【Java】文件流 BUG

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【Java】文件流 BUG

一、问题

1.1、环境
电脑环境：Windows 10；
开发工具：IntelliJ IDEA；
数据库环境：Redis 3.2.100
JDK环境: Jdk1.8；

1.2、问题
Check the return value of the “read” call to see how many bytes were read.

二、解答

1、源代码

try {String fileName = deFileName(templateName, request);URL fileUrl = new URL(tempFileUrl);HttpURLConnection httpUrl = (HttpURLConnection) fileUrl.openConnection();t();// 以流的形式下载文件。InputStream fis = new InputStream());byte[] buffer = new byte[fis.available()];ad(buffer);fis.close();
}catch (Exception e){(e.getMessage(), e);
}

这段代码的问题在哪里？
我们先来看看 read(byte[] by)这个方法的定义：

/*** Reads some number of bytes from the input stream and stores them into* the buffer array <code>b</code>. The number of bytes actually read is* returned as an integer.  This method blocks until input data is* available, end of file is detected, or an exception is thrown.** <p> If the length of <code>b</code> is zero, then no bytes are read and* <code>0</code> is returned; otherwise, there is an attempt to read at* least one byte. If no byte is available because the stream is at the* end of the file, the value <code>-1</code> is returned; otherwise, at* least one byte is read and stored into <code>b</code>.** <p> The first byte read is stored into element <code>b[0]</code>, the* next one into <code>b[1]</code>, and so on. The number of bytes read is,* at most, equal to the length of <code>b</code>. Let <i>k</i> be the* number of bytes actually read; these bytes will be stored in elements* <code>b[0]</code> through <code>b[</code><i>k</i><code>-1]</code>,* leaving elements <code>b[</code><i>k</i><code>]</code> through* <code>b[b.length-1]</code> unaffected.** <p> The <code>read(b)</code> method for class <code>InputStream</code>* has the same effect as: <pre><code> read(b, 0, b.length) </code></pre>** @param      b   the buffer into which the data is read.* @return     the total number of bytes read into the buffer, or*             <code>-1</code> if there is no more data because the end of*             the stream has been reached.* @exception  IOException  If the first byte cannot be read for any reason* other than the end of the file, if the input stream has been closed, or* if some other I/O error occurs.* @exception  NullPointerException  if <code>b</code> is <code>null</code>.* @see        java.io.InputStream#read(byte[], int, int)*/public int read(byte b[]) throws IOException {return read(b, 0, b.length);}

这里大致翻译一下：

1、从输入流中读取一定数量的字节，并且保存在缓存。
2、读到的返回值，是整数。
3、直到读取可读的文件到达末尾，或者异常才会终止。
4、如果数组长度是 0，则读取不到字节，返回值是 0；否则，至少读取 1 个字节。
5、流被读取到末尾时，则没有任何有效字节，返回 -1；否则，则会被正常读取，并存储到 b 数组中。
6、真正读取到的字节的长度，跟数组的长度是一致的。

所以，我们这里要判断，文件读取完毕了，再关掉输入流。修改后的代码如下：

try {URL fileUrl = new URL("tempFileName");HttpURLConnection httpUrl = (HttpURLConnection) fileUrl.openConnection();t();// 以流的形式下载文件。InputStream fis = new InputStream());byte[] buffer = new byte[fis.available()];while (true) {int read = ad(buffer);//判断是不是读到了数据流的末尾 ，防止出现死循环。if (read == -1) {break;}}fis.close();
}catch (Exception e){(e.getMessage(), e);
}

完毕~