Java URL类踩坑指南（一）
2017-11-28 本文已影响0人
java部落
背景介绍
最近再做一个RSS阅读工具给自己用，其中一个环节是从服务器端获取一个包含了RSS源列表的json文件，再根据这个json文件下载、解析RSS内容。核心代码如下：
class PresenterImpl(val context: Context, val activity: MainActivity) : IPresenter {
private val URL_API = "https://vimerzhao.github.io/others/rssreader/RSS.json"
override fun getRssResource(): RssSource {
val gson = GsonBuilder().create()
return gson.fromJson(getFromNet(URL_API), RssSource::class.java)
}
private fun getFromNet(url: String): String {
val result = URL(url).readText()
return result
}
......
}
之前一直执行地很好，直到前两天我购买了一个vimerzhao.top的域名，并将原来的域名vimerzhao.github.io重定向到了vimerzhao.top。这个工具就无法使用了，但在浏览器输入URL_API却能得到数据：
image
那为什么URL.readText()没有拿到数据呢？
不支持重定向
可以通过下面代码测试：
import java.net.*;
import java.io.*;
public class TestRedirect {
public static void main(String args[]) {
try {
URL url1 = new URL("https://vimerzhao.github.io/others/rssreader/RSS.json");
URL url2 = new URL("http://vimerzhao.top/others/rssreader/RSS.json");
read(url1);
System.out.println("=--------------------------------=");
read(url2);
} catch (Exception e) {
e.printStackTrace();
}
}
public static void read(URL url) {
try {
BufferedReader in = new BufferedReader(
new InputStreamReader(url.openStream()));
String inputLine;
while ((inputLine = in.readLine()) != null) {
System.out.println(inputLine);
}
in.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
得到结果如下：
<html>
<head><title>301 Moved Permanently</title></head>
<body bgcolor="white">
<center><h1>301 Moved Permanently</h1></center>
<hr><center>nginx</center>
</body>
</html>
=--------------------------------=
{"theme":"tech","author":"zhaoyu","email":"dutzhaoyu@gmail.com","version":"0.01","contents":[{"category":"综合版块","websites":[{"tag":"门户网站","url":["http://geek.csdn.net/admin/news_service/rss","http://blog.jobbole.com/feed/","http://feed.cnblogs.com/blog/sitehome/rss","https://segmentfault.com/feeds","http://www.codeceo.com/article/category/pick/feed"]},{"tag":"知名社区","url":["https://stackoverflow.com/feeds","https://www.v2ex.com/index.xml"]},{"tag":"官方博客","url":["https://www.blog.google/rss/","https://blog.jetbrains.com/feed/"]},{"tag":"个人博客-行业","url":["http://feed.williamlong.info/","https://www.liaoxuefeng.com/feed/articles"]},{"tag":"个人博客-学术","url":["http://www.norvig.com/rss-feed.xml"]}]},{"category":"编程语言","websites":[{"tag":"Kotlin","url":["https://kotliner.cn/api/rss/latest"]},{"tag":"Python","url":["https://www.python.org/dev/peps/peps.rss/"]},{"tag":"Java","url":["http://www.codeceo.com/article/category/develop/java/feed"]}]},{"category":"行业动态","websites":[{"tag":"Android","url":["http://www.codeceo.com/article/category/develop/android/feed"]}]},{"category":"乱七八遭","websites":[{"tag":"Linux-综合","url":["https://linux.cn/rss.xml","http://www.linuxidc.com/rssFeed.aspx","http://www.codeceo.com/article/tag/linux/feed"]},{"tag":"Linux-发行版","url":["https://blog.linuxmint.com/?feed=rss2","https://manjaro.github.io/feed.xml"]}]}]}
HTTP返回码301，即发生了重定向。可在浏览器上这个过程太快以至于我们看不到这个301界面的出现。这里需要说明的是URL.readText()是Kotlin中一个扩展函数，本质还是调用了URL类的openStream方法，部分源码如下：
.....
/**
* Reads the entire content of this URL as a String using UTF-8 or the specified [charset].
*
* This method is not recommended on huge files.
*
* @param charset a character set to use.
* @return a string with this URL entire content.
*/
@kotlin.internal.InlineOnly
public inline fun URL.readText(charset: Charset = Charsets.UTF_8): String = readBytes().toString(charset)
/**
* Reads the entire content of the URL as byte array.
*
* This method is not recommended on huge files.
*
* @return a byte array with this URL entire content.
*/
public fun URL.readBytes(): ByteArray = openStream().use { it.readBytes() }
所以上面的测试代码即说明了URL.readText()失败的原因。
不过URL不支持重定向是否合理？为什么不支持？还有待探究。
