wrapped byte array.
     */
    String toHex();

    /**
     * Returns the <a href="http://en.wikipedia.org/wiki/Base64">Base 64</a>-formatted String representation of the
     * underlying wrapped byte array.
     *
     * @return the <a href="http://en.wikipedia.org/wiki/Base64">Base 64</a>-formatted String representation of the
     *         underlying wrapped byte array.
     */
    String toBase64();

    /**
     * Returns {@code true} if the underlying wrapped byte array is null or empty (zero length), {@code false}
     * otherwise.
     *
     * @return {@code true} if the underlying wrapped byte array is null or empty (zero length), {@code false}
     *         otherwise.
     * @since 1.2
     */
    boolean isEmpty();

    /**
     * Utility class that can construct ByteSource instances.  This is slightly nicer than needing to know the
     * {@code ByteSource} implementation class to use.
     *
     * @since 1.2
     */
    public static final class Util {

        /**
         * Returns a new {@code ByteSource} instance representing the specified byte array.
         *
         * @param bytes the bytes to represent as a {@code ByteSource} instance.
         * @return a new {@code ByteSource} instance representing the specified byte array.
         */
        public static ByteSource bytes(byte[] bytes) {
            return new SimpleByteSource(bytes);
        }

        /**
         * Returns a new {@code ByteSource} instance representing the specified character array's bytes.  The byte
         * array is obtained assuming {@code UTF-8} encoding.
         *
         * @param chars the character array to represent as a {@code ByteSource} instance.
         * @return a new {@code ByteSource} instance representing the specified character array's bytes.
         */
        public static ByteSource bytes(char[] chars) {
            return new SimpleByteSource(chars);
        }

        /**
         * Returns a new {@code ByteSource} instance representing the specified string's bytes.  The byte
         * array is obtained assuming {@code UTF-8} encoding.
         *
         * @param string the string to represent as a {@code ByteSource} instance.
         * @return a new {@code ByteSource} instance representing the specified string's bytes.
         */
        public static ByteSource bytes(String string) {
            return new SimpleByteSource(string);
        }

        /**
         * Returns a new {@code ByteSource} instance representing the specified ByteSource.
         *
         * @param source the ByteSource to represent as a new {@code ByteSource} instance.
         * @return a new {@code ByteSource} instance representing the specified ByteSource.
         */
        public static ByteSource bytes(ByteSource source) {
            return new SimpleByteSource(source);
        }

        /**
         * Returns a new {@code ByteSource} instance representing the specified File's bytes.
         *
         * @param file the file to represent as a {@code ByteSource} instance.
         * @return a new {@code ByteSource} instance representing the specified File's bytes.
         */
        public static ByteSource bytes(File file) {
            return new SimpleByteSource(file);
        }

        /**
         * Returns a new {@code ByteSource} instance representing the specified InputStream's bytes.
         *
         * @param stream the InputStream to represent as a {@code ByteSource} instance.
         * @return a new {@code ByteSource} instance representing the specified InputStream's bytes.
         */
        public static ByteSource bytes(InputStream stream) {
            return new SimpleByteSource(stream);
        }

        /**
         * Returns {@code true} if the specified object can be easily represented as a {@code ByteSource} using
         * the {@link ByteSource.Util}'s default heuristics, {@code false} otherwise.
         * <p/>
         * This implementation merely returns {@link SimpleByteSource}.{@link SimpleByteSource#isCompatible(Object) isCompatible(source)}.
         *
         * @param source the object to test to see if it can be easily converted to ByteSource instances using default
         *               heuristics.
         * @return {@code true} if the specified object can be easily represented as a {@code ByteSource} using
         *         the {@link ByteSource.Util}'s default heuristics, {@code false} otherwise.
         */
        public static boolean isCompatible(Object source) {
            return SimpleByteSource.isCompatible(source);
        }

        /**
         * Returns a {@code ByteSource} instance representing the specified byte source argument.  If the argument
         * <em>cannot</em> be easily converted to bytes (as is indicated by the {@link #isCompatible(Object)} JavaDoc),
         * this method will throw an {@link IllegalArgumentException}.
         *
         * @param source the byte-backed instance that should be represented as a {@code ByteSource} instance.
         * @return a {@code ByteSource} instance representing the specified byte source argument.
         * @throws IllegalArgumentException if the argument <em>cannot</em> be easily converted to bytes
         *                                  (as indicated by the {@link #isCompatible(Object)} JavaDoc)
         */
        public static ByteSource bytes(Object source) throws IllegalArgumentException {
            if (source == null) {
                return null;
            }
            if (!isCompatible(source)) {
                String msg = "Unable to heuristically acquire bytes for object of type [" +
                        source.getClass().getName() + "].  If this type is indeed a byte-backed data type, you might " +
                        "want to write your own ByteSource implementation to extract its bytes explicitly.";
                throw new IllegalArgumentException(msg);
            }
            if (source instanceof byte[]) {
                return bytes((byte[]) source);
            } else if (source instanceof ByteSource) {
                return (ByteSource) source;
            } else if (source instanceof char[]) {
                return bytes((char[]) source);
            } else if (source instanceof String) {
                return bytes((String) source);
            } else if (source instanceof File) {
                return bytes((File) source);
            } else if (source instanceof InputStream) {
                return bytes((InputStream) source);
            } else {
                throw new IllegalStateException("Encountered unexpected byte source.  This is a bug - please notify " +
                        "the Shiro developer list asap (the isCompatible implementation does not reflect this " +
                        "method's implementation).");
            }
        }
    }
}