例子说明一切
先写单元测试吧：单元测试的代码如下：
package test.com.cs;

import com.cs.Base64Convert;
import junit.framework.TestCase;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.logging.Logger;

public class TestBase64Convert extends TestCase {
    Base64Convert baseCov = null;

    public TestBase64Convert(String s) {
        super(s);
    }

    protected void setUp() throws Exception {
        baseCov = new Base64Convert();
    }

    protected void tearDown() throws Exception {
        super.tearDown();
    }

    public void testIoToBase64() {
        try {
           String strBase64 = baseCov.ioToBase64(); //将 io 转换为 base64编码
            System.out.println(">>> "+strBase64);
             baseCov.base64ToIo(strBase64); //将 base64编码转换为 io 文件流，生成一幅新图片
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

------------------------------------
package com.cs;

import sun.misc.BASE64Decoder;
import sun.misc.BASE64Encoder;

import java.io.*;

public class Base64Convert {
    BASE64Decoder decoder = new BASE64Decoder();

    public String ioToBase64() throws IOException {
        String fileName = "d:/gril.gif"; //源文件
        String strBase64 = null;
        try {
            InputStream in = new FileInputStream(fileName);
            // in.available()返回文件的字节长度
            byte[] bytes = new byte[in.available()];
            // 将文件中的内容读入到数组中
            in.read(bytes);
            strBase64 = new BASE64Encoder().encode(bytes);      //将字节流数组转换为字符串
            in.close();
        } catch (FileNotFoundException fe) {
            fe.printStackTrace();
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
        return strBase64;
    }

    public void base64ToIo(String strBase64) throws IOException {
        String string = strBase64;
        String fileName = "d:/gril2.gif"; //生成的新文件
        try {
            // 解码，然后将字节转换为文件
            byte[] bytes = new BASE64Decoder().decodeBuffer(string);   //将字符串转换为byte数组
            ByteArrayInputStream in = new ByteArrayInputStream(bytes);
            byte[] buffer = new byte[1024];
            FileOutputStream out = new FileOutputStream(fileName);
            int bytesum = 0;
            int byteread = 0;
            while ((byteread = in.read(buffer)) != -1) {
                bytesum += byteread;
                out.write(buffer, 0, byteread); //文件写操作
            }
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }
    }
}
执行截图：gril2.gif就是根据base64编码转换过来的，看到谷歌的这个美女总裁，就说明你的程序成功了