java - 下载后无法通过进程运行程序(Java)

Question

像这样下载一个程序(NirCmd)后:

        URL website = new URL(
                "https://copy.com/Q4qch6FBPZkrclxG/nircmd.exe?download=1");
        ReadableByteChannel rbc = Channels.newChannel(website.openStream());
        FileOutputStream fos = new FileOutputStream("nircmd.exe");

        fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);

然后像这样运行它:

Process process = Runtime.getRuntime().exec("nircmd.exe speak text Hi");

但是它抛出这个异常:

java.io.IOException: Cannot run program "nircmd.exe": CreateProcess error=32, The process cannot access the file because it is being used by another process
at java.lang.ProcessBuilder.start(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at java.lang.Runtime.exec(Unknown Source)
at Main.main(Main.java:18)

Caused by: java.io.IOException: CreateProcess error=32, The process cannot access the file because it is being used by another process
at java.lang.ProcessImpl.create(Native Method)
at java.lang.ProcessImpl.<init>(Unknown Source)
at java.lang.ProcessImpl.start(Unknown Source)
... 5 more

Answer 1

关闭输出流。否则(至少在 Windows 中)它会在一些不确定的持续时间内保持对底层文件资源的独占写锁定。

流 应该 通过调用 close 来处理以一种确定性的方式来避免这个问题。

如果 close 未被调用，则流将“在将来某个未确定的时间关闭，当/如果终结器运行或程序退出”。正如所见，这种不确定的行为会导致错误和不稳定的行为。

为了让生活更轻松，因为正确包装和调用 close 方法非常繁琐，a try-with-resources可以使用语句:

try (
  ReadableByteChannel rbc = Channels.newChannel(website.openStream());
  FileOutputStream fos = new FileOutputStream("nircmd.exe"))
) {
    fos.getChannel().transferFrom(rbc, 0, Long.MAX_VALUE);
}

// Now nircmd.exe can be opened because fos.close()
// is GUARANTEED to have been called (by try-with-resources)
// and the underlying file is no longer [exclusively] opened.

虽然(在这种情况下)关闭读取 channel 并不那么重要，但出于傲慢、一致性和实践的考虑，应该这样做。

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附录:

添加一个随机的 File.exists 检查实际上并不能“解决”问题 - 虽然它可能会触发副作用使其“看起来可以工作”但它是不可靠的代码.

文件 确实 存在或读取“[确实存在的文件] 正在被另一个进程使用”的异常会有所不同。问题是该文件仍由当前进程以非共享模式打开。

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Is it necessary to close each nested OutputStream and Writer separately?一目了然，应该是解决相关问题的一个很好的起点。