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作者：wuxinliulei
链接：https://www.zhihu.com/question/20304015/answer/71645651
来源：知乎
著作权归作者所有。商业转载请联系作者获得授权，非商业转载请注明出处。

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.ByteBuffer;

/**
 *检验一个字符串是否是经过Base64处理过的
 * 
 */
public class TestBase64
{
    public static final char[] BASE64_CODE = new char[] { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
            'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4',
            '5', '6', '7', '8', '9', '+', '/', '=' };

    public static final int MAX_BUFF_SIZE = 4000000;

    public static boolean doCheck() throws IOException
    {
        final File file = new File("src.txt");
        final BufferedReader br = new BufferedReader(new InputStreamReader(new FileInputStream(file)));
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        String line = null;
        String lastLine = null;
        while ((line = br.readLine()) != null)
        {
            lastLine = line;
            final byte[] bytes = line.getBytes();
            tryAllocate(buffer, bytes.length);
            buffer.put(bytes);
        }

        // 检查最后两个字节
        final byte[] lastLineBytes = lastLine.getBytes();
        // 等号个数
        int equalsNum = 0;
        for (int i = lastLineBytes.length - 1; i >= lastLineBytes.length - 2; i--)
        {
            if (lastLineBytes[i] == '=')
            {
                equalsNum++;
            }
        }

        final byte[] src = buffer.toString().getBytes();
        for (int i = 0; i < src.length - equalsNum; i++)
        {
            final char c = (char) src[i];
            if (c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c >= '0' && c <= '9' || c == '+' || c == '/')
            {
                continue;
            }
            return false;
        }

        if ((src.length - equalsNum) % 4 != 0)
        {
            return false;
        }

        return true;
    }

    public static ByteBuffer tryAllocate(ByteBuffer buffer, int length)
    {
        if (length > buffer.remaining())
        {
            buffer.flip();
            return ByteBuffer.allocate(roundup(buffer.limit() + length)).put(buffer);
        }
        return buffer;
    }

    public static int roundup(int length)
    {
        if (length > MAX_BUFF_SIZE)
        {
            throw new IllegalArgumentException("length too large!");
        }

        int capacity = 16;
        while (length < capacity)
        {
            capacity = capacity << 1;
        }
        return capacity;
    }

}

4、

一篇文章彻底弄懂Base64编码原理的超链接

网址为
https://blog.csdn.net/wo541075754/article/details/81734770
这篇博客详细的介绍了Base64编码原理