踩坑笔记 >> InputStream.read(byte[]) 造成死循环

写在前面

在 Java 中流的一系列操作，可能会感到既熟悉又陌生。熟悉是因为很基础且出镜率很高，陌生对大多数 程序员 平时工作中很少写相关的代码。

～～ 我是很少写～～

回归正题，本章我不不是探讨流，主要来说下造成‘标题’问题的原因。

问题很简单，稍微看下源码或者debug下就可以找到问题所在，这是一些细节问题，既然出现了在此做下记录，给自己一个警惕。

场景引入

今天微信上突然收到前同事一段这样的问题描述

在读取文件时如果文件为空、导致进入while死循环，

并附上一段代码。

/**
     * 将文件数据流写入到zip流中
     * 
     * @param fileName
     * @param inputStream
     * @param outputStream
     * @throws IOException
     */
    public static void zipInputStream(String fileName, InputStream inputStream, ZipOutputStream outputStream)
            throws IOException {
        try {
            BufferedInputStream bInStream = new BufferedInputStream(inputStream);
            outputStream.putNextEntry(new ZipEntry(fileName));
            byte[] buffer = new byte[inputStream.available()];
            int r = 0;
            while ((r = bInStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, r);
            }
            outputStream.closeEntry();
        } catch (IOException e) {
            throw e;
        } finally {
            if (null != inputStream) {
                try {
                    inputStream.close();
                } catch (IOException e) {
                    throw e;
                }
            }
        }
    }

咋一看这段代码也没啥问题！

我们一点点的来分析下

首先陷入while死循环的条件 bInStream.read(buffer)) != -1

while ((r = bInStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, r);
}

看下源码中的描述，

/**
     * Reads up to <code>len</code> bytes of data from this input stream
     * into an array of bytes. If <code>len</code> is not zero, the method
     * blocks until some input is available; otherwise, no
     * bytes are read and <code>0</code> is returned.
     * <p>
     * This method simply performs <code>in.read(b, off, len)</code>
     * and returns the result.
     *
     * @param      b     the buffer into which the data is read.
     * @param      off   the start offset in the destination array <code>b</code>
     * @param      len   the maximum number of bytes 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  NullPointerException If <code>b</code> is <code>null</code>.
     * @exception  IndexOutOfBoundsException If <code>off</code> is negative,
     * <code>len</code> is negative, or <code>len</code> is greater than
     * <code>b.length - off</code>
     * @exception  IOException  if an I/O error occurs.
     * @see        java.io.FilterInputStream#in
     */
    public int read(byte b[], int off, int len) throws IOException {
        return in.read(b, off, len);
    }

上面描述有这么一段，如果byte[]数组的len为0则不做任何操作直接返回0。

看到这问题基本就可以定位了，在看代码中byte[]的定义。

byte[] buffer = new byte[inputStream.available()];

看到这小伙们就笑了 inputStream.available() ，你读的是一个空文件可不是为0。

/**
     * Returns an estimate of the number of bytes that can be read (or
     * skipped over) from this input stream without blocking by the next
     * caller of a method for this input stream. The next caller might be
     * the same thread or another thread.  A single read or skip of this
     * many bytes will not block, but may read or skip fewer bytes.
     * <p>
     * This method returns the result of {@link #in in}.available().
     *
     * @return     an estimate of the number of bytes that can be read (or skipped
     *             over) from this input stream without blocking.
     * @exception  IOException  if an I/O error occurs.
     */
    public int available() throws IOException {
        return in.available();
    }

描述的很清楚，返回此输入流下一个方法调用可以不受阻塞地从此输入流读取（或跳过）的估计字节数。

说到这基本就反应过来，再说就有点啰嗦了。

总结

像这种问题其实和技术和能力没多大关系、主要是细心和经验。其实定义一个程序员的牛逼与否在于他踩过的坑是否足够多。

既然说到这那就顺便温故下InputStream的几个方法吧！

方法摘要

以次简单的问题来对InputStream 温故。

如有分析的错误的地方、欢迎指正