Android 解码 Base64 图片

开发中需要做图形验证码校验的功能，验证码由后端生成，然后传输到前端显示。

为了传输方便，我们不能直接使用 JPG 或者 PNG 格式，而是采用 Base64 格式。

什么是 Base64？

Base64 是网络上最常见的用于传输 8Bit 字节码的编码方式之一，Base64 就是一种基于 64 个可打印字符来表示二进制数据的方法。

Base64 编码要求把 3 个 8 位字节（3*8=24）转化为 4 个 6 位的字节（4*6=24），之后在 6 位的前面补两个 0，形成 8 位一个字节的形式。如果剩下的字符不足 3 个字节，则用 0 填充，输出字符使用 =，因此编码后输出的文本末尾可能会出现 1 或 2 个 =。

为了保证所输出的编码为可读字符，Base64 制定了一个编码表，以便进行统一转换。编码表的大小为 2⁶=64，这也是 Base64 名称的由来。

这样也就了解 Base64 的编码方式了。

Android 端拿到 Base64 后，是不能够像 JPG 或者 PNG 图片一样直接显示的，还需要对其进行转码操作。

在网上搜了一下，发现 Android SDK 就为我们提供了相应的 API：

拿来即用，发现怎么也解不了，把从服务器获取到的 Base64 放到测试网站上查看了一下，发现是可以加载的，那就肯定是上面这种加载方法有问题了。

我的用法是，从服务器获取到 Base64 的字符串，Base64 的字符串格式如下：

iVBORw0KGgoAAAANSUhEUgAAAFAAAAAeCAMAAACMnWmDAAAAGFBMVEUAAABQUFAAAAAAAAAAAAAAAAAAAAAAAABiRp8mAAAACHRSTlMA/wAAAAAAACXRGJEAAAmJSURBVHjaAX4JgfYAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAQEBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAEBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAQEBAQEBAQEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAQEBAQEBAQEBAQAAAAAAAAAAAAAAAAAAAAEBAQEBAQAAAAAAAAAAAAAAAAABAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAQEBAQEBAQEBAQAAAAAAAAAAAAAAAAAAAAEBAQEBAQAAAAAAAAAAAAAAAAEBAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQEBAQAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQEBAQAAAAAAAAAAAAAAAQABAQAAAAAAAAAAAAAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAQEBAQABAQEAAAAAAAAAAAAAAAAAAAAAAQEBAQEBAQAAAAAAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAQEBAQEBAQEBAAAAAAAAAAAAAAAAAAABAQEAAQEBAQAAAAAAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAQEBAQEBAQEBAQAAAAAAAAAAAAAAAAABAQEAAQEBAQAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAAAAAQEBAQAAAQEBAQAAAAAAAAAAAAAAAAABAQEAAQEBAQAAAAAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAAAAAQEBAQAAAQEBAQAAAAAAAAAAAAAAAAEBAQAAAQEBAQAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAEBAQAAAQEBAQAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAEBAQAAAQEBAQAAAAAAAAAAAAABAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAQEBAQAAAAAAAAAAAAAAAQEBAAAAAQEBAQAAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAQEBAQAAAAAAAAAAAAAAAQEBAQEBAQEBAQEAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAQEBAQAAAAAAAAAAAAAAAQEBAQEBAQEBAQEAAAAAAAAAAAEBAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAQEBAQAAAAAAAAAAAAAAAQEBAQEBAQEBAQEAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQEBAQEBAQAAAAAAAAAAAAAAAQEBAQEBAQEBAQEAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAEBAQEBAQEBAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAQEAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAQEBAQEAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAQEBAQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAB5zgFrGnILCQAAAABJRU5ErkJggg== 

由于其并没有上面示例的 Base64 的解析头 data:image/png;base64,…，所以我便自作主张给它添加上去了，而且在使用一些 Base64 解码工具时，也要求有该部分内容才能成功解码，所以我一直认为这样做并没有错。

后来真的想不到原因就抱着试一下的心态把这部分内容去掉了，发现居然可以用，被网上的那些人给坑惨了。

所以正确的使用方式应该如下：

public static void setImageViewWithBase64(ImageView imageView, String base64) {
    if (TextUtils.isEmpty(base64)) {
        throw new NullPointerException("The string of Base64 is empty");
    }
    if (base64.startsWith("data:image/png;base64,") || base64.startsWith("data:image/*;base64,") || base64.startsWith("data:image/jpg;base64,")) { 
        base64 = base64.split(",")[1];
    }
    byte[] decodedString = Base64.decode(base64, Base64.DEFAULT);
    Bitmap decodedByte = BitmapFactory.decodeByteArray(decodedString, 0, decodedString.length);
    imageView.setImageBitmap(decodedByte);
}

为了能够正常显示图片，这里先对 Base64 的字符串进行判空，如果为空则抛出 NullPointerException 异常，接着再看这个字符串是否以 Base64 的解析头开头，如果是，就把这部分去掉，最后再把 Base64 解码转为 Bitmap，并显示到 ImageView 中去。

即使我们对 Base64 字符串进行了判空和去掉解析头的操作，这个字符串仍然有可能是错误的，即它有可能不是一个符合 Base64 规范的字符串，别担心，这种情况下 Base64 的库会帮我们抛出 IllegalArgumentException 的异常。